INTRODUCTION.

General Principles.

WHEN a glass tube of considerable size, perfectly clean and dry, is rubbed briskly with a dry hand, and immediately held over small pieces of paper, straws, feathers, or other light bodies, it will attract them, and after retaining them in contact with it for some time, repel them; and this attraction and repulsion will be alternately repeated several times.

If after rubbing the tube, the knuckle be presented to the closed end, a snapping noise will be heard, and the finger will receive a slight shock. When this experiment is made in the dark, a luminous spark will appear at the moment the snap is heard, between the finger and the tube.

Many other substances possess the property of attracting light bodies and emitting sparks, when rubbed with certain other substances, as amber, sealing-wax, rosin, &c. As amber was first observed to possess

(A) The attracting power of amber when rubbed, is said to have been known to Thales the Milesian philosopher, 600 years before Christ. General principles from this a portion of the electric power, and are thus made capable of producing the same appearance as the electric. Thus if a metallic rod or wire pointed at one end and rounded at the other, be attached by the pointed extremity to an excited electric, or even placed very near this, the rounded extremity will attract light bodies, and emit sparks. As these substances are found to convey or conduct the electric power to any distance in proportion to their length, they are called conductors.

It is found that all bodies in nature are either electrics or conductors. Neither of these classes of bodies are, however, perfect electrics or conductors, as there are few electrics which may not under some circumstances be made to act as conductors, and on the other hand, many conductors may be so far excited as to become in some measure electrics per se (b). The following table exhibits the electrics and conductors arranged according to their degree of electrical or conducting power.

**Electrics.**

- Glass and all vitrifications, even the metallic vitrifications. - All precious stones, of which the most transparent are the best. - Amber. - Sulphur. - All resinous substances. - Wax. - Silk. - Cotton. - Several dry and external animal substances, as feathers, wool, hair, &c. - Paper. - White sugar, and sugar-candy. - Air, and other permanently elastic fluids. - Oils. - Dry and complete oxides of metallic substances. - The ashes of animal and vegetable substances. - Dry vegetable substances. - Most hard stones, of which the hardest are the best.

**Conductors.**

- Gold. - Silver. - Copper. - Platina. - Brasses. - Iron. - Tin. - Quicksilver. - Lead. - Semi-metals, more or less. - Metallic ores, more or less. - Charcoal, either of animal or of vegetable substances. - The fluids of an animal body. - Water (especially salt water), and all fluids, excepting the aerial, and oils. - The effluvia of flaming bodies. - Congealed water, viz. ice or snow. - Most saline substances, of which the metallic salts are the best. - Several earthy or stony substances. - Smoke. - The vapour of hot water. - Electricity pervades also such a vacuum, or absence of air, as is caused by the best air-pump; but not the perfect absence of air, or the torricellian vacuum, formed by boiling the quicksilver in a barometer tube.

Many of the substances given in the above table are found to change their nature under certain circumstances. Thus, among the electrics, glass heated to redness, melted resins, baked wood when very hot, and heated air, are tolerably good conductors; and glass, which is usually the best electric, is sometimes from causes which have not been well ascertained a very bad electric. The excitability of glass vessels is found to differ according to the degree of rarefaction of the included air; when this is rarefied as much as possible, the external surface of the vessel cannot be excited, while the internal surface exhibits strong marks of electric power; but when the included air is considerably condensed, the internal surface shows no marks of electric power, while the external is much more excitable than usual.

Among the conductors, the conducting power of charcoal varies in proportion to the degree of heat to which it has been exposed in the making, as, when imperfectly burned, it is a bad conductor. Indeed it is worth remarking here, that wood is capable of being made an electric or a conductor several times alternately according to its state. When fresh cut, it is a good conductor; thoroughly dried by baking, it becomes, as we have seen an electric; burned to charcoal, it is again a conductor; but when reduced to ashes, it is once more made an electric.

Ice (c) is placed among the conductors: but in an experiment of M. Acharde, it appeared that when distilled water was gradually frozen, so that one side of the vessel retained its fluid, and therefore permitted the air to escape, the ice thus produced would not conduct, but on the contrary became a very good electric, and was employed as such. Snow is a much worse conductor than ice. Water is a conductor, and so are the secondary salts; it is found that when water is impregnated with a salt, its conducting power is much increased.

We have, after Mr Cavallo, placed the salts among conductors; but in strict propriety, this conducting power must be confined to salts in the state of crystals, as it has been proved by decisive experiments, that salts,

(b) The difference between electrics and conductors was first observed by Mr Stephen Grey in 1759, but the terms conductor and electric per se, were first employed in this present sense by Dr Defaguliers.

(c) The conducting power of common ice was first shown by M. Jallabert, professor of philosophy at Geneva; there seem, however, to have been various opinions respecting this fact till it was fully ascertained by Dr Priestley. Vid. Priestley's History of Electricity, Part viii. sect. 4. General salts, when deprived of their water of crystallization, become non-conductors. The conducting power of crystallized salts is therefore probably owing to the water which they contain.

Electricities are called non-conductors, as they do not readily transmit electric power; they may hence be employed to check the passage of this power, or to confine its influence. When a body communicates with a conducting substance, as the earth, a table, the human body, &c., the electric power easily passes off; but when it is supported by an electric, the power may be retained for a considerable time. In this latter case the body is said to be inflamed.

We have seen (1, 3), that fire or light appears to issue from an excited electric; and, this appearance is stronger in proportion to the size of the electric, and the degree of friction which it has undergone. When a rounded body, as the knuckle or a metallic ball, is presented to the excited electric, the fire appears to dart from it in a spark; but if the presented body be pointed, the fire will appear to issue in a stream composed of luminous rays. These rays will take a different direction, according to the substance with which the electric is rubbed, and other circumstances which will be explained hereafter. In the case of the glass tube rubbed with the hand, when a pointed body, as a needle, or wire, is presented to the tube, the luminous rays will appear like a star around the point. The same appearance will take place on presenting a point to a stick of sealing-wax rubbed with any metallic body, as a piece of tin foil; but when the sealing-wax is rubbed with a piece of woollen cloth, the rays will appear to issue from the point in a pencil diverging towards the wax. In some experiments which will afterwards be described, the stream of fire appears in an evident current in a direction from the electric in some cases, as in the tube excited as above, and towards the electric in others, as in the wax rubbed with the woollen cloth.

Positive and negative electricity, when glass is rubbed with most substances, positive electricity is excited, and negative when resinous bodies are rubbed with most substances, the former is often called vitreous, and the latter resinous electricity.

The difference of these two states of the electric power may be further illustrated by the following simple experiment.

Let a piece of glass (A, B, fig. 1.) be fixed in a wooden pedestal C. Through the upper extremity A, pass a wire A, D, with a rounded end at D, and from this end suspend two very fine silk threads a, b. These threads in the usual state of the instrument will hang in the parallel position a, b, but if the end of the wire to which they are attached be presented to the excited tube, the threads will diverge from each other, and take a position as at c, d. If in this diverging state they are presented to an excited stick of sealing-wax, they will collapse into their original position. Again, the threads presented first to the excited sealing-wax will diverge, but presented in this state to the excited tube will collapse, thus showing that these two states are opposite to each other, each destroying the effect produced by the other.

The following table shows what kind of electricity will be excited by rubbing various electrics with different bodies.

| Substance | Positive | Negative | |--------------------|----------|----------| | Every substance | | | | with which | | | | it has been | | | | hitherto tried | | | | Smooth glass | Positive | | | Rough glass | Positive | | | Tourmalin | Positive | | | Hare's skin | Positive | | | White silk | Positive | | | Black silk | Positive | | | Sealing-wax | Positive | | | Baked wood | Positive | |

It appears from (3) that the power of producing electrified electrical appearances may be communicated from any bodies excited electric to a conductor. The more perfect the conductor, the more easily does it receive the electric power. Electrics may also be made to receive this power from excited electrics, but it is communicated to these with more difficulty than to conductors. When any body, whether electric or conductor, is made to exhibit electrical phenomena, either by being excited, or by communication, it is said to be electrified. PART I.

OF THE GENERAL PHENOMENA OF EXCITED ELECTRICITY.

WHEN an electric is once excited, it will retain the electric power for a longer or shorter time according to its situation and nature. If it communicates freely with conductors, it will lose it sooner in proportion as these are more perfect; but if it be inflected, it will continue in an electrified state for a considerable time.

Electrics may be excited in various modes; the greatest number of them by friction, as glass, precious stones, silk, sulphur, sealing-wax, amber, &c.; some by melting, and being allowed to cool, as sulphur, wax; or simply by heating and cooling, as the tourmaline. We shall here give an account of the general appearances exhibited by the principal electrics when excited in these several modes.

Friction, as we have observed, is the most usual method of exciting electrics. These may be rubbed either by other electrics, or by conductors, but in some cases they are best excited by being rubbed with the most perfect conductors. Thus glass rubbed with silk, exhibits signs of electricity, but these are much stronger if the silk be covered with some metallic substance, as an amalgam of zinc. Dust or moisture is found very much to diminish the excitability of electrics; but oil, or any fat substance increases it. The appearances shown by electrics excited by friction, differ somewhat according to the nature of the electric, and the substance employed as a rubber; we shall describe the most remarkable of them, as they will serve hereafter to illustrate and explain the experiments which are to be introduced in the following parts of this article.

CHAP. I. Of the Phenomena produced by excited glass.

Dr William Gilbert, a native of Colchester in Essex, and a physician in London, who published in the year 1600 a valuable treatise "De Magnete," was the first, we believe, who observed the electrical property of glass when rubbed; but he discovered little more than that like amber it attracted and repelled light bodies. He found that the most transparent glass was the best electric. In the beginning of the eighteenth century, Mr Hawkesbee, to whom electricity is indebted for many improvements, made the first rational experiments on the electric power of glass. He contrived to fix a hollow globe of glass in a wooden frame, so that it could be whirled round while he rubbed it by applying his dry hand to the surface. He observed that when the air within the globe was considerably rarefied, a strong light appeared in the inside on applying his hand to the globe, and when the air was restored to its natural density, a light appeared also on the outside, appearing as if sticking to his fingers or other bodies held near the globe.

Having exhausted another globe of glass, he observed, that on bringing this near his excited globe, a light appeared within the former, and became very brilliant if the exhausted globe was kept in motion, but died away in a short time if it was suffered to remain at rest.

He coated more than half of the inside of a globe with sealing-wax of various thicknesses, and after exhausting the globe, he set it in motion. On applying his hand as a rubber, he was surprised to see the exact shape of his hand appearing on the concave surface of the wax, and that even where the coating of wax was interposed between his hand and the opposite side, though the wax was in some places an eighth of an inch in thickness.

Pitch or common sulphur melted answered as well as wax, but he could not produce these appearances by using melted flowers of sulphur. When he employed a very thick coating of common sulphur, he observed that there was a much greater light within the globe; but he could not so easily distinguish the figure of his hands.

On admitting a small quantity of air into the globe, the light diminished, and on the coating of sealing wax it entirely disappeared. While the globe continued exhausted, the coated part of it showed some attraction for light bodies, but if there was no wax, the globe would not attract at all; on admitting the air, the power of mechanical attraction was greater on the coated than on the uncoated part.

Glass in any form is capable of excitation, but it is more easy, as well as more convenient, to employ a vessel or plate of glass than a solid rod or mass of that substance; and the thinner the vessel or plate is, the more easily it is excited. When a tube, plate, or vessel of glass is excited, it is found that one side is electrified positively and the other negatively. Both smooth and rough glass may be employed to produce electrical phenomena, but they require different rubbers. The best rubber for smooth glass is black oiled silk spread with an amalgam of zinc, made in the proportion of four or five parts of mercury to one of zinc. The best rubber for rough glass is soft new flannel. The amalgam of zinc may be most conveniently made in the following manner. Place the zinc over the fire in an iron ladle; and when the ladle is red hot, put a small quantity of tallow or suet on the zinc, which will immediately melt. It is best not to allow the zinc to melt without the addition of some fatty matter, as this metal is very easily oxidized or calcined, and thus a great part of it would be rendered unfit for the required purpose; this inconvenience is prevented by the fat which covers the surface of the melted metal, and protects it from the action of the air. When the zinc is melted, add the mercury, previously heated to the degree of boiling water; stir the mixture a little, and allow it to cool. Lastly, rub it well in a glass mortar, so as to unite the fat with it, which will prevent it from becoming hard by keeping, and will also preserve it longer from oxidation.

Mr Canton, who was the first person that employed an amalgam to increase the effect of friction on glass tubes, Chap. I.

General Phenomena.

It had been observed by Mr Hawkesbee, that on shaking mercury in a glass vessel, in the dark, a considerable light was produced, and that this was much more remarkable when the air in the vessel was considerably rarefied. He called the light which he conceived to be emitted from the mercury, mercurial phosphorus.

Mr Cavallo found that, by shaking mercury in a glass tube hermetically sealed, and in which the air was pretty much rarefied, the tube was sensibly electrified on the outside; but the electricity produced was not constant, nor in proportion to the agitation. From this observation he was led to make some experiments, the results of which are very curious.

He prepared several tubes such as are represented at fig. 2, Plate CLXXXVII., about 3 inches long, and with glass tubes.

They were closed at one end, and contained each three fourths of an ounce of mercury, which being made to boil, the air within the tube was rarefied and the open end was then hermetically sealed. Having made the tube clean and a little warm, he caused the mercury to flow from the one end to the other, by gently elevating and depressing either end, alternately, while the tube was held nearly in a horizontal position. The tube was thus rendered electrical, so that the end where the mercury stood was electrified positively (p) and all the remaining part of the tube negatively. If the mercury was made to flow from the positive end to the negative, by elevating the former, the end to which it flowed became positive, while the rest of the tube acquired a negative electricity; but if in elevating the positive end where the mercury stood, that end were not touched with the hand, it became negative only in a slight degree, and if the mercury was made to flow back to it, and again retire from it, still without touching it, it became positive; whereas by touching it while elevating it, it was rendered strongly negative. The electric power was always strongest at the positive end. The electric power at either end was made much more apparent by coating each end for about two inches with tin foil, as represented in the figure, so that the tubes would sometimes emit sparks on being brought near a conductor.

We have seen (6) that when an electric is once excited, it retains the electric power for some time. Glass is one of the most remarkable electrics in this respect.

Mr Canton procured some very thin glass balls, about an inch and a half in diameter, with slender tubular stems of eight or nine inches in length. He electrified these balls in the inside, or semi-positive, and then sealed the stems hermetically. On examining them after some time, he found that they showed no signs of electricity; but on holding them at a small distance from the fire, they became strongly electrical, and still more so as they cooled. On repeatedly heating them, he found that the electric power diminished, but it was not impaired by keeping them for a week under water. One of them which he had heated several times before immersing it in water, and again several times after lying for a week in water, still retained a considerable degree of electric power at the end of above a month; and even at the end of six years they had not entirely lost it.

Mr Henley having electrified a small bottle, observed that it showed signs of electricity seventy days after, though it had stood all that time in a cupboard.

On the 5th February he excited a glass cylinder; and from that time till the 10th of March following, various methods were employed to destroy its electricity. These always succeeded at the time, and the cylinder lost all signs of electricity; but these signs returned again without any fresh excitation, and on the 10th of March the cylinder still retained considerable electric power. The marks of electricity sometimes became stronger or weaker, or even quite disappeared and returned without any evident cause. The electricity was generally strongest when the wind was northerly, or when it had returned after having been destroyed by flame; it was generally weakest when there was a fire in the room where it was kept, or when the door was left open. He repeated the excitation, but not always with the same success; for some times the cylinder would lose all signs of electricity in a fortnight, and at others in twelve hours, till it was again excited.

*Phil. Trans. lxvii.

Chap. II. Of the Phenomena produced by excited Silk.

Silk was first discovered to be an electric in the year 1729 by Mr Stephen Grey, while making experiments with his friend Mr Wheeler. These gentlemen attempted to conduct the electric power to a great distance by means of silk lines, as Mr Grey had done before by means of packthread; but they were disappointed, as they found that the silk refused to conduct.

(d) The method of distinguishing between positive and negative electricity will be more fully explained hereafter, as well as the modes in which either may be produced at pleasure. But it may be proper here to show a simple mode of distinguishing these two states of the electric power, which may be done by means of the instrument described in (8). The electricity shown by excited polished glass was said to be positive; and it appeared that the threads of the instrument separated when brought near an excited tube, as also when brought near excited sealing wax, the electricity of which is negative. If, therefore, when the threads are made to diverge by excited glass, they diverge still farther, or remain stationary on being made to approach any other electrified body, the electricity of this last is positive; but if they collapse, it is negative. Again if the threads, when made to diverge by excited sealing wax, diverge still farther, or remain stationary on being made to approach another electrified body, the electricity of this is negative; but if they collapse, it is positive. General but seemed rather to retain the electric power; no experiments of any consequence were however made on this subject, till 1759, when Mr Symmer presented to the Royal Society a series of observations which he had made on silk stockings.

He had been accustomed to wear two pairs of silk stockings; a black and a white. When these were put off both together, no signs of electricity appeared; but on pulling off the black ones from the white, he heard a snapping or cracking noise, and in the dark perceived sparks of fire between them. To produce this and the following appearances in great perfection, it was only necessary to draw his hand several times backward and forward over his leg with the stocking upon it.

When the stockings were separated and held at a distance from each other, both of them appeared to be highly excited; the white stocking positively, and the black negatively. While they were kept at a distance from each other, both of them appeared inflated to such a degree, that they exhibited the entire shape of the leg. When two black or two white stockings were held in one hand, they would repel one another with considerable force, making an angle seemingly of 30 or 35 degrees. When a white and black stocking were presented to each other, they were mutually attracted; and if permitted, would rush together with surprising violence. As they approached, the inflation gradually subsided, and their attraction of foreign objects diminished, but their attraction of one another increased; when they actually met, they became flat, and joined close together like as many folds of silk. When separated again, their electric virtue did not seem to be in the least impaired for having once met; and the same appearances would be exhibited by them for a considerable time. When the experiment was made with two black stockings in one hand, and two white ones in the other, they were thrown into a strange agitation, owing to the attraction between those of different colours, and the repulsion between those of the same colour. This mixture of attractions and repulsions made the stockings catch at each other at greater distances than otherwise they would have done, and afforded a very curious spectacle.

When the stockings were suffered to meet; they stuck together with considerable force. At first Mr Symmer found they required from one to 12 ounces to separate them. Another time they raised 17 ounces, which was 20 times the weight of the stocking that supported them; and this in a direction parallel to its surface. When one of the stockings was turned inside out, and put within the other, it required 20 ounces to separate them; though at that time ten ounces were sufficient when applied externally. Getting the black stockings new dyed, and the white ones washed, and whitened in the fumes of sulphur, and then putting them one within the other, with the rough sides together, it required three pounds three ounces to separate them. With stockings of a more substantial make, the cohesion was still greater. When the white stocking was put within the black one, so that the outside of the white was contiguous to the inside of the black, they raised nine pounds wanting a few ounces; and when the two rough surfaces were contiguous, they raised 15 pounds one pennyweight and a half. Cutting off the ends of the thread and the tufts of silk which had been left in the inside of the stockings, was found to be very unfavourable to these experiments.

Mr Symmer also observed, that pieces of white and black silk, when highly electrified, not only cohered with each other, but would also adhere to bodies with broad and even polished surfaces, though these bodies were not electrified. This he discovered accidentally; having, without design, thrown a stocking out of his hand, which stuck to the paper-hangings of the room. He repeated the experiment, and found it would continue hanging near an hour. Having stuck up the black and white stockings in this manner, he came with another pair highly electrified; and applying the white to the black, and the black to the white, he carried them off from the wall, each of them hanging to that which had been brought to it. The same experiments held with the painted boards of the room, and likewise with the looking-glass, to the smooth surface of which both the white and the black silk appeared to adhere more tenaciously than to either of the former.

Similar experiments, but with a greater variety of circumstances, were afterwards made by Mr Cigna on Turin, upon white and black ribbons. He took two white silk ribbons just dried at the fire, and extended them upon a smooth plain, whether a conducting or electric substance was a matter of indifference. He then drew over them the sharp edge of an ivory ruler, and found that both ribbons had acquired electricity enough to adhere to the plain; though while they continued there, they showed no other sign of it. When taken up separately, they were both negatively electrified, and would repel each other. In their separation, electric sparks were perceived between them; but when again put on the plain, or forced together, no light was perceived without another friction. When by the operation just now mentioned they had acquired the negative electricity, if they were placed, not upon the smooth body on which they had been rubbed, but on a rough conducting substance, they would, on their separation, show contrary electricities, which would again disappear on their being joined together. If they had been made to repel each other, and were afterwards forced together, and placed on the rough surface above mentioned, they would in a few minutes be mutually attracted; the lowermost being positively and the uppermost negatively electrified.

If the two white ribbons received their friction upon the rough surface, they always acquired contrary electricities. The upper one was negatively, and the lower one positively electrified, in whatever manner they were taken off. The same change was instantaneously produced by any pointed conductor. If two ribbons, for instance, were made to repel, and the point of a needle drawn opposite to one of them along its whole length, they would immediately rush together.

The same means which produced a change of electricity in a ribbon already electrified, would communicate electricity to one which had not as yet received it; viz. laying the unelectrified ribbon upon a rough surface, and putting the other upon it; or by holding it parallel to an electrified ribbon, and presenting a pointed General pointed conductor to it. He placed a ribbon that was not quite dry under another that was well dried at the fire, upon a smooth plain; and when he had given them the usual friction with his ruler, he found, that in what manner forever they were removed from the plain, the upper one was negatively and the lower one positively electrified.—If both ribbons were black, all these experiments succeeded in the same manner as with the white. If, instead of the ivory ruler, he made use of any skin, or a piece of smooth glass, the event was the same; but if he made use of a stick of sulphur, the electricities were in all cases the reverse of what they had been, before the ribbons were rubbed, having always acquired the positive electricity. When he rubbed them with paper either gilt or not gilt, the results were uncertain. When the ribbons were wrapped in paper gilt or not gilt, and the friction was made upon the paper laid on the plain above mentioned, the ribbons acquired both of them the negative electricity. If the ribbons were one black and the other white, whichever of them was laid uppermost, and in whatever manner the friction was made, the black generally acquired the negative and the white the positive electricity.

He also observed, that when the texture of the upper piece of silk was loose, yielding, and retiform like that of a flocking, so that it could move and be rubbed against the lower one, and the rubber was of such a nature as could communicate but little electricity to glass, the electricity which the upper piece of silk acquired did not depend upon the rubber, but upon the body on which it was laid. In this case, the black was always negative and the white positive. But when the silk was hard, rigid, and of a close texture, and the rubber of such a nature as would have imparted a great degree of electricity to glass, the electricity of the upper piece depended on the rubber. Thus, a white silk flocking rubbed with gilt paper upon glass became negatively, and the glass positively, electrified. But if a piece of silk of a firmer texture was laid upon a plate of glass, it was always electrified positively, and the glass negatively, if it was rubbed with sulphur, and for the most part if it was rubbed with gilt paper.

If an electrified ribbon was brought near an insulated plate of lead, it was attracted, but very feebly. On bringing the finger near the lead, a spark was observed between them, the ribbon was vigorously attracted, and both together showed no signs of electricity. On the separation of the ribbon, they were again electrified, and a spark was perceived between the plate and the finger.

When a number of ribbons of the same colour were laid upon a smooth conducting substance, and the ruler was drawn over them, he found, that when they were taken up singly, each of them gave sparks at the place where it was separated from the other, as did also the last one with the conductor; and all of them were negatively electrified. If they were all taken from the plate together, they cohered in one mass, which was negatively electrified on both sides. If they were laid upon the rough conductor, and then separated singly, beginning with the lowermost, sparks appeared as before, but all the ribbons were electrified positively, except the uppermost.—If they received the friction upon the rough conductor, and were all taken up at once, all the intermediate ribbons acquired the electricity, either of the highest or lowest, according as the separation was begun with the highest or the lowest. If two ribbons were separated from the bundle at the same time, they clung together, and in that state showed no sign of electricity, as one of them alone would have done. When they were separated, the outermost one had acquired an electricity opposite to that of the bundle, but much weaker.

A number of ribbons were placed upon a plate of metal to which electricity was communicated by means of a glass globe, and a pointed conductor held to the other side of the ribbons. The consequence was, that all of them became possessed of the electricity opposite to that of the plate, or of the same, according as they were taken off; except the most remote, which always kept an electricity opposite to that of the plate.

* Mem. of the Acad. of Turin, for 1763.

Chap. III. Of the Phenomena produced by excited Paper.

1. When a single leaf of writing paper, after being warmed, is laid on a table, and rubbed briskly with a piece of caoutchouc (elastic gum or India rubber) it becomes strongly electrical; on attempting to remove it from the table, it is found to adhere as if it were beamed with some gluey substance; and if, before it is quite separated, it be suffered to return to the table, it will fly back with considerable force, and will adhere almost as strongly as at first.

2. On separating it from the table immediately after rubbing, it will be strongly attracted by the table or any substance presented to it, and remain in contact for a considerable time.

3. When the knuckle is presented to the paper on its being first taken from the table, a snapping noise is heard, which is more perceptible if the knuckle be made to pass successively over different parts of the paper. If this experiment is made in the dark, sparks will be seen to accompany the snapping noise.

4. On employing a double piece, or two pieces of paper, these appearances will be increased. On attempting to separate the two pieces of paper, they are found to adhere strongly together, and their separation is accompanied with a crackling noise, similar to that produced by the application of the knuckle but not so loud. When quite separated, on being brought again within some inches of each other they are strongly and mutually attracted, and if, while separated, one of them be held between the other, and some contiguous substance, it will be alternately attracted by that substance, and the other piece, according as it is nearer the one or the other.

5. Placing a piece of clean new flannel between the paper and the table, or between the folds of the paper, does not appear to diminish the electrical appearances produced; but rubbing the paper with flannel produces no remarkable signs of electricity.

6. It is not necessary that the paper be rubbed on a table to produce these appearances; a book will answer as well, but with this difference, that if the book be in boards, the paper will produce no crackling when the knuckle is applied to it; but when the paper is double, General the separation of the folds will be attended with the same crackling as before; whereas when the book is bound in leather, a fling sheet when rubbed will produce the crackling on the application of the knuckle, while the double piece will produce it only when its folds are separated. The adhesion of the paper to the books is in both cases much lighter than its adhesion to the table, and in the case of the book in boards it is scarcely perceptible.

7. White paper of all kinds seems capable of producing these appearances, when rubbed with caoutchouc; but blotting paper whether white or red produces them in a very inferior degree, probably on account of the weakness of its texture not allowing it to be rubbed with sufficient force.

In general, the flouter the texture of the paper, the stronger will be the sparks and the attraction.

8. Paper does not appear to retain its electricity for any great length of time; in general, it ceases to show any remarkable signs of electric power about 10 or 15 minutes after being excited.

9. Other substances besides caoutchouc may be employed as rubbers for the excitation of paper, especially the dry hand, but none succeed so well as caoutchouc.

The electric property of paper was first discovered by Mr Grey. The paper employed by him was the kind called white pressing paper, which is of the same nature with card paper. Not only did this paper, when made as hot as his fingers could bear, produce a light when drawn briskly through his fingers; but when his fingers were held near it, a light issued from them also, attended with a crackling noise.

CHAP. IV. Phenomena produced by the Tourmalin.

The electrical power of this stone, so far at least as respects its attraction of light bodies, was known to the ancients; as Theophratus speaks of a stone by him called lycurium, which agrees in all respects with the tourmalin, and which he says attracted straws, ashes, and even small cuttings of iron and copper.

Nothing more seems to have been known of this stone till the year 1756, when M. Æpinus made a set of experiments on this stone, which were printed in the History of the Academy of Sciences and Belles Lettres of Berlin for that year.

In 1758, the duc de Noya, in conjunction with M. Daubenton and Adamson made some experiments on the tourmalin, but they do not seem to have been so accurate as those of M. Æpinus.

Soon after this stone was introduced to the notice of the English, by Dr Heberden, who procured from Holland several, with which Æpinus's experiments were repeated by Messrs Wilson and Canton.

But the most complete series of experiments on the tourmalin were made by Dr Priestley, and of these we shall here give a detailed account, as they comprise nearly all that is known on the subject.

1. To ascertain the kind of electricity produced, he had always at hand a stand of baked wood with four arms projecting from it. Three of these were of glass, having threads of fine silk as it comes from the worm fastened to them, and at the end of each thread a small piece of down. From the other arm hung a fine thread about 9 or 10 inches long, while a brass arm suspended a pair of pith-balls. At the other extremity of this arm, which was pointed, a jar could be placed, to receive the electricity, and by the repulsive power of it keep the balls equally diverging with positive or negative electricity; or sometimes he suspended the balls in an uninfluenced state within the influence of large charged jars: and lastly, he had always a fine thread of trial at hand, by which he could discover whether the stone was electrical or not before he began his experiments (f).

2. Before he began any experiments on the stone, also, he never failed to try how long the fine threads, which he used as electrometers, would retain their virtue; and found this to be various in various cases. When the threads would retain their electric virtue for a few minutes, he preferred them; but when this was not the case, he had recourse to the feathers, which never failed to retain it for several hours. They might be touched without any sensible loss of power, though they received their virtue very slowly. In the experiments now to be related, he made use of Dr Heberden's large tourmalin, whose convex side became positive and the flat side negative in cooling; and in all of them, when the positive or negative side of the tourmalin is mentioned, it is to be understood that which is positive or negative in cooling.

3. From Mr Wilcke's experiments on the production of spontaneous electricity, by melting one substance within another, he first conjectured that the tourmalin might collect its electricity from the neighbouring air: To determine which the following experiment was made. Part of a pane of glass was laid on the standard bar of an excellent pyrometer, and upon that glass the tourmalin was placed. This bar was heated by a spirit lamp, so that the increase or decrease of heat in the tourmalin could thus be exactly determined. In this situation he observed, that whenever he examined the tourmalin, the glass had acquired an electricity contrary to that side of the stone which lay upon it, and equally strong with it. If, for example, the flat side of the stone had been presented to a feather electrified positively, as the heat was increasing, it would repel it at the distance of about two inches, and the glass would attract it at the same or a greater distance; and when the heat was decreasing, the stone would attract, and the glass repel it at the distance of four or five inches. The case was the same whichever of the sides was presented, as well as when a shilling was fastened with sealing-wax upon the glass; the electricity both of the shilling and glass being always opposite to that of the stone. When it came to the turn, the electricity was very quickly reversed; so that

(f) Dr Priestley's method will be better understood, after the reader has perused Chap. I. III. and XIII. of Part III. General that in less than a minute the electricity would be contrary to what it was before. In some cases, however, viz. where the convex surface of the tourmalin was laid upon the glass or flinting, both of these became positive as well as the stone. This he supposed to be owing to the stone touching the surface on which it lay only in a few points, and that its electricity was collected from the air; which supposition was verified: for, getting a mould of Paris plaster made for the tourmalin, and heating it in the mould, fastened to a slip of glass, he always found the mould and glass possessed of an electricity contrary to that of the stone, and equally strong with it. During the time of cooling, the mould seemed to be sometimes more strongly negative than the stone was positive; for once, when the stone repelled the thread at the distance of three inches, the mould attracted it at the distance of near six.

4. On substituting another tourmalin instead of the piece of glass; it was observed, that when one of the tourmalins was heated, both of them were electrified as much as the tourmalin and glass had been. If the negative side of a hot tourmalin was laid upon the negative side of a cold one, the latter became positive, as would have been the case with a piece of glass. On heating both the tourmalins, though fastened together by cement, they acquired the same power that they would have done in the open air.

5. As the tourmalins could not in this case touch in a sufficient number of points, it was now thought proper to vary the experiment by cooling the tourmalin in contact with sealing-wax, which would fit it with the utmost exactness. On turning the stone, when cold, out of its waxen cell, it was found positive, and the wax negative; the electricity of the stone being thus contrary to what would have happened in the open air. The other side, which was not in contact with the wax, acquired the same electricity that it would have done though the stone had been heated in the open air; so that both sides now became positive. In like manner the positive side of the stone, on being cooled in wax, became negative.

6. On attempting to ascertain the state of the different sides of the tourmalin during the time it was heating in wax, many difficulties occurred. It was found impossible in these cases to know exactly when the stone begins to cool; besides, that in this method of treatment it must necessarily be some time in the open air before it can be presented to the electrometer; and the electricity of the sides in heating is by no means so remarkable as in cooling. In the experiments made with the tourmalin, when its positive side was buried in wax, it was generally found negative, though once or twice it seemed to be positive. On cooling it in quicksilver contained in a china cup, it always came out positive, and left the quicksilver negative; but this effect could not be concluded to be the consequence of applying the one to the other, because it is almost impossible to touch quicksilver without some degree of friction, which never fails to make both sides strongly positive though it be quite cold, and especially if the stone be dipped deep into it. At last, supposing that the stone would not be apt to receive any friction by simple pressure against the palm of the hand, he was induced to make the experiment, and found it fully to answer his expectations; for thus, each side of the stone was affected in a manner directly contrary to what would have happened in the open air.

7. Fastening the convex side of the large tourmalin to the end of a stick of sealing-wax, and pressing it against the palm of the hand, it acquired a strong negative electricity, contrary to what would have happened in the open air. Thus it continued till it had acquired all the power it could receive by means of the heat of the hand; after which it began to decrease, though it continued sensibly negative to the very last. On allowing the stone to cool in the open air, its negative power constantly increased till it became quite cold.

8. On heating the same flat side by means of a hot poker held near it, and then just touching it with the palm of the hand when so hot that it could not be borne for any length of time, it became positive. Letting it cool in the air it became negative, and on touching it again with the hand it became positive; and thus it might be made alternately positive and negative for a considerable time. At last, when it became so cool that the hand could bear it, it acquired a strong positive electricity, which continued till it came to the same degree of heat.

9. The wax was removed from the convex, and fastened to the flat side of the stone; in which circumstances it became weakly positive after receiving all the heat the hand could give it. On letting it cool in the open air it grew more strongly positive, and continued to till it was quite cold; and thus the same side became positive both with heating and cooling.

10. On heating the convex side by means of a poker, and pressing it against the palm of the hand as soon as it could be borne, it became pretty strongly negative; though it is extremely difficult to procure any appearance of negative electricity from this side; and care must be taken that a slight attraction of the electrified feather, by a body not electrified, be not mistaken for negative electricity.

11. On covering the tourmalin when hot with oil and tallow, no new appearances were produced; nor did the heating of it in boiling oil produce any other effect than lessening the electricity a little; and the event was the same when the tourmalin was covered with cement made of bees-wax and turpentine. On making a small tourmalin very hot, and dropping melted sealing-wax upon it, so as to cover it all over to the thickness of a crown piece, it was found to act through this coating nearly, if not quite, as well as if it had been exposed to the open air. Thus a pretty deception may be made: for if a tourmalin be inclosed in a stick of wax, the latter will seem to have acquired the properties of the stone.

12. On letting the stone cool in the vacuum of an air-pump, its virtue seemed to be diminished about one half, owing no doubt to the vacuum not being sufficiently perfect.

13. On fixing a thin piece of glass opposite and parallel to the flat side of the tourmalin, and about a quarter of an inch distance from it, in an exhausted receiver, the glass was so slightly electrified, that it could not be distinguished whether it was positive or negative.

14. On laying the stone upon the standard bar of the the pyrometer, and communicating the heat to it by means of a spirit lamp, it was extremely difficult to determine the nature of the electricity while the heat was increasing to 70°; during which time the index of the pyrometer moved about one seventh part of an inch. But if the stone was taken off the bar, and an electrified thread or feather presented to that side which had lain next it, the convex side was always negative, and the flat one positive.

14. To determine what would be the effect of keeping the tourmalin in the very same degree of heat for a considerable time together, it was laid upon the middle of the bar, to which heat was communicated by two spirit lamps, one at each extremity; and making the index move 45 degrees, it was kept in the same degree for half an hour without the least sensible variation; and it was observed, that the upper side, which happened to be the convex one, was always electrified in a small degree, attracting a fine thread at the distance of about a quarter of an inch. If in that time it was taken off the bar, though ever so quick, and an electrified feather presented to it, the flat side, which lay upon the bar, was negative, and the upper side very slightly positive, which appeared only by its not attracting the feather. On putting a piece of glass between the stone and standard bar, keeping it likewise in the same degree of heat, and for the same space of time as before, the result was the same; the glass was slightly electrified, and of a kind opposite to that of the stone itself. To avoid the inconvenience of making one side of the stone hotter than another, which necessarily took place when it was heated on the pyrometer, the following method was used. By means of two rough places which happened to be in the stone, it was tied with a silk thread which touched only the extreme edge of it; and thus being perfectly insulated, it might be held at any distance from a candle, and heated to what degree was thought necessary; while, by twisting the string, it was made to present its sides alternately, and thus the heat was rendered very equal in both. After being made in this manner so hot that the hand could scarce bear it, it was kept in that situation for a quarter of an hour. Then, with a bundle of fine thread held for some time before in the same heat, the electricity which it had acquired by heating was taken off, and it was found to acquire very little, if any; whence appeared the justness of an observation of Mr Canton's, that it is the change of heat, and not the degree of it, that produces the electric property of this stone.

15. On heating the stone suddenly, it may sometimes be handled and pressed with the fingers several times before any change takes place in the electricity which it acquires by heating, though it begins to cool the moment it is removed from the fire. In this case, however, the stone must be heated only to a small degree. When the heat is three or four times as great as is sufficient to change the electricity of the two sides, the virtue of the stone is the strongest, and appears to be so when it is tried in the very neighbourhood of the fire. In the very centre of the fire the stone never fails to cover itself with ashes attracted to it from every quarter; whence it acquired its name in Dutch.

16. The tourmalin often changes its electricity very slowly; and that which it acquires in cooling never fails to remain many hours upon it with very little diminution. It is even possible, that in some cases the electricity acquired by heating may be so strong as to overpower that which is acquired by boiling; so that both sides may show the same power in the whole operation. "I am very certain (says the Doctor), that in my hands both the sides of Dr Heberden's large tourmalin have frequently been positive for several hours together, without any appearance of either of them having been negative at all. At this time I generally heated the tourmalin, by presenting each side alternately to a red hot poker, or a piece of hot glass, held at the distance of about half an inch, and sometimes I held it in the focus of a burning mirror; but I have since found the same appearance when I heated it in the middle of an iron hoop made red hot. The stone in all these cases was fastened by its edge to a stick of sealing-wax. This appearance I have observed to happen the oftener when the iron hoop has been exceedingly hot, so that the outside of the stone must have been heated some time before the inside; and also I think there is the greatest chance of producing this appearance, when the convex side of the stone is made the hotter of the two. When I heat the large tourmalin in this manner, I seldom fail to make both sides of the stone positive till it be about blood-warm. I then generally observe a ragged part of the flat side towards one end of the stone become negative first, and by degrees the rest of the flat side; but very often one part of the flat side will, in this method of treatment, be strongly positive half an hour after the other part is become negative."

CHAP. V. Phenomena produced by excited sulphur.

Sulphur is one of those electrics which may be made to exhibit electrical appearances by being melted and melted on sufferer to cool again. Dr Gilbert had shown that fulphur might be rendered electric by friction; but the first person who demonstrated its excitability by melting, was Mr Wilcke, of Rothenburg in Lower Saxony, who first called this spontaneous electricity.

He melted some crude sulphur in an earthen vessel, and left it to cool after placing the vessel on a conducting substance. On taking out the sulphur when cool, he found it strongly electrical, but this was not the case when the vessel was placed on an electric.

He then melted sulphur in glass vessels, and found that both the glass and the sulphur became electrical, but the former acquired a positive, and the latter a negative electricity. When glass vessels were employed, it did not matter whether they were placed on electrics or conductors, except that the electricity produced, was stronger in the former case, and still stronger when the glass was coated with some metallic substance. The electricity of the sulphur was not produced till it began to contract, and was the strongest when the greatest degree of contraction had taken place. The electricity of the glass was always weakest when that of the sulphur was strongest, and the former was the strongest possible when the sulphur was shaken out before it had begun to contract.

He found that when melted sulphur was poured into vessels of rough glass, or into hollowed cakes of sulphur, no electricity was produced.

Mr Wilcke also made experiments of the same kind with melted sealing-wax, and found that when this was was left to cool in vessels of smooth glass or of wood, the sealing-wax acquired a negative, and the glass or wood a positive electricity; but when it was cooled in cups of sulphur, the sealing-wax became electrified positively, and the sulphur negatively.

Epinus made some experiments on melted sulphur which he cooled in metal cups. On examining them after the sulphur was cold, he found that while the sulphur remained in the cups, neither of them showed any signs of electricity; but the moment they were separated, both appeared strongly electrical. The marks of electricity disappeared however on replacing the sulphur in the cups, and returned on their being again separated. When separated, the sulphur was electrified positively, and the cups negatively; but if, before replacing the sulphur in the cups, the electricity of either was taken off, the sulphur and cups when together, would show signs of that electricity that had not been taken off.

It must be remarked here that though the electricity of the sulphur, sealing-wax, &c. in the above experiments appears to be the consequence of their cooling after being melted, it is in fact excited by a degree of friction which these substances undergo by their contraction while cooling in the cups, or by being touched with the hand in making the experiment; for it is found that if they are cooled under circumstances that prevent all friction, a very small degree of which is sufficient to excite these bodies, no electricity is produced. This appears from experiments made by M. M. Van Marum and Van Troostwyck, for the purpose of ascertaining this point, an account of which is contained in the 33rd volume of Rozier's Journal, to which we must refer our readers.

The durability of the electric power in excited sulphur is so remarkable, that Mr Grey, from some experiments which he made on this and similar substances, was led to suppose it perpetual. In particular, he poured melted sulphur into a conical drinking glass, and when it was cold he found, that on taking off the glass, the sulphur never failed to attract light bodies, and that in every state of the atmosphere; and in fair weather the glass would also attract.

Mr Henly, who repeated Mr Grey's experiments, says, he has never known the sulphur fail of showing signs of electricity on the removal of the glass.

Although it be true that sulphur, as well as rosin, sealing-wax, amber, and silk, retain the electric power for a considerable time, this is, however, continually diminishing, and at length disappears altogether.

Other substances, as well as sulphur and sealing-wax, become electrical by cooling after being melted. Mr Henly observed that chocolate, when first from the mill, as it cools in the tin pans into which it is received, becomes strongly electrical, and retains this property for some time after being taken out of the pans, but loses it by handling. If melted again, and left to cool as before, its electricity returns, though in a less degree; and thus it may be renewed once or twice, but still in a much smaller degree than before. But if before pouring it into the pan, it be well mixed with a little olive oil, it becomes again strongly electrical.

When a stick of sealing-wax is broken across, each piece becomes electrified at the extremities that were contiguous, the one positively and the other negatively.

When wood that is hard and pretty dry, is cut or shaved, the shavings are rendered electrical. This fact was first observed by Mr William Wilson, who, from wood, a number of experiments, draws the following conclusions.

From these experiments it appears, that when very dry wood is scraped with a piece of window glass, the shavings are always positively electrified. And if chipped with a knife, the chips are positively electrified if the wood is hot, the edge of the knife not very sharp, and negatively electrified if the wood is quite cold. But if the edge of the knife is very keen, the chips will be negatively electrified whether the wood is hot or cold.

The greatest number of trials was made with the insulated knife, which was always electrified contrarily to the chips; but the surface of the wood where the chips were cut from was very seldom electrified, and when it was it, was always but weakly so, and of the same denomination as that of the weakest of the other two. Mr Wilson repeatedly found that if a piece of dry and warm wood is suddenly split asunder, the two surfaces which were contiguous are electrified, one side positive and the other negative.

Powders, either of electrics or conductors, are rendered electrical by dropping them on an insulated metallic plate.

The method, as described by Mr Cavallo, is as follows:

"Inflating a metal plate upon an electric stand, such as a wine glass, and connect with it a cork-ball electrometer; then the powder required to be tried, being held in a spoon, or other thing, at about six inches above the plate, is to be let fall gradually upon it. In this manner the electricity acquired by the powder, being communicated to the metal plate, and to the electrometer, is rendered manifest by the divergence of the threads; and its quality may be ascertained in the usual manner; to be hereafter described.

"It must be observed, that if the powder is of a conducting nature, like the amalgam of metals, sand, &c., it must be held in some electric substance, as a glass phial, a plate of sealing-wax, or the like. Sometimes the spoon that holds the powder may be insulated, in which case, after the experiment, the spoon will be found possessed of an electricity contrary to that of the powder.

"In performing these experiments, care must be taken to render the powders, and whatever they are held in, as free from moisture as possible; sometimes it being necessary to make them very warm, otherwise the experiment is apt to fail. The following are the particulars which have been observed with this method, which, however, are neither numerous, nor often repeated; but they may suffice to excite the curiosity of those persons who have leisure and the opportunity of repeating them more at large and in a greater variety.

"Powder of rosin, whether it be let fall from paper, glass, or a metal spoon, electrifies the plate strongly negative; the spoon, if insulated, remaining strongly positive. Flower of sulphur produces the same effect, but in a little less degree. Pounded glass, let fall from..." Electrical from a piece of paper, made dry and warm, electrifies the plate negatively, but not in so strong a degree as rosin. If it be let fall from a brass cup, it electrifies the plate positively, but in a very small degree.

"Steel-filings let fall either from a glass phial or paper, electrify the plate negatively; but brass-filings, treated in the same manner, electrify the plate positively. The amalgam of tin-foil and mercury, gunpowder, or very fine emery, electrify the plate negatively, when they are let fall upon it from a glass phial. Quick-silver, from a glass phial, electrifies the plate positively.

"Soot from the chimney, or the ashes of common pit-coal mixed with small cinders, electrify the plate negatively, when let fall from a piece of paper."

M. Volta discovered, that when water and some other fluids are reduced to a state of vapour, by throwing the fluid on some lighted coals placed in an inflated crucible, the vapour shows signs of positive electricity, while the coals it is leaving are negatively electrified; and hence it is concluded, that all fluids in the act of evaporation become electrical, the vapour being electrified positively, and the body which it is leaving negatively; and again, that when vapour becomes condensed into a fluid, it becomes negatively electrified, leaving the bodies with which it was last in contact in a state of negative electricity.

Some conductors arranged in certain ways will produce electrical appearances without friction, or communication with any electric except the air.

Thus if a plate of zinc, a plate of silver, or of copper, and a piece of woollen cloth moistened with some

PART II.

OF ELECTRICAL APPARATUS.

CHAP. I. Of the Construction of Electrical Machines.

WE shall first lay down the general principles on which the construction of an electrical machine and the adjusting of its several parts depends; and shall afterwards describe some of the more important machines which are now in use.

The principal parts in an electrical machine are the electric, the engine by which it is to be set in motion, the rubber, and the prime conductor.

Several substances have, at various times been employed as electrics, as sulphur (v), rosin, polished glass, and rough glass; and they have been used of various form as globes, spheroids, cylinders, &c. The reason of this variety of form seems to be that experience had not shown what form was the most convenient; but the different substances were employed for the purpose of producing a positive or negative electricity as the nature of the experiment or the fancy of the operator might require.

But as this purpose is better answered by insulating the rubber, or allowing it to communicate freely with conductors, polished glass is almost the only substance at present employed as the electric of a machine. Globes of glass are sometimes used, but the most convenient forms are cylinders and plates.

The most convenient size for globes is from nine to twelve inches diameter. They are made with one neck, which is cemented to a strong brass cap, in order to adapt them to a proper frame. The most convenient cement for holding together the parts of electrical apparatus is made by melting together, over a gentle fire, two parts of rosin, two of bees-wax, and one of powdered red ochre. This cement is much better than rosin alone, as it serves the purposes of insulation equally well, and is much less brittle. Globes were formerly

(f) The first person who constructed anything like an electrical machine was Otto Guericke, burgomaster of Magdebourg, who lived in the latter end of the 17th century. He formed a globe of sulphur by melting this substance in a glass globe, which he then broke away from it, little imagining that the glass itself would have answered his purpose much better. Vid., Experimenta Magdeburgica. Electrical Apparatus.

Plates of glass.

Cylinders to be preferred.

Chap. I. ELECTRICITY.

Electrical formerly much more used than at present; their great advantage appears to be, that by making the electric revolve on an axis nearly perpendicular, the upper part is more completely insulated; but one great disadvantage attends this motion, namely, that as the pressure is applied at a distance from the fulcrum, it in time loosens the adhesion of the strongest cement.

Plates of glass are much in fashion on the continent; and they seem to attribute to this form much of the wonderful power of their machines, as of that at Haarlem, to be hereafter mentioned. Perhaps the greatest advantage of plates is that the friction may be applied to both surfaces at once; but it may be doubted, whether this be not an imaginary advantage, and this form is attended with several material inconveniences; as, 1st, Plates cannot bear any great pressure of the rubber; 2d, They cannot be inflated without very complicated machinery; 3d, As they are fixed by the centre, and the friction is applied towards the circumference, if much force be employed, there will be great danger of breaking the plate, or at least of loosening it, and thus disturbing the equability of its motions; and, 4thly, They are much more expensive than any other form, and hence, as they are much exposed to injury, the replacing of them becomes a very serious object.

The ingenious Mr Cuthbertson has contrived to obviate some of these disadvantages, and his plate machines are very conveniently managed, as well as very powerful in their effect.

On the whole, the cylindrical form seems preferable to any other, and this is now almost universally employed. The cylinders should be blown as light as possible, consistently with sufficient strength, and their surface should be as equable and free from knots or protuberances as may be; for these not only render the cylinder more liable to be broken, but prevent the risk of the rubber from being closely applied to every part of the surface. To avoid these inequalities, the cylinder should be blown at the time when the glass is in the most complete state of fusion, and this is found to be the case, when the pot is about half emptied, which happens at the London glass-houses on Wednesdays and Thursdays.

The cylinders are usually made of the best flint-glass, but it is not determined which is the best kind. In size they vary from eight inches long and four in diameter to two feet long and one foot in diameter, which is perhaps as large as they can be conveniently blown. Very small cylinders are, however, of little use, and it may be doubted whether the diameter should not be greater in proportion to their length than what is above assigned. It is of great consequence that the cylinders should have been properly annealed, or that they should be brought very gradually from the temperature of the glass-house to that of the external air; as when they have been too suddenly cooled, they are apt to fly in pieces in the act of whirling, to the great annoyance both of the experimenter and the spectators.

Cylinders are made with two necks; and the openings of these should be so wide as to admit the hand to clean the inner surface of the glass, which is sometimes filled by condensed vapour. These necks are cemented as above directed, to caps of brass, which are much superior to wooden caps, as they may be made much more smooth and equal.

Brass caps have been objected to on account of the conducting power of the metal; but this objection is absurd, as the insulation depends on the distance between the cap and the cushion, which, as will be mentioned presently, should be as great as possible. Indeed wood, if ever so well dried, is but a very imperfect insulator, and the hardest can never be so completely polished as a metallic substance. The brass cap should be composed of two parts; one a ring to be cemented round the neck of the cylinder, with an aperture sufficient to admit of the introduction of the hand within the glass, and with a surface as extensive as possible, that the adhesion of the cement may be the more complete; the other a head or lid of brass completely polished, to be screwed into the ring, and with an orifice into which the winch or the pin on which the other end of the cylinder is supported may be screwed.

It has been thought of advantage to line the inside of the glass with some electric substance, as wax, rosin, globes &c.; this has been thought by some to increase the excitability of the glass. It seems ascertained, however, that if such a coating does not make a good cylinder better, it at least often improves a bad one. The composition most approved for coating globes or cylinders is formed of four parts of Venice turpentine, one part of rosin, and one of bees-wax, melted together and kept boiling over a gentle fire for about two hours, frequently stirring it. When a vessel is to be coated with this composition, a sufficient quantity of it, broken into small pieces, is to be put within the globe or cylinder, which is then held to the fire to melt the composition; and by constantly turning it round, the coating is to be spread equally over the surface to about the thickness of a sixpence. In doing this, care must be taken to heat the glass very gradually and equally, otherwise it is liable to be broken during the operation.

The electric is set in motion either by a simple means of winch, or by means of multiplying wheels. The former moving the electric, as being more simple, and consequently less liable to produce disorder in the motion of the machine, is generally to be preferred. The handle of the winch is sometimes made of glass, but this is unnecessary; for the glass does not shorten the interval, which is most favourable to the diffusion of the electric power.

Multiplying wheels were much more common formerly than at present. The usual method of employing these is, to fix a wheel on one side of the frame of the machine, which is turned by a winch, and has a groove round its circumference.

Upon the brass cap of the neck of the glass globe, or one of the necks of the cylinder, is fixed a pulley, whose diameter is about the third or fourth part of the diameter of the wheel; then a string or strap is put over the wheel, and the pulley; and by these means, when the winch is turned, the globe or cylinder makes three or four revolutions for one revolution of the wheel. The principal inconvenience attending this construction, is, that the string is sometimes so very slack, that the machine cannot work. To remedy this, the wheel should be made moveable with respect to the electric, so that, by means of a screw, it may be fixed at the proper distance; or else the pulley should have... Electrical have several grooves of different radiuses on its circumference.

The chief advantage of multiplying wheels, is that the arm of the operator is less fatigued by turning the machine, when these are employed, than when a simple winch is used; and as by these the motion of the electric is rendered quicker, it is supposed by some that its electric power is proportionally increased.

In some machines, instead of the pulley or string described above, there are used a wheel and pinion, or a wheel and an endless screw. This machinery requires considerable nicety in its construction, is apt to produce an unpleasant rattling, and unless frequently oiled, the constant friction of the parts against each other soon wears them away, so as to render the motion very unsteady.

Rubber.

The rubber (c), by which the electric is to be excited, consists of two parts. One part is a cushion, which is usually made of a piece of red badger skin, stuffed with hair or flannel. The cushion is either fixed to a piece of wood well rounded at the edges, and fastened to a support of glass, or some other insulating substance; or where two conductors are employed, it is fixed to one of these. The cushion should be made as hard as the bottom of an ordinary hair-chair, and should be so adapted to the surface of the cylinder, as to press equally on it in every part. For this purpose it is generally provided with a spring, by which means it may be the better adapted to any inequalities of the surface of the glass; in the usual construction of the cushion the spring is placed without, but Mr. Jones, instrument-maker in London made, what he considers as a great improvement on the mode of placing it. This consists in a spring placed within the rubber itself; the action of which is found to be better suited for adapting the rubber to the inequalities of the glass, than that placed entirely without the rubber. It consists of a piece of flexible iron or brass, represented edgewise by A, fig. 3.; and it is evident that it acts in a much more parallel and uniform manner than the former, which is constantly changing the pressure of the line of contact between the rubber and cylinder while it passes from the under to the upper side, thus rendering the effect insufficient and uncertain.

The length of the cushion should not exceed one-third of the length of the cylinder; for if it were longer, the inflation would be much less complete, since one end of the conductor (when the rubber is fixed to a conductor) must always be nearer to the hand by two or three inches than the cushion.

The other part of the rubber consists of a piece of black Persian silk as broad as the length of the cushion, and reaching from it over nearly one half of the cylinder. It should be sewed upon a wire, bent at both ends, and these ends are adapted to holes made on the upper edge of the wood to which the cushion is fastened; or it may be glued to the edge of the cushion; but the former mode of fixing it is to be preferred, as it can then be easily removed.

The rubber should be inflated in the most perfect manner; as, when inflation is not required, it may be easily taken off by a chain or wire hung upon it, and thus communicate with the earth or with any electrified body; but where there is no contrivance for inflating the rubber, it is impossible to perform many of the most curious experiments. In short, to construct the rubber properly, it must be made in such a manner, that the tide it touches in whirling may be as perfect a conductor as it can be made, in order to supply electricity as quick as possible; and the opposite part should be as perfect a non-conductor as possible, in order that none of the electric power accumulated upon the glass may return back to the rubber; which has been found to be the case when the rubber was not made in a proper manner (H).

Of late, a considerable improvement in the rubber Wolf's has been made by M. Wolff, of Hanover. The construction and advantage of his rubbers, as applied to a plate machine similar to that of Van Marum, of which an account will be given by and by, is thus described by the author in a paper in Gilbert's Annalen der Physik for 1802, and translated in Nicholson's Journal, for February 1804, from which we have copied them.

The four rubbers are made of dry walnut wood soaked in amber varnish, and are 5½ inches long, 1½ broad, and a little more than one quarter of an inch thick. The metallic plate that communicates with the leather covered with amalgam, is only 1½ inch broad; and is fixed externally to the centre of the piece of wood. The rubbers are pressed towards the glass by means of a spring. They are covered with a piece of thick woollen, upon which is a piece of fine neat's leather. After the leather is fastened to the wood, it is wetted, and pressed between two boards, where it is kept till it is again dry. Thus it is rendered very flat, and its edge very sharp, and all its parts will apply to the surface of the glass. This piece of leather is covered with another a little broader, the rough surface of which is towards the glass, and its lower edge on the side towards which the plate moves; and its lower edge on the other side from which the plate moves, being likewise very sharp. The piece of silk is applied with accuracy to this leather. Before it is fastened on, it is heated, and bemarred first with butter of cacao, then with a large quantity of Kienmayer's amalgam (I); and after it is fastened on, it is compressed in conjunction with the wood, or pressed strongly against the machine. The leather is next covered with amber varnish, amalgam is spread over this, and after the varnish is dry, it is smoothed with a burnisher. This is repeated several times. The whole being very dry, and the rubber being pressed so as to touch the glass in all points,

(g) For a long time the only rubber employed was the dry hand of the experimenter, till the middle of the 18th century, when M. Winckler, professor at Leipzig, introduced the cushion. It was long after this, however, before electricians could be persuaded that any rubber was better than the clean dry human hand. Vid. Prießnitz's Hiil. part i. loc. 7.

(h) The improvement of the silk flap was first introduced by Dr. Nooth. Vide Phil. Transf. vol. lxiii.

(i) He adds to this amalgam as much silver, as the mercury can dissolve in conjunction with the zinc. Electrical points, the leather coated with amalgam (k) is covered with a piece of fine white paper, as long as the leather, and half an inch broader, so as to cover the frame that fastens the silk to the leather; and the paper is fastened to the wood above or below, accordingly as it is on the ascending or descending side of the plate.

Dry paper is known to be capable of acquiring a high state of electricity, which induced me to try this substance as an immediate rubber. The following are the advantages, that by my experiments, repeated and varied in a great number of ways, I have found paper employed as a rubber to possess over every other known substance.

1. The glass is not rendered dull by the friction, as happens at length, and by frequent using, when it is in immediate contact with the amalgam.

2. By the immediate contact of the amalgam, the glass frequently contracts breaks here and there, that occasion a circulation of the fluid. This cannot take place in the construction I propose.

3. Neither the glass nor the silk can be foiled. It is well known, that the cleanliness of the glass, as well as of the rubber and the whole machine in general, is of importance in producing an intense degree of electricity. It is true, that it has been proposed to apply the amalgam to the glass instead of the rubbers; but the greater effect, that seems to be produced by this last method, is only apparent, and consists entirely in the circulation of the fluid on the glass, while far from exciting or accumulating more of the fluid, this process and the circulation disperse it.

4. The amalgam on the leather does not require to be frequently renewed. The dust of the amalgam, that is deposited on the edges of the paper, is injurious only when accumulated there in sufficient quantity to be conveyed to the glass, from which however it may easily be removed.

5. The return and passage of sparks to the rubbers are rendered more difficult, as the paper sufficiently covers the borders of the rubbers, that are turned toward the axis.

6. In my construction the rubbers may be larger than in the usual way, and in reality they are larger in proportion in my machine than in Van Marum's. No spark passes the axis, unless the air be very damp. I am persuaded, that, by adopting my construction, the rubbers of a plate of 32 inches, such as Van Marum's is, may be eleven inches instead of nine, in which case there would still be two inches for the diameter of the piece of wood that fastens the plate to the axis, and three inches for the distance from this piece to the rubbers; which I think would be sufficient in these circumstances; and the friction being on a larger surface of the plate, the effect must naturally be much greater. I shall try this alteration of the rubbers on large plates of Bohemian glass, as well as on English cylinders of 18 inches diameter, and 21 inches long. The electrical apparatus result I have already obtained with a small cylinder gives me reason to hope much more complete success with a large one.

7. With my rubbers the friction may be rendered much greater, than with those the amalgam of which is in immediate contact with the glass, and foils it; besides, the plate turns with an uniform friction.

8. The activity of the machine is extraordinarily increased by this construction. The greater freedom with which the plate moves, even under a greater pressure, and the paper's preventing the glass from being foiled, would be sufficient to produce this effect; even if the greater pressure alone did not occasion a more powerful effect than can be obtained from common machines.

The last part of a machine which we are to describe is the prime conductor.

This is a cylindrical tube, usually made of brass, copper or tinned iron, the two first of which are much the best, as they admit of more nicety in the construction, especially of being better polished. When required very large, the cylinder may be made of pate-board covered with tin-foil or gold-leaf. It is of great consequence that the conductor be made perfectly free from points or edges, and where holes are made in it, as is commonly done, for the purpose of experiment, these should have their edges perfectly smooth and even. The cylinder is closed at both ends by spherical lids or covers, made so as to fit with the greatest accuracy, but so as to be taken off, if requisite. These ends are sometimes made larger than the rest of the cylinder; but this is unnecessary, and it is much better that they should form with it one smooth and uniform surface. In some machines the conductor is placed at right angles to the glass cylinder, but it is now usually placed parallel to it. At the end or side opposite the glass, are fixed a row of metallic points, for receiving the electric power; these are generally either fixed immovably in the side or end of the conductor, or are fixed along a separate piece of strong brass wire, which is made to shut into two holes in the conductor, so that the points can be removed at pleasure. Mr Reid contrived to fix them to rings turning on an axis, the ends of which were forced into holes made in the conductor, so that the points rested on the glass, with which they were thus in perpetual contact, without disturbing its motion. It is certainly of great advantage to have the points as near the glass as possible, but this mode of fixing them is attended with the inconvenience of multiplying the protuberances on the surface of the conductor.

The size of the conductor is of some consequence; in general its length should equal that of the glass cylinder including its necks, and its diameter should be about one-third of that of the cylinder. It should be inflated

(k) The amalgam mentioned by M. Wolff is formed of two parts of mercury, one part of purified zinc, and one of pewter. The zinc and pewter are melted together, and, before the mixture is quite cool, the mercury is added. The whole is then poured into a close box, shaken for some time and left to cool on a marble slab. When nearly cold, it is reduced to powder in a glass or earthen mortar, taking care not to triturate it so long as to make it turn gray. The Baron de Kienmayer, the author of this amalgam, has given a particular account of its preparation and uses in the 33rd vol. of Rozier's Journal. p. 96. q. v. Electrical insulated by being fixed on a pillar of glass covered with sealing-wax. For this purpose, the sealing-wax may be dissolved in alcohol (spirit of wine), and thus applied to the glass pillar; but it is better to heat the glass gradually, and then rub it over with the sealing-wax till it is covered to a sufficient thickness. Where there are two conductors, one of them supports the rubber, and is called the negative conductor; this is not furnished with points: the other, which is what we have just described, is placed opposite and parallel, on the other side of the glass, and is called the positive conductor (1).

It is proper to have several brass balls furnished with stalks, some straight, and others curved, which may be fitted into the holes in the surface of the conductor.

The balls should be of various sizes, and should be made to screw upon the ends of the stalks, some of which should be terminated by points. It is convenient also that some of the stalks be made with a joint, so that the ball or point can be placed in any position.

Electrical machines should be furnished with one or more chains, by which, when inflation is not required, either of the conductors may be made to communicate with the table or with the floor.

There is also attached to the electrical machine, a stool with four glass legs or feet, for the purpose of inflating various bodies in the course of experiment, and hence called the inflating stool. This stool should be made either of baked wood or thick glass, and should be sufficiently large to support an ordinary chair, or at least so large that a person can easily stand on it.

**Chap. II. Description of some particular Electrical Machines.**

The first machine which we shall describe is one invented by Dr Priestley, which has been considered by some as so ingenious, that it has been called a universal electrical machine.

It is thus described by Dr Priestley in his history. The frame consists of two strong boards of mahogany of the same length, parallel to one another, about four inches apart; and the lower is an inch on each side broader than the upper. In the upper board is a groove, reaching almost its whole length. One of the pillars, which are of baked wood, is immovable, being let through the upper board, and firmly fixed in the lower, while the other pillar slides in the groove above mentioned, in order to receive globes or cylinders of different sizes; but it is only wanted when an axis is used. Both the pillars are perforated with holes at equal distances, from the top to the bottom; by means of which globes may be mounted higher or lower according to their size; and they are tall to admit the use of two or more globes at a time, one above the other. Four of moderate size may be used, if two be fixed on one axis; and the wheel has several grooves for that purpose.

If a globe with only one neck be used, a brass arm electrical with an open socket, is necessary to support the axis beyond the pulley; and this part is also contrived to be put higher or lower, together with the brass socket in which the axis turns. The axis is made to come quite through the pillar, that it may be turned by another handle, without the wheel, if the operator chooses. The frame being screwed to the table, may be placed nearer to, or farther from the wheel, as the length of the string requires, in different states of weather. The wheel is fixed in a frame by itself, by which it may have a situation with respect to the pulley, and be turned to one side, so as to prevent the string from cutting itself. The hinder part of this frame is supported by a foot of its own.

The rubber consists of a hollow piece of copper lined with horse hair, and covered with a baile skin. It is supported by a socket, which receives the cylindrical axis of a round and flat piece of baked wood, the opposite part of which is inserted into the socket of a bent steel spring. These parts are easily separated; so that the rubber, or piece of wood that serves to inflate it, may be changed at pleasure. The spring admits of a twofold alteration of position. It may be either slipped along the groove or moved in a contrary direction; so as to give it every desirable position with respect to the globe or cylinder; and it is besides furnished with a screw, which makes it press harder or lighter, as the operator chooses.

The prime conductor is a hollow vessel of polished copper, in the form of a pear, supported by a pillar, and a firm basis of baked wood, and it receives its fire by means of a long arched wire, or rod of very soft brass, easily bent into any shape, and raised higher or lower, as the globe requires; and it is terminated by an open ring, in which are hung some sharp-pointed wires playing lightly on the globe when it is in motion. The body of it is furnished with holes and sockets, for the insertion of metallic rods, to convey the fire wherever it is wanted, and for many other purposes convenient in a course of electrical experiments. The conductor is by this means steady, and yet may be easily put into any situation. It collects the fire perfectly well, and (what is of the greatest consequence though but little attended to) retains it equally everywhere.

When positive electricity is wanted, a wire or chain, as is represented in the figure, connects the rubber with the table, or the floor. When negative electricity is wanted, that wire is connected with another conductor, such as is represented at fig. 5, while the conductor at fig. 4 is connected by another wire or chain with the table. If the rubber be made tolerably free from points, the negative power will be as strong as the positive.

In short, the capital advantages of this machine are, that glass vessels, or any other electric body, of any size or form, may be used, with one neck or two necks.

(1) M. Boze, professor at Wittemburg, first employed a prime conductor; his conductor was a tube of iron or tin, which he inflated at first by its being held by a man standing on cakes of rosin, and afterwards by suspending it by silk lines, horizontally before the globe. For a long time a gun-barrel was employed as a prime conductor. Electrical necks at pleasure; and even several of them at the same time if required. All the essential parts of the machine, the globe, the frame, the wheel, the rubber, and conductor, are quite separate; and the position of them to one another may be varied in every manner possible. The rubber has a complete inflation, by which means the operator may command either the positive or negative power, and may change them in an instant. The conductor is steady and easily enlarged, by rods inserted into the holes, with which it is furnished, or by the conjunction of other conductors in order to give larger sparks, &c. The wheel may be used or not at pleasure; so that the operator may either fit or stand to his work, as he pleases; and he may with the utmost ease both manage the wheel and his apparatus.

This machine is figured in Plate CLXXXVII, fig. 4, where

- a. Represent the two boards of the frame. - b. One of the pillars. - c. The brafs arm with the open socket. - d. The axis on which the globe turns. - e. The frame to which the wheel is fixed. - f. The rubber; g. The piece of baked wood; h. The steel spring; i. The screw. - k. The prime conductor; l. The rod or wire; m. The points. - n. The wire for connecting the rubber with the table.

Next to Dr Priestley's machine, we shall describe one which was invented by Dr Ingenhousz, in which a plate of glass is employed instead of a globe or cylinder.

There is a circular plate of glass, about a foot in diameter, perforated in the centre by an iron axis, upon which it is turned vertically by means of a winch. It has four cushions, each above two inches long, which are situated at the opposite ends of its vertical diameter. It moves in a frame composed of a bottom board about a foot square, or a foot long and about six inches broad, upon which are raised two other smaller boards, parallel to each other, and fastened together at the top by a small wooden cross bar. By these upright boards, the axis of the plate is supported, and to them the cushions are fastened. When the machine is used, the bottom of the frame is fastened to the table by an iron crank. The conductor in this machine is made of hollow brass; and is furnished with branches extending from its extremities, and approaching very near the circumference of the plate.

An improvement on this machine is thus described by Mr Walker in his Lectures on Familiar Philosophy.

"It is made of a round plate of thick looking-glass, (fig. 6, Plate CLXXXVII.) This plate turns on an axis a, supported by the mahogany frame c c, by the handle g. The rubbers are of red leather stuffed with curled hair, and nailed to thin strips of wood, d d, one on each side of the glass, and made to press the glass very close by the screws x x; to these rubbers are attached oiled silk curtains, z z, on both sides of the glass. The conductor, w w w, is of brass and fixed to the frame, c c, by the glass supporter q, which inflates the conductor w, and terminates in the two knobs, s s; into these knobs are screwed small cylinders of brass, with a number of points that nearly touch the glass, and receive the electric matter from it; they cannot be seen in the drawing, being behind the curtains. For exciting positive electricity in all kinds of weather and situations, this is the most powerful and convenient machine ever yet invented."

A very powerful machine, in which plates of glass are employed, is that in Teyler's museum at Haarlem, constructed by Mr John Cuthbertson. It consists of a circular plate of glass, each 65 inches in diameter, and made to turn upon the same horizontal axis, at the distance of 7½ inches from one another. These plates are excited by eight rubbers, each 15½ inches long. Both sides of the plates are covered with a resinous substance to the distance of 16½ inches from the centre, both to render the plates stronger, and likewise to prevent any of the electricity being carried off by the axis. The prime conductor consists of several pieces, and is supported by three glass pillars 57 inches in length. The plates are made of French glass, as this is found best next to the English flint which could not be procured of sufficient size. The conductor is divided into branches which enter between the plates, but collect the fluid by means of points only from one side of the plate. The force of two men is required to work this machine; but when it is required to be put in action for any length of time, four are necessary.

By this machine Van Marum made his experiments on metals, &c., which will be mentioned hereafter.

Within these few years, Dr Van Marum has constructed a new machine, of smaller dimensions, but of much greater proportional power than the preceding. It is thus described in Nicholson's Journal. Fig. 75, Pl. CXC. exhibits a perspective view of the machine, and fig. 76, 77, 78, 79, a section, exclusive of the cushions. In the view it may be observed that the cushions are each separately inflated upon pillars of glass, and are applied nearly in the direction of the horizontal diameter of the plate, instead of the vertical diameter as heretofore. The ball diametrically opposite to the handle is the prime conductor, and the semicircular piece with two cylindrical ends serves, in the position of the drawing, to receive the electricity from the plate. By the happy contrivance of altering the position of this semicircular branch from vertical to nearly horizontal, the cylindrical ends may be placed in contact with the cushions, and the prime conductor instantly exhibits negative electricity. But as it is necessary that the cushions should communicate with the ground when the positive power is wanted, and that they should be inflated when the negative power is required, there is another semicircular branch applied to the opposite side of the plate nearly at right angles to the first. That is to say, when positive electricity is wanted, this second branch denoted by I, I in the section fig. 76, is placed nearly horizontal, and forms a communication from the cushions to the ground through a metallic rod from K behind the mahogany pillar which supports the axis; but when, on the contrary, the negative power is wanted, and the branch from the prime conductor is placed in contact with the cushions, this other branch from the axis is put into the vertical situation, and carries off the electricity emitted from the plate of glass. The axis of the plate B h, fig. 76, is supported by a single column A, which for that purpose is provided with a bearing-piece K, on which two brafs collar-pieces DD, represented more at large and in face in figs. 78, are fixed, and carry the axis itself. The whole of fig. 76 is reduced to one sixtieth of its real dimensions, unless contracted by the shrinking of the paper after printing; to obviate which, it may be remarked that the diameter of the plate is 34 English inches. The axis has a counterpoise O, of lead, to prevent too great friction in the collar D nearest the handle. The arc of the conductor EE, which carries the two small receiving conductors FF, is fixed to the axis G, which turns in the ball H. On the other side of the plate is seen the other arc II, of brafs wire, half an inch in diameter, fixed to the extremity of the bearing-piece K, so that it may be turned in the same manner as the arc EE. The two receiving conductors FF are six inches long, and two and a half inches in diameter. The double line P represents a copper tube terminating in a ball Q. It moves like a radius upon the stem R of the ball S, which being screwed into the conductor H, serves to confine the arm P in any position which may be required. The diameter of the ball S is only two inches, which, together with certain other less rounded parts of this apparatus, may serve to show that considerable electricity from this machine is less disposed to escape than if it had proceeded from a cylinder. The diffusion of electricity along the glass supports is prevented by a kind of cap T, of mahogany, which affords an electrical well or cavity underneath, and likewise effectually covers the metallic caps into which the glass is cemented. The lower extremity of the cap is guarded in the same manner by a hollow piece or ring V, of mahogany, which covers the metallic socket into which the glass is cemented. The three glass pillars are set in sliding-pieces, as marked on the platform of fig. 75, which are nine inches long.

The rubbers of this machine differ in no essential particular from those described by the inventor in the Journal de Physique for February 1791; and the apparatus for applying them is described in the same work for April 1789. Fig. 77 represents a section of this judicious piece of mechanism seen from above, and one-fourth of the real size. A metallic sliding-piece bb, is fitted into a corresponding face, on the ball Z, which is one of those fixed on the top of the glass pillars near the circumference of the glass plate in fig. 75. To this is affixed the piece dd, which terminates in two hinges gg, that allow the springs ee to move in the plane of the horizon. The pieces gg represent the wood-work of the cushions attached to the extremities of the springs by the hinges hh. The springs are regulated by the bolt and screw ii. The two cushions are thus made to apply to the plate equally through their whole length; the actions on the opposite sides of the plate are accurately the same; and the play of the hinges gg prevents the plate from being endangered by any strain in the direction of its axis. It is certain that, before this adequate provision was made to secure these essential requisites, it was impracticable to apply the cushions to a plate with the same safety and effect as to cylinders, which possess much strength from their figure. An ingenious workman will probably find little difficulty in constructing these rubbers from this description and drawing; but the most precise information respecting every circumstance and dimensions is to be found in the letters above quoted.

The inner extremities of the cushions are defended by the plates of gum-lac YY, which cover the three sides or edges, and prevent their attracting the electric power from the ends of the receiving conductor.

The part of the axis which moves between the collars is made of steel. The middle of the non-conducting part of the axis is a cylinder of walnut-tree aa, baked until its insulating power is equal to that of glass, and then soaked in amber varnish, while the wood still remains hot. The two extremities of this cylinder, which are of a less diameter, are forced, by strong blows, with a mallet, into the stout brafs caps b and c, in which they are retained by three iron screws dd. The cylinder aa, and the brafs caps, are covered with a layer of gum-lac eee, to preserve the insulating state of the wooden cylinder more perfectly, and to prevent the cap b from throwing flashes to the rubbers. The bottom of the cap b is screwed home on the capped extremity of the steel axis b. The base of the cap c, which is four inches in diameter, terminates in an axis one inch thick, and two in length; the extremity of which is formed into a screw. The glass plate is put on this projecting part, and secured in its place by a nut of box-wood, forced home by a key, applied in the holes ii. Two rings of felt are applied on each side of the glass, to defend its surface from the contact of the wood and the metal; and the central hole in the glass, which is two inches in diameter, contains a ring of box-wood, which prevents its immediate application to the axis.

As it is necessary that the axis G should be parallel to the axis of the plate, in order that the conductors FF may move parallel to the plate itself, the pillar M is rendered adjustable by three bearing screws RR at the bottom, which react against the strong central screw T, and this is drawn downwards by its nut. The conductors FF are also adjustable by the sliding-pieces vv, and the binding-screws ww, which also afford an adjustment to bring the axis of each small conductor parallel to the face of the glass plate. A similar adjustment may be observed at the extremities of the arc II.

Fig. 79 represents a section of the moving part of the branch II, one-half of its real size. A brafs plate uu is screwed to the face of the capital K by three iron screws ss. To this is screwed another ring dd, which affords a groove for the moveable ring yy, into which the arms II are fixed. This is accordingly applied in its place before the ring dd is fixed.

The wooden part of the rubbers GG, fig. 77, is covered with thin plates of iron, excepting the surface nearest to the glass. The intention of this is to maintain a more perfect communication between the rubbed part of the cushion and the earth or negative conductor, as the case may be.

The plates of gum-lac YY, are applied to the rubbers, each by means of a thin plate of brafs, to which they are affixed by heat. There are two wires riveted in the plates, which are thrust into correspondent holes in the wooden part of the cushion.

The mahogany column A ends in a square ss, upon which Electrical which the piece K is fitted and firmly applied, by means of the screw and nut exhibited in the section.

The following description of a useful machine is taken from Mr Cavalla, who considers it as one of the most complete with which he is acquainted.

The frame of this machine consists of the bottom board ABC, which when the machine is to be used, is fastened to the table by two iron clamps, one of which appears in the figure near C. Upon the bottom board are perpendicularly raised two strong wooden pillars KL, and AH, which support the cylinder, and the wheel. From one of the brass caps of the cylinder FF, an axle of steel proceeds, which passes quite through a hole in the pillar KL, and has on this side of the pillar a pulley I, fixed upon its square extremity. Upon the circumference of this pulley there are three or four grooves, in order to suit the variable length of the string ab, which goes round one of them, and round the groove of the wheel D. The other cap of the cylinder has a small cavity, which fits the conical extremity of a strong screw, that proceeds from the pillar H. The wheel D, which is moved by the handle E, turns round a strong axle, proceeding from almost the middle part of the pillar KL.

The rubber G of this machine is on each end two or three inches shorter than the cylinder (i.e., the cylinder exclusive of the necks), and it is made to rub about one-tenth part of the cylinder's circumference, or rather less; it consists of a thin quilted cushion of silk, stuffed with hair, and fastened by silk strings upon a piece of wood, which is properly adapted to the surface of the cylinder. And to the lower extremity of the cushion, or rather of the piece of wood to which the cushion is tied, a piece of leather is fastened, which is turned over the cushion, i.e., stands between it and the surface of the cylinder, and to the extremity of which a piece of silk or oiled silk is fastened, which covers almost all the upper part of the cylinder. Upon this leather, which reaches from the lower to almost the upper extremity of the cushion, some of the amalgam is to be worked, so as to be forced as much as possible into its substance: if mica gold is to be tried, then the leather should be new, and wherein no other amalgam has been put. This rubber is supported by two springs, screwed to its back, and from which it may be easily unscrewed, when occasion requires it. The two springs proceed from the wooden cap of a strong glass pillar, perpendicular to the bottom board of the machine. This pillar has a square wooden base, that slides in two grooves in the bottom board ABC, upon which it is fastened by a screw. In this manner the glass pillar may be fastened at any required distance, and in consequence the rubber may be made to press harder or lighter upon the cylinder. The rubber in this manner is perfectly insulated; and, when insulation is not required, a chain with a small hook may be hanged to it, so as to have a regular communication with the piece of leather; the chain then falling upon the table, renders the rubber uninsulated.

Fig. 7 represents the prime conductor AB belonging to this machine. This is of hollow brass, and is supported by two glass pillars varnished, that by two brass sockets are fixed in the board CC. This conductor receives the electric power through the points of the collector L, which are set at about half an inch distance from the surface of the cylinder of the machine.

If the handle E of the wheel, be turned (and on account of the rubber it should be turned always in the direction of the letters abc) this machine standing in the situation that is represented in the figure, will give positive electricity, i.e., the prime conductor will be electrified positively. But if a negative electricity be required, then the chain must be removed from the rubber, and hung to the prime conductor; for in this case the electricity of the prime conductor will be communicated to the ground, and the rubber remaining insulated, will appear strongly negative. Another conductor, equal to the conductor AB, may be connected with the insulated rubber, and then the operator may obtain as strong negative electricity from this, as he can positive from the conductor AB.

The next machine which we shall mention is one invented by Mr Nairne, which is chiefly employed for medical purposes; but a modification of which, to be presently described, will answer for most purposes of electrical experiment better than any other.

The cylinder in Mr Nairne's machine is about twelve inches long and seven in diameter; it turns upon two wooden pieces cemented on the top of two strong glass pillars, BB. These pillars are made fast into the bottom board of the machine, which is fastened to the table by means of a crank. There are grooves made in the under part of the bottom of the crank, through which the pieces FE slide. On these pieces the pillars stand by which the two conductors are supported; and in order to place these conductors nearer to the cylinder, or remove them farther from it, the pieces on which they stand are moveable inwards or outwards, and may be fixed by the two screws LL. The rubber is fastened to the conductor R; and consists of a cushion of leather stuffed, having a piece of silk glued to its under part. This last being turned over the surface of the cushion, and thus interposed between it and the glass, goes over the cylinder, and almost touches the pointed wires which are fixed on the other conductors. The conductors are of tin covered with black lacquer, each of them containing a large coated glass jar, and likewise a smaller one, or a coated tube, which are visible when the caps NN are removed. To each conductor is fixed a knob O, for the occasional suspension of a chain to produce positive or negative electricity. The part of the winch C, which acts as a lever in turning the cylinder, is of glass. Thus every part of the machine is insulated, the cylinder itself and its brass caps not excepted. And to this the inventor has adapted some flexible conducting joints, a discharging electrometer, and other utensils necessary for the practice of medical electricity.

A modification of this machine is represented at fig. 9.

a, the handle of the cylinder. b, the negative, and c, the positive conductor. d, the silk flap of the rubber.

Mr Reid's portable machine, as improved by Mr Reid's Lane, is the last which we shall describe, and is represented at fig. 10. A is the glass cylinder, moved vertically by means of the pulley at the lower end of the axis. Electrical axis. This pulley is turned by a large wheel B, which lies parallel to the table. There are three pulleys of different dimensions marked in the figure; one of which revolves four times for every revolution of the large wheel B. The conductor C, is furnished with points to collect the fluid, and is screwed to the wire of a coated jar D, which stands in a socket between the cylinder and the wheel. This figure also shows how Mr Lane's electrometer, to be afterwards described, may be adapted to this machine.

A great many other machines have been described in the Philosophical Transactions, Journal de Physique, and in various books on electricity; but those of which we have given an account are the most material.

**CHAP. III. General directions for using the Electrical Machine.**

It is of the greatest consequence that the machine, as well as the table on which it stands, and everything in its neighbourhood, be perfectly free from dust; it is therefore necessary to begin by wiping every part of the machine, &c., with a clean, dry, soft linen cloth. If the weather is not warm and dry, it will be proper also to place the machine for some time before the fire, that it may be perfectly free from moisture. The cylinder if used lately and not cleaned, may have contracted spots of dirt or grease; in which case it must be rubbed with a soft rag dipped in spirit of wine. In short, very much depends on the machine being quite free from dirt and moisture.

The conductors are now to be fixed in a proper situation, so that the rubber of the negative conductor may press closely to the cylinder on one side, and the points of the positive conductor may approach on the other as near to it as possible, without touching. Then while the cylinder is made to revolve, the amalgam is to be applied to it, where it is not covered with the silk; this is best done by means of a piece of leather to which the amalgam has been previously fastened, which is a better method than by spreading it on the rubber. As the amalgam is liable to oxidation from exposure to the air, it is proper to scrape the surface of it before it is applied to the cylinder; and if any old amalgam has been left on the cushion of the rubber, this should also be scraped before using the new.

After having made these arrangements, on whirling the cylinder in contact with the rubber, without bringing any conducting body near the former, or inflating the latter, we will perceive in the dark a stream of fire issuing from the place of contact between the rubber and the cylinder, and adapting itself to the form of the cylinder, so as to involve it in a blue flame mixed with bright sparks; the whole making a very perceptible whizzing and snapping noise. If the finger is brought near the cylinder in this situation, the flame and sparks will leave the cylinder and strike the finger; and this phenomenon will continue as long as the globe continues to be whirled round.

On applying the prime conductor, the light will vary, and be perceptible only upon the points presented to it by the cylinder; but if the finger is now brought near the conductor, a very small spark will strike it, and that at a greater or smaller distance, according to the strength of the machine. This spark, when the electricity is not very strong, appears like a straight line of fire; but if the machine acts very powerfully, it will put on the appearance of zig-zag lightning, throwing out other sparks from the corners, and strike with such force as to give considerable pain to those who receive it.

If these appearances do not take place, or if they take place only in a slight degree, soon after the applying the amalgam, spread a little oil on the palm of your hand, and let it slightly touch the cylinder as it moves round; in general this is instantly followed by a copious emission of sparks, numerous torrents of which will now pass from the edge of the silk to the knuckles. Sometimes, however, after using all these precautions, the machine does not act well, and in this case the rubber should be examined, to see if something is not wrong there. The rubber should be removed from the glass pillar or the negative conductor, to which it is fastened, by taking out the screws by which it is usually secured; it is then to be brought near the fire so that the silk may be perfectly dried, after which a little tallow or fuel should be rubbed upon the cushion, and it should then be replaced in its situation. If the silk of the rubber is fitted to the cushion by means of a wire as described in (42.) it will only be necessary to take out this wire, in order to dry the silk.

While both conductors remain insulated, the machine will not continue to act long, or at least its action will be much less powerful; but if the negative conductor or rubber be made to communicate with the floor or a moist wall, it will in general continue its action for any length of time required.

The weather is found to have considerable influence on the action of an electrical machine; in wet weather it will neither act to powerfully, nor for so long a time as when the weather is moderately warm and dry, unless perpetual care be taken to keep every part of it warm and clean. Very hot dry weather is also unfavourable to the action of the machine, and when this happens, even the floor of the room may be too dry to serve as a conductor; it is then necessary to connect the rubber by a chain which communicates with some moist surface, as a cellar, a pump, or the like.

Mr Nicholson lays down the following directions for Mr Nicholson's directions for preparing the machine for experiment.

Clean the cylinder, and wipe the silk.

Grease the cylinder, by turning it against a greased creasing the power of leather till it is uniformly obscured. I use the tallow the cylinder.

Turn the cylinder till the silk flap has wiped off so much of the grease as to render it semitransparent.

Put some amalgam on a piece of leather, and spread it well so that it may be uniformly bright. Apply this against the turning cylinder. The friction will immediately increase, and the leather must not be removed until it ceases to become greater.

Remove the leather, and the action of the machine will be very strong.*

**CHAP. IV. An Enumeration of some other Parts of 1799. an Electrical Apparatus to be described hereafter.**

There are many other parts of the electrical apparatus; but these we can only enumerate here, as their description and use will come more properly to be explained. Chap. I. ELECTRICITY.

Principles explained under the principles on which they are constructed.

Electricity illustrated by experiment.

Fig. 12. and 13. represent different forms of coated jars or Leyden phials employed for the accumulation of the electric power, and the usual forms of the discharging rod.

Fig. 14. shews one of the most approved forms of the electrical battery.

Fig. 15. represents a stand supporting four electrometers for ascertaining the presence and measuring the degree of electricity.

Fig. 17. exhibits the usual form of the quadrant electrometer invented by Mr Henly, placed on the end of the prime conductor.

Fig. 18. represents Mr Bennet's gold-leaf electrometer.

Fig. 19. shews Mr Cavallo's pocket electrometer.

Fig. 20. represents Mr Henly's universal discharger; and fig. 30. a press belonging to it.

Fig. 31. and 32. shew an outline of Mr Morgan's discharging rod.

Fig. 42. represents Mr Nicholson's instrument for distinguishing positive and negative electricity.

Fig. 67. gives a view of Lane's electrometer.

Fig. 68. and 69. represent Mr Brooke's electrometer as made by Mr Adams.

Fig. 70. represents Cuthbertson's compound or universal electrometer.

Fig. 71. and 72. are two views of Dr Robison's comparable electrometer.

Fig. 73. illustrates the mode of using the electrophorus.

Fig. 80. is a figure of an electrical machine in which silk is employed as an electric instead of glass.

Fig. 85. represents Bennet's doubler, and fig. 86. Nicholson's revolving doubler.

The rest of the apparatus figured in the plates will be enumerated and fully described in the succeeding parts of the article.

Besides the apparatus which we have described and enumerated, the electrician should have several glass tubes, some smooth and others of rough glass, sticks of sealing wax, a piece of yellow amber, &c., for exciting positive and negative electricity, when these two states are to be observed or compared.

It is of some consequence that an electrician should have some mechanical tools; as he may often be required to renew or repair parts of his apparatus, either to save expense, or when he is at a distance from a skilful workman. For this purpose, few tools are necessary. The principal are a turner's lathe, for turning caps, balls, pedestals, &c.; a blow-pipe with a proper lamp, for bending tubes, or opening and closing such as are of large diameter; a few files of various degrees of fineness, and various forms, as flat, half-round, rat-tail, &c.

PART II.

AN EXPERIMENTAL ILLUSTRATION OF THE PRINCIPLES OF ELECTRICITY.

We propose, in this part, to describe the principal phenomena of communicated electricity; and to illustrate these by experiments, which we shall, as nearly as can be done, class under certain general heads or principles. After recounting the experiments which illustrate each head, we shall describe the construction and explain the uses of the several electrical instruments which depend on the principle laid down. We shall also take an opportunity, in this part, of tracing the origin and progress of the more important discoveries which have been made in the experimental part of the science.

As it must be supposed that the reader is at present unacquainted with the theory of electricity, the principles to which the several experiments in this part are referred, will be merely such facts or general phenomena as have been observed in the course of experiment, independently of theory. In the following part of this article, we shall endeavour more accurately to illustrate these phenomena, and explain them according to the most generally received theory of electricity.

Chap. I. Of Electrical Attraction and Repulsion, and the Instruments which depend on them.

An electrified body attracts bodies brought near it, and after holding them in contact with it for some time, again repels them.

Vol. VII. Part II.

Experiment 1.—Suspend a downy feather by a filken thread; on making it approach within a few inches of the prime conductor, while the cylinder is set in motion, it will be attracted to the conductor, and almost immediately repelled; and thus alternately attracted and repelled, as long as the machine continues to be worked.

This experiment may be thus varied in a pleasing manner. Take a glass tube, no matter whether smooth or rough, and, after rubbing it, present it to a downy feather; this will, as in the former instance, be instantly attracted, and be retained for a short time in contact with the tube; when it will be repelled. If, at the time of its repulsion, the tube be held in the air at a distance from surrounding objects, the repelled feather will float above the tube, and may be driven about the room as long as it does not touch any object in its neighbourhood. If one person hold a smooth glass tube, and another a rough tube or a stick of sealing wax, and a feather be let loose between them when excited, the feather will leap from one to the other, and thus the two persons will seem to drive it between them like a shuttlecock, whence this experiment is called the electrical shuttlecock.

Experiment 2.—Let there be two metallic plates, one as Dancing c, fig. 20., supported by a stand, so that it may be placed on a table, &c., the other d provided with a hook, by which it may be hung by a chain to the prime conductor.

Principles of Electricity illustrated by experiment.

Electricity at some distance from the other plate. Then cut some small figures of men or other objects in paper, or what is better, form them out of the dry pitch of elder, or of rushes, and lay them on the lower plate. On working the machine the figures will rise from the lower plate, and move perpetually from the one plate to the other as represented in the figure.

Exper. 3.—Let a solid rod of glass, as a, fig. 21, be made to pass through a bell b, perforated for the purpose, and let one end of the rod be fixed in a wooden foot, while the other supports two metallic arms, c d e f, crooking each other, and knobbled at their extremities. From each extremity let a small bell without a clapper be suspended by a metallic wire, and from each arm, at a little distance from the extremities, let the clappers of these bells be suspended by filken threads. On connecting the top of the stand with the prime conductor, and setting the machine in motion, the clappers will begin to move between the central bell and the other four so as to ring the whole five.

Here the bells receive the electric power from the prime conductor, and being electrified, attract and repel the clappers which hang freely between them.

Exper. 4.—Tie a small body, as for instance a light piece of cork, to a silk thread about eight inches long, and holding the thread by its end, let the small body hang at the distance of about eight inches from the side of the prime conductor electrified. This small body, if the electrification of the conductor is not strong, will not be attracted. But if a finger or any conducting substance be presented to that side of the small body which is farthest from the prime conductor, then the small body will immediately move toward the prime conductor; and when this body has touched the prime conductor, it will be instantly repelled from it, on account of the repulsion existing between bodies possessed of the same kind of electricity.

Indeed, if this insulated body be very near to the prime conductor, or the prime conductor strongly electrified, then the small body will be attracted without presenting to it any conducting substance; or the natural fluid belonging to that body will be all crowded on that side of it which is nearest to the prime conductor.

If this small body, instead of the silk, be suspended by a linen thread, it will be attracted at a much greater distance, than in the other case.

Bodies in the same state of electricity, i.e., which are all electrified positively, or all negatively, have a tendency to repel each other.

Exper. 1.—Stick a downy feather into one of the holes of the prime conductor. When the cylinder is moved the feather will begin to swell, and its plumes will separate to a considerable distance from each other.

This experiment may be varied, by placing the representation of a human head upon the prime conductor. When the cylinder is moved, the hair of the head will bristle up and stand erect as represented in Plate CLXXXVII fig. 22.

Exper. 2.—Let small balls made of cork or the pith of elder well dried, be suspended from the prime conductor by threads of an equal length. While the cylinder continues at rest, the balls will touch each other, but as soon as the machine is set in motion they will repel each other to a greater or less distance, according as the electric power produced is stronger or weaker.

It is not necessary that the threads be in contact with the prime conductor, for if the balls be brought near the conductor while the machine is in motion, they will recede from each other as before.

The same effect will be produced whether the balls are hung from the positive or the negative conductor.

From the circumstance observed in the above experiments we deduce the following important corollary.

Objects brought near an electrified body are electrified Corollary by position.

The communication of electricity from an electrified body, to another which is not in contact with it, but is only in its vicinity, may for the present be conceived by remarking that these bodies are surrounded with air. Air, although an electric, is not a very perfect electric, but is more or less also a conductor, especially when it is moist. When a body is electrified it communicates to the air in contact with it a portion of its electric power, and thus the air becomes electrified, and of course imparts to the bodies, which are surrounded by it a degree of electricity; and this the more easily as it is in a better conducting state.

The apparent action of the air in communicating electricity to a body which is surrounded by it, may be illustrated by the following experiments.

Inflate in a horizontal position a metallic rod about two feet long, having blunt ends, and at one of its ends suspend an electrometer, like that represented in fig. 116; then bring within three or four inches distance of its other end an excited glass tube. On the approach of the tube, the balls of the electrometer will open, and if you present towards them a body positively electrified, you will perceive that they diverge with positive electricity. If the tube be removed, the balls come together again, and no electricity remains in them, or in the metallic rod. But if while the tube is near one end of the rod, and the balls diverge with positive electricity, the other end of the rod, viz. that from which the electrometer hangs, be touched with some conductor, the cork balls will come immediately together, and they will remain so when the conductor has been removed—remove now the excited glass tube, and the balls will immediately diverge with negative electricity—which shows that the rod remains electrified negatively.

If the above experiment be made with an electric negatively electrified (for instance, a rod of sealing-wax instead of the excited glass tube) then the apparent electricities in the rod will be just the reverse of what they were before; for in this case, that end of the rod to which the electric has been presented, will be positive, and the opposite end negative; which opposite end, if touched in this state with some conducting substance, will acquire some electric power from that substance; and when, after that substance has been removed, the excited electric is also removed, the rod will remain positive.

In making this experiment, care must be taken that the end of the rod be very blunt, and that the electric be not very powerfully excited; otherwise a spark may pass from this to the rod, which renders the experiment precarious. Take two rods of metal, each about a foot long, furnished with knobs at both ends; and, either by silk lines or by inflating stools, inflate them, so that they may stand horizontally in one direction, and about a quarter of an inch distance from one another. To the middle of each of these rods hang an electrometer, like that represented in fig. 16. This done, take an excited glass tube, and bring it to about three inches distance from the knob of one of the rods; on doing which, the electrometers of both rods will appear electrified: keep the tube in that situation for about two seconds, then remove it. The rods now will remain electrified, as appears by the electrometers; the first, viz. that to which the excited tube had been presented, remaining negative, and the other positive.

In this experiment, if, instead of the glass tube, an electric, negatively excited, be brought near the end of one rod, then that rod will be electrified positively, and the other negatively.

This is all that we can properly explain at present with respect to the agency of the air in the production of electrical phenomena. We shall take occasion to consider this subject more fully in a future part of this article, when we shall see that a variety in the state of the air produces considerable diversity in the phenomena.

On the principle of electric repulsion and the above corollary depend the action of several instruments which are of great use in electrical experiments, and which we shall now describe.

Instruments which are employed to ascertain the presence of electricity are called electroscopes. As they are generally employed to measure the degree of electricity produced, they are also called electrometers, and by this name we shall in future distinguish them.

The first electrometer appears to have been constructed by the abbe Nollet; it consisted of two threads of silk, which, as has been shown, recede from each other on the approach of an electrified body. He observed the angle of their divergency by its shadow cast on a board placed behind them. Mr Watzlitz this electrometer by appending small weights to the threads.

Mr Canton contrived an electrometer which is the foundation of those which are now in common use. He got a pair of balls turned in a lathe out of the dry pith of elder; these he hung by threads of the finest linen, and kept them in a narrow box with a sliding cover, where they were disposed that the threads could lie straight. When he was to use it, he held the box by the extremity of the cover, and allowed the balls to hang freely from a pin to which they were fixed.

Fig. 15 represents a stand supporting the electrometers DD, CC. B is the basis of it, made of common wood. A is a pillar of wax, glass, or baked wood. To the top of the pillar, if it be of wax or glass, a circular piece of wood is fixed; but if the pillar be of baked wood, that may constitute the whole. From this circular piece of wood proceed four arms of glass, or baked wood, suspending at their ends four electrometers, two of which, DD, are silk threads about eight inches long, suspending each a small downy feather at its end. The other two electrometers, CC, are those with very small balls of cork, or of the pith of elder; and they are constructed in the following manner:—

A stick of glass about six inches long, covered with sealing-wax, and shaped at top in a ring: from the lower extremity of this stick of glass proceed two fine linen threads (m) cc, about five inches long, each suspending a cork or pith-ball d, about one-eighth of an inch in diameter. When this electrometer is not electrified, the threads cc hang parallel to each other, and the cork-balls are in contact; but when electrified, they repel one another, as represented in the figure. The glass stick ab serves for an insulating handle, by which the electrometer may be supported, when it is used without the stand AB.

Another species of the above electrometer is represented in fig. 16; which consists of a linen thread, having at each end a small cork-ball. The electrometer is suspended by the middle of the thread on any conductor proper for the purpose, and serves to show the kind and quantity of its electricity.

Fig. 17 represents the quadrant electrometer of Mr Henly, one of the most useful instruments of the kind yet discovered, as well for measuring the degree of electricity of any body, as to ascertain the quantity of a charge before an explosion; and to discover the exact time the electricity of a jar changes, when without making an explosion, it is discharged by giving it a quantity of the contrary electricity. The pillar LM is generally made of wood, the graduated arch NOP of ivory, the rod RS is made of very light wood, with a pith ball at the extremity; it turns upon the centre of the semicircle, so as always to keep near its surface; the extremity of the stem LM may either be fitted to the conductor or the knob of a jar. When the apparatus is electrified, the rod is repelled by the stem, and moves along the graduated arch of the semicircle, so as to mark the degree to which the conductor is electrified, or the height to which the charge of the jar is advanced.

Beccaria recommends fixing the index between two semicircles, because when it is placed over one only, the electricity of this repels and counteracts the motion of the index. Other improvements and variations have been made in this instrument, which will be described hereafter.

The first account of Mr Henly's electrometer was given in the Phil. Trans. vol. lxiii. by Dr Priestley, who speaks of it in very high terms in a letter to Dr Franklin. He considers it as a perfect instrument for measuring degrees of electricity, but it will appear hereafter that this is not the case.

The scale in Mr Henly's quadrant is divided into equal parts; but M. Achard has already shewn that when this is the case, the angle at which the index is suspended by the electric repulsion is not a true measure of the repulsive force; to estimate which truly, scale he demonstrates that the arc of the electrometer should

(m) These threads should be wetted in a weak solution of salt. Principles of Electricity illustrated by experiment.

The balls of the ordinary electrometer may be made of pith or cork, but the latter must be very smooth and well polished. They are best made in a turner's lathe. They may be made of any shape, provided they are regular and free from edges. A very convenient electrometer is made of two long, slender pieces of ruff pith, made and appended to short threads of flax. These may easily be hung parallel to each other, whereas in the usual ball-electrometers the threads to which the balls are hung form an angle with each other. This parallelism of the threads is of advantage, and was considered of so much consequence by Lord Stanhope (better known to electricians by the title of Lord Mahon) that he was at great pains to suspend his balls in a parallel position.

Of all the instruments by which it has been attempted to measure electricity, none have been found to answer the purpose better than that invented by Mr Benet, and which is represented in fig. 18. It consists of two slips of gold leaf, a a, suspended in a glass cylinder b. The foot, c, may be made of wood or metal, and the cap, d, should be of metal; the latter being made flat at top for the convenience of putting anything upon it that is to be electrified. The cap is about an inch wider than the diameter of the glass, and its rim about three quarters of an inch broad, hanging parallel to the glass to keep it sufficiently inflated, and to turn off the rain, when the instrument is employed in experiments on atmospheric electricity. Within this is another circular rim about half as broad as the former, lined with silk or velvet, so that it may be made to fit the outside of the glass exactly, while the cap may be easily taken off to repair any damage done to the gold leaf. From the centre of the cap hangs a thin tube somewhat longer than the depth of the inner rim, in which a small peg, f, is placed, which may be taken out occasionally. To this peg, which is rounded at one end and flat at the other, two slips of gold leaf are fastened with paste, gum water, or varnish. They are about a fifth part of an inch broad and two inches long, and are generally made tapering to a point. In one side of the cap is a small tube, g, to place wires in; h, h, are two long pieces of tin-foil fastened with varnish on opposite sides of the internal surface of the glass, where the slips of gold leaf may be expected to strike, and in connexion with the foot of the instrument. The upper end of the glass is covered and lined with sealing-wax as low as the outer rim, in order to make the insulation more complete.

An improvement on this electrometer is to make the cylinder pretty long, and to have a small additional tube of gum lac on the end of it. The slips of tinfoil reach almost to the edge of the outer rim, and are sharply pointed at the top, widening in the middle and decreasing in breadth again as they descend.

The great advantage of this instrument over the electrometers which we have described above is its extreme sensibility, which will appear from the following examples.

1. On putting powdered chalk into a pair of bellows and blowing it upon the cap, this was electrified positively when the nozzle of the bellows was about six inches from it; but at the distance of three feet from the nozzle, the same stream electrified the cap negatively. Thus it appears that the electricity may be changed from positive to negative merely from the circumstance of this stream of chalk being more widely diffused in the air. It may also be changed by placing a bunch of fine wire, silk, or feathers, in the nozzle of the bellows: and it is likewise negative when the air is blown from a pair of bellows wanting the iron pipe, so that it may come out in a larger stream; but this last experiment succeeded best when the air was damp. There is likewise a remarkable difference between the experiments in which the electricity is positive and that in which it is negative; the former being communicated to the cap with some degree of permanency, so that the slips of gold leaf continue for some time to diverge; but the latter being only momentary, and the slips collapsing as soon as the cloud of chalk is dispersed. The greater permanency of the electricity in the former case is owing to some of the chalk sticking to the cap when the nozzle of the bellows is very near it.

2. A piece of chalk drawn over a brush, or powdered chalk put into the brush, and projected upon the cap, electrifies it negatively; but its electricity is not communicated.

3. Powdered chalk blown with the mouth or bellows from a metal plate placed upon the cap, communicates to the cap a permanent positive electricity. If the chalk is blown from the plate either insulated or not so, that the powder may pass over the cap, if not too far off, the electricity communicated is also positive; or if a brush be placed upon the cap and a piece of chalk be drawn over it, the slips of gold-leaf when the hand is withdrawn gradually open with positive electricity as the cloud of chalk disperses.

4. Powdered chalk falling from one plate to another placed upon the instrument electrifies it negatively.

Other methods of producing electricity with chalk and other powders have been tried; as projecting chalk from a goose wing, chalking the edges of books and clapping the leaves of the book suddenly together, afflicting the powder upon the cap, all which electrified it negatively; but the instrument being placed in a dusty road, and the dust struck up with a stick near it, electrified it positively. Breaking the glass-pearl upon a book electrified it negatively, but when broken in water it did not electrify it.

Wheat flour and red lead produced a strong negative electricity in all cases where the chalk produced a positive electricity. The following powders were like chalk: red ochre, yellow rosin, coal ashes, powdered crocus metallorum, aurum molaicum, black-lead, lamp-black (which was only sensible in the two first methods), powdered quick-lime, umber, lapis calaminaris, Spanish brown, powdered sulphur, flowers of sulphur, iron filings, rust of iron, land. Rosin and chalk, separately alike, were charged by mixture; this was often tried in dry weather, but did not succeed in damp; white-lead also sometimes produced positive and sometimes negative electricity when blown from a plate.

If a metal cup be placed upon the cap with a red hot coal in it, and a spoonful of water be thrown in, it electrifies it negatively; and if a bent wire be placed in the cap, with a piece of paper fastened to it to increase its surface, the positive electricity of the ascending Principles of electricity may be tried by introducing the paper into it.

The sensibility of this electrometer may be considerably increased by placing a candle on the cap. By this means, a cloud of chalk, which in the other case only just opens the gold-leaf, will cause it to strike the fides for a long time together; and the electricity which was not before communicated, now passes into the electrometer, causing the gold-leaf to repel after it is carried away. Even sealing-wax by this means communicates its electricity at the distance of 12 inches at least, which it would scarcely otherwise do by rubbing upon the cap.

A cloud of chalk or wheat flour may be made in one room, and the electrometer with its candle be afterwards leisurely brought from another room, and the cloud will electrify it before it comes very near. The air of a room adjoining to that wherein the electrical machine was used, was very sensibly electrified, which was perceived by carrying the instrument through it with its candle.

No sensible electricity is produced by blowing pure air, by projecting water, by smoke, flame, or explosions of gun-powder.

A book was placed upon the cap, and struck with silk, linen, woollen, cotton, parchment, and paper, all which produced negative repulsion; but when the other side of the book was struck with silk, it became positive; this side, struck at right angles with the former, was again negative; and by continuing the strokes which produced positive, it changed to negative for a little while; and by flopping again, became positive. No other book would do the same, though the sides were scraped and chalked, upon a supposition that altering the surface would produce it. At last, one side of a book was moistened, which changed it; whence it was concluded, that one edge of the book had lain in a damp place; which conjecture was farther confirmed by all the books becoming positive in damp weather, and one of them being dried at the fire again became negative.

When the cap is approached with excited sealing-wax, the gold-leaf may be made to strike the fides of the glass more than twelve times; and as the sealing-wax recedes, it strikes nearly as often; but if it approaches much quicker than it recedes, the second number will sometimes be greater.

The quantity of electricity necessary to cause a repulsion of the gold-leaf is so small, that the sharpest points or edges do not draw it off without touching; hence it is unnecessary to avoid points or edges in the construction of this instrument.

To the experiments on blowing powders from a pair of bellows, it may be added, that if the powder is blown at about the distance of three inches upon a plate which is moistened or oiled, its electricity is contrary to that produced by blowing upon a dry plate. This shows that the electricity of the streams of powder rising out of the bellows is only contrary to the more expanded part, because it is within the influence of its own atmosphere; for when this is destroyed by the adhesion of the powder to the moistened place, it is negative when the bellows are positive, as it was before positive when the more expanded cloud was negative.

This instrument is also free from an inconvenience which attends the electrometers in which cork or pith balls are employed. In these, when the balls are electrified, they are very apt to adhere together for some time before the repulsion takes place, and then they often separate with a jerk so as to recede from each other farther than they ought to do, and thus make the electricity produced appear greater than it really is; whereas the flips of gold-leaf in Bennet's electrometer do not adhere together, and separate equally and gradually.

This instrument is, however, not without its defects, as the delicate texture of the gold-leaf renders it very difficult to fasten the flips, so as to keep them entire, and also prevent the instrument from being easily removed from one place to another. Mr Cavalli proposes poles to remedy these defects in the following manner: When the flips are cut and are lying upon paper, or on the leather cushion upon which they are cut, make them equal in length, by measuring them with a pair of compasses, and cutting off a suitable portion from the longest; then cut two bits of very fine gilt paper, each about half an inch long, and a quarter of an inch broad, and by means of a little wax, stick one of them to one extremity of each flip of gold-leaf, so as to form a kind of letter T. This done, hold up in the fingers of one hand, one of those pieces of paper with gold-leaf suspended to it, and hold the other with the fingers of the other hand; then bringing them near to each other, and having adjusted them properly, viz. so as to let them hang parallel and smooth, force the pieces of paper which now touch each other, between the two sides of a fort of pincers made of brass wire, or of very thin and hammered brass plate, which pincers are fastened to the under part of that piece which forms the top or cover of the glass vessel. As these gold flips are very apt to be spoiled, we should keep several of them ready cut in a book, each having a cross piece of paper fastened to one extremity, so that in case of accident, a new pair of gold flips may be soon put between the aperture of the above-described pincers; and by this means the electrometer is rendered, in a certain manner, portable.

Mr Cavalli describes an electrometer which is nearly as sensible as Mr Bennet's, and is not liable to the inconveniences above mentioned. It is represented at fig. 19.

The case or handle of this electrometer is formed by a glass tube, about three inches long, and three-tenths of an inch in diameter, half of which is covered with sealing-wax. From one extremity of this tube, i.e. that without sealing wax, a small loop of silk proceeds, which serves occasionally to hang the electrometer on a pin, &c. To the other extremity of this tube a cork is adapted, which, being cut tapering on both ends, can fit the mouth of the tube with either end. From one extremity of this cork, two linen threads proceed, a little shorter than the length of the tube, suspending each a little cone of pith of elder. When this electrometer is to be used, that end of the cork which is opposite to the threads is pushed into the mouth of the tube; then the tube forms the insulated handle of the pith electrometer, as represented fig. 19, c. But when the electrometer is to be carried in the pocket, then the threads are put into the tube, and the cork stops it, as represented at b. The peculiar advantages Principles of stages of this electrometer are, its convenient small size, Electricity its great sensibility, and its continuing longer in good order than any other we have yet seen.

Represents a case to carry the above-described electrometer in. This case is like a common tooth-pick case, except that it has a piece of amber fixed on one extremity A, which may occasionally serve to electrify the electrometer negatively, and on the other extremity it has a piece of ivory fastened upon a piece of amber BC. This amber BC serves only to inflate the ivory, which, when inflated, and rubbed against woollen cloths, acquires a positive electricity; and it is therefore useful to electrify the electrometer positively.

There are many other electrometers employed by electricians; but these cannot properly be described at present, as they are constructed on principles which have not yet been explained. They will be noticed in their proper place.

The electric power forces a fluid to flow in a stream through a capillary tube, through which, when not electrified, it would only pass in drops.

Exper. 1.—Suspend a small metallic bucket full of water from the prime conductor, and place in the water a glass syphon, the diameter of whose tube is so small that the water will only drop from it. Now set the cylinder of a machine in motion, and the water will begin to flow in a full stream from the end of the syphon. The stream will sometimes be subdivided, and if the experiment is made in the dark, the water will appear luminous.

Exper. 2.—Dip a sponge in water, and suspend it from the prime conductor. The water which before only dropped from the sponge, will now flow very fast, and appears in the dark like fiery rain.

The effect of electricity on water flowing through capillary tubes, was first observed by M. Boze, but was more accurately investigated by the Abbé Nollet. He found that the stream of water through a capillary tube, was accelerated in the inverse ratio of the diameter of the tube; but that if the diameter of the tube, was less than a line; the stream was not sensibly accelerated. The important application which the able thought he could make of this experiment will be seen hereafter.

When an inflated vessel is electrified, and an inflated body, such as a ball-electrometer, is suspended within the cavity of the vessel, the body shows no signs of electrical attraction or repulsion.

The experiment by which this principle is to be illustrated, is called the electrical well, and is thus described by Mr Cavallo.

"Place upon an electric stool a metal quart mug, or some other conducting body nearly of the same form and dimension; then tie a short cork-ball electrometer, of the kind represented fig. 16. (x), at the end of a silk thread proceeding from the ceiling of the room, or from any other proper support, so that the electrometer may be suspended within the mug, and no part of it may be above the mouth; this done, electrify the mug, by giving it a spark with an excited electric or otherwise, and you will see that the electrometer, whilst it remains in that inflated situation, even if it be made to touch the sides of the mug, is not attracted by it, nor does it acquire any electricity; but if, whilst it stands suspended within the mug, a conductor standing out of the mug be made to communicate with, or only presented to it, then the electrometer acquires an electricity contrary to that of the mug, and a quantity of it, which is proportionable to the body with which it has been made to communicate; and it is then immediately attracted by the mug.

If, by raising the silk thread a little, part of the electrometer, i.e. of its linen threads, be lifted just above the mouth of the mug, the balls will be immediately attracted; for then, by the action of the electricity of the mug, it will acquire a contrary electricity, by giving to, or receiving the electric power from, the air above the cavity of the mug."

This experiment may be made in greater perfection by employing a globular glass vessel, with a narrow neck just sufficient to admit the electrometer, which should be fastened to a crooked glass rod, so that it may be presented to any part of the cavity. The outside of the vessel should be smeared with some clammy substance, as syrup or treacle, and may be inflated by placing it on a wine glass. The balls presented to the outside when the vessel is electrified, will be repelled, but presented to any part of the inside, they will show no signs of electricity, unless touched with some substance, as a wire, while within the cavity; when, on being taken out, they will repel each other.

This experiment was invented by Dr Franklin, and is called by him the electrical cup.

CHAP. II. Of the diversities exhibited by the electric power in its passage from pointed surfaces, and from obtuse surfaces.

When the electric power passes between an electrified body and a pointed conductor, a luminous stream is produced, attended with a current of air from the point.

Exper. 1.—Fix a metallic point in the prime conductor, and let the machine in motion. No crackling, but rather a hissing noise, will be heard, and a light will appear as if issuing from the point, and on holding the hand near it, a strong blast of air will be found to proceed from it. On holding another point at the distance of about half an inch from the point in the prime conductor, a stream of light will be seen passing between them, attended with a crackling noise. This current of air will be sufficiently strong to turn any light bodies which are freely suspended, and in this way the following pleasing experiments may be made.

Exper. 2.—Cut a round flat piece of cork, with the edges very smooth, and stick a number of small crow quills into the circumference, with the feather ends as represented in fig. 23.; pass a needle through the centre of the cork, and suspend this needle by a small magnet m; on holding the cork near the point

(N) Instead of the electrometer, there may be used any other kind of small conducting body; but that seems best adapted to such experiments. Chap. II.

Principles of point, the current of air will make it move round with great swiftness.

Exper. 3.—Let four arms of wire, with their extremities pointed and turned all in the same direction, be stuck in the circumference of a small circular piece of light wood, supported on a pointed wire, as represented in fig. 24. On bringing the wires near the point in the conductor, while the machine is in motion, they will move swiftly round as before, and in the dark, a beautiful circle of fire will be produced by the light issuing from the points. If figures of dogs, horses, &c. formed of elder pith, be stuck on the points, they will appear as if pursuing each other, thus forming what Mr Kinnerley called the electrical horse-race.

Exper. 4.—Fix eight bells near the edge of a circular board supported on four feet, as represented fig. 25, having a glass pillar e in the centre, terminated by a point g. On this point place the pointed wires used in the last experiment, hanging from one of them as d, a small glass clapper by a filken thread; and connecting the apparatus by a chain h, proceeding from the prime conductor. On setting the machine in motion, the wire will move round, and the clapper ring the bells.

Exper. 5.—By this motion of circulating points we may in some measure imitate the revolutions of the heavenly bodies, forming what is called the electrical orrery. Let a single wire, with the extremities pointed and turned as before, be nicely balanced on a point; fix a small glass ball over its centre, as a, fig. 26, to represent the sun. At one extremity of the wire, let a small wire be soldered perpendicularly, and on this balance another small wire with its ends pointed and turned, and having a small pith ball w in its centre to represent the earth, and a smaller ball of the same kind at one of the angles for the moon. Let the whole be supported upon a glass pillar, and be conducted by a chain proceeding from the prime conductor to the wire supporting the glass ball. Now, when the machine is put in motion, the wires will turn round, so that the ball representing the earth will move round the central ball, and the little ball at the angle of the smaller wire will at the same time revolve about the earth.

Exper. 6.—The power exerted by electricity upon points, may under some circumstances be made to counteract the power of gravitation. Let an inclined plane be formed of two parallel wires fastened by their extremities to four pillars of solid glass, M, N, O, P, fig. 27, fixed in a board so that the two at one end shall be higher than the other two. Then fix a wire with its ends pointed and turned in the same direction, at right angles upon a wire axis. When this axis is laid upon the inclined plane, it will of course roll to the bottom, but if, when it has nearly arrived there, the machine be put in motion, the wire will return up the plane, revolving on its axis.

Exper. 7.—Take a small lock of cotton, extended in every direction as much as can conveniently be done, and by a linen thread about five or six inches long, or by a thread drawn out of the same cotton, tie it to the end of the prime conductor; then set the machine in motion, and the lock of cotton on being electrified, will immediately swell, by repelling its filaments from one another, and will stretch itself towards the nearest conductor. In this situation let the cylinder be kept in motion, and present the end of your finger, or the knob of a wire, towards the lock of cotton, which will then immediately move towards the finger, and endeavour to touch it; but take with the other hand a pointed needle, and present its point towards the cotton, a little above the end of the finger, and the cotton will be observed immediately to shrink upwards, and move towards the prime conductor. Remove the needle, and the cotton will come again towards the finger. Present the needle, and the cotton will shrink again.

When the electric power passes between an electrified body and a conductor whose surface is obtuse, a luminous spark is produced, attended with an explosion, and these appearances are more or less strong in proportion as the surfaces are more or less obtuse.

Exper. 1.—When the prime conductor is situated in its proper place, and electrified by whirling the sparks cylinder, if a metallic wire with a ball at its extremity, or the knuckle of a finger, be presented to the prime conductor, a spark will be seen to issue between them, which will be more vivid, and will be attended with a greater or less explosion, according as the ball is larger. The strongest and most vivid sparks are drawn from that end or side of the prime conductor, which is farthest from the cylinder. The sparks have the same appearance whether they be taken from the positive or the negative conductor; they sometimes appear like a long line of fire reaching from the prime conductor to the opposed body, and often (particularly when the spark is long, and different conducting substances in the line of its direction) it will have the appearance of being bent to sharp angles in different places, exactly resembling a flash of lightning.

The figure of the spark varies with the superficial dimensions of the part from which it is taken. If it be drawn from a ball of two or three inches in diameter, it will have the appearance of a straight line; but if the ball from which it is drawn be much smaller, as half an inch in diameter, it will assume the zig-zag appearance above mentioned.

We have just seen that when the electric power passes from a point to a point, there are no sparks, but a luminous stream appears; but if the point be obliterated, by being thrust back so as to be on a level with the surface of the conductor, by being held between the fingers &c., the sparks will appear as before.

The length of the spark, or the distance through the length of air which it strikes from the conductor, depends on several circumstances; as, the length and diameter of the conductor; the termination of the surface from which, or to which, the spark passes; the dimensions of the cylinder; and the position of the conductor.

1. If the conducting body be increased in length only, the distance of the spark will be shortened. This fact was very early observed by Dr Priestley. He found that a spark from the end of a wire several yards in length, and about one fourth of an inch in diameter, was not longer than one taken from a conductor two feet in length, and two inches in diameter. Signor Volta found, that when he connected several rods, eight feet long, and half an inch in diameter, suspended at the distance of eight inches from each other, the spark drawn from them was not so long as one drawn from a conductor of the same length, but of twelve inches diameter. Mr. Brook of Norwich connected nearly twenty rods of wood covered with tin-foil, near seven feet long, and three-fourths of an inch in diameter, at about a foot from each other, so that the whole apparatus resembled a large gridiron, which was suspended from the ceiling by glass rods. From so large an extent of conducting surface no spark exceeding six inches could be drawn; whereas from a conductor eight feet long, and five inches in diameter, sparks may often be drawn above nine inches long, with the same cylinder.

2. If the diameter of the conductor be increased in proportion to its length, the spark is not so long as when it is shortened, while the diameter is increased. A conductor twelve feet long, and eight inches in diameter, does not yield a spark above half the length that may be drawn from a conductor of the same diameter, only six feet long.

3. The spark will strike to a greater distance, according as the cylinder is smaller in proportion to the conductor. A much longer and more violent spark was drawn from Mr. Brook's gridiron conductor with a cylinder only four inches in diameter than from a conductor five feet long, and five inches in diameter, with the same cylinder.

4. The length of the spark is greater from a ball of moderate dimensions than from the surface of the prime conductor.

5. The spark will be longer when the conductor is placed parallel to the cylinder, than when it is at right angles with the cylinder.

The sound of the spark varies according as the surfaces between which it strikes are more or less obtuse. It is louder when the spark is taken from the prime conductor, than when taken from a ball annexed to it; and it is loudest of all when the spark strikes from one flat surface to another: a straight spark is always louder than one of the zig-zag appearance. If the spark be made to pass from one end of a glass tube (clothed at both ends and very dry) to the other, the sound is entirely hushed.

When the spark is received by the knuckle it produces a sensation which is, more or less painful. It is more pungent when received from the prime conductor than from a ball attached to it. The spark produces a more painful sensation in proportion as it is shorter.

The most remarkable circumstance attending the electric spark is the light (60) produced in its passage through the air. The sparks which usually pass between the rubber of the negative conductor, through the cylinder to the points of the positive conductor, are of a beautiful light blue colour; but when the spark passes between the prime conductor and a ball of the diameter of an inch, its edges are purplish, and from these diverge several ramifications of a purple or indigo colour. If the balls between which the spark passes are not more than half an inch distance from each other, a continued stream of the most brilliant light will be produced, attended with a whizzing noise. If the distance of the balls from each other be increased, sparks equally brilliant will be produced; but their succession will be less quick, and no continued stream will appear.

The light emitted from electrified conductors is more copious and brilliant in proportion as their surfaces are more extended. If a person standing on an inflating stool, and connected with the prime conductor of a machine in motion, hold a flat plate of metal, as a pewter plate, while another person standing on the floor holds another plate, large flames of vivid light will appear between the plates, so as to illuminate a dark room.

Soon after the cylinder is set in motion, and sparks begin to issue, a peculiar odour may be perceived; and if the machine acts well, this is very powerful. It is difficult to describe this odour, but it seems to resemble that of phosphorus.

CHAP. III. Of charging and discharging the Leyden Phial; with directions for the construction of Jars and Batteries.

The electric power is communicated to electric vessels with difficulty, unless their surfaces be covered with some conducting substance; but it may be accumulated on them in a much greater degree than on conductors.

Exper. 1.—Take a common tumbler or glass dancing jar, and having placed a brass ball in one of the holes of the prime conductor, let the machine in motion, and let the balls touch the inside of the tumbler; while the ball touches only one point, no more of the surface of the glass will be electrified, but by moving the tumblers about so as to make the ball touch many points successively, all these points will be electrified, as will appear by turning down the tumbler over a number of pith or cork balls placed on a table. These balls will immediately begin to fly about, thus showing the electric attraction and repulsion illustrated in (61). This experiment is commonly called the experiment of the dancing balls, and is represented at fig. 28.

Exper. 2.—Let a glass jar, either cylindrical, such as is represented at fig. 12, Plate CLXXXVII., or with the mouth as wide a mouth as possible, be covered on both its inside and outside surfaces to within two inches of the top, with tin foil fastened on by means of gum water. The jar is then said to be coated. Fit to the mouth of the jar a piece of baked wood, through the centre of which pass a wire, whose lower extremity is terminated by a number of other wires, which must be made to touch the inside coating, while its upper extremity projects an inch or two above the mouth of the jar, and terminates in a metallic ball a. This ball should be perforated so as to receive a wire supporting a quadrant electrometer.

The jar being thus prepared, let the knob a communicate with the prime conductor, and let it remain charging while the cylinder is in motion till the ball c of the electrometer stands nearly horizontal; the jar is then said to be charged. It may be removed from the conductor.

(o) The first person who seems to have observed the electric light was Otto Guericke. He appears indeed only to have had a glimpse of it; and the first who perceived it in any great degree was Dr Wall, on rubbing a pretty large piece of amber. Vid. Philos. Trans. abridged, vol. ii. Let there be provided a curved brass rod, terminating at each end in a knob, and furnished with a glass handle, such as Def; if now one of the knobs, as e, be made to approach the ball a of the jar, while the other knob f touches the outside coating, a considerable explosion will take place, and the jar will lose its electricity, as will appear by the ball of the electroscope falling into a perpendicular situation.

The jar is then said to be discharged, and the rod Def is called a discharging rod.

A jar or phial of glass thus constructed is, for a reason which will presently appear, called a Leyden jar or phial.

In this experiment, the jar is not charged to its utmost height. If, instead of stopping when the index of the quadrant is nearly at right angles, we persist in charging, there will soon appear several luminous streams passing from the prime conductor across the cylinder to the cushion. Presently an explosion will take place from the phial, and this is called its spontaneous discharge. If the phial is not very strong, it will probably either be broken, or on examination will be found perforated in some part. If the glass be very thin, a spontaneous discharge will soon take place, attended with a harsh cracking noise, and the phial will certainly be cracked. A spontaneous discharge happens much more readily when the neck of the phial is very small, and consequently the wire comes very near the uncoated part of the glass.

If the uncoated part of the glass be moist or dusty, the jar will not receive a charge, so that it is necessary to be very careful in cleaning the jar before using it. When the uncoated part is made very hot, the spontaneous discharge is much accelerated.

The appearance of the uncoated part of the jar, when the discharge is made in the dark, is very curious. A great number of luminous streams will be seen pouring over the edge of the jar from the inside to the outside.

The force of the explosion depends very much on the termination of the extremity of the discharging rod. If this be terminated by a large ball, the noise will be much greater than when the ball is small; if it be terminated by a small obtuse surface, a hissing noise is heard before the explosion, and this is faint. But if the rod terminate in a point, no explosion will take place, but the jar will be silently discharged.

In the above experiment the jar is charged positively, it having been in contact with the positive conductor; but if it be connected with the negative conductor, the jar will be charged negatively. This will be more fully illustrated by and by.

As the accumulation of the electric power by means of coated jars forms one of the most important discoveries which have been made in this science, we shall here relate the method in which the discovery was made.

This discovery was accidental, and was the result of an experiment made in the end of the year 1745 by M. Van Kleift, dean of the cathedral in Cammin, who sent the following account of it to Dr Leibekuhn at Berlin.

When a nail or a piece of thick brass-wire, &c., is put into a small apothecary's phial, and electrified, remarkable effects follow; but the phial must be very dry or warm. I commonly rub it once before hand with a finger, on which I put some pounded chalk. If a little mercury, or a few drops of spirits of wine, be put into it, the experiment succeeds the better. As soon as this phial and nail are removed from the electrifying glass, or the prime conductor to which it has been exposed, is taken away, is taken away, it throws out a pencil of flame so long, that with this burning machine in my hand, I have taken above sixty steps in walking about my room. When it is electrified strongly, I can take it into another room, and there fire spirits of wine with it. If, while it is electrifying, I put my finger, or a piece of gold, which I hold in my hand, to the nail, I receive a shock which stuns my arms and shoulders.

A tin tube, or a man placed upon electrics, is electrified much stronger by this means than in the common way. When I present this phial and nail to a tin tube, which I have, fifteen feet long, nothing but experience can make a person believe how strongly it is electrified. Two thin glasses have been broken by the shock of it. It appears to me extraordinary, that when this phial and nail are in contact with either conducting or non-conducting matter, the strong shock does not follow. I have cemented it to wood, metal, glass, sealing-wax, &c., when I have electrified without any great effect.

M. Van Kleift communicated this experiment to several of his acquaintances, but they all for some time failed in their attempts to repeat it.

An experiment of a similar kind was soon after made at Leyden by Mr Cuneus. Making an attempt to communicate the electric power to water, contained in a phial, in which was a nail, happening to hold his glass in one hand, while he disengaged it from the prime conductor with the other, when he imagined that the water had received as much electricity as it was capable of acquiring, he was surprised with a sudden shock in his arms and breast, which he had not in the least expected.

This experiment was afterwards repeated in the presence of M. M. Allamand and Muffchenbroeck with similar results; and as it was at Leyden that the experiment was made with the greatest success, and afterwards improved, it obtained the name of the Leyden experiment, and a phial so constructed as to exhibit similar phenomena, has been ever since called a Leyden phial.

Indeed the philosophers of Leyden seem to have views of some merit in this discovery, which with them does which led not appear to have been merely accidental. The views to this discovery, which are said to have led to it were as follows:

Professor Muffchenbroeck and his friends, observing that electrified bodies, exposed to the common atmosphere, which is always replete with conducting particles of various kinds, soon lost their electricity, and were capable of retaining but a small quantity of it, imagined, that were the electric bodies terminated on all sides by original electrics, they might be capable of receiving a stronger power, and retaining it a longer time. Glass being the most convenient electric for this purpose, and water the most convenient non-electric, they first made progressive improvements with water in close bottles.

For a long time water and spirit of wine were the only conductors employed in this experiment; but it was

Principles of was soon found by Dr Watson, that the experiment succeeded better when the outside of the glass was coated with some metallic leaf, as sheet-lead, or tin-foil, while the phial contained some water within; and after this there was a natural transition to the use of an internal as well as external metallic coating, and thus the Leyden phial was completed in its present form (p).

A number of coated jars having their internal coatings connected together by metallic wires, constitute what is called a battery. Fig. 14. represents an electrical battery of the most approved form, containing nine jars. The bottom of the box is covered with tin-foil to connect the exterior coatings; the inside coatings of the jars are connected by the wires \(a b c, d e f, g h i\), which meet in the large ball \(A\); a ball \(B\) proceeds from the inside, by which the circuit may be conveniently completed. In one side of the box, near the bottom, is a hole through which a brass hook passes, and which communicates with the metallic lining of the box, and consequently with the outside coating of the jars. To this wire or chain is occasionally connected when a discharge is made; and for the more convenient making of this discharge, a ball and wire, \(B\), proceed to a convenient distance from the centre of the ball \(A\). When the whole force of the battery is not required, one, two, or three jars may be removed, only by pressing down the wires belonging to them, until their extremities can slip out of their respective holes in the brass ball, and then turning them into such a posture that they cannot have any communication with the battery. The number of jars represented in this figure is rather small for some purposes; but it is better to join two or three small batteries, rather than have a single large one, which is inconvenient on account of its weight and unwieldiness.

As coated jars form one of the most expensive parts of an electrical apparatus, it is of consequence that the electrician should himself be able to adjust them for experiment, and repair the coating, &c., when injured. We shall therefore give particular directions for the preparation of jars and batteries. The circumstances necessary to be attended to, respect principally the form of the coated electric, the substance employed as an electric, and the conductor employed as a coating.

For most experiments the best form is that of a cylindrical jar, in which the mouth is large enough to admit the introduction of the hand. A phial of this form is much more easily coated, cleaned, or repaired, than one of any other form. Mr Cuthbertson used to make his jars entirely cylindrical, but now he is of opinion that it is better to have the mouth a little contracted, and he has of late always made his jars of this latter form. For illustrating the theory of coated electrics, as we shall see hereafter, plates are the most convenient, and they are also useful in some experiments. Dr Robison prefers bottles of a globular form to any other, and he commonly employed the balloons used in distillation, which he says make excellent jars. The principles of bottles employed for holding mineral acids also make electricity very good jars, but they are rather inferior to the bottles illustrated, as having very thick bottoms. For ordinary purposes, where a glass-bottle is at a great distance, common green glass bottles or apothecary's phials with the mouths as wide as possible, will answer very well.

With respect to the electric employed for this purpose, glass is to be preferred on many accounts, and of employed this the best kind, as flint or crystal; but the expense here becomes a very considerable object, especially as the jars of a battery are very apt to break by reason of the inequality of their strength; for it should seem that the force of the electric power in a battery is equally distributed among all the bottles, without any regard to their capacities of receiving a charge singly considered. Thus if we express the quantity of charge which one jar can easily receive, by the number 10, we ought not to connect such a jar in a battery with one whose capacity is only 8; because the whole force of electricity expressed by 10 will be directed also against that whose capacity is only 8, so that the latter will be in danger of being broken. It will be proper, therefore, to compare the bottles with one another in this respect before putting them together in a battery. Besides the consideration of the absolute capacity which each bottle has of receiving a charge, the time which is taken up in charging it must also be attended to, and the jars of a battery ought to be as equal as possible in this respect as well as the former. The thinner a glass is, the more readily it receives a charge, and vice versa; but it does not follow from thence, as was formerly imagined, that on account of its thinness it is capable of containing a greater charge than a thicker one. The reverse is actually the case; and though a thick glass cannot be charged in such a short time as a thin one, it is nevertheless capable of containing a greater degree of electric power. In fact, if the glass be thinner than one-eighth of an inch, the phial will not bear any considerable charge. If the thickness of the glass be very great, no charge can indeed be given it; but experiments have not yet determined how great the thickness must be which will prevent any charge. Indeed it is observed, that though a thick glass cannot be charged by a weak electrical machine, it may be by a more powerful one, whence it seems reasonable to suppose that there is no real limit of this kind; but that if a machine could be made sufficiently powerful, glasses of any thickness might be charged.

Glass is attended with one considerable inconvenience; that it is very apt to attract moisture, and therefore the jars acquire perpetual care in wiping before they are used; and this, when a large battery is employed, becomes a very troublesome operation. It is the uncoated part of the jar which is injured by the moisture, for it is found, that if the coating be moist, the jar is more easily and more completely charged.

Electricians

(p) Dr Watson was indebted for the hint of a metallic coating to Dr Bevis, who was also the first electrician that employed a plate of glass coated on both sides in performing the experiments with coated electrics. Hence the coated plate is often called, especially by the continental electricians, Bevis's plate, or square, le carreau de Bevis. Electricians have often endeavoured to find some other electric which might answer better than glass for this purpose, at least be cheaper; but except Father Beccaria's method, which may be used very well, no remarkable discovery has been made relating to this point. He took equal quantities of very pure colophonium and powder of marble fitted exceeding fine, and kept them in a hot place a considerable time, where they became perfectly free from moisture; he then mixed them, and melted the composition in a proper vessel over the fire, and when melted, poured it upon a table, upon which he had previously stuck a piece of tin-foil, within two or three inches of the edge of the table. This done, he endeavoured with a hot iron to spread the mixture as equally as possible, and to the thickness of one-tenth of an inch all over the table; he afterwards coated it with another piece of tin-foil, reaching within about two inches of the edge of the mixture; in short he coated a plate of this mixture as he would a plate of glass. This coated plate seems, from what he says, to have had a greater power than a glass plate of the same dimensions: even when the weather was not very dry, and if it is not liable to be easily broken by a spontaneous discharge, it may be conveniently employed in place of glass; for it does not very readily attract moisture, and consequently may hold a charge better and longer than glass, besides when broken, it may be again repaired by means of a hot iron, whereas a broken plate or vessel of glass can seldom be employed again.

Talc, or Muscovy glass, is one of the most convenient electrics for the purposes of coating. It is not very apt to contract moisture, and will retain a charge for a very considerable time.

A very convenient portable phial may be constructed of sealing wax in the following manner: Procure a phial made of tin-plate, or white-iron (as it is called in Scotland), with a long neck; cover the outside of this phial with sealing wax as far as the neck, and coat the sealing wax to within a little of the neck with tin-foil. In this phial it is evident that the sealing wax is the electric, of which the tin-foil forms the outer and the tin-plate the inner coating.

When plates or jars having a sufficiently large opening are to be coated, the best method is to coat them with tin-foil on both sides, which may be fixed upon the glass with varnish, gum water, bees wax, &c.; but in case the jars have not an aperture wide enough to admit the tin-foil, and an instrument to adapt it to the surface of the glass, brads filings, such as are sold by the pin-makers, may be advantageously used; and these may be stuck on with gum water, bees wax, &c., but not with varnish, for this is apt to be set on fire by the discharge. Care must be taken that the coating do not come very near the mouth of the jar, for that will cause the jar to discharge itself. If the coating is about two inches below the top, it will in general do very well; but there are some kinds of glass, especially ringed glass, that when coated and charged have the property of discharging themselves more easily than others, even when the coating is five or six inches below the edge.

It is much more difficult to coat vessels of a globular form than plates or cylindrical jars; but the former may be coated with tolerable ease by attending to the method of cutting the tin-foil. This should be cut into the form of guillets as in covering a globe or in making a balloon; and they should be patted on, so as to overlap each other about half an inch. After having coated the sides of a balloon in this manner, the bottom is to be covered with a circular piece of tin-foil. The thinner the foil, the better it is adapted for the inside coating; and it may readily be applied by first patting it upon paper, and then patting either the paper or the foil next the glass.

In coating plates of glass it is better to cut the tin-foil into circular pieces, as it is found that a circular space is capable of giving as great a charge to the glass, as a square coating of the same breadth, and a spontaneous discharge does not so readily take place from the circular edge, as from the edges of a square coating.

Mr Brooke discovered, that when jars were coated with tin-foil first patted upon paper, they were rendered much less liable to be broken by the discharge. As coating, the trials which led to this discovery afford a useful lesson to the young electrician, we shall relate them in his own words.

"In making electrical experiments, and in particular those in which the Leyden phial is concerned, a method of preserving the bottles or jars from being struck through by the electric power, is very desirable; but I do not know that it has hitherto been accomplished. The number of them that have been destroyed in many of my experiments, have led me to various conjectures to preserve them: at the same time I have been obliged to make use of bottles instead of open-mouthed jars. And as coating the former withinside is very troublesome, it has put me on thinking of some method more easy, quicker, and equally firm and good, as with tin-foil. With respect to the new method of coating, I failed; though something else presented itself rather in favour of the former: therefore, introducing the process here will not be of very great use; unless in saving another the trouble of making use of the same method, or giving a hint towards the former so as to succeed with certainty. My aim was to find something that should be quick and clean, and not easy to come off with the rubbing of wires against it, and yet a good conductor. My first essay was with a cement of pitch, rosin, and wax, melted together; into which, to make it a good conductor, I put a large proportion of finely fitted brass filings. When this mixture was cold, I put broken pieces of it into the bottle, and warmed the bottle till it was hot enough to melt the cement in it so as to run, and cover the bottle withinside; then I coated the outside with tin-foil, as is commonly done, and now it was fit for use or ready to be charged, to which I next proceeded; and I believe I had not made more than four or five turns of the winch, before it spontaneously struck through the glass with a very small charge. I then took off the outside coating, and flopped the fracture with some of my common cement, after which I put the coating on again; and in as little time as before, it was struck through again in a different place; and thus I did with this bottle five or six times; sometimes it struck through the glass in four different places. This made me consider what it might be that facilitated the spontaneous striking through the glass, and likewise what might retard it. I had, long before, Principles of before, thought that jars or bottles appeared to be struck through with a much less charge, just after their being coated, or before they were dry, than when they had been coated long enough for the moisture to be evaporated from the paste, with which I mostly lay on the tin-foil, and could only consider the dry paste as a kind of mediator between the tin-foil and the glass, or in other words, that the moisture in the paste was a better conductor and more in actual contact with the glass than the paste itself when dry. And the coating the bottles with the heated cement, though long afterward, did not alter my former idea; for it appeared as if the hot cement, with the conducting substance in it, might be still more in actual contact with the glass than the moisture in the paste. On these probabilities I had to consider what might act as a kind of mediator more effectually than the dry paste, between the glass and the tin-foil. It occurred, that common writing paper, as being neither a good conductor nor insulator, might be serviceable by being first pasted smoothly to the tin-foil, and left to dry. The paper then being pasted on one side, having the tin-foil on the other, I put them on the glass together with the tin-foil outward, and rubbed them down smooth. This succeeded so well that I have never since had any struck through that were thus done, either common phials or large bottles, which contain near three gallons each, though some of the latter have stood in the battery in common use with the others for a long time. And as I have never had one struck through that has been prepared in this way, I am much less able at present to tell how great a charge they will bear before they are struck through, or whether they will be struck through at all.*

The mineral acids serve very well for an inside coating to jars; but their use is attended with some risk, from their corrosive quality.

The wire through which the charge is made, should not be less than the fourth of an inch in diameter; it should be terminated by a metallic ball, at least one inch in diameter.

If the phials be intended to be frequently removed from one place to another, the charging wire must be fastened so as to be always steady in the centre of the phial. For this purpose, some employ a piece of wood, to fit the mouth of the phial like a lid, but the length of insulation which separates the coating from the phial is thus diminished, and consequently, as we shall see hereafter, the phial is more liable to a gradual spontaneous discharge, so that it is much more difficult to charge it. The wire is best fastened below the edge of the inner coating, and in this way Mr Cuthbertson constructed his jars, the mouth being left entirely open.

When the phial is not to be removed from the situation in which it is charged, the wire may be fastened to the conductor.

Batteries may be formed either of plates or jars. A very commodious battery may be made in the following manner. Select a number of plates of the best crown glass that are very flat and thin; coat them on each side with a circular piece of tin-foil patted in the middle of the plate, so as to leave a margin sufficiently wide to prevent a spontaneous discharge; let a narrow lip of tin-foil pass from the circumference of the coating on each side, and lay the plates upon each other so that these lips may coincide. Let the lips be connected at their ends by a wire which touches them all; then if one of these lips is connected with the prime conductor, and the other with the ground, the whole may be charged or discharged together. If we wish to have a number of these plates connected so as to form a perpetual battery, they may be cemented by covering the tin-coated margins with melted pitch, and pressing the plates down on each other while the pitch is soft till the coatings touch each other. But if we desire to make use only of a few of the plates at a time, and to vary their number, they may be placed upon their edges in an open frame; and when we wish to make a break in the chain of plates, this may easily be done by placing one of them at right angles to the rest.

A very convenient battery may be formed in this way with coated plates of Mucovoy glass; but great caution is necessary in the use of such plates, as they are very easily broken by a spontaneous discharge, and it is not easy to discover where the crack has happened.

Mr Brooke of Norwich, constructed his batteries, which appear to have been very powerful, of green Brooke's glass bottles. Some of them, like that represented in mode of the figure, had only nine of these bottles; but when a greater power was wanted, more were added. Jarseries would have been preferred to bottles, on account of their being more easily coated by reason of their wide mouths; but being less easily procured, he was content to put up with this inconvenience. The mean size of these bottles was about eight inches in diameter; they were coated ten inches high, and made of the thickest and strongest glass that could be procured, weighing from five pounds and a half to seven pounds each. In the construction of a battery of twenty-seven bottles, he disposed of them in three rows; nine of the stoutest and best composing the first row, nine of the next in strength being disposed in the second, and the third containing the nine weakest. All of these were of green glass, but not of the same kind. Some of those which stood in the foremost row, were composed of a kind very much like that of which Frontignac wine bottles are made; and our author remarks, that this kind of glass seems to be by much the best, as being both harder and stronger, and less liable to break by a high charge. The second and third rows of the battery consisted of bottles whose diameter was from six and a half to ten inches, and which were coated from eight and a half to eleven inches high; none of their mouths being larger than an inch and a half, nor less than three quarters of an inch.

All the bottles of this battery, as well as the single ones which he commonly made use of in his experiments, were coated both on the inside and outside with lips of tin-foil from three-eighths to three-fourths of an inch wide, laid on with paste of flour and water, at the distance of about a flip between each.

Mr G. Morgan lays down the following requisites as essential in the construction of a battery.

1. Its connecting wires should be perfectly free from all points and edges.

2. The connecting wires should be easily moveable, so that when accident has lessened the number of phials, Principles of the number of wires may be reduced so as to correspond with the remaining quantity of glass.

3. The phials should not be crowded; for in such a case, if necessity should oblige us to employ phials of different heights or sizes, the tin-foil of the higher ones, being in contact with the uncoated glass of the lower ones, the inflation will thus be rendered less complete.

4. The size of the phials should not be large; for though an increase of magnitude lessens the trouble of cleaning, it at the same time increases the expense of repairing damages which frequently occur.

5. The several wires should be fixed very steadily, or in such a manner as not to admit of any shaking.

6. The battery should take up the least possible room; for as it increases in size, so is the probability increased of its being exposed to the influence of surrounding conductors.

The first electrical battery appears to have been constructed in the year 1746 by Mr Gralath, a German. Dr Franklin constructed a battery consisting of eleven plates of common window glass, and with this he made most of the experiments which will be mentioned hereafter. The construction of the battery was greatly improved by Dr Priestley, who formed them of considerable size and power. In his history he describes and figures one consisting of sixty-four jars, each ten inches high, and two inches and a half in diameter, and the whole battery containing 32 square feet of coated surface.

But the most complete electrical batteries are those made by Mr Cuthbertson for Teyler's museum at Haarlem. Of these batteries there are two, differing in their magnitude and mode of construction, but allowed to be equally perfect. The first was completed in the year 1784, and is composed of 135 jars in nine boxes, which may be used separately or combined, as the nature of the experiment requires. Each box is a separate battery of itself; and the description of one box will be sufficient for explaining its construction and use. Each box contains 15 jars; each jar is 11 inches high, and six inches in diameter, contracted at the mouth to four inches, and coated so as to contain 140 square inches; and thus the whole battery will contain about 132 square feet of coated surface. Each box is divided into 15 partitions, five of which are in the length and three in the breadth; the height of the sides of the box being somewhat lower than the coating of the jars, as are also the partitions in which they stand. The lid of the box is made without hinges, for the convenience of releasing it from the box, that it may be removed while experiments are performed. It is taken off by lifting it upwards. The outside coatings of the jars are connected by means of crofs wires passing under the bottom of each jar; and those on the inside by means of a brass frame, bearing 15 brass balls, fixed upon the frame above the centre of each jar. All these balls, excepting the four at the corners, have wires screwed to them and hanging downwards into the inside of each jar; but the wires of the four corner jars are screwed to a foot, which is cemented to the bottom of each in the inside. Upon these wires the whole frame rests, and is kept in its proper position. The four corner balls have holes, which receive the ends of the wires, and terminate at a proper height from the jars. By this contrivance the inside connecting frame may at any time be easily removed. It is according to the above principles of construction that Mr Cuthbertson forms his present batteries, excepting that he has increased the size of the jars, so as to make one battery contain about 17 square feet; and he engages to prove by experiment, that the batteries of his construction are far superior to any others. Teyler's second grand battery was finished by Mr Cuthbertson in 1789. This is the largest and most complete battery that was ever made. It consists of 100 jars of the same shape with that of those already described, only that they are so enlarged in size, that each of them contains 5½ square feet of coated surface, instead of 140 inches, and the whole battery contains 550 square feet of coating; and for convenience, it is put into four separate cases, each containing 25 jars in the form of a square, five on each side. The boxes are lined with lead on the inside for forming the outside communication; each jar has a perpendicular stand resting upon its bottom, and supported from falling sideways by three stays on the inside. Upon the top is screwed a three-inch brass globe, from which proceeds a brass tube about one inch in diameter, to a large brass globe, supported by the middle jar at a proper height, so as to keep the inside communication properly arranged.

Various expedients have been thought of to repair Method of jars when cracked, and enable them to bear another charging charge, but they seem to have been attended with very little success. Mr Brooke found that when any of his bottles was broken by the discharge, it might be conveniently mended and made serviceable in the following manner. "Take of Spanish white, eight ounces; heat it very hot in an iron ladle, to evaporate all the moisture; and when cool sift it through a lawn sieve; and three ounces of pitch, three quarters of an ounce of rosin, and half an ounce of bees wax; heat them all together over a gentle fire, stirring the whole frequently for near an hour; then take it off the fire, and continue the stirring till it is cold and fit for use." The bottles cemented with this composition, however, were not judged to be sufficiently strong to stand in their original place, but were removed to the second or third row, as it was apprehended they could best sustain the charge.

In relating the experiment of charging and discharging a Leyden phial, we have briefly described the discharging charging rod. Discharging rods are made of various rod forms and dimensions; fig. 13 represents one of the most common forms. It is convenient that the legs should move upon a hinge, so that the balls may be placed at a greater or lesser distance as occasion may require; the extremities of the legs should terminate in points, and the balls be made to screw on and off at pleasure.

Fig. 29. represents Mr Henry's universal discharger; Mr Henry's instrument of very extensive use in forming communications between jars, or directing the shock through any particular substance. AB is a flat board fifteen inches long, four broad, and one thick, and forming the basis of the instrument. DC are two glass pillars cemented in two holes upon the board AB, and furnished at their tops with brass caps; each of which has a turning joint, and supports a spring tube, through which the wires EF and ET slide. Each of these caps is composed of three pieces of brass, connected with each other in such a manner, that the wire EF, besides its Principles of its sliding through the socket, has two other motions, viz. a horizontal one, and a vertical one. Each of the wires is furnished with an open ring at one end, and at the other has a brass ball; which, by a short spring socket, is slipped upon its pointed extremity, and may be removed from it at pleasure. HG is a circular piece of wood five inches diameter, having a lip of ivory inlaid on its surface, and furnished with a strong cylindrical foot, which fits the cavity of the socket I. This socket is fixed in the middle of the bottom board, and has a screw at K; by which the foot of the circular board is made fast at any required height.

Fig. 30. is a small press belonging to this instrument. It consists of two oblong pieces of wood, which are forced together by the two screws, a, a. The lower end has a circular foot equal to that of the circular table H. When this press is to be used, it must be fixed into the socket I, in place of the circular board HG; which in that case is to be removed.

Mr G. Morgan gives the following rules for the construction and use of discharging instruments.

1. They should be constructed so as to allow no other passage to the electric power, than that of the intended circuit.

2. The conducting wires of the instrument should be made to come into contact with the inner surface of the coated electric as speedily as possible; for when approached gradually part of the charge is taken off previous to the explosion, the power of which is thus greatly diminished.

3. The operator should not be within the atmosphere of the conductor at the time of making the discharge.

4. The discharging instrument and the inside of the charged surface should be separated as rapidly as they were connected.

On these principles the instrument employed by Mr Morgan, in his experiments on the conducting power of various substances, is constructed, and is thus described by him.

A and B, fig. 31. are two brass wheels, whose diameter is four or five inches; they are connected by an axis, which is made to turn easily in a collar, fixed upon the glass stem DM. The wires DC, and EF, are screwed into the circumference of the wheels, but on sides directly opposite to each other. The length of these wires is regulated by the distances at which the discharging rod is fixed from the conducting body, and their direction is perpendicular to the axis of the wheels. Two other wires are to be fixed perpendicularly to the planes of the wheels, to the circumference of which they are screwed as nearly as possible, but at opposite points, so that they may strike objects lying in the same line, parallel to the axis at the distance of half a revolution from each other. The length of these last wires is regulated by the distances at which they join the metallic or other connection that is formed with the outside of the coated phial.

The mode of using this discharging rod is as follows. When C is brought into contact with the conductor, it receives the electric power, and conveys it through G into the outside of the coated surface. The motion of C is not stopped by the contact, but the continuance of it brings E into the same contact by which the residue of the jar is conveyed through K to the outside. The glass stem should penetrate deeply into each of the caps, for the whole apparatus will be otherwise loosened and put out of order, by the necessary rapidity of the motion and the conglomeration of parts attending it.

If C, in its circumvolutions, strike against an immoveable body in connexion with the conductor, it is frequently stopped, and then its ball is injured, or a change unfavourable to the accuracy of the experiment takes place.

To prevent these inconveniences, C, fig. 32. strikes the ball A which is connected with the brass tube that penetrates into the conducting substance, with an elastic wire bent into the form of a spring. The points and edges of this instrument are rendered impotent by fattening a box to the brass tube, so that the ball A may move backwards and forwards in the hollow of it, when struck by C. The box should be made of hard wood, and its edge carefully turned and well polished.

When a coated jar has been discharged, either spontaneously, or by a discharger, there is still a portion of the charge remaining, sufficient to give a slight shock, as will be found by grasping the outside with one hand, and with the other touching the ball of the wire. As this remaining charge, especially in large jars or batteries, is often so considerable as to give a pretty severe shock, it is therefore proper to caution the experimenter, not to touch the outside of the jar or battery, or any conductor which communicates with the inside at the same time.

Every machine will not charge jars equally well, but the power of charging will depend much on the goodness of the cylinder.

In a battery it sometimes happens, that one or more of the jars is more apt than the rest to undergo a spontaneous discharge, and in this case, the whole of the battery will be discharged at the same time, although the other jars, without this accident, would have contained a much higher charge.

To remedy the inconvenience of some of the jars in Mr Nairne's battery bursting at the time of the discharge, Mr Nairne proposed that the discharge should not be made through a perfect conductor of a shorter circuit than a battery five feet; and this method he found so effectual, that from being after he adapted it, he was able to discharge a battery broken by for a hundred times without breaking a single jar, a discharge which before was continually happening. It must be observed, however, as will appear soon, that when the circuit through which the discharge is made, is considerably lengthened, the force of the discharge is also proportionally diminished. Hence in many experiments, where it is necessary to employ the highest possible charge, this method of diminishing the risk of breaking the jars is inadmissible.

If a Leyden phial, or any other coated electric, be insulated or placed so that its external coating has no communication with conducting bodies, it cannot be charged. Place a Leyden phial on the insulating stool, or on a wine glass turned mouth downwards; connect the knob of the jar, or its outside coating, with the prime conductor, by means of a chain, and let the machine in motion. It will then be observed that the quadrant electrometer on the knob will soon rise to 90°, seeming to indicate that the jar is charged. On taking off the connection between the jar and the prime conductor, and endeavouring to discharge the jar by means of the discharging rod, or by the hands, it will however appear If now the outside of the jar, still standing on the inflator, be connected with the floor, table, &c., by a chain, and then charged, the result will be very different as the jar will then receive its usual charge.

If a jar be inflated, and one side of it, instead of being connected with the earth, be connected with the insulated rubber, while the other side communicates with the prime conductor, the jar will be charged, and perhaps in a more expeditious manner.

To make the above experiment in a clearer and more satisfactory manner, place the jar upon the stool as before, and with its wire not in contact, but at about half an inch distance from the prime conductor; hold the knob of another wire at such a distance from the outside coating of the jar, as the knob of the jar is from the prime conductor, then let the winch of the machine be turned, and it will be observed, that whenever a spark comes from the prime conductor to the wire of the jar, another spark passes from the outside coating of the jar to the knob of the wire presented towards it. In this manner the jar becomes charged.

If instead of the knobbed wire, a pointed wire be presented to the outside of the jar, the point will appear illuminated with a star; and if instead of presenting any wire to the jar, a point be connected with its coating, the point will appear illuminated with a brush of rays which will last as long as the jar is charging.

If the knob of another jar be presented to the outside coating of the inflated jar in the above experiment, it will also be charged.

The charge of a coated jar, or any coated electric, resides in the jar, or electric, and not in the coating.

Take an uncoated phial, and, for a coating on the outside, stick a piece of tinfoil with a little tallow or bees wax, so that it can just adhere to the glass; and for an inside coating put into the jar a quantity of small shot or of mercury; stop the mouth of the jar with a perforated cork, through which insert a knobbed wire, so as to communicate with the shot or the mercury. Then hold the phial thus coated, by its outside coating, and charge it by presenting the knob of the wire to the prime conductor. When it is charged, turn it upside down, so that the wire, and the mercury or shot within the jar, may fall into a dry glass vessel; then remove also the outside coating. During this operation the phial does not lose its charge; and if the shot or mercury be examined, it will be found that they are not more electrified than would happen to any other inflated body of equal conducting power, after having been in contact with the prime conductor. Now replace the outside coating on the phial, and pour into it the shot or mercury; then touch with one hand the outside coating, and with the other introduce a knobbed wire within the phial so as to touch the inside non-electric, and you will feel a shock, which will prove that the jar has lost very little of its charge by removing the coatings.

The same experiment may be more conveniently made by laying a pane of glass upon a metal plate, and covering an equal part of the upper surface with tin foil, having a silk thread fastened to one of its sides, by which it may be easily taken off when the glass is charged, and as easily replaced when required.

This important fact, that the charge in a coated electric resides in the electric and not in the coating, was ascertained by Dr Franklin.

When he first began his experiments upon the Leyden phial, he imagined that the electric power was all accumulated in the substance of the non-electric in contact with the glass; but he afterwards found by the following ingenious analysis of the bottle, that the power of giving a shock lay in the glass itself, and not in the coating.

In order to find where the strength of the charged bottle lay, he placed it upon glass; then first took out the cork and the wire, and finding the virtue was not in them, he touched the outside coating with one hand, and put the finger of the other into the mouth of the bottle, when the shock was felt quite as strong as if the cork and the wire had been in it. He then charged the phial again, and pouring out the water into an empty bottle inflated, expected that if the force resided in the water it would give the shock, but he found it gave none. He then judged that the electric fire must either have been lost in decanting, or must remain in the bottle, and the latter he found to be true; for filling the charged bottle with fresh water, he found the shock, and was satisfied that the power of giving it resided in the glass itself.

He made the same experiment with panes of glass, laying the coating on lightly, and charging it as he had before charged the water in the bottle, and the result was the same in both. This experiment is more satisfactory than the former; because when the water is poured out of the phial, there still remains a thin coating of the fluid, which might be thought to contain the power of giving a shock.

A charged jar may be gradually discharged by making a conducting body communicate alternately with the outside and the inside coating.

Experiment 1.—Fig. 33 represents an electric jar having a wire, CDE, fastened on its outside, which is fixed so as to have its knob E as high as the knob A. Plate B is the figure of a spider formed out of a piece of cork slightly burned, with a few short threads run through it to represent its legs. This spider is to be fastened at the end of a silk thread, proceeding from the ceiling of the room, or any other support, so that the spider may hang midway between the two knobs AE, when the jar is not charged. Let the place of the jar upon the table be marked; then charge the jar, by bringing its knob A in contact with the prime conductor, and replace it in its marked place. The spider will now begin to move from knob to knob, and continue this motion for a considerable time, sometimes for several hours.

This experiment is one of the earliest that were made by Dr Franklin and his friends, and is described by Dr Franklin in one of his letters to Collinson.

Experiment 2.—Let a coated jar be inflated by passing it through a ring fixed upon a glass stand, as represented at fig. 34. From the ball a of the wire which communicates with the inside coating suspend a wire to which Electricity illustrated by experiment.

These states of electricity are usually distinguished by means of the common pith-ball electrometer.

Experiment.—Set the machine in motion, while both conductors are insulated, or without connecting either the prime conductor or the rubber with contiguous bodies. We have before remarked (41), that the prime conductor was called the positive, and that to which the rubber is adapted the negative conductor; so that they are so in these circumstances may be demonstrated according to the explanation given in note (d).

On presenting a pith-ball electrometer to the cylinder whose electricity we have agreed to call positive, the balls will diverge, and will continue to diverge when brought near that side of the prime conductor which is most remote from the cylinder; but being carried to the other conductor, they will instantly collapse; thus showing that the electricity of the rubber is opposite to that of the other conductor or of the cylinder, i.e., that it is negative. This may be known in another way. The balls presented to the rubber will diverge with negative electricity, but being brought near the other conductor in this divergent state, they will collapse.

But should a more precise method be required to determine the quality of the electricity of an electrified body, the following may be used:—First, electrify one of the electrometers C, placed upon the stand fig. 15, either positively, or negatively, at pleasure: touch it, for instance, with an excited glass tube, so that its balls may repel, and stand about two inches distant from one another; then touch the other electrometer C with the electrified body, that you desire to examine, so that it may be possessed of the same degree of electricity: lastly, take either of the two electrometers by the top of the glass handle, disengage it from the arm of the stand, and bring it near the other electrometer; if then the balls of one electrometer repel those of the other, you may conclude that they are possessed of the same kind of electricity; but if they attract each other, you may conclude that they have been electrified with contrary electricities; and as you know the electricity of that electrometer, which was first electrified, you will of course know the electricity of the other electrometer, i.e., of the electrified body, with which it was touched.

The above experiment may also be made with the single-thread electrometers; for if they are brought near to one another, when their feathers are electrified, they will, if possessed of the same electricity, repel each other, or, if possessed of contrary electricities, they will attract each other.

While the conductors are thus insulated, if a pointed body (as for instance, the point of a needle or pin) be light presented to the back of the rubber, at the distance of about two inches, a lucid pencil of rays will appear to proceed from the point presented, and diverge towards the rubber.

If another pointed body be presented to the prime conductor, it will appear illuminated with a star; but... If a pointed wire, or other pointed conducting body, be connected with the prime conductor, it will throw out a pencil of rays.

F. Beccaria remarks, that if two equally sharp points are approached to a prime conductor, they will appear luminous at only half the distance at which one of them would have done.

From this experiment may be learned the method of distinguishing the quality of the electricity of an electrified body, by the appearance of the electric light; for if a needle, or any other pointed body, be presented in the dark, with the point towards a body strongly electrified, it will appear illuminated with a star, when that body is electrified positively, and with a pencil or brush, when it is electrified negatively (q).

Here it is proper to remark, that when two points (one of which is connected with the prime conductor, or the rubber) are opposed to one another, the appearance of light in both is pretty much the same. Mr Wilcke remarks, that when a point not electrified, is opposed to another point electrified positively, the cones of light, which otherwise would appear upon them, disappear; but that if a positive cone be opposed to a negative cone, they both preserve their own characteristic properties.

Mr Nicholson has given us some valuable observations on the different appearances of the electric light, when proceeding from bodies electrified positively or negatively.

"The escape of negative electricity from a ball," says Mr Nicholson, "is attended with the appearance of straight sharp sparks with a hoarse or chirping noise. When the ball was less than two inches in diameter, it was usually covered with short flames of this kind, which were very numerous.

"When two equal balls were presented to each other, and one of them was rendered strongly positive, while the other remained in connection with the earth, the positive brush or ramified spark was seen to pass from the electrified ball: when the other ball was electrified negatively, and the ball, which before had been positive, was connected with the ground, the electricity exhibited the negative flame, or dense, straight, and more luminous sparks from the negative ball; and when the one ball was electrified plus and the other minus, the signs of both electricities appeared. If the interval was not too great, the long zig-zag spark of the plus ball struck the straight plane of the minus ball, usually at the distance of about one third of the length of the latter from its point, rendering the other two thirds very bright: sometimes, however, the positive spark struck the ball at a distance from the negative flame. These effects are represented in Plate CLXXXVIII, figs. 35, 36, and 37.

"Two conductors of three quarters of an inch diameter, with spherical ends of the same diameter, were laid parallel to each other, at the distance of about two inches, in such a manner as that the ends pointed in opposite directions, and were six or eight inches asunder. These, which may be distinguished by the letters P and M, were successively electrified, as the balls were in the last paragraph. When one conductor P was positive, fig. 39, it exhibited the sparks of that electricity at its extremity, and struck the side of the other conductor M. When the last-mentioned conductor M was electrified negatively, fig. 38, the former being in its turn connected with the earth, the sparks ceased to strike as before, and the extremity of the electrified conductor M exhibited negative signs, and struck the side of the other conductor. And when one conductor was electrified plus and the other minus, fig. 40, both signs appeared at the same time, and continual streams of electricity passed between the extremities of each conductor, to the side of the other conductor opposed to it.

"In drawing the long spark from a ball of four inches diameter, I found it of some consequence that the stem should not be too short, because the vicinity of the large prime conductor altered the disposition of the electricity to escape: I therefore made a set of experiments, the result of which showed, that the disposition of balls to receive or emit electricity, is greater when they stand remote from other surfaces in the same plate; and that between this greatest disposition in any ball, whatever may be its diameter, every possible less degree may be obtained by withdrawing the ball towards the broader or less convex surface out of which its stem projects, until at length the ball, being wholly deprived beneath that surface, loses the disposition entirely.

From these experiments it follows, that a variety of balls is unnecessary in electricity: because any small ball, if near the prime conductor, will be equivalent to a larger ball whose stem is longer.

"From comparing some experiments made by myself many years ago with the present set, I considered a point as a ball of an indefinitely small diameter, and which I constructed an instrument consisting of a brass ball of six inches diameter, though the axis of which a stem, points carrying a fine point, was screwed. When this stem is illustrated fixed in the prime conductor, if the ball be moved on its axis in every direction, it causes the fine point either to protrude through a small hole in its external surface, or to withdraw itself; because by this means the ball runs along the stem. The disposition of the point to transmit electricity may thus be made equal to that of any ball whatever, from the minutest size to the diameter of six inches. See fig. 41, A.

"The effect of a positive surface appears to extend farther than that of a negative; for the point acts like a focus for a ball, when considerably more prominent, if it be positive, than it will if negative."

Fig. 42 represents an instrument invented by Mr Nicholson for distinguishing positive from negative electricity. It consists of two metallic balls, A, B, which may be placed at a greater or less distance from each other, by means of a joint at C, on which the two branches CA, CB move. These branches are of positive glass covered with varnish. A short point proceeds from one of the balls B towards the other A. If the two balls be placed near a body which is electrified, so that

(q) The pencil of light exhibited by a point positively electrified was first seen by Mr Grey, though the difference of the two states was not in his time correctly ascertained. Principles of Electricity illustrated by experiment.

For supposing that the electricity passes from A to B, there will be a certain distance of the balls at which a spark will pass between the balls; but this distance will be much shorter when the electricity is passing from B to A. It is evident that this instrument will be of use only when the electricity to be examined is sufficiently strong to give sparks.

The appearances of positive and negative electricity are sufficiently distinct in almost every experiment which can be made with the exhibition of electric light. Paper is a good substance for observing the visible passage of the electric power. If a strong positive electric stream be let fall on the flat side of an uninflated sheet of paper, it will form a beautiful star about four inches in diameter, consisting of very distinct radii not ramified. Negative electricity, in perfectly similar circumstances, throws many pointed brushes to the paper, but forms no star upon it. This experiment is by Mr Nicholson, and the cylinder of the machine employed in making it was seven inches in diameter*.

Chap. V. Of the different states of electricity possessed by the two surfaces of a charged electric.

The opposite surfaces of a charged electric are in opposite states, i.e., one positive, and the other negative.

Exper. 1. Inflate a coated phial, such as is described in fig. 34, without the bells, and charge it by holding the knob a to the positive conductor, while the knob b communicates with the table. When the phial is charged, hold a pith-ball electroscope to the knob a, and the balls will diverge with positive electricity, as will appear by presenting them in their diverging state to excited sealing-wax, when they will collapse. Now hold the balls to the knob b, which communicates with the outer coating of the phial, and they will diverge with negative electricity, as will appear by presenting them to an excited glass tube.

If the jar be charged at the negative conductor, these appearances will be reversed; the balls presented to the knob a will diverge with negative electricity, and presented to b, they will diverge with positive electricity.

Exper. 2. Fix a pointed wire into a hole in the knob of the inflated phial, and fix another wire in the positive conductor. Hold the knob a to the point in the positive conductor, and on turning the cylinder in the dark, a pencil of luminous rays will be seen diverging from the point in the conductor to the knob a, while a similar pencil of rays, diverges from the wire fixed in the knob b.

If the wire is fixed in the negative conductor, a luminous star will appear at each point.

Exper. 3.—Fix a pointed wire into a hole in the knob a, while another pointed wire is fixed in b, as in the last experiment. Present the wire in the knob a in the dark, to the positive conductor, and a luminous star will appear at the point a, while the point at b throws out a pencil of luminous rays.

If the point at a be presented to the negative conductor, the luminous pencil will appear at a and the luminous star at b.

Exper. 4.—Fig. 43. is an electric jar which serves to illustrate the contrary states of the sides of a Leyden phial while charging: BB is the tin foil coating; C, a stand which supports the jar; D, a socket of metal, carrying the glass rod EF, a bent brass wire pointed at each end, and fixed at the end of the rod G; this rod is moveable in the spring tube N at pleasure: that tube being fixed by a socket on the top of the glass rod E, the jar is charged by the inside wire, which communicates with the different divisions of the inside coating by horizontal wires.

Place the jar at the conductor as usual; and when charging, a luminous star will appear upon the upper point of the wire at F, clearly showing, according to the commonly received opinion, that the point is then receiving the electric power. From the upper ring of the coating B, on the outside of the jar, a stream or pencil of rays will at the same time fly off, beautifully diverging from the lower point of the wire E upon the bottom ring of the coating of the jar. When the appearances cease, which they do when the jar is charged, let a pointed wire be presented to the conductor: this will soon discharge the jar filently; during which the point will be illuminated with a small spark, while the upper point of the wire will throw off a pencil of rays diverging towards the upper ring of the coating.

When a charged electric is discharged, the electric course of power passes from the positive to the negative surface.

Exper. 1.—When a jar has been charged at the positive conductor, take a discharging rod, furnished with pointed extremities, and hold it in such a position, that the point shall be at the distance of about an inch from the knob of the jar, while the other point shall be at nearly the same distance from the outside coating. In this way the jar will be filently discharged, and if the experiment be made in the dark, a luminous star will appear at that point which is held to the knob of the jar, and a luminous pencil at the point which is held to the outer coating.

If the jar has been charged at the negative conductor, the appearance of the light at the points will be reversed; a luminous pencil will now appear at the point which is held to the knob of the jar, and a luminous star at that which is held at the outer coating.

Exper. 2.—Remove the circular piece of wood GH, by the direction of the universal discharger, fig. 29, fix the wires; EF, rectification FT, so that its knobs FT may be about two inches distant from one another. Then fix upon the socket of the board, from which the board was removed, a small lighted wax taper so that its flame may be just in the middle between the knobs FT. When the apparatus is thus disposed, if the outside of a charged jar be connected by means of a chain or other conducting substance, with one of the wires, and the knob of the jar be brought to the other wire, it will be observed, that, on making the discharge which must pass between the knobs FT, the flame of the taper will be driven in the direction of the electric power, i.e. it will be blown towards the knob of that wire which communicates with that surface of the jar which is negatively electrified.

Exper. 3.—Fig. 44 and 45, of Plate CLXXXIX, represent a small phial coated on the outside, about three inches Principles of Electricity.

Electricity phial, cemented a brass cap, having a hole with a valve, and from the cap a wire proceeds a few inches within the phial, terminating in a blunt point. When this phial is exhausted of air, a brass ball is to be screwed on the brass cap, so as to defend the valve, and prevent any air from getting into the exhausted glass. This phial exhibits clearly the direction of the electric power, both in charging and discharging; for if it be held by its bottom, and its brass knob be presented to the prime conductor positively electrified, you will see that the electric power causes a pencil of rays to proceed from the wire within the phial, as represented fig. 45, and when it is discharged, a star will appear in the place of the pencil, as represented in fig. 44. But if the phial be held by the brass cap, and its bottom be touched with the prime conductor, then the point of the wire, on its inside, will appear illuminated with a star when charging, and with a pencil when discharging. If it be presented to a prime conductor electrified negatively, all these appearances, both in charging and discharging, will be reversed.

This experiment of the Leyden vacuum, as it is called, is an invention of the late Mr Henry.

Exper. 4.—Fig. 46 represents an electric jar, whose exterior coating is made up of small pieces of tin-foil placed at a small distance from each other. This jar is to be charged in the usual manner, when small sparks will pass from one piece of tin-foil to the other, in various directions, forming a very pleasing spectacle. The separation of the tin-foil is the cause of this visible passage, from the outside to the table; and the experiment is similar in appearance to that mentioned. If the jar be discharged by bringing a pointed wire gradually to the knob T, the unsealed part of the glass between the wire and knob will be agreeably illuminated, attended by a cracking noise of the sparks. If the jar be suddenly discharged, the whole outside will be illuminated. The jar, used in these experiments, must be very dry.

Exper. 5.—Fig. 47 represents two jars, or Leyden phials, placed one over the other, by which various experiments may be made in order to elucidate the theory of electricity. Bring the outside coating of the bottle A in contact with the prime conductor, and turn the machine till the bottle is charged; then place one ball of the discharging rod upon the coating of B, and with the other touch the knob of the jar A; an explosion will follow; now place one ball of the discharger on the knob A, and bring the other ball to its coating, and you have a second discharge. Again, apply one ball of the discharger to the coating of B, and carry the other to the coating of A, and it will produce a third discharge. A fourth is obtained by applying the discharger from the coating of A to its knob.

The outer coating of the upper jar communicating with the inside of the under one, conveys the electric power from the conductor to the large jar which is therefore charged positively; the upper jar does not charge, but when a communication is formed from the outside of A to the inside of B, part of the electric power on the inside of A will be conveyed to the negative coating of B, and the jar will be discharged. The second explosion is occasioned by the discharge of the jar A; but as the outside of this communicates by conducting substances with the positive inside of the jar B, if the ball of the discharging rod remains for a little time after the discharge on the knob of A, part of the electric power of the inside of A will escape, and be replaced by an equal quantity on the outside from the jar B, by which means A is charged a second time; the discharge of this produces the third, and of B the fourth explosion.

Mr Brooke of Norwich brings the following experiments to prove that the opposite surfaces of an electric, while charging, are not necessarily in opposite states of electricity.

1. Let two pound phials be coated with tin-foil on their outsides, and filled to convenient height with common shot, to serve as a coating within side, as well as to keep a wire steady in the phials without a stopple in the mouth of them. Let each phial be furnished with a wire about the size of a goose-quill, and about ten inches long, and let each wire be sharpened a little at one end, that it may the more easily be thrust down into the shot, so as not to touch the glass anywhere at the mouth of the phials, yet so as to stand steadily in them. Let a metallic ball about six or seven eighths of an inch diameter be screwed on at the other end of each wire; also, let there be in readiness a third wire, fitted up like those for the phials, except that another ball of nearly the same size as the former may occasionally be screwed on at the sharpened end of it. I say, instead of suspending the phials from the prime conductor, let one of those above described be charged at the prime conductor, and then set it aside, but let it be in readiness in its charged state; then let the other be placed upon a good insulating stand, and let the third wire also be laid upon the stand, so that its ball, or some part of the wire, may touch the coating of the phial. Let the sharpened end of this wire project five or six inches over the edge of the stand; all of these being now placed close to the edge of a table, hang a pair of cork balls on the sharpened end of the wire, and make a communication from the prime conductor to the ball on the wire on the bottle: on working the machine, the sharpened end of the wire will permit the bottle to be charged although it be insulated; and if the wire be very finely pointed, the bottle may be charged nearly as well as if it were not insulated: I say, on working the machine, the phial will charge, and the cork balls will immediately repel each other; but whilst this phial is charging, take the first phial, which having been previously charged at the same prime conductor in the hand, and while the second phial is charging, present the ball of the first to the cork balls, and they will all repel each other. This plainly proves that the outside of the second bottle is electrified plus at the time that it is charging, the same as the inside of the first; and the inside of both the bottles will readily be allowed to charge alike, that is plus or positive.

2. Let the second bottle in the last experiment be wholly discharged, and charge it again as before (the first bottle yet remaining charged); and whilst it is charging, let the ball of the first approach the cork balls contiguous with the second, and they will, as before, all repel each other; withdraw the ball of the first, and lo... Principles of long as the machine continues to charge the second bot- tle higher, the cork balls will continue to repel each other; but cease working the machine, and the cork balls will cease to repel each other till they touch, and will then very soon repel each other again; then let the ball in the first phial approach the cork balls, and they will now be attracted by it, instead of being repelled as above, as in the last experiment. This also plainly shows, that both sides of a Leyden phial are alike at the time it is charging; and at the same time evidently shows, that the difference of the two sides does not take place till after the bottle is charged, or till the machine ceases to charge it higher.

3. In this experiment, let both of the former bottles be discharged, then let one of them be placed upon the insulating stand. Let a ball be put on over the sharpened end of the third wire, and let it be laid on the stand as before, so as to touch the coating of the phial; place the other phial on the table, so that its ball or wire may touch the ball on the third wire, or any part of the wire itself; make a communication from the ball on the wire of the first phial to the prime conductor; then, by working the machine, both bottles will soon become charged. As soon as they are pretty well charged, and before the machine ceases working, remove the second phial from the third wire; after the second phial is removed, cease working the machine as soon as possible; take the third wire, with its two balls, off the stand with the hand, and lay it on the table, so that one of its balls may touch the outside coating of the second phial; remove the first phial off the stand, and place it on the table so as to touch the ball at the other end of the third wire; then with an insulated discharging rod, make a communication from the ball in one bottle to the ball in the other. If the outside of the first phial be negative at the time it is charging, the inside of the second will be the same, and making the above communication would produce an explosion, and both bottles would be discharged; but the contrary will happen, for there will be no explosion, nor will either of the bottles be discharged, although there be a complete communication between their outsides, because the inside of them both will be positive. This is a proof, that considering one side of a phial to be positive and the other negative at the time they are charging is a mistake; as well as that, if any number of bottles be suspended at the tail of each other, all the intermediate surfaces or sides do not continue so.

4. Here also let the apparatus be disposed as in the last experiment, till the bottles are highly charged, then with a clean stick of glass, or the like, remove the communication between the balls of the first phial and the prime conductor, before the machine ceases working; then, with an insulated discharging rod, make a communication from the outside to the inside of the first phial; a strong explosion will take place on account of the excess withinside, notwithstanding they are both positive.

5. This experiment being something of a continuation of the preceding one, immediately after the last explosion takes place, discharge the prime conductor of its electricity and atmosphere; then touch the ball in the first phial with the hand, or any conducting substance that is not insulated; then will the inside coating of the first phial, which at first was so strongly positive, be in principles of the same state as the outside coating of the second, having a communication with the hand, the floor, &c., with experience each other; that is negative, if anything can properly be called negative or positive that has a communication with the common stock: but a pair of cork-balls that are electrified either plus or minus will no more be attracted by either the inside coating of the first phial or the outside coating of the second, than they will by the table on which they stand, or a common chair in the room, while they continue in that situation. Remove the aforeaid communication from the ball of the first phial; touch the ball in the second, as before in the first, or discharge the bottle with the discharging rod, and the ball in the first bottle will immediately become negative; with a pair of cork balls electrified negatively, approach the ball in the first phial, and they will all repel each other, or if the cork-balls be electrified positively, they will be attracted. All these circumstances together serve fully to prove what has already been said, not only that the inside of the first phial, which was so strongly positive, may be altered so as to become in the same state as the outside of the second, without discharging the phial, or any more working the machine; but that it may be fairly changed, from being positively charged to being negatively charged. If a pair of cork-balls are now hung on to the ball of the wire in this phial, by the help of a stick of glass, they will repel each other, being negatively electrified. Make a communication from the outside of the bottle to the table, and replace the communication from the prime conductor to the ball in the bottle; then, upon moderately working the machine to charge the bottle, the cork-balls will cease to repel each other till they touch, and will soon repel each other again by being electrified positively.

Here the working the machine anew, plainly shows that the inside of the first bottle, which was positive, was changed to negative.

The following observations and experiments on the Milner's Leyden phial, are taken from a little work by Dr Thomas Milner.

An electric power communicated to any insulated substances has been named simple electrification; the Leyden phial, in order to distinguish this particular state from that of the charged phial; but it will appear whether this distinction ought to be retained or not, by taking a comparative view of both these cases. And, if the changes which an electrical power in general is capable of making in the electrical state of any substance contained within the sphere of its influence, be taken into consideration, and compared with those which have been observed in the charged phial, it is apprehended that they will not appear to be different in any material circumstance.

1. In the charged phial, when the inside has either kind of electricity communicated to it, the outside is found to possess a contrary power. It appears also that either kind of electricity always produces the other on any conducting substance placed within the sphere of influence. And as the same effect is also produced on electrics themselves, in the same situation, and as some portion of the air, supposing no other substance to be near enough, must be unavoidably exposed to such influence, it necessarily follows, that neither power can exist. When a plate of coated glass has been charged, and principles of the circuit between the coatings has been completed, by the mediation of a good conducting substance, no part of the coated surface is supposed to retain any part of the charge; but, according to the commonly received doctrine, the whole of it is said to be discharged; or in other words, to be brought into its natural state. This however is not really the case, as will evidently appear from the following experiment; the design of which is to show the effects produced by charging and discharging a plate of glass.

Let the middle of a piece of crown window glass, seven inches square, be placed between two circular plates of brass, about the 16th part of an inch thick, and five inches in diameter. In order to enable these plates to retain a greater degree of power, it will be proper to terminate each of them with a round bead the third part of an inch thick; and the whole of the bead should be formed on one side of the plate, that the other side may remain quite flat, and apply well to the surface of the glass. Let the whole be inflated about four inches above the table, and in a horizontal position, by fastening one end of a cylindrical piece of some good insulating substance to the middle of the under plate, the other end of it being fixed in any convenient stand. Let a like insulating item be fastened to the middle of the upper plate. Let a brass chain, which may easily be removed, reach from the under plate to the table. In the last place bend a piece of brass wire into such a shape, that it may stand perpendicularly on the upper plate; and let the upper extremity of this wire be formed into a hook, that it may be removed at any time by the assistance of a silk string, without destroying the inflation of the plate.

The glass being thus coated with metal on both sides, and having also a proper communication with the table, will admit of being charged; and both coatings may be separated from the glass, and examined apart, without destroying the inflation of either: for the upper coating may be separated by the means of its own proper stem; and the under coating may be separated by taking hold of the corners of the glass, and lifting the glass itself. As glass readily attracts moisture from the atmosphere, it will therefore be necessary to warm it in the beginning, and to repeat it several times in the course of the experiment, unless the air should be very dry.

Excite a smooth glass tube, of the common size, by rubbing it with silk, and apply it repeatedly to the bent wire, until the glass be well charged. Then remove the chain, which reaches from the lower plate to the table, and also the charging wire from the upper plate, by laying hold of its hook with a silk string. It necessarily follows, from considering the quality of the power employed in the present case, that the upper surface of the glass, together with the upper coating, must be electrified positively; and that the under surface and coating must be electrified negatively; but as it is designed in this experiment to examine the powers of charged glass, that no virtue may be imputed to the glass but what really belongs to it, let both coatings be separated from it; and after they have been brought to their natural state, by touching them with a conducting body not inflated, let the glass be replaced between them; and whatever effects may be now produced must be ascribed solely Principles of solely to the powers of the charged glass. On bringing a finger near the upper coating, a small electrical spark will appear between that coating and the finger, attended with a snapping noise. Apply a finger in the same manner to the under coating, and the same thing will happen. This effect cannot be produced twice, by two succeeding applications to the same coating; but it may be repeated several hundred times over, in a favourable state of the atmosphere, by alternate applications to the two coatings; and the powers of the glass will be thus gradually weakened.

This part of the experiment may be explained, by observing that the contrary electricities have a natural tendency to produce and to preserve each other, on the opposite sides of a plate of glass; and therefore, the increase or decrease of power, on the other side: and as in charging a plate of glass positively, no gradual addition of electric matter can be made to the upper surface, without a proper conveyance for a proportionate part to pass away from the lower surface; so in this method of uncharging it, the electric power cannot be gradually taken away from the upper surface, without adding a proportionate part to the under surface: one operation is the reverse of the other, and so are the effects; one case being attended with an increase and the other with a decrease of power.

Let the glass be again fully charged, and after bringing both coatings to their natural state, as before, let the glass be replaced between them; and on touching the upper coating with a finger, and then separating it from the upper and positive surface of the glass by the insulating item, this coating will acquire a weak negative power, which will be sufficient to produce a small spark while the glass is in full force, though after the power of the glass has been reduced, it will give little or no spark: but in both cases, on touching the coatings alternately two or three times, the negative power of this coating, when separated from the positive surface of the glass, will be considerably increased, as to produce strong negative sparks.—This effect may now be repeated several times, by only touching the upper coating, but the sparks will grow weaker every time; and they may be restored again to nearly their former strength, by alternate applications to both coatings, as before. The same things will also happen to the under coating, in the same circumstances; but with this difference, that the power of the under coating, on being separated from the under and negative surface of the glass will be positive. And thus a long succession of both positive and negative sparks may be produced in favourable weather, or at any time by keeping the glass moderately warm.

It appears from this part of the experiment, that each of the surfaces of the charged glass has a power of producing a contrary electricity in the coating in contact with it, by a momentary interruption of the insulation. It necessarily follows, in producing these effects, that more electrical matter must have passed away from the upper coating, at the time of touching it, than the same coating could receive from the upper surface of the glass; and therefore the upper coating, by losing some of its natural quantity, will be negatively electrified; and also that more electric matter must have been added to the under coating at the time of touching it, than the under surface of the glass could receive from it; and therefore the under coating, by receiving some addition to its natural quantity, will be positively electrified. It appears further, that the greatest degree of this influential power, which may be consistent with the circumstances of the case, will be produced in either coating by taking care at the same time to bring the opposite coating into a like state of influential electricity; and thus it is evident, that the influential powers of the two coatings have the same relation to each other, as the contrary powers of the glass itself, and will therefore always increase or decrease together.

The glass being again well charged, as at first, let a brass wire bent in the form of a staple be brought into contact with the upper and lower coating at the same time. By this the common discharge will be made; but the equilibrium of the coated glass will be only restored in part; for a considerable degree of attraction will happen at the same time between the upper coating and the glass, which has frequently been strong enough to lift a piece of plate glass weighing ten ounces. Neither coating will now show the least external sign of electricity while it is in contact with the glass: but on separating either of them from it, if care be taken to preserve their insulations, the upper coating will be strongly electrified negatively, and the under coating will be strongly electrified positively. Let then both coatings be brought to their natural state, by touching them when separated from the glass, with a conducting body not insulated, and let the glass be replaced between them as before. In this state of things, on touching the upper coating only, and separating it from the glass, it will not be capable of giving any spark; but on touching the coatings alternately five or six times, it will then give a weak spark; and this may now be repeated several times by only touching the upper coating: but on a second application of the bent wire to both coatings at the same time, a second discharge may be perceived, though much weaker than the first, and the coatings will be again brought into the same electrical state as immediately after the first discharge. This may frequently be repeated; and a considerable number of strong negative sparks may be taken from the coating when it is separated from the positive surface of the glass. If the glass in replacing it between the two plates be turned upside down, the electrical powers of both coatings will be changed by the next application of the discharging wire to complete the circuit; and a succession of strong positive sparks may be taken from the coating when it is separated from the negative surface of the glass.

It appears from this part of the experiment, that the coated part of the charged glass was not brought into its natural state by completing the circuit between the coatings, but that it still retained a degree of permanent electricity; that the powers of both coatings were actually changed at the time of the first discharge; and that a succession of the same powers may be produced in the coatings, without renewing the least application of electricity to the glass itself.

The whole quantity of electric power added to the glass in charging it, is evidently distinguished into two parts in this experiment. The first part, which is by far the most considerable, appears to have been readily communicated from one surface of the glass to the other. Principles of other, along the bent wire, when it was first brought into contact with both coatings at the same time. The second part of the charge appears to be more permanent, and remains still united with the glass, notwithstanding the circuit has been completed (r). This permanent electricity, as well as the other, must be positive on the upper surface, and negative on the lower surface: because, in the present experiment, the charge was given by a smooth glass tube excited with a silk rubber. Now, the influence of the opposite and permanent powers on the different sides of the glass (each side having a tendency to bring the coating in contact with it into a state of electricity contrary to its own) must affect each other, in causing part of the electric matter naturally belonging to the upper coating to pass away from it to the under coating, along the discharging wire, and at the same time the discharge to pass the same way. The upper coating, therefore, by losing some part of its natural quantity, must be negatively electrified; and the under coating, by receiving an addition to its natural quantity, must be positively electrified. The whole quantity of electric matter, which the influence of the permanent electricity of the glass is capable of taking from one coating and of adding to the other, bears but a small proportion to the whole charge: and therefore the second and every subsequent discharge must be considerably weaker than the first.

It appears from several of the preceding experiments, that a considerable degree of influential power may be produced at some distance by an electric in full force; and therefore a small excited body of a cylindrical shape was sufficient to answer that purpose: but when the excited electric has been so far weakened that it cannot communicate its own power, nor produce this influential power in any body, unless it be brought very near or in contact with it, bodies of a cylindrical form must then act to great disadvantage, and a small degree of power only can be produced; because the strength of the influential electricity in this case will be in proportion to the surfaces of the electric and conducting bodies, which are brought near together, or in contact with each other; and therefore a plate of glass in the same circumstances, whether its permanent power be derived from excitation or communication, is enabled from its shape to produce a considerable degree of the influential powers in the coatings in contact with it.

It has been very properly recommended to use a particular kind of rubber, and to attend to the state of it, in order to excite glass well; but it will not be necessary to pay the least regard to these circumstances in the following experiments, in which a method will be shown of charging a small phial and a plate of glass at the same time, by a gradual accumulation of power; that power being entirely derived from the glass itself, and with no other degree or kind of friction than is necessarily connected with the form of the experiment.

Place a circle of tin-foil five inches in diameter on the table, between a soft piece of baize and the middle of the same plate of glass that was used in the last experiment, which will thus be coated on the under side: and in order to preserve a proper communication with this coating, let a fillet of tin-foil reach from it beyond the extremity of the glass. The same insulated metal cover is to be used for the upper coating as before. Let a thin ounce phial of glass be filled with brass filings, and coated with tin-foil on the outside to about one inch from the top. Let a large brass wire, the fifth part of an inch in diameter, pass through the cork of the phial into the filings, about an inch of it being left above the cork, and let the upper extremity of this wire be well rounded. This experiment requires, that the whole construction should be well warmed at first; and it will be necessary to repeat it at proper intervals, unless the atmosphere should be very dry.

Taking hold of the wire of the phial with one hand, let it be placed on the upper surface of the glass, and its bottom carried in contact over the middle of the upper surface, as far as the tin-foil coating reaches on the under side: and during this part of the operation, a finger of the other hand must be kept in contact with the fillet of tin-foil. Then lifting the phial by the wire with one hand, let it be placed on the insulated metal cover, suspended in the air with the other hand; and after shifting the hand from the wire to the coating, let the bottom of the phial be placed on the end of the tin-foil fillet. Place the insulated metal cover on the middle of the glass, and touch it with a finger of one hand, while the other hand touches the tin-foil fillet. Now lift the insulated cover by its stem, and bring the head of the cover in contact with the wire of the phial, and a very small spark of light will appear between them. Let this be repeated in the same manner about fifteen times, taking care to preserve a proper communication between the coating and the floor. Then taking hold of the phial by the coating, let it be replaced on the insulated cover while it is suspended in the air; and after shifting the hand from the coating to the wire, let it be again placed on the middle of the glass: and let the bottom be again carried in contact over the middle of the glass, holding the wire in one hand, while the other has a proper communication with the tin-foil coating. Let the phial be again returned to the tin-foil fillet as before, and let the insulated cover be applied repeatedly to the wire, immediately after every separation from the glass; and a brighter spark, together with a weak snapping, will now attend each application, if it be carefully observed to touch the cover with one hand before every separation, while the other hand rests on the fillet of tin-foil. By proceeding in this manner, after the third application of the phial to the glass, a very

(r) Some new terms seem to be wanted in order to express with precision the different parts of the charge. And if that part of it which cannot be destroyed by completing the circuit, should be called the permanent part of the charge, or more simply the charge; then might the other part, or that which may be destroyed by completing the circuit, be named the firecharge. Principles of very weak shock will be felt in those fingers which are used in completing the circuit of the glass; and after repeating two rounds more in the manner before mentioned, the phial will be fully charged. By applying the coating of the phial when it is in full force to the upper surface as before, the glass plate will get the greatest power it is thus capable of receiving, and will then give a shock as high as the elbows. After this, on attempting to lift the insulated cover, the glass itself will generally be lifted at the same time, with the tin-foil coating adhering to the under surface; but by continuing the separations of the cover from the glass, a succession of strong negative sparks may be produced by the influence of the upper surface; and by turning the glass over, and leaving the tin-foil coating on the baize, a succession of strong positive sparks may be produced by the influence of the other side.

This experiment may be performed more steadily by placing the glass, together with the tin-foil coating and baize, on a plate of metal about one-tenth of an inch thick, and of the same square as the glass. The whole may be fastened together by two small hoidsails placed at the opposite corners, which will prevent the glass from being lifted. This plate of metal will be useful in another view; for after it has been sufficiently warmed, by retaining heat well, it will help to keep the glass dry, and consequently fit for use so much the longer. But when it shall be required to show the contrary powers of the opposite sides of the glass, it will be more convenient not to fasten the parts together, and the whole may be kept sufficiently steady, by the operator's keeping down one corner of the glass with a finger, and by placing a proper weight on the opposite corner.

The bottom of the phial cannot be carried in contact over the glass without producing some little degree of friction; from which the power in this experiment is originally derived. The cover will appear on examination to be electrified negatively after every separation from the glass; but as it was touched in completing the circuit between the coatings before every separation, it necessarily follows, that the cover can have only an influential electricity, and consequently that the permanent power of the upper surface of the glass must be positive. The negative power of the cover is communicated to the wire of the phial, by which the inside is electrified negatively and the outside positively; and both these powers will increase with every application, because the circumstances of the phial are favourable to its charging. The phial must be insulated every time it is required to shift the hand from the wire to the coating, or from the coating to the wire; for without this precaution the phial would be discharged. By applying the outside of the phial to the upper surface of the glass, in the manner above mentioned, the phial will be partly discharged on the surface; and though it must be therefore weakened, the power of the glass will be increased, and consequently enabled to produce a proportionally stronger effect on the brass cover, which by the next round of applications will give the phial a stronger charge than it had before. And thus a very small degree of original power is first generated, and then employed in forming two different accumulations: and by making each of these subservient to the increase of the other, the phial is at last fully charged, and the glass plate acquires such a degree of charge, as to give a pretty smart shock; and after, it remains capable, by the influence of its permanent powers, of producing a succession of positive and negative sparks on the opposite surfaces.

The contrary charge may be given to the phial by taking hold of the coating, and carrying the wire in contact over the middle of the upper surface of the glass, and by applying the power of the insulated cover to the coating; for if the operation be conducted in every other respect in the same manner as before, then will the inside be electrified positively, and the outside negatively. The powers of the glass plate will be the same as they were in the former case.

After the phial has been fully charged negatively, by the process of the last experiment, let it be insulated; and taking hold of the wire, let the bottom be held uppermost, and let the hand which holds it rest on the fillet of tin-foil. Apply the insulated cover to the glass, and after touching it with a finger of the other hand, separate it from the glass; and on bringing it towards the coating of the phial, a strong spark will pass between them. After repeating this between 20 and 30 times, the powers of the phial will be destroyed; and by continuing the same operation, they will be inverted; for the inside will be at last fully charged positively, and the outside negatively.

The same effect may be produced by turning the glass over, and by repeatedly applying the influential electricity, produced on that side, to the wire of the phial.

When the phial has been fully charged negatively, as in the last experiment, take hold of the coating of the phial with one hand, and while the other hand rests on the tin foil fillet, apply the wire to the middle of the upper surface of the glass, as far as the tin-foil coating extends on the other side. By this the powers of the glass plate will be changed.

Another, and perhaps a better method of applying the phial, is to place the insulated cover on the surface of the glass, and then holding the phial by the coating in one hand, to apply the wire to the cover, while the other hand touches the fillet of tin-foil; by which a shock will be given, and the same change of powers will be produced in an instant, which before took up some little time. On lifting the insulated cover by its stem immediately after the shock, it will be negative, or have the same power as the inside of the phial; but on replacing the cover, and completing the circuit of the glass plate, the surcharge will be destroyed; another shock will be felt; and the power of the cover, after the next separation, will be positive, or contrary to that of the inside of the phial. Apply this positive power to the wire of the phial as before; and after 15 applications, the powers of the phial will be destroyed; and by still proceeding in the same manner, the powers of the phial will be changed, and the inside will be fully charged positively, and the outside negatively, by 60 applications.

These effects may also be produced by a single application of the coating of the phial to the other side of the glass plate; and by repeated applications of the influential electricity, produced on the same side, to the coating of the phial.

If it were simply the object in this experiment to change Principles of change the powers of the phial, the operation might then be considerably shortened, by completing the circuit of the phial, and consequently destroying the whole by expert furlage; but it was intended to show what effects might be produced, by opposing the contrary powers to each other; and by doing this it appears that either side of the glass plate can destroy the powers of the phial, and give it a contrary charge; that either side of the phial can also change the powers of the glass plate; and that the powers of the glass plate, thus inverted, can again destroy the powers of the phial, and give it a full charge of the contrary electricity.

Here it may be observed, that, in some cases, the quality of the power may be determined by observation alone. When the phial employed in the two last experiments has been fully charged, it may be known whether the inside be positive or negative from the light which appears at the wire, or from the hissing noise which attends it: for when the phial has been fully charged positively, if the room be sufficiently darkened, a bright luminous appearance may be seen, diverging in separate rays to the distance of an inch, attended with an interrupted hissing noise; and both the light and the noise continue a very short time. But when the phial is fully charged negatively, a weaker and more uniform light appears, which does not extend itself more than the sixth part of an inch, and is attended with a closer and more uniform hissing; and this noise and light always continue longer than the former. Even positive and negative sparks, passing between the insulated cover and a finger, may be distinguished from each other: for the positive sparks are more divided, give less light, make a weaker snapping noise, and affect the finger less sensibly than the negative.

The strongest sparks which can be produced in these experiments, are those that pass between the coating of the phial and the insulated cover, when they possess the contrary powers; but they will be more particularly vigorous if the coating be positive, and the insulated cover negative.

**Chap. VI. Miscellaneous Experiments with charged Electrics.**

Sig. Cigna made some curious experiments on the adhesion of electrified plates of glass. He laid two of these plates well dried, one upon the other as one piece, the lowermost of them being coated on the outside; and, when they were insulated, he alternately rubbed the uppermost plate with one hand, and took a spark from the coating of the lower with the other till they were charged; when the coating and both the plates adhered firmly together. Giving a coating to the other side, and making a communication between the two coatings, the usual explosion was produced. But, though the united electric was thus discharged, the plate still cohered, and though no sign of electricity appeared while they were united, they were, when separated, found possessed of opposite states of electricity.

If the two plates were separated before they were discharged, and the coating of each was touched, a spark came from each, and when they were again placed together, they cohered as before, but were not capable of giving a flock.

If plates of glass, thus coated and electrified, be separated in the dark, flashes of light will be perceived between them. By laying the plates together again, and again separating them successively, the appearance of these luminous flashes may be repeated several times, but always in a weaker degree than the first.

Mr Symmer made several experiments of the same kind before Sig. Cigna. He found that when the two plates were coated only on one side, they were charged as one plate, and the uncoated sides adhered together; but when they were coated each on both sides, they became charged distinctly from each other, and did not adhere.

Mr Henley, in describing an experiment of this kind, makes the following observation. "Crown glass, that is, the glass commonly used for sash-windows, though so much thinner, succeeds in this experiment as well as the plate-glass; but what is very remarkable, the Dutch plates, when treated in the same manner, have each a positive and a negative surface, and the electricity of both surfaces of both plates is exchanged for the contrary electricity in the discharge. If a clean, dry, uncoated plate of looking-glass be placed between the coated looking-glass plates, or between the plates of crown-glass, it appears after charging, to be negatively electrified on both sides; but if it be placed between the Dutch plates it acquires, like them, a positive electricity on one surface, and a negative electricity on the other."

A very curious and elegant experiment on the Leyden phial was made by Professor Richman of Petersburgh, whose unfortunate death will be hereafter related.

He coated both sides of a pane of glass, within two or three inches of the edge, and fastened linen threads to the upper part of the coating, on both sides; which, when the plate was not charged, hung down in contact with the coating: but setting the plate upright and charging it, he observed, that when neither of the sides was touched by his finger, or any other conductor communicating with the earth, both the threads were repelled from the coating, and stood at an equal distance from it; but when he brought his finger or any other conductor to one of the sides, the thread hanging to that side fell nearer to the coating, while the thread on the opposite side receded as much; and that when his finger was brought into contact with one of the sides, the thread on that side fell into contact with it likewise, while the thread on the opposite side receded twice the distance at which it hung originally; so that the two threads always hung so as to make the fame angle with one another.

One of the most diverting experiments with charged electrics, is that which Dr Franklin calls the Magic Picture, and which he describes in the following manner. Having a large mezzotinto print (supposed of the king), with a frame and glass; take out the print and cut a panel out of it, near two inches distant from the frame all round. If the cut be through the picture, it is not the worse. With thin paste or gum-water, fix the board that is cut off on the inside of the glass, pressing it smooth and close, then fill up the vacancy by gilding. Principles of gilding the glass well with gold or brass leaf. Gild Electricity likewise the inner edge of the back of the frame all round, except the top part, and form a communication between that gilding and the gilding behind the glass; then put in the board and that side is finished. Turn up the glass, and gild the foreside exactly over the back gilding; and when it is dry, cover it, by pasting on the panel of the picture that has been cut out, observing to bring the correspondent parts of the board and picture together, by which the picture will appear of a piece as at first, only part is behind the glass, and part is before. Lastly, hold the picture horizontally by the top, and place a little moveable gilt crown on the king's head.

If now the picture be moderately electrified, and another person take hold of the frame with one hand, so that his fingers touch its inside gilding, and with the other hand endeavour to take off the crown, he will receive a severe shock, and fail in the attempt. The operator who, to prevent it from falling holds the picture by the upper end, where the inside of the frame is not gilt, feels nothing of the shock, and may touch the face of the picture with impunity, which he pretends to be a test of his loyalty. If a ring of persons take a shock among them, the experiment is called the conspirators*.

On the same principle that the wires of phials charged differently, will attract and repel differently, is made an electrical wheel, which, Dr Franklin says, turns with considerable strength, and of which he gives the following description. A small upright shaft of wood passes at right angles through a thin round board, of about twelve inches diameter, and turns on a sharp point of iron, fixed in the lower end; while a strong wire in the upper end, passing through a small hole in a thin brass plate, keeps the shaft truly vertical. About thirty radii of equal length, made of soft-glass, cut in narrow lips, issue horizontally from the circumference of the board; the ends most distant from the centre, being about four inches apart. On the end of every one a brass thimble is fixed.

If now the wire of a bottle electrified in the common way, be brought near the circumference of this wheel, it will attract the nearest thimble, and so put the wheel in motion. That thimble, in passing by, receives a spark, and thereby being electrified is repelled, and so driven forwards; while a second being attracted, approaches the wire, receives a spark, and is driven after the first; and so on till the wheel has gone once round; when the thimbles before electrified approaching the wire, instead of being attracted as they were at first, are repelled, and the motion presently ceases.

But if another bottle which had been charged through the coating, be placed near the same wheel, its wire will attract the thimble repelled by the first, and thereby double the force that carries the wheel round; and not only taking out the electric power that had been communicated by the thimbles to the first bottle, but even depriving them of their natural quantity, instead of being repelled when they come again towards the first bottle, they are more strongly attracted; so that the wheel mends its pace, till it goes with great rapidity, 12 or 15 rounds in a minute, and with such strength, that the weight of 100 Spanish, with which it was once loaded, did not seem in the least to retard its motion. This is called an electrical jack, and if a large bottle of wine were spilt on the upper shaft, it would be carried round before a fire, with a motion fit for roadding.

But this wheel, like those driven by wind, moves by a foreign force, viz. that communicated to it by the bottles.

The self-moving wheel, though constructed on the same principles, appears more surprising. It is made of a thin round plate of window glass, seventeen inches in diameter, well gilt on both sides, to within two inches of the circumference. Two small hemispheres of wood are then fixed with cement, to the middle of the upper and under sides, centrally opposite, and in each of them a thick strong wire, eight or ten inches long, together making the axis of the wheel. It turns horizontally on a point at the lower end of its axis, which rests on a bit of brass cemented within a glass salt cellar. The upper end of its axis passes through a hole in a thin brass plate, cemented to a long and strong piece of glass; which keeps it fix or eight inches distant from any non-electric, and has a small ball of wax or metal on its top.

In a circle on the table which supports the wheel, are fixed twelve small pillars of glass, at about eleven inches distance, with a thimble on the top of each. On the edge of the wheel is a small leaden bullet, communicating by a wire with the upper surface of the wheel; and about six inches from it, is another bullet, communicating, in like manner, with the under surface. When the wheel is to be charged by the upper surface, a communication must be made from the under surface with the table.

When it is well charged it begins to move. The bullet nearest to a pillar moves toward the thimble on that pillar, and passing by, electrifies it, and then pushes itself from it. The succeeding bullet, which communicates with the other surface of the glass, more strongly attracts that thimble, on account of its being electrified before by the other bullet, and thus the wheel increases its motion, till the resistance of the air regulates it. It will go half an hour, and make one minute with another, twenty turns in a minute, which is six hundred turns in the whole, the bullet of the upper surface giving in each turn, twelve sparks to the thimbles, which makes seven thousand two hundred sparks, and the bullet of the under surface receiving as many from the thimble, these bullets moving in the time near two thousand five hundred feet. The thimbles are well fixed, and in so exact a circle, that the bullets may pass within a very small distance of each of them.

If instead of two bullets you put eight, four communicating with the upper surface, and four with the under surface, placed alternately, (which eight at about six inches distance, complete the circumference) the force and swiftness will be greatly increased, the wheel making fifty turns in a minute, but then it will not continue moving so long.

These wheels may be applied perhaps to the ringing chimes, and moving light made orreries*.

Mr Cavalli gives the following description of an instrument which he calls the self-charging Leyden phial.

Take a glass tube of about eighteen inches in length, and an inch, or an inch and a half, in diameter. It is immaterial Principles of Electricity illustrated by experiment.

Coat the inside of it with tin-foil, but only from one open extremity of it about as far as its middle; the other part, which remains uncoated, we shall call the naked part of the instrument. Put a cork into the aperture of the coated end, and let a knopped wire pass through the cork, and come in contact with the coating. The instrument being thus prepared, hold it in one hand by the naked part, and with the other hand clean and dry-rub the outside of the coated part of the tube; but after every three or four strokes you must remove the rubbing hand, and must touch the knob of the wire, and in so doing a little spark will be drawn from it. By this means the coated end of the tube will gradually acquire a charge, which may be increased to a considerable degree. If then you grasp the outside of the coated end of the tube with one hand, and touch the knob of the wire with the other hand, you will obtain a shock, &c.

In this experiment the coated part of the tube answers the double office of electrical machine and of Leyden phial; the naked part of it being only a sort of handle to hold the instrument by. The friction on the outside of the tube accumulates a quantity of positive electricity upon it, and this electricity forces out of the inside a quantity of electricity also positive. Then by taking the spark from the knob, this inside electricity, which is by the coating communicated to the knob through the wire, is removed, consequently the inside remains undercharged or negative, and of course the positive electricity of the outside comes closer to the surface of the glass, and begins to form the charge. By farther rubbing and taking the spark from the knob this charge is increased, &c.

Instead of a tube, this instrument may be constructed with a pane of glass, in which case it will be rather simpler, but it cannot be managed so easily, nor of course can it be charged so high as the tube. A piece of tin-foil must be pasted in the middle of only one surface of the pane, leaving about two inches and a half or three inches of uncoated glass all round. This done, hold the glass by a corner, with the coated side from you, and with the other hand rub its uncoated side, and take the spark from the tin-foil alternately, until you think that the glass may be sufficiently charged; then lay the glass with its uncoated side flat upon one hand, and on turning the tin-foil with the other hand you will receive the shock.

**Chapter VII. Of the chemical effects of the Electric Spark.**

**Exper. 1.—To fire rosin.** Wrap some cotton wool, containing as much powdered rosin as it will hold, about one of the knobs of a discharging rod. Then having charged a Leyden jar, apply the naked knob of the rod to the external coating, and the knob enveloped by the cotton to the ball of the wire. The act of discharging the jar will set fire to the rosin.

A piece of phosphorus or camphor wrapped in cotton wool, and used in the same way, will be much more easily inflamed.

**Exper. 2.—To fire spirits.** Hang a small ball with a stem to the prime conductor, so that the ball may project below the conductor. Then warm a little ardent spirit, by holding it a short time over a candle in a metallic spoon; hold the spoon about an inch below the ball, and let the machine in motion. A spark will soon issue from the ball and set fire to the spirits.

This experiment succeeds in the very same manner, whether the conductor is electrified positively or negatively, i.e., whether the spark be made to come from the conductor or from the spoon; it being only in consequence of the rapid motion of the spark that the spirits are kindled.

It will perhaps scarce necessary to remark, that the more inflammable the spirits are, the more proper they will be for this experiment, as a smaller spark will be sufficient to inflame them; therefore rectified spirit of wine is better than common proof spirit, and ether is better than either.

This experiment may be varied different ways, and may be rendered very agreeable to a company of spectators. A person, for instance, standing upon an electric stool, and communicating with the prime conductor, may hold the spoon with the spirits in his hand, and another person, standing upon the floor, may set the spirits on fire, by bringing his finger within a small distance of it. Instead of his finger, he may fire the spirits with a piece of ice; when the experiment will seem much more surprising. If the spoon is held by the person standing upon the floor, and the insulated person brings some conducting substance over the surface of the spirit, the experiment succeeds as well.

Mr Winckler says, that oil, pitch, and sealing-wax, might be lighted by electric sparks, provided those substances were first heated to a degree next to kindling. To these it must be added, that Mr Graham fired the smoke of a candle just blown out, and lighted it again; and that Mr Boze fired gunpowder, melting it in a spoon, and fired the vapour that rose from it.

This experiment will succeed better with a charged jar.

**Exper. 3.—To fire hydrogenous gas.** Provide a bottle of strong glass with two necks, as a, fig. 48. Let a hydrogenous gas be fitted to each neck c, d; one of which is furnished with a cock, and through the other e, CLXXXIX. a glass tube s is passed, containing a wire projecting beyond the tube at one end, which is terminated by a knob n, while the other passing within the bottle turns round so as to come within an inch of the brass through which the glass tube passes. The bottle being thus prepared, fill it with water, and throw up into it equal parts of hydrogen gas and common air, or three parts of hydrogen and one of oxygen gas; fix in the cork, and shake the bottle so as to mix the gases well together. Then bring the knob n, near the knob of a charged jar, or a ball of the prime conductor, and the hydrogen will be inflamed with a loud report.

In general the cork will be forced out by the explosion; but if this should not be the case, an opportunity is afforded of proving that the gases have disappeared, and water has been produced by the experiment. On taking out the cork below the surface of water, the water will rush in, and fill the bottle, thus shewing that the gases have disappeared.

To prove the production of water, it is necessary that the bottle should have been filled with mercury before

Principles of Electricity illustrated by experiment.

The first person who fired inflammable bodies by the electric spark was Dr Ludolf of Berlin, in 1774, who, by sparks excited by the friction of a glass tube, kindled the ethereal spirits of Frobenius*. Mr Gordon of Erfurd, produced so strong a spark from the back of a cat, as to fire spirit of wine†.

Exper. 4.—It has been proposed by Sig. Volta to apply the burning of hydrogen gas to economical purposes, in what he called the inflammable air lamp.

A, fig. 49, is a glass globe for containing the gas; B a glass basin or reservoir for holding water; D a cock to form a communication between the water and the gas. The water passes into the globe through the metal pipe g g, which is fixed to the upper part of the reservoir A; at r is a cock to cut off or open a communication between the air and the jar K. N is a small pipe to hold a piece of wax taper; L a brass pillar, on the top of which is a ball of the same metal; a, is a pillar of glass with a socket at the top, in which slides the wire b, having a ball screwed on the end of it. F, is a cock by which the globe is filled with hydrogen gas, and which afterwards serves to confine the gas and what water falls from B into A.

To use this instrument, having filled the globe with gas, and the reservoir A with water, turn the cocks D and r, and water will fall into the globe, forcing up a quantity of gas, which will rise through the pipe K. If now an electric spark be made to pass from the ball m to that marked n, it will set fire to the inflammable gas which passes through the pipe K. To extinguish the lamp, first shut the cock r, and then D.

The gas is obtained in the usual way from diluted sulphuric acid and iron filings, and the globe is to be filled in the following manner. Having previously filled it with water, place the foot A in a tub of water so that it may be covered, and that the bent glass tube through which the gas is to be introduced, may pass commodiously below the foot. When the gas has driven out nearly all the water, turn the cock F, and the lamp is ready for use.

Exper. 5. To fire gunpowder.—Fix a small cartridge on a metallic wire which is fitted to a glass or wooden handle; make a communication between the wire and the ground; then present the cartridge to the knob of a charged Leyden phial, and the gunpowder will be fired.

Fig. 50, represents a small cannon, with an ivory touch-hole fitted with a brass pin furnished with a round head. Gunpowder may be fired from this cannon by the electric shock, in the following manner. Charge the cannon with gunpowder as usual; then fill the touch-hole with powder, ram it well down, and push into it the brass pin so that its end may be near the bottom of the hole. Now make a communication between the outside of a large charged jar, or a battery, and the body of the cannon; then, placing one ball of a discharging rod on the head of the pin, which passes down the touch-hole of the cannon, and bring the other to the knob of the jar, and the discharge will fire the cannon.

The electric spark decomposes most of the compound gases, and forms new compounds with their component principles.

The first who examined the action of electricity on the gases, was Dr Priestley. In the course of his experiments on air, he found that by means of the electric spark, he could convert the blue colour of a vegetable infusion into red. The instrument used in this experiment, was a glass tube about four or five inches long, and one or two tenths of an inch in diameter in the gases, the inside; a piece of wire was put into one end of the tube, and fixed there with cement; a brass ball was fixed on the top of this wire; the lower part of the tube Dr Priestley was filled with water, tinged blue with a piece of turnery—fole or archil. This was easily effected by fitting the tube in a vessel of the tinged water, then placing it under a receiver on the plate of an air-pump; exhausting the receiver in part, and then, on letting in the air, the tinged liquor rose in the tube, and the elevation would be in proportion to the accuracy of the vacuum; now taking the tube and vessel from under the receiver, he threw strong sparks on the brass ball from the prime conductor.

When Dr Priestley made this experiment, he perceived, that after the electric sparks had been passed between the wire and the liquor for about a minute, the upper part of the liquor began to look red; in two minutes it was manifestly fo, and the red part did not readily mix with the rest of the liquor. If the tube was inclined where the sparks were passed through it, the redness extended twice as far on the lower side as on the upper. In proportion as the liquor became red, it advanced nearer to the wire, so that the air through which the sparks were passed, was diminished; the diminution amounted to about one fifth of the whole space; after which a continuation of the electric sparks produced no sensible effect.

To determine the cause of the change of colour, Dr Priestley expanded the air in the tube by means of an air-pump, till it expelled all the liquor, and admitted fresh blue colour in its place; but after this, electricity produced no sensible effect on the air or on the liquor; so that it was clear, that the air had been decomposed, and something of an acid nature had been produced. The result was the same with wires of different metals. It was also the same, when by means of a bent tube, the sparks were made to pass from the liquor in one leg of the tube to the liquor in the other. The air thus diminished, was in the highest degree noxious.

In passing the electric spark through different gases, it appears of different colours. In carbonic acid gas, the spark is very white; in hydrogenous gas, and ammoniacal gas, it appears of a purple or red colour.

Dr Priestley found that the electric spark passed through any kind of oil, produced an inflammable gas. He tried it with oil of olives, oil of turpentine, and essential oil of mint. The electric spark when passed through ether, produces the same effect.

He found that the electric spark when passed through ammoniacal gas, increases the bulk of this gas; so that, by making about two hundred shocks pass through a given quantity of it, the original quantity was sometimes increased one fourth. If water was admitted to this gas, it absorbed the original quantity, and left about as much gas as was generated by the electricity, and this was a strongly inflammable gas. Dr Priestley found, that on passing slight electric shocks for about an hour, through an inch of carbonic acid gas, confined in a glass tube one-tenth of an inch in diameter, when water was admitted to it, only one fourth of the air was absorbed.

He likewise found, when the electric spark was passed through carbonated hydrogen gas, that the inside of the tube in which the gas was confined, was covered with a blackish substance.

Dr Priestley took the simple electric spark from a conductor of moderate size, for the space of five minutes without interruption, in a quantity of carbonated hydrogen gas, without producing any change in the inside of the glass; when immediately after, passing through it only two shocks of a common jar, each of which might be produced in less than a quarter of a minute with the same machine in the same state, the whole of the inside of the tube was completely covered with the black matter.

A large phial, about an inch and a half wide, being filled with this gas, the explosions of a very large jar containing more than two feet of coated surface, had no effect upon it; from which it seems, that in these cases the force of the shock was not able to decompose the gas.

Several valuable experiments were made by the Hon. Henry Cavendish, of which he gave an account in the 73rd volume of the Phil. Trans.

The apparatus used in making the experiments was as follows. The air, through which the spark was intended to be passed, was confined in a glass tube M, bent at an angle, as in fig. 51, which, after being filled with quicksilver, was inverted into two glasses of the same fluid, as in the figure. The air to be tried, was then introduced by means of a small tube, such as is used for thermometers, bent in the manner represented by ABC, fig. 52, the bent end of which, after being previously filled with quicksilver, was introduced, as in the figure, under the glass DEF, inverted into water, and filled with the proper kind of air, the end C of the tube being kept stopped by the finger; then, on removing the finger from C, the quicksilver in the tube descended in the leg BC, and its place was supplied with air from the glass DEF. Having thus got the proper quantity of air into the tube ABC, it was held with the end C uppermost, and stopped with the finger; and the end A, made smaller for that purpose, being introduced into one end of the bent tube M, fig. 51, the air, on removing the finger from C, was forced into that tube by the pressure of the quicksilver in the leg BC. By these means he was enabled to introduce the exact quantity of soap-lees, or any other liquor which he wanted to be in contact with the air.

In one case, however, in which he wanted to introduce air into the tube many times in the same experiment, he used the apparatus represented in fig. 53, consisting of a tube AB of a small bore, a ball C, and a tube DE of a larger bore. This apparatus was first filled with quicksilver, and then, the ball C and the tube AB were filled with air, by introducing the end A under a glass inverted into water, which contained the proper kind of air, and drawing out the quicksilver from the leg ED by a siphon. After being thus furnished with air, the apparatus was weighed, and the end A introduced into one end of the tube M, and kept there during the experiment; the way of forcing air out of this apparatus into the tube, being by thrusting down the tube ED a wooden cylinder, of such a size as almost to fill up the whole bore, and by occasionally pouring quicksilver into the same tube, to supply the place of that pushed into the ball C. After the experiment was finished, the apparatus was weighed again, which showed exactly how much air had been forced into the tube M, during the whole experiment; it being equal in bulk to a quantity of quicksilver, whose weight was equal to the increase of weight of the apparatus.

The bore of the tube M used in most of the following experiments, was about one-tenth of an inch; and the length of the column of air, occupying the upper part of the tube, was in general from one and a half to three-quarters of an inch.

In order to force an electrical spark through the tube, it was necessary, not to make a communication between the tube and the conductor, but to place an inflated ball at such a distance from the conductor, as to receive a spark from it, and to make a communication between that ball and the quicksilver in one of the glasses, while the quicksilver in the other glass communicated with the ground.

When the electric spark was made to pass through common air, included between short columns of a solution of litmus, the solution acquired a red colour, and the air was diminished conformably to what was observed by Dr Priestley. When lime-water was used instead of the solution of litmus, and the spark was continued till the air could be no farther diminished, not the least cloud could be perceived in the lime-water; but the air was reduced to two thirds of its original bulk; which is a greater diminution than it could have suffered by mere phlogistication, as that is very little more than one-fifth of the whole.

The experiment was next repeated with some impure oxygen gas. The gas was very much diminished, but without the least cloud being produced in the lime-water, nor was any cloud produced when carbonic acid gas was let up to it; but on the further addition of a little caustic ammonia, a brown sediment was immediately perceived.

Hence we may conclude that the lime-water was saturated by some acid formed during the operation; as in this case it is evident that no earth could have been precipitated by the carbonic acid gas alone, but that the caustic ammonia, on being added, would unite with the carbonic acid, and thus becoming a carbonate would precipitate the lime by double affinity; whereas, if the lime had not been saturated with an acid, it would have been precipitated on the addition of carbonic acid gas. As to the brown colour of the sediment, it was probably owing to some of the mercury having been dissolved.

When the impure oxygen gas was confined by soap lees, the diminution proceeded rather faster than when it was confined by lime-water; for which reason, as well as on account of this lixivium containing a large quantity of alkali in proportion to its bulk, it seemed better adapted than lime-water for experiments designed to investigate the nature of the acid produced. Accordingly some experiments were made to determine of what degree of purity the oxygen gas should be, in order Electricity

Principles of order to be diminished most readily and in the greatest degree; and it was found that when good oxygen gas was employed, the diminution was but small; when perfectly pure azotic gas was used, no sensible diminution took place; but when five parts of pure oxygen gas, and three of common air were employed, almost the whole of the gases were made to disappear. It must be considered that common air consists of one part of oxygen gas mixed with between three and four of azotic gas, so that a mixture of five parts of pure oxygen gas and three of common air, was nearly the same thing as seven parts of oxygen gas and three of azotic gas.

Having made these previous trials, Mr Cavendish introduced into the tube a little soap lees, and then let up some oxygen gas and common air, mixed in the above proportions, which rising to the top of the tube M, distributed the soap-lees in the two legs of the tube, as fast as the air contained in it was diminished by the electric spark; continuing to add more of the same mixture till no further diminution took place; after which a little pure oxygen-gas, and then a little common air were added, in order to see whether cessation of diminution was not owing to some imperfection of the proportion of the two kinds of air to each other, but without effect. The lixivium being then poured out of the tube, and separated from the mercury, seemed to be perfectly neutralized, as it produced no change on the colour of paper tinged with the juice of blue flowers. Being evaporated to dryness, a small quantity of salt was left, which was evidently nitre, as appeared by the manner in which paper impregnated with a solution of it burned.

For more satisfaction, he tried this experiment over again, on a larger scale. About five times the former quantity of soap lees were now let up into a tube of a larger bore; and a mixture of oxygen gas and common air, in the same proportions as before, being introduced by the apparatus represented in fig. 53. The spark was continued till no more air could be made to disappear. The liquor when poured out of the tube, smelled evidently of nitrous acid. This fact was found by the manner in which paper, dipped into a solution of it, burned, to be true nitre. It appeared by the test of muriate of baryta, to contain no more sulphuric acid than the soap-lees themselves often contain, which is in general very little; and there is no reason to think that any other acid entered into it, except the nitric.

(T) It was found by C. Monge, who carefully examined the gas produced by passing electric sparks through carbonic acid gas, that it had been rendered inflammable; and that the mercury employed to confine the gas, as well as the wires between which the sparks passed, were oxidated. C. Monge supposed that the carbonic acid employed had undergone no change, but that the water held in solution by it had been decomposed; thus accounting for the oxidation of the metals, and the generation of inflammable gas.

M. Theodore de Saussure, not considering C. Monge's experiments as decisive, repeated them on a larger scale. He caused to circulate for 18 hours, electric sparks in the bulb of a matra which contained 13 cubic inches of pure carbonic acid gas, and without any mixture of water superabundant to that which it might naturally hold in solution. The mercury in which the inverted matra was immersed rose to about the half of its neck. After electrification the metallic fluid was found oxidated black, as had been observed by Monge and Priestley; but his conductors, which were of copper, were not sensibly altered. The elastic fluid had experienced a small dilatation, which appeared to him not to exceed the tenth part of a cubic inch. He then made about a grain of water to pass in contact with the aeriform gas contained in the matra. He let it remain there for several days, without perceiving... The gas obtained from sulphuric acid and charcoal was diminished a little, and black spots were formed on the inside of the glass receiver. Afterwards it was observed, that only one-eighth part of the electrified gas was absorbed by water. It extinguished a candle, and had very little smell.

Muriatic acid gas seemed to oppose in great measure the passage of the electric sparks, since they would not pass through a greater length than 2½ inches of this air. It was considerably diminished, but the rest was readily absorbed by water.

Fluoric acid gas was neither diminished, nor any other way sensibly altered, by the electric sparks.

Ammoniacal gas, extracted from pure ammonia, was at first almost doubled in bulk; then it was diminished a little; after which it remained without any augmentation or diminution. It became unabsorbable by water; and by the contact of flame it exploded, like a mixture of hydrogen gas and a good deal of common air.

Common air was lastly tried, and it was found to give a little faint redness to the tincture of turnsole; becoming, at the same time sensibly deoxidized. The experiment was repeated thrice at different times, and in each time after the electrification it was examined by the admixture of nitrous gas in Mr Fontana's eudiometer, and it was compared with the same gas not electrified; the latter always suffering the greatest diminution. In the first experiment the diminutions were \( \frac{1}{4} \) and \( \frac{1}{8} \); in the second, \( \frac{1}{6} \) and \( \frac{1}{8} \); and in the last, \( \frac{1}{8} \) and \( \frac{1}{8} \).

On attempting to repeat Mr Cavendish's experiment described above, in which he produced the nitric acid by a mixture of oxygen with azotic gas; instead of a syphon, the Doctor made use of a glass tube one-sixth part of an inch in diameter, closed at one end, into which an iron wire, \( \frac{1}{3} \) of an inch in diameter, had been inserted: into this tube, filled with mercury, and fixed in a vertical position, was introduced the air with which the experiment was to be tried. The oxygen gas was obtained from red precipitate, and had been thoroughly purified by alkaline salts, from any acid it might have contained. With a mixture of five parts of this and three of common air, the tube was filled to the height of three inches, to which was added five-twelfths of an inch of lixivium, of the same kind with that used by Mr Cavendish. The result was, that, after transmitting through the tube a continued stream of the electric sparks during 15 minutes, two inches of the air were absorbed by the lixivium: more air being introduced into the tube till it was filled to the height of three inches, when it was again electrified. This process was repeated till 8½ inches of air had been absorbed by the lixivium: this was now examined, and found to be, in some degree, impregnated with the nitric acid; but it was very far from being saturated. With the same lixivium, of which a quarter of an inch remained in the tube, the experiment was continued till 14 inches more of air had been absorbed; but its diminution was not perceived to decrease, though the lixivium had now absorbed 77 measures of air, each equal to its own; whereas, in the experiment related by Mr Cavendish, only 38 measures of air were absorbed by the alkali. But notwithstanding this greater absorption, the lixivium was yet far from being saturated.

The experiment was repeated with oxygen gas, obtained from minium, moistened with the sulphuric acid; seven parts of this were mixed with three of azotic gas, and lixivium added to the height of one-eighth of an inch. Here, as in the former experiment, the diminution continued without any decrease; and the lixivium, after it had absorbed 22½ inches, and consequently 178 times its own measure of air, was very far from being saturated with the nitric acid.

On this Dr Van Marum wrote to Mr Cavendish; and finding, by his answer, that this gentleman had used oxygen gas, obtained from a black powder produced by shaking mercury with lead, he requested to be informed of the process by which it is generated; but Mr Cavendish, not choosing to communicate this at present, he determined to defer the repetition of the experiment till this ingenious philosopher should have published his mode of obtaining the oxygen gas used in it.

Our author then goes on to some experiments made

perceiving any dilatation in the volume of the gases, the residue of the operation. He then moistened with a drop of water, which he introduced, the whole inside of the matras; but in vain: the mercury constantly remained at the same height. He, however, found, on absorbing by potash the residuum of the acid gas, that a cubic inch of carbonic acid gas had disappeared, and had been replaced by a quantity nearly equal, or rather superior, to the inflammable gas. The 20 cubic centimetres, occupied in the neck of the matras, a column four inches in length; and the acid gas, had the supposed explanation been just, would have been dilated through all that space. He then thought that this inflammable gas did not arise from the decomposition of the water, but from that of the carbonic acid itself, by the metal. He indeed found that this gas was not hydrogen gas, but carbonous gas perfectly pure. He burnt 100 parts of it on mercury with about a third of oxygen gas. He did not perceive water after this combustion, which left for residuum 77 parts of carbonic acid gas.

The dilatation which the latter experiences by electrization may be explained by the different densities of the carbonous gas and the carbonic acid gas. He was not able to verify the observation of C. Monge reflecting the dilatation experienced by the carbonic acid gas, after electrization over mercury.

If it was not possible to reduce entirely the acid gas into carbonous gas by these processes, it was because the first stage of metallic oxidation prevented an obstacle to further oxidation, by preventing the points of contact. The development of the carbonous gas produced therefore an analogous effect.

It results then from his observations, that the change which carbonic acid gas undergoes by electrization does not arise from the decomposition of the water, but from the partial decomposition of the carbonic acid gas, which becomes carbonous gas, giving up a part of its oxygen to the metal introduced in those experiments. Oxygen gas obtained the week before from red precipitate, being placed over mercury, and electrified for 30 minutes, was diminished by one-fifth, the surface of the quicksilver soon began to be oxidated, and towards the end of the experiment the glass tube was lined with the oxide as to cease to be transparent. By introducing a piece of iron, the electric stream was made to pass through the air without immediately touching the mercury; yet this was equally oxidated.

Two inches and three quarters of the same kind of gas being placed over water, and electrified in the same manner during half an hour, lost a quarter of an inch; and being suffered to stand 12 hours in the tube, was found to have lost one-eighth of an inch more. This was very nearly the same diminution of the gas that had taken place when it was electrified over mercury; but, in this case, the process appears to have been more slow. The gas remaining after these experiments, being tried by the edimeter, did not differ from unelectrified oxygen gas taken from the same receiver.

To determine whether the gas retained any of the acid employed in its production, the Doctor repeated the experiment with gas obtained from red precipitate, confined by an infusion of turpentine, but could not perceive in it the least change of colour. He also electrified gas obtained from minium and the sulphuric acid, placed over some diluted acetate of lead; but this was not rendered at all turbid.

Three inches of azotic gas being electrified, during the first five minutes were augmented to 3½ inches, and in the next 10 minutes to 3¾ inches: some lixivium was then introduced to try whether this would absorb it; but upon being electrified 15 minutes, the column rose to the height of 3½ inches. It was suffered to stand in the tube till the next day, when it was found to have sunk to its original dimensions.

Nitrous gas, confined by lixivium, being electrified during half an hour, lost three quarters of its bulk; the lixivium appeared to have absorbed a great deal of nitric acid; and the gas remaining in the tube did not seem to differ from common azotic gas. Some of the same nitrous gas, confined by lixivium, was, by standing three weeks, diminished to half its bulk, and this residuum also proved to be azotic gas.

Hydrogen gas obtained from steel filings and the diluted sulphuric acid, being confined by an infusion of turpentine, was electrified for 10 minutes without any change of colour in the infusion, or any alteration in the bulk of the air. The tube being filled with the same air to the height of 2½ inches, and placed in diluted acetate of lead, was exposed to the electric stream during 12 minutes, in which time the enclosed gas rose to five inches; but the acetate remained perfectly clear. Three inches of inflammable gas, obtained from a mixture of alcohol and sulphuric acid, on being electrified for 15 minutes, rose to 10 inches; thus dilated, it lost all its inflammability, and when nitrous gas was added, no diminution ensued.

A column of ammoniacal gas obtained by heat from pure ammonia, three inches high, was electrified four minutes, and rose to six inches, but did not rise higher when electrified ten minutes longer. It appears that this air is not expanded more by the powerful electric stream from this machine than by the common spark. Water would not absorb this electrified air, which was in part inflammable.

The tube, being filled to the height of an inch with ammoniacal gas, and inverted in mercury, was electrified four minutes; in which time the tube was filled with eight inches of gas, which proved to be equally inflammable, and as little absorbed by water as the ammoniacal gas.

The following experiment is very curious. Two balloons, made of the allantoides of a calf, were filled periment with hydrogen gas, of which each contained about two cubic feet. To each of these was suspended, by a filken thread about eight feet long, such a weight as was just sufficient to prevent it from rising higher in the air; they were connected, the one with the positive, the other with the negative conductor, by small wires about 30 feet in length; and being kept near 20 feet asunder, were placed as far from the machine as the length of the wires would admit. On being electrified, these balloons rose up in the air as high as the wire allowed, attracted each other, and uniting as it were into one cloud, gently descended.

The rarefaction of air by the electric explosion, is well illustrated by an experiment of Mr Kinnerley, described by Mr Cavallo, Fig. 54. Plate CLXXXIX. This instrument, which the inventor, Mr Kin-nerley, calls the electrical air thermometer, it being very useful to observe the effects of the electric explosion upon air. The body of this thermometer consists of a glass tube AB, about ten inches long, and nearly two inches diameter, and closed air-tight at both ends by two brass caps. Through a hole in the upper cap, a small tube HA, open at both ends, is introduced in some water at the bottom B of the large tube. Through the middle of each of the brass caps, a wire FG, EI, is introduced, having a brass knob within the glass tube, and by sliding through the caps, they may be set at any distance from one another. This instrument is, by a brass ring C, fastened to the pillar of the wooden stand CD, that supports it. When the air within the tube AB is rarefied, it will press upon the water at the bottom of the tube, which will consequently rise in the cavity of the small tube; and as this water rises higher or lower, so it shows the greater or less rarefaction of the air within the tube AB, which has no communication with the external air.

If the water, when this instrument is to be used, is all at the bottom of the large tube, (i.e. none of it is in the cavity of the small tube) it will be proper to blow with the mouth into the small tube, and thus cause the water to rise a little in it; where, for better regulation, a mark may be fixed.

Bring the knobs GI of the wires IE, FG, into contact with one another, then connect the ring E or F, with one side of a charged jar, and the other ring with the other side, by which operation a shock will be made to pass through the wires FG, IE, i.e. between the knobs EI. In this case you will observe, that the water in the small tube is not at all moved from the mark.

Put the knobs GI, a little distant from one another, and send a shock through them as before, and you will Principles of Electricity illustrated by experiment.

If the experiment be made in a room, where the degree of heat is variable, then proper allowance must be made for this circumstance, in estimating the event of the experiment; for the electrical air thermometer is affected by heat or cold in general, as well as by that caused by an electric spark.

In the year 1789, Messrs Paets, Van Troostwyk, and Deiman, the three associated Dutch chemists, as they are generally called, sent a letter to M. de la Mettrie, giving an account of some experiments, which they, assisted by Mr Cuthbertson, had made on the effect of passing a stream of electricity for a considerable time through water. Their letter was printed in the Journal de Physique for that year; but the account is too long to be inserted here; we shall, therefore, copy the following succinct account of the experiment by Dr Pearson.

The apparatus employed was a tube 12 inches in length, and its bore was one-eighth of an inch in diameter, English measure; which was hermetically sealed at one end, and, while it was sealing, an inch and a half of gold or platina wire was introduced within the tube, and fixed into the closed end, by melting the glass around the extremity of the wire. Another wire of platina, or of gold, with platina wire at its extremity, immersed in quicksilver, was introduced at the open end of the tube, which extended to within five-eighths of an inch of the upper wire, which, as was just said, was fixed into the sealed extremity (u).

The tube was filled with distilled water, which had been freed from air by means of Cuthbertson's last improved air-pump, of the greatest rarefying power. As the open end of the tube was immersed in a cup of quicksilver, a little common air was let into the convex part of the curved end of the tube, with the view of preventing fracture from the electrical discharges.

The wire which passed through the sealed extremity was set in contact with a brass insulated ball; and this insulated ball was placed at a little distance from the prime conductor of the electrical machine. The wire of the lower or open extremity, immersed in quicksilver, communicated by a wire or chain with the exterior coated surface of a Leyden jar, which contained about a square foot of coating; and the ball of the jar was in contact with the prime conductor.

The electrical machine consisted of two plates of 31 inches in diameter, and similar to that of Teyler. It possessed the power of causing the jar to discharge itself 25 times in 15 revolutions. When the brass ball and that of the prime conductor were in contact, no air or gas was discharged from the water by the electrical discharges; but on gradually increasing their distance from one another, the position was found in which gas was discharged, and which ascended immediately to the top of the tube. By continuing the discharges, gas continued to be discharged, and ascend, till it reached nearly to the lower extremity of the upper wire; and then a discharge occasioned the whole of the gas to disappear, a small portion excepted, and its place was consequently supplied by water.

The residuary portion of gas being let out after each experiment, and the discharges being continued in the same water, this residuary gas was left in smaller and smaller quantity; so that after four experiments, probably made on the same day, it did not amount to more than 1/80th of the bulk of gas which had been produced. If it had been possible to pass electric sparks through this very small quantity of gas a second time, or oftener, it was supposed it would have been diminished still more. But when the tube had been left for a night only filled with water, the residuary gas was in greater quantity than after the last experiment the preceding day (x).

It was concluded that the gas produced by the electrical discharges was oxygen and hydrogen gas, from decomposed water:

1. Because no other gas hitherto known instantly disappears on passing through it an electric spark. 2. The gas obtained must have been the oxygen and hydrogen of decomposed water, because they were in exactly those proportions in which by combination they reproduce water; the trifling residue being considered to be merely a portion of air which had been dissolved in the water. 3. Liquids which are not compounded of hydrogen and oxygen, as sulphuric and nitric acids, afforded gas by the electric discharges, but which did not disappear on

(u) In another part of Mr Van Troostwyk's memoir it is stated that the distance was an inch and a quarter from the end of the upper wire to the top of the lower wire; and that the distance between the insulated ball and prime conductor was at first three-fourths of an inch, but that afterwards it was increased to an inch. Although the wire fastened into the top of the tube was said to be an inch and a half in length, it is observed, that when a column of three-eighths of an inch of air was collected, it was almost at the extremity of the upper wire. From these and other inaccuracies, it will be made appear, that no one, from the account published, has been able to repeat the experiment.

(x) In at least fifty experiments I have never seen the residue of gas less than one-fortieth of the gas produced, although the water had been freed from air by the most effectual means. But Mr Schurer (Annales de Chimie, tom. v. p. 276.) testifies that he saw Mr Van Troostwyk make the experiment; and that after it was repeated many times, on the same parcel of water, there was no residue at all. I have very good grounds for believing, that this is one of the number of inaccuracies in the account published of this subject. Dr Pearson repeated the above experiments; and has given an ample detail of the manner in which he conducted his experiments, and of their result. Our limits will not permit us to give the paper of this ingenious chemist at length: we shall, therefore, present our readers with a brief abstract of it, referring them for the original to Nicholson's Journal for September, October, and November 1797, or the Philosophical Transactions for the same year.

Dr Pearson remarks that electric discharges may be employed in two manners to decompose water, viz. by what has been termed the interrupted explosion, which was Mr Van Troolwyk's method, and the uninterrupted or complete explosion.

The Doctor lays down the following requisites for succeeding in this experiment by the interrupted explosion:

1. The electrical machine must possess sufficient power. 2. Dr Pearson employed a plate machine, constructed by Cuthbertson, which he considers as preferable to a cylindrical machine. 3. The Leyden jar must have a sufficient quantity of coated surface. The Doctor found by experience that the proper quantity was about 150 or 160 square inches, with a proportional prime conductor. 4. The distance between the insulated ball and the prime conductor must always be less than the distance between the extremities of the wires. 5. The extremities of the upper and under wire within the tube must be at a certain distance from one another. The distance which the doctor generally found to answer best, was about five-eighths or seven-eighths of an inch. 6. The upper wire fixed into the closed extremity of the tube must be of a proper length and thickness. The diameter of the upper wire cannot perhaps be too small, and the smaller the diameter of the tube, the longer this wire may be. 7. The tubes must be of a proper length and diameter. The Doctor found the proper length to be nine or ten inches, exclusive of the curved part. The diameter should not be more than one-eighth, or less than one-twelfth of an inch.

To succeed by the complete or uninterrupted explosion, Dr Pearson used the following apparatus:

1. A tube about four or five inches in length, and one-fifth or one-sixth of an inch in diameter; one end of which was mounted with a brass cap, and into the other, which was hermetically sealed, was fitted a platinum wire of about 1/40th of an inch in diameter, extending into the brass cap, so as to be almost in contact with it. 2. He also employed a tube five inches long and half an inch wide, either blown into a funnel at one end, or having a brass funnel fitted to it, and inverted in a brass dish; a wire, such as the last, is sealed into the other end, and nearly touches the brass dish.

The proper distance between the wire and dish must be found by trials. In the Doctor's experiments it was about one-twentieth of an inch.

3. The Leyden jar employed must contain about 150 square inches of coating. 4. The distance between the insulated ball and the prime conductor was about half an inch.

From his experiments Dr Pearson draws the following conclusions.

The mere concussion by the electric discharges, appears to extricate not only the air dissolved in water, which can be separated from it by boiling and the air-pump, but also that which remains in water, notwithstanding these means of extricating it have been employed.

The quantity of this air varies in the same, and in different waters, according to circumstances. New-River water from the cistern yielded one-fifth of its bulk of air, when placed by Mr Cuthbertson under the receiver of his most powerful air-pump; but in the same situation, New-River water taken from a tub exposed to the atmosphere for some time yielded its own bulk of air. Hence the gas procured by the first one, two, or even three hundred explosions in water containing its natural quantity of air, is diminished very little by an electric spark.

The gas thus separable from water, like atmospheric air, consists of oxygen and nitrogen, or azotic gas; which may be in exactly the same proportions as in atmospheric air: for the water may retain one kind of gas more tenaciously than the other; and on this account the air separated may be better or worse than atmospheric air at different periods of the process for extricating it.

With regard to the gas, which instantly disappears on passing through it an electric spark, its nature is shown by (a) this very property of thus diminishing; and by the following properties:

(b) A certain quantity of nitrous gas instantly disappeared, apparently composing nitrous acid, on being added to the gas (a).

Oxygen gas being added to the residue after saturation with nitrous gas, and an electric spark being applied to the mixture of gasses, well dried, a considerable diminution immediately took place, and water was produced.

(c) Combustion from hydrogen and oxygen gas took place when the tube was about three-fourths full of gas, which was confirmed by passing an electric discharge, under the same circumstances, through a mixture of hydrogen and oxygen gas.

(d) Combustion from hydrogen and oxygen gas took place when the points of the compasses were accidentally applied to the part of the tube containing gas; which was confirmed by passing a discharge, under the same circumstances, through a mixture of hydrogen and oxygen gas, while the points of the compasses were applied to the tube.

(e) The observations made of the kindling of gas, in small quantities, from time to time, during the process of obtaining it, particularly while it was ascending in chains of bubbles, or was adhering to the funnel of the tube, confirm the evidence in favour of this gas being hydrogen and oxygen gas.

The electric spark fuses and oxidizes metals. The metals by first experiment to ascertain the action of electricity on the electric spark. Principles of metals was, we believe, made by Dr Franklin. The method in which he made the spark fuse metals was by illustrating putting thin pieces of them between two panes of glass bound together, and sending an electric shock through them. Sometimes the piece of glass by which they were confined, would be shattered to pieces by the discharge, and be broken into a kind of coarse sand, which once happened with pieces of thick looking-glass; but if they remained whole, the piece of metal would be melting in several places where it had lain between them, and instead of it, a metallic stain would be seen on both the glasses, the stains on the under and upper glasses being exactly similar in the minutest stroke.

A piece of gold-leaf used in this manner appeared not only to have been melted, but even vitrified, as the Doctor thought, or otherwise so driven into the pores of the glass, as to be protected by it from the action of the strongest aqua-regia. Sometimes he observed that the metallic stains would spread a little wider than the breadth of the thin pieces of metal. True gold, he observed, made a darker stain, somewhat reddish, and silver a greenish tinge.

Mr Cavallo gives the following directions for fusing metallic wires.

Connect with the hook, communicating with the outside coating of a battery, containing at least thirty square feet of coated surface, a wire, that is about one-fiftieth part of an inch thick, and about two feet long; the other end of it must be fastened to one end of the discharging rod; this done, charge the battery, and then by bringing the discharging rod near its wires, send the explosion through the small wire, which, by this means, will be made red hot, and melted, so as to fall upon the floor in different glowing pieces. When a wire is melted in this manner, sparks are frequently seen at a considerable distance from it, which are red hot particles of the metal, that by the violence of the explosion are scattered in all directions. If the force of the battery is very great, the wire will be entirely dispersed by the explosion, so that none of it can be afterwards found.

By repeating this experiment with wires of different metals, and the same force of explosion, it will be found that some metals are more readily fused than others, and some not at all affected; which shows the difference of their conducting power. If it be required to melt such particles of metals, that cannot easily be drawn in wires, as ores, grains of platinum, &c., they may be set in a train upon a piece of wax; this train may be inserted in the circuit, and an explosion may be sent through it, which, if it be sufficiently strong, will melt the metallic particles, as well as the wires; or, if the quantity to be tried be large enough, it may be confined in a small tube of glass.

If a wire be stretched by weights, and a shock be made to pass through it, so as to render it just red hot, the wire after the explosion will be found considerably increased in length, but if the wire be left loose it will be found after a similar explosion considerably shortened.

If a wire be melted upon a piece of glass, the glass will after the explosion be found marked with all the prismatic colours.

The wire may be formed into globules by inclosing it in a glass tube about a quarter of an inch in diameter, and sending the charge of a battery through it. The wire thus melted, will run into globules, which will adhere to the inner surface of the tube, and may be easily separated from it. On examination they will be found to be hollow, and are the metal in its least state of oxidation.

Some nicety is required in this experiment, as if the charge be too small, the globule will not be well formed, and if it be too great, the metal will be so much oxidated as to be diffused in smoke.

If a piece of metal be fixed upon each of the knobs of the universal discharger, or upon the extremities of the wires that support these knobs, so that their surfaces may come sufficiently near each other for the charge of a battery to be passed between them, and if a discharge be then made, a spot and coloured circles will be formed upon each metallic surface, which are evidently owing to a partial oxidation of the metal.

In order to exhibit coloured rings upon the surface of metals, place a plain piece of any of the metals upon one of the wires of the universal discharger, and upon the other wire fix a sharp-pointed needle, with the point just opposite to the surface of the metal; then connect one wire of the discharger with the outside of a battery, and the other with the discharging rod, &c. In this manner, if explosions be repeatedly sent either from the point to the piece of metal, or from the latter to the former, they will gradually mark the surface of the piece of metal opposite to the point, with circles, consisting of all the prismatic colours, which are evidently occasioned by laminae of the metal, raised by the force of the explosions.

These colours appear sooner, and the rings are closer to one another, when the point is nearer to the surface of the metal. The number of rings is greater or less, according as the point of the needle is more sharp or more blunt; and they are represented equally well upon any of the metals.

The point of the needle is also coloured to a considerable distance; the colours upon it returning in circles, though not very distinctly. This is an experiment of Dr Priestley.

But the most splendid experiments on the fusion of metals by electricity have been made by Dr Van Marum's experiments. He first tried the effect of a battery containing 130 square feet of coated surface. With this extraordinary power, he melted an iron wire 15 feet long and \( \frac{1}{4} \) of an inch in diameter; and another time melted a wire of the same metal 25 feet long and \( \frac{1}{4} \) of an inch in diameter.

He afterwards added to the battery 90 jars, each of the same size with the former, so that his grand battery now formed a square of 15 feet, and contained 225 square feet of coated glass. He caused wires of different metals to be drawn through the same hole, of one-thirty-eighth part of an inch in diameter, and observed how many inches of each could be melted by the explosion of his battery; taking care in all these experiments to charge it to the same degree as ascertained by his electrometer. The results were as follows:

| Metal | Inches | |-------------|--------| | Lead | 120 | | Tin | 120 | | Iron | 5 | | Gold | 3\(\frac{1}{2}\) | | Silver, copper, and brass | Not quite a quarter of an inch. |

These These several lengths of wire, of the same diameter melted by equal explosions, indicate according to our author, the degree in which each metal is fusible by the electrical discharge; and if these be compared with the fusibility of the same metals by fire, a very considerable difference will be observed. According to the experiments of the academicians of Dijon, to melt tin required a heat of 172 degrees of Reaumur's thermometer.

| Lead | 230 | |--------|-----| | Silver | 430 | | Gold | 563 | | Copper | 630 | | Iron | 696 (v) |

Thus tin and lead appear to be equally fusible by electricity, but not by fire; and iron, which by fire is less fusible than gold, is much more so by the electrical explosion.

When iron wire is melted by the explosion of the battery, the red hot globules are thrown to a very considerable distance, sometimes to that of 30 feet; it is however remarkable, that the thicker the wire is which is melted, the further are the globules dispersed; but this is accounted for, by observing, that the globules formed by the fusion of the thinner wires, being smaller, are less able to overcome the resistance of the air, and are therefore sooner stopped in their motion.

Two pieces of iron wire being tied together, the fusion extended no further than from the end connected with the inside coating of the jars to the knot; though wire of the same length and thickness, when in one continued piece, had been entirely melted by an equal explosion.

When a wire was too long to be melted by the discharge of the battery, it was sometimes broken into several pieces, the extremities of which bore evident marks of fusion; and the effect of electricity in shortening wire, was very sensible in an experiment made on 18 inches of iron wire \( \frac{1}{10} \)th of an inch in diameter, which by one discharge lost a quarter of an inch of its length. An explosion of this battery through very small wires, of nearly the greatest length that could be melted by it, did not entirely discharge the jars. On transmitting the charge through 50 feet of iron wire of \( \frac{1}{10} \)th of an inch in diameter, the doctor found that the residuum was sufficient to melt two feet of the same wire; but this residuum was much less when the wire was of too great a length to be melted by the first discharge. After an explosion of the battery through 180 feet of iron wire of equal diameter with the former, the residuum was discharged through 12 inches of the same wire which it did not melt, but only blued.

Twenty-four inches of leaden wire \( \frac{1}{10} \)th of an inch in diameter, were entirely oxidized by an explosion of this battery; the greater part of the lead rose in a thick smoke, the remainder was struck down upon a paper laid beneath it, where it formed a stain which resembled the painting of a very dark cloud. When shorter wires were oxidated, the colours were more varied. In Dr Van Marum's work a plate is given of a stain made by the oxidation of this wire, in which the cloud appears variously shaded with different tints of green, gray, and brown, in a manner of which no adequate description can give an idea.

On discharging the battery through 8 inches of tin wire \( \frac{1}{10} \)th of an inch in diameter, extended over a sheet of paper, a thick cloud of blue smoke arose, in which a number of filaments of oxide of tin were discernible; at the same time a great number of red hot globules of tin, falling upon the paper, were repeatedly thrown up again into the air, and continued thus to rebound from its surface for several seconds. The paper was marked with a yellowish clouded stain immediately under the wire, and with streaks or rays of the same colour issuing from it in every direction; some of these formed an uninterrupted line, others were made up of separate spots. In order to be certain that the colour of these streaks was not caused by the paper being scorched, the experiment was several times repeated, when a plate of glass and a board covered with tin were placed to receive the globules. These, however, were stained exactly like the paper. On oxidating five inches of the same kind of wire, the red hot globules were thrown obliquely to the height of four feet, which afforded an opportunity of observing that each globule, in its course, diffused a matter like smoke, which continued to appear for a little time in the parabolic line described by its flight, forming a track in the air of about half an inch in breadth.

Dr Van Marum attributes the clouded stain, immediately under the wire, to the instantaneous oxidation of its surface; whereas the remainder of the metal is melted into globules, which while they retain their glowing heat, continue to be superficially oxidated, and during the process, part with this oxide in the form of vapour.

Phenomena something similar to the above, were observed on the oxidation of a wire of equal parts of tin and lead, eight inches long, and \( \frac{1}{10} \)d of an inch in diameter. This also was melted into red globules, which were repeatedly driven upwards again from the paper on which they fell, and marked it with streaks of the same kind, but of a brown colour, edged with a yellow tinge. Some of these globules, though apparently not less hot, moved with less velocity than others, and were soon stopped in their course by their burning a hole in the paper. In this case a yellow matter was seen to rise from their surface to the height of one or two lines, and extended itself to the width of a quarter of an inch. This matter continued during five or six seconds,

(v) According to the experiments of Mr Wedgwood with his Pyrometer, the following are the degrees of heat computed in degrees of Fahrenheit's scale that are required to fuse certain metals.

| Metals | Degrees | |--------------|---------| | Brass | 380° | | Swedish copper | 458° | | Fine silver | 471° | | Fine gold | 523° | | Cast iron | 3997° |

Vid. Phil. Trans. vol. lxxii. Principles of seconds, to issue from the globules, and formed on their electricity surface a kind of efflorescence, resembling the flowers of sulphur produced by the foifa-terra. The globules, from which this efflorescence had issued, were found to be entirely hollow, and to consist only of a thin shell. When this mixed metal is oxidated with a less charge of battery, it leaves a stain upon the paper, something similar to that made by lead, and does not run into globules.

Dr Van Marum has also given plates of the stains made upon paper by the oxidation of iron, copper, brass, silver, and gold. Those made by copper and brass wire, are uncommonly beautiful, and are variegated with yellow, green, and a very bright brown. Eight inches of gold wire, of \( \frac{1}{8} \)th of an inch diameter, were, by the explosion reduced to a purple substance, of which a part rose like a thick smoke, and the remainder on the paper, left a stain diversified with different shades of this colour. Gold, silver, and copper, cannot easily be melted into globules. Our author has once accidentally succeeded in this; but it required a degree of electrical force so very particular, that the medium between a charge, which only broke the wire into pieces, and one which entirely oxidated it, could not be ascertained by the electrometer.

Dr Van Marum found, as might be expected, that the electric spark did not oxidate metals when confined in any gas which did not contain oxygen. On exposing wires of lead, tin, and iron, to the electric spark from the discharge of a battery, while the metals were confined in air deprived of oxygen, by the burning of inflammable bodies in it, he found that the first was reduced to a fine powder, which upon trial with nitric acid appeared to be merely lead; the two other metals were melted into small globules. He found that in general metals were not more highly oxidated in pure oxigenous gas than in common air, except that lead was reduced to a fine yellow oxide, perfectly resembling mastic.

In nitrous gas, oxidation took place as easily as in common air or in oxigenous gas.

His method of making these experiments was as follows. He confined the gas in which he was to subject the metal to the explosion, in a glass cylinder six inches high and four inches in diameter, closed at the upper end with a brass plate; from the centre of this plate was suspended the wire on which the experiment was made. The cylinder was set in a pewter dish filled with water; and to prevent its being broken by the expansion of the air, its lower edges were supported by two pieces of wood half an inch high. The lower end of the wire rested on the dish, which was connected with the outside coating of the battery.

On submitting metallic wires to the action of the electric spark while confined in water, he found that the water was decomposed, the metal being oxidated, and a portion of impure hydrogenous gas being discharged (z).

**Exper.—To burn a metallic wire in oxygen gas, by the electric spark.**

The apparatus for this experiment is represented at fig. 55. It consists of a glass jar for holding the gas, fitted to the bottom C, so that it may easily be taken out. Into the bottom is fastened a brass knob B, and a wire passes through the top of the jar furnished with a ball at A, and a knob within the jar as D, into which the piece of wire, twisted in a spiral form, is to be inserted.

The jar, thus fitted up with the wire, is to be filled with oxygen gas, obtained from the black oxide of manganese as described under Chemistry; and on passing the charge of a small Leyden phial through the wire A, an explosion will take place between the knob B, and the extremity of the small wire, by which this will be inflamed, producing a most brilliant and beautiful appearance.

When the electric spark is passed through a metallic oxide, the oxide is reduced to the metallic state.

This was effected by Sign. Beccaria, by making the spark pass between two surfaces of the oxide. In this way he reduced several of the metallic oxides, among others, that of zinc. He also obtained pure mercury, from the red sulphuret or cinnabar.

The electric spark renders bodies luminous, and makes opaque substances appear transparent.

**Exper. i.—Connect one end of a chain with the outside of a charged phial, and let the other end lie on the**

(z) Although there was good reason to suppose that the powders produced in the above experiments were real oxides of the metals, yet they had not been proved to be so by any satisfactory experiment. Dr Van Marum and his ingenious coadjutor (Mr Cuthbertson), began a set of direct experiments for the purpose of ascertaining this point; but the doctor was soon discouraged by the breaking of apparatus, and nothing satisfactory was done. Since Mr Cuthbertson's return to London, he has carried into execution a series of experiments which he had projected in Holland, and by these he has fully proved that metals exploded by the electric spark absorb oxygen from the air and become oxidated, more readily than when fused by ordinary fire. We cannot pretend to give anything like an account of these experiments in this note; they are published at length in Nicholson's Journal for July 1801. The following are Mr Cuthbertson's general conclusions.

"From the result of the foregoing experiments, it may be safely concluded that all the ductile metals can by electric discharges be sublimed and converted into proper oxides, by absorbing the oxygen from the atmosphere, and although some of the metals resist the action of common fire, and require different solvents to convert them into oxides, yet they all yield to the action of electricity.

It is remarkable that platinum, though it resists the action of common fire, is more easily fused by electric discharges than copper, silver, or gold, and seems to be as greedy of oxygen as any of the other metals; but these experiments have not been sufficiently extensive to settle the last mentioned property.

It is well known that all metals which are fusible by common fire, absorb oxygen in different degrees, and likewise in different proportions, according to the degrees of heat employed; this seems to take place also when they are sublimed by electric discharges, but the proper degree of discharge for each metal remains for investigation." Place the end of another piece of chain at the distance of about a quarter of an inch from the former; and set a glass decanter of water on the separated ends. On making the discharge, the water will appear perfectly luminous.

The electric spark may be rendered visible in water, in the following manner. Take a glass tube of about half an inch in diameter, and fix inches long; fill it with water, and to each extremity of the tube adapt a cork, which may confine the water; through each cork insert a blunt wire, so that the extremities of the wires within the tube may be very near one another; then on connecting one of these wires with the coating of a small charged phial, and touching the other wire with the knob of it; by which means the shock will pass through the wires, and cause a vivid spark to appear between their extremities within the tube. The charge in this experiment must be very weak, or there will be danger of bursting the tube.

Exper. 2.—Fig. 56. represents a mahogany stand, so constructed as to hold three eggs at greater or smaller distance, according to the position of the sliding pieces. A chain C is placed at the bottom, in such a manner as to touch the bottom of the egg at B with one end, and with its other the outside coating of a charged jar. The sliding wire A at top is made to touch the upper egg; and the distance of the eggs asunder should not exceed the quarter or eighth part of an inch. The electric spark, being made to pass down by means of the discharging rod through the wire and ball at A, will in a darkened room render the eggs very luminous and transparent.

Exper. 3.—Place an ivory ball on the prime conductor of the machine, and take a strong spark, or send the charge of a Leyden phial through its centre, and the ball will appear perfectly luminous; but if the charge be not passed through the centre, it will pass over the surface of the ball and fizzle it. A spark made to pass through a ball of boxwood, not only illuminates the whole, but makes it appear of a beautiful crimson, or rather fine scarlet colour.

Exper. 4.—Gold-leaf or Dutch metal may be rendered luminous by discharging a small Leyden phial through it. A strip of gold leaf, one-eighth of an inch in breadth, and a yard long, will frequently be illuminated throughout its whole extent, by the explosion of a jar containing two gallons. This experiment may be beautifully diversified, by laying the gold or silver leaf on a piece of glass, and then placing the glass in water; for the whole gold leaf will appear most brilliantly luminous in the water, by exposing it thus circumstanced to the explosion of a battery.

Exper. 5.—The natural, or what answers better, the artificial Bolognian stone reduced to powder, (commonly called Canton's phosphorus) may be illuminated by the electric spark in a more perfect manner than by the rays of the sun. The method of making this experiment is thus related by Mr Cavallo.

Put some of this powder in a clear glass phial, and stop it with a glass stopper, or a cork and sealing-wax. If this phial be kept in a darkened room (which for this experiment must be very dark) it will give no light; but let two or three strong sparks be drawn from the prime conductor, when the phial is kept at about two inches distant from the sparks, so that it may be exposed to that light, and this phial will receive the principles of light, and afterwards will appear illuminated for a considerable time.

This powder may be stuck upon a board by means of the white of an egg, so as to represent figures of planets, letters, or any thing else, at the pleasure of the operator, and these figures may be illuminated in the dark, in the same manner as the above-described phial.

A beautiful method of expressing geometrical figures with the above powder, is to bend small glass tubes, of about the tenth part of an inch diameter, in the shape of the figure desired, and then to fill them with the phosphoric powder. These may be illuminated in the manner described; and they are not so subject to be spoiled, as the figures represented upon the board frequently are.

The best method of illuminating this phosphorus, and that Mr W. Canton generally used, is to discharge a small electric jar near it.

Paper, after being made dry and rather hot, marble, oyster shells, and most calcareous substances, especially when burned to lime, have the property of being illuminated by the light given by the discharge of a jar, though not so much as the above-mentioned powder.

Put the extremities of two wires upon the surface of a card, or other body of an electric nature, so that they may be in one direction, and about one inch distance from one another; then, by connecting one of the wires with the outside of a charged jar, and the other wire with the knob of the jar, the shock will be made to pass over the card or other body. If the card be made very dry, the lucid track between the wires will be visible upon the card for a considerable time after the explosion. If a piece of common writing paper be used instead of the card, it will be torn by the explosion into very small bits.

When the electric discharge is passed through a lump of sugar, the sugar is rendered perfectly luminous, and will retain the light for a considerable time.

Exper. 6.—But the most remarkable instance of the penetrability of the electric light, is that related by Dr Priefley. "I laid a chain (says he), which was in contact with the outside of a jar, lightly on my finger, and sometimes kept it at a small distance by means of a thin piece of glass. If I made the discharge at the distance of about three inches, the electric fire was visible on the surface of the finger, giving it a sudden concussion, which seemed to make it vibrate to the very bone; and when it happened to pass on that side of the finger that was opposite the eye, the whole seemed, in the dark, perfectly transparent."

The following is Mr Cavallo's method of making this curious experiment.

Let the extremities of two wires, one of which proceeds from the outside of a charged jar, and another from one branch of the discharging rod, be laid on a table at the distance of one-tenth of an inch from each other; then put the thumb just upon that interruption, pressing it flat down. This done, bring the discharging rod in contact with the knob of the jar, and on making the discharge, the spark which necessarily happens under the thumb will illuminate it in such a manner that the bone and the principal blood-vessels may be easily discerned in it. In this experiment the operator need not be afraid of receiving a shock; for the discharge of the jar passes from wire to wire, and only affects the thumb with a sort of tremor, which is far from being painful.

We have before related Mr Hawkesbee's experiment by which he rendered sealing-wax transparent. Signior Beccaria effected the same by making an electric explosion pass between two plates of sealing-wax, on which some brass-dust was sprinkled. The whole was rendered perfectly luminous and transparent.

**Exper. 7.**—Fig. 57 represents an instrument composed of two glass tubes CD, one within another, and closed with two-knobbed brass caps A and B. The innermost of these is a spiral row of small round pieces of tin-foil stuck upon its outside surface, and lying at about one-thirtieth of an inch from each other. If this instrument be held by one of the extremities, and its other extremity be presented to the prime conductor, every spark that it receives from the prime conductor will cause small sparks to appear between all the round pieces of tin-foil stuck upon the innermost tube; which in the dark affords a pleasing spectacle, the tube appearing encompassed by a spiral line of fire.

Fig. 58 represents several spiral tubes placed round a board, in the middle of which is screwed a glass pillar, and on the top of this pillar is cemented a brass cap with a fine steel point. In this a brass wire turns, having a brass ball at each end, nicely balanced on the wire. To make use of this apparatus, place the middle of the turning wire under a ball proceeding from the conductor, so that it may receive a succession of sparks from the ball; then push the wire gently round; and the balls in their relative motions will give a spark to each tube, and thereby illuminate them down to the board, which from its brilliancy and rapid motion, affords a most beautiful and pleasing sight.

**Exper. 8.**—The small pieces of tin-foil may be stuck on a flat piece of glass ABCD, fig. 59, so as to represent various fanciful figures. Upon the same principle is the word LIGHT produced, in luminous characters.

It is formed by the small separations of the tin-foil pasted on a piece of glass fixed in a frame of baked wood, as represented fig. 60. To use this, the frame must be held in the hand, and the ball G presented to the conductor. The spark then will be exhibited in the intervals composing the word; from whence it passes to the hook at h, and thence to the ground by a chain. The brilliancy of this is equal to that of the spiral tubes.

Though many of the following experiments on electric light, may not with strict propriety belong to this chapter, we shall relate them here for the sake of uniformity.

Mr G. Morgan, in the Philosophical Transactions for 1785, has given a series of propositions respecting the electric light, and illustrated them by experiments; we shall here give the substance of his paper nearly in his own words.

I. There is no fluid or solid body, in its passage through which the electric light may not be rendered luminous.

This proposition has been fully illustrated by the foregoing experiments.

II. The difficulty of making any quantity of the electrical light visible in any body, increases as the conducting power of that body increases.

**Exper. 1.**—In order to make the contents of a jar luminous in boiling water, a much higher charge is necessary, than would be sufficient to make it luminous in cold water, which is universally allowed to be the worst conductor.

**Exper. 2.**—There are various reasons for believing the acids to be very good conductors; if, therefore, into a tube filled with water, and circumstanced as has been already described, a few drops of either of the mineral acids are poured, it will be almost impossible to make the light visible in its passage through the tube.

**Exper. 3.**—If a string, whose diameter is one-eighth of an inch, and whose length is six or eight inches, is moistened with water, the contents of a jar will pass through it luminously; but no such appearance can be produced by any charge of the same jar, provided the same string be moistened with one of the mineral acids.

To the preceding instance we may add the various instances of metals which will conduct the electric power without any appearance of light, in circumstances the same with those in which the same force would have appeared luminous in passing through other bodies, whose conducting power is less.

III. That the ease with which the electric light is rendered visible in any particular body, is increased by increasing the rarity of the body. The appearance of a spark, or of the discharge of a Leyden phial, in rarefied air, is well known. But we need not rest the truth of the preceding observation on the several varieties of this fact; similar phenomena attend the rarefaction of ether, of spirits of wine, and of water.

**Exper. 4.**—Into the orifice of a tube, 48 inches long, and two-thirds of an inch in diameter, cement an iron ball, so as to bear the weight which presses upon it when the tube is filled with quicksilver, leaving only an interval at the open end, which contained a few drops of water. Having inverted the tube, and plunged the open end of it into a basin of mercury, the mercury in the tube stood nearly half an inch lower than it did in a barometer at the same instant, owing to the vapour which was formed by the water. But through this rarefied water, the electrical spark passed as luminously as it does through air equally rarefied.

**Exper. 5.**—If, instead of water, a few drops of spirits of wine are placed on the surface of the mercury, phenomena, similar to those of the preceding experiment, will be discovered, with this difference only, that as the vapour in this case is more dense, the electrical spark, in its passage through it, is not quite so luminous as it is in the vapour of water.

**Exper. 6.**—Good ether, substituted in the room of the spirits of wine, will press the mercury down so low as the height of 16 or 17 inches. The electric spark, in passing through this vapour, (unless the force be very great indeed,) is scarcely luminous; but if the pressure on the surface of the mercury in the basin, be gradually lessened by the aid of an air-pump, the vapour will become more and more rare, and the electric spark, in passing through it, more and more luminous.

**Exper. 7.**—It has not been discovered, that any vapour does escape from the mineral acids when exposed in vacuo. To give them, therefore, greater rarity. Principles of Electricity or tenacity, different methods are found necessary.

Electricity With a fine camel-hair pencil, dipped in the sulphuric, nitric, or the muriatic acid, draw upon a piece of glass a line, about one-eighth of an inch broad. In some instances, you must extend this line to the length of 27 inches, and you will find that the contents of an electric battery, consisting of ten pint phials coated, will pass over the whole length of this line with the greatest brilliancy. If, by widening the line, or by laying on a drop of the acid, its quantity be increased in any particular part, the charge, in passing through that part, will not appear luminous. Water, spirits of wine, circumstanced similarly to the acids in the preceding experiments, will be attended with similar, but not equal effects; because, in consequence of the inferiority of their conducting power, it will be necessary to make the line, through which the charge passes, considerably shorter.

IV. The brilliancy or splendour of the electric light, in its passage through any body, is always increased by lessening the dimensions of that body; that is, a spark, or the discharge of a battery, which we might suppose equal to a sphere one quarter of an inch in diameter, will appear much more brilliant, if the same quantity is compressed into a sphere one-eighth of an inch in diameter. This observation is the obvious consequence of many known facts; if the machine be large enough to afford a spark, whose length is nine or ten inches, this spark may be seen sometimes forming itself into a brush, in which state it occupies more room, but appears very faintly luminous; at other times, the same spark may be seen dividing itself into a variety of ramifications, which float into the surrounding air. A spark, which in the open air cannot exceed one quarter of an inch in diameter, will appear to fill the whole of an exhausted receiver, four inches wide and eight inches long: but in the former case it is brilliant, and in the latter it grows fainter and fainter, as the size of the receiver increases. This observation is further proved by the following experiments.

Exper. 8.—To an insulated ball, four inches in diameter, fix a silver thread, about four yards long. This thread, at the end which is removed from the ball, must be fixed to another insulated substance. Bring the ball within the striking distance of a conductor, and the spark, in passing from the conductor to the ball, will appear very brilliant; the whole length of the silver thread will appear faintly luminous at the same instant. When the spark is confined within the dimensions of a sphere, one-eighth of an inch in diameter, it will be bright; but when diffused over the surface of air which received it from the thread, its light will be so faint as to be seen only in a dark room. If you lessen the surface of air which receives the spark, by shortening the thread, it will not fail to increase the brightness of the appearance.

Exper. 9.—To prove that the faintness of the electric light in vacuo, depends on the enlarged dimensions of the space through which it is diffused; we have nothing more to do than to introduce two pointed wires into the vacuum, so that the fluid may pass from the point of the one to the point of the other; when the distance between them is not more than the one-tenth of an inch, in this case we shall find a brilliancy as great as in the open air.

Exper. 10.—Into a Torricellian vacuum, 36 inches long, convey as much air as will fill two inches only of the exhausted tube if it were inverted in water; this quantity of air will afford resistance enough to condense the light as it passes through the tube into a spark, 38 inches in length. The brilliancy of the spark in condensed air, in water, and in all substances through which it passes with difficulty, depends on principles similar to those which account for the preceding facts.

V. That in the appearances of electricity, as well as in those of burning bodies, there are cases in which all the rays of light do not escape; and that the most refrangible rays are those which escape first or most easily. The electrical brush is always of a purple or bluish hue. If you convey a spark through a Torricellian vacuum, made without boiling the mercury in the tube, the brush will display the indigo rays. The spark, however, may be divided and weakened, even in the open air, so as to yield the most refrangible rays only.

Exper. II.—To an insulated metallic ball, four inches in diameter, fix a wire a foot and half long; this wire should terminate in four ramifications, each of which must be fixed to a metallic ball half an inch in diameter, and placed at an equal distance from a metallic plate, which must be communicated by metallic conductors with the ground. A powerful spark, after falling on the large ball at one extremity of the wire, will be divided in its passage from the four small balls to the metallic plate. When you examine the division of the spark in a dark room, you will discover some little ramifications, which will yield the indigo rays only; indeed at the edges of all weak sparks, the same purple appearance may be discovered. You may likewise observe, that the nearer you approach the center of the spark, the greater is the brilliancy of its colour.

VI. That the influence of different media on electrical light, is analogous to their influence on solar light, and will help us to account for some very singular appearances.

Exper. 12.—Let a pointed wire, having a metallic ball fixed to one of its extremities, be forced obliquely into a piece of wood, so as to make a small angle with the surface of the wood, and to make the point lie about one-eighth of an inch below the surface. Let another pointed wire, which communicates with the ground, be forced in the same manner into the same wood, so that its point likewise may lie about one-eighth of an inch below the surface, and about two inches distant from the point of the first wire. Let the wood be insulated, and a strong spark, which strikes on the metallic ball will force its passage through the interval of wood which lies between the points, and appear as red as blood. To prove that this appearance depends on the wood's absorption of all the rays but the red; when these points were deepest below the surface, the red only came to the eye through a prism; when they were raised a little nearer the surface, the red and orange appeared; when nearer still, the yellow; and so on, till, by making the spark pass through the wood very near its surface, all the rays were at length able to reach the eye. If the points be only one-eighth of an inch below the surface of soft deal wood, the red, the orange, and the yellow rays will appear as the spark passes through it; but when the points are at an equal depth. Principles of depth in a harder piece of wood, (such as box) the yellow light will be low, and perhaps the orange will disappear. As a farther proof that the phenomena, thus described, are owing to the interposition of the wood, as a medium which absorbs some of the rays, and suffers others to escape; it may be observed, that when the spark strikes very brilliantly on one side of the piece of deal, on the other side it will appear very red. In like manner, a red appearance may be given to a spark which strikes brilliantly over the inside of a tube, merely by spreading some pitch very thinly over the outside of the same tube.

Exper. 13.—If into a Torricellian vacuum, of any length, a few drops of ether are conveyed, and both ends of the vacuum are stopped up with metallic conductors, so that a spark may pass through it; the spark in its passage will assume the following appearances. When the eye is placed close to the tube, the spark will appear perfectly white; if the eye is removed to the distance of five or seven yards, the colour of the spark will be reddish. These changes evidently depend on the quantity of medium through which the light passes, and the red light of a distant candle, or a beclouded sun.

Exper. 14.—Dr Priestley long ago observed the red appearance of the spark when passing through hydrogen gas; but this appearance is very much diversified by the quantity of medium, through which you look at the spark. When at a very considerable distance, the red comes to the eye unmixed; but if the eye is placed close to the tube, the spark appears white and brilliant. In confirmation, however, of some of these conclusions, you must observe, that by increasing the quantity of sparks which are conveyed through any portion of hydrogen gas, or by condensing that gas, the spark may be entirely deprived of its red appearance, and made perfectly brilliant. All weak explosions and sparks, when viewed at a distance, bear a reddish hue. Such are the explosions which have passed through water, spirits of wine, or any bad conductor, when confined in a tube whose diameter is not more than an inch. The reason of these appearances seems to be, that the weaker the spark or explosion is, the less is the light which escapes; and the more visible the effect of any medium, which has a power to absorb some of that light.

Chalk, oyster-shells, together with those phosphoric bodies, whose goodness has been very much impaired by long keeping, when finely powdered, and placed within the circuit of an electrical battery, will exhibit, by their scattered particles, a shower of light; but these particles will appear reddish, or their phosphoric power will be sufficient only to detain the yellow, orange, and red rays. When spirits of wine are in a similar manner brought within the circuit of a battery, a similar effect may be discovered; its particles diverge in several directions, displaying a most beautiful golden appearance. The metallic oxides are, of all bodies, those which are rendered phosphoric with the greatest difficulty; but even these may be scattered into a shower of red luminous particles by the electric stroke.

Vol. VII. Part II.

The following experiments are given by Mr Cavalli to illustrate the appearance of the electric light in rarefied air.

Exper. 1.—Fig. 61 represents a prime conductor, invented by Mr Henry, which shows clearly the direction of the electric power passing through it, from whence it is called the luminous conductor (a). The middle part EF of this conductor, is a glass tube about eighteen inches long, and three or four inches in diameter. To both ends of this tube the hollow brass pieces FD, BE, are cemented air-tight, one of which has a point C, by which it receives the electric power, when let near the excited cylinder of the electrical machine, and the other has a knopped wire G, from which a strong spark may be drawn; and from each of the pieces FD, BE, a knopped wire proceeds, within the cavity of the glass tube. The brass piece FD, or BE, is composed of two parts, i.e., a cap F cemented to the glass tube, and having a hole with a valve, by which the cavity of the glass tube may be exhausted of air; and the ball D, which is screwed upon the cap F. The supporters of this instrument are two glass pillars fastened in the bottom board H, like the prime conductor represented fig. 61. When the glass tube of this conductor is exhausted of air by means of an air-pump, and the brass ball is screwed on, as represented in the figure, then it is fit for use, and may serve for a prime conductor to an electrical machine.

If the point C of this conductor is set near the excited cylinder of the machine, it will appear illuminated with a star; at the same time the glass tube will appear all illuminated with a weak light; but from the knopped wire, that proceeds within the glass from the piece FD, a lucid pencil will issue out, and the opposite knob will appear illuminated with a star or round body of light, which, as well as the pencil of rays, is very clear, and discernible among the other light, that occupies the greatest part of the cavity of the tube.

If the point C, instead of being presented to the cylinder, be connected with the rubber of the machine, the appearance of light within the tube will be reversed; the knob which communicates with the piece FD appearing illuminated with a star, and the opposite with a pencil of rays.

If the wires within the tube EF, instead of being furnished with knobs, be pointed, the appearance of light is the same, but it seems not so strong in this, as in the other case.

Exper. 2.—Take a glass tube of about two inches conducting diameter, and about two feet long; fix to one of its glass tube ends a brass cap, and to the other a stop-cock, or a valve; then by means of an air-pump exhaust it of air. If this tube be held by one end, and its other end be brought near the electrified prime conductor, it will appear to be full of light, whenever a spark is taken by it from the prime conductor; and much more so, if an electric jar be discharged through it.

This experiment may also be made with the receiver of an air-pump.—Take, for instance, a tall receiver, clean and dry, and through a hole at its top insert a wire,

(4) An instrument much like this conductor was some years ago invented by Dr Watson, with which he made several original experiments upon the electric light. Principles of wire, which must be cemented air-tight. The end of Electricity the wire, that is within the tube, must be pointed, but illustrated by experiment not very sharp; and the other end must be furnished with a knob. Put this receiver upon the plate of the air-pump, and exhaust it. If now the knob of the wire at the top of the receiver be touched with the prime conductor, every spark will pass through the receiver in a dense and large body of light, from the wire to the plate of the air-pump.

It must be observed, that when the air is very much rarefied, the electric light in it is less dense, though more diffused; and contrarywise.

Exper. 3.—Take a phial nearly of the shape and size of a Florence flask, such as is represented at fig. 62.

Fix a stop-cock or a valve to its neck, and exhaust it of air as much as it is possible with a good air-pump. If this glass be rubbed in the common manner used to excite electrics, it will appear luminous within, being full of a flashing light, which plainly resembles the aurora borealis, or northern light. This phial may also be made luminous by holding it by either end, and bringing the other end to the prime conductor; in this case all the cavity of the glass will instantly appear full of flashing light, which remains in it for a considerable time after it has been removed from the prime conductor.

Instead of the above-described glass vessel, a glass tube, exhausted of air and hermetically sealed, may be used, and perhaps with greater advantage. The most remarkable circumstance of this experiment is, that if the phial or tube, after it has been removed from the prime conductor (and even several hours after its flashing light hath ceased to appear) be grasped with the hand, strong flashes of light will immediately appear within the glass, which often reach from one of its ends to the other.

Exper. 4.—GI, fig. 63, represents the receiver with the plate of an air-pump. In the middle of the plate IF, a short rod is fixed, having at its top a metal ball B nicely polished, whose diameter is nearly two inches. From the top of the receiver another rod AD with a like ball A proceeds, and is cemented air-tight in the neck C; the distance of the balls from one another being about four inches, or rather more. If, when the receiver is exhausted of air, the ball A be electrified positively, by touching the top D of the rod AD with the prime conductor or an excited glass tube, a lucid atmosphere appears about it, which, although it consists of a feeble light, is yet very conspicuous, and very well defined; at the same time the ball B has not the least light. The atmosphere does not exist all round the ball A, but reaches from about the middle of it, to a small distance beyond that side of its surface, which is towards the opposite ball B. If the rod with the ball A be electrified negatively, then a lucid atmosphere, like the above described, will appear upon the ball B, reaching from its middle to a small distance beyond that side of it, that is towards the ball A; at the same time the negatively electrified ball A remains without any light.

The operator in this experiment must take care not to electrify the ball A too much, as, in that case, a spark will pass from one ball to the other, and the desired effect will not be produced. A little practice, however, will render the experiment very easy and familiar.

This elegant experiment is the invention of Sig. Becaria.

Fig. 64. and 65. represent a curious appearance of the electric light. In fig. 65, the light is seen streaming from a wire within the exhausted receiver of an air-pump. If in this state of things, the hand or a finger be applied to the external part of the receiver, part of the light will approach the finger, as represented in fig. 64.

The electric spark produces changes on most artificial colours.

Mr Cavallo made several experiments on substances Mr Cavallo painted with various colours. They were occasioned by his having observed that an electric spark sent over the surface of a card, made a black stroke upon a red spot, from which he was induced to try the effect of sending shocks over cards painted with different water colours. The force employed was generally about one foot and a half of charged surface; and the shocks were sent over the cards while the latter were in a very dry state.

"Vermilion was marked with a strong black track, about one tenth of an inch wide. This stroke is generally single, as represented by AB, fig. 66. Sometimes it is divided in two towards the middle, like EF; and sometimes, particularly when the wires are set very distant from one another, the stroke is not continued, but interrupted in the middle, like GH. It often, although not always happens, that the impression is marked stronger at the extremity of that wire from which the electric sparks issue, as it appears at E, supposing that the wire C communicates with the positive side of the jar; whereas, the extremity of the stroke, contiguous to the point of the wire D, is neither so strongly marked, nor surrounds the wire so much, as the other extremity E.

"Carmine received a faint and slender impression of a purple colour.

"Verdegris was shaken off from the surface of the card; except when it had been mixed with strong gum-water, in which case it received a very faint impression.

"White lead was marked by a long black track, not so broad as that on vermilion.

"Red lead was marked with a faint mark much like carmine.

"The other colours I tried were orpiment, gamboge, sap green, red ink, ultramarine, prussian blue, and a few others which were compounds of the above; but they received no impression.

"It having been intimated, that the strong black mark, which vermilion receives from the electric shock, might possibly be owing to the great quantity of sulphur contained in that mineral, I was induced to make the following experiment. I mixed together equal quantities of orpiment and flower of sulphur; and with this mixture, by the help, as usual, of very diluted gum-water, I painted a card; but the electric shock sent over it left not the least impression.

"Deferring of carrying this investigation on colours, a little farther, with a particular view to determine something relative to the properties of lamp black and oil, I procured some pieces of paper painted on both sides..." Chap. VIII.

Principles of fides with oil colours; and fending the charge of two feet of coated glafs over each of them, by making the interruption of the circuit upon their surfaces, I observ- ed that the pieces of paper painted with lamp black, prussian blue, vermilion, and purple brown, were torn by the explosion; but white lead, Naples yellow, Eng- lish ochre, and verdigrife, remained unharmed.

"The same shock fent over a piece of paper painted very thickly with lamp black and oil left not the least impression. I fent the shock also over a piece of paper unequally painted with purple brown, and the paper was torn where the paint lay very thin, but re- mained unhurt where the paint lay evidently thicker. These experiments I repeated several times, and with some very little variation, which naturally produced different effects; however, they all seem to point out the following propositions.

"1. A coat of oil-paint over any substance, defends it from the effect of such a shock as would otherwise injure it; but by no means defends it from any electric shock whatever.

"2. No one colour seems preferable to the others, if they are equal in substance, and equally well mixed with oil; but a thick coating does certainly afford a better defence than a thinner one.

"By rubbing the above mentioned pieces of paper, I find that the paper painted with lamp-black and oil is more easily excited, and acquires a stronger electricity, than the papers painted with the other colours; and perhaps on this account it may be, that lamp black and oil might resist the shock somewhat better than the other paints.

"It is remarkable that vermilion receives the black impression when painted with linseed oil nearly as well as when painted with water. The paper painted with white lead and oil receives also a black mark; but its nature is very singular. The track when first made, is almost as dark as that marked on white lead painted with water; but it loses its blackness, and in about an hour's time (or longer, if the paint is not fresh) it appears without any darkness; and when the painted paper is laid in a proper light appears only marked with a colourless track, as if made by a finger nail. I fent the shock also over a piece of board, which had been painted with white lead and oil four years before, and the explosion marked the black track upon this also; this track, however, was not so strong, nor vanished so soon, as that marked upon the painted paper; but in about two days time it also vanished entirely.

CHAP. VIII. Of the Mechanical Effects of the Electric Power.

The electric power in its passage through the air, drives light bodies before it.

Sig. Beccaria put a narrow piece of silver leaf between two plates of wax, laying it across them, but so that it did not quite reach one of the sides. The discharge being made through this strip of metal, by bringing a wire opposite to the silver at the place where it was discontinued; the silver was found melted, and part of it dispersed all along the track that the electric spark took between the plates of wax, from the silver to the wire.

The following experiment shows the force of the elec- An experiment given by Mr. Cavallo to prove the direction of the electric power in the discharge of a Leyden phial, will afford a good illustration of our present position.

Bend a card, length-ways, over a round ruler, so as to form a channel, or semicircular groove (B): lay this card upon the circular board E of the universal discharger, and in the middle of it put a pith-ball of about half an inch diameter; then at equal distances, about half or three quarters of an inch from the pith-ball, lay the two brass knobs DD. The card being perfectly dry, and rather hot, if you connect, by means of a chain or otherwise, the outside of a charged jar with one of the wires C, and bring the knob of the jar to the other wire C, you will observe, that on making the discharge, which must pass between the knobs DD, and over the card, &c., the pith-ball is always driven in the direction of the electric power; i.e., it is pushed towards that knob which communicates with the negative side of the jar.

It must be observed, that in this experiment the charge of the jar must be just sufficient to pass through the interval in the circuit; the card, or piece of baked wood, must be very dry and clean; and, in short, the disposition of the apparatus, and the performance of this curious experiment, require a degree of nicety that can only be obtained by practice. Without great precaution, it sometimes fails; but when the operator has once got it to succeed, and follows exactly the same method of operating, he may be sure that the event of the experiment will be constantly as above described.

By the electric explosion, paper, pasteboard, card, thin glass, and other non-conducting substances, may be perforated or broken.

Exper. 1.—Take a card, a quire of paper, or the cover of a book, and keep it close to the outside coating of a charged jar; put one knob of the discharging rod upon the card, quire of paper, &c., so that between the knob and coating of the jar the thickness of that card or quire of paper only is interposed; lastly, by bringing the other knob of the discharging rod near the knob of the jar, make the discharge, and the electric spark will pierce a hole (or perhaps several) quite through the card or quire of paper. This hole has a bur raised on each side, except the card, &c., be pressed hard between the discharging rod and the jar. If this experiment be made with two cards instead of one, which however must be kept very little distant from one another, each of the cards, after the explosion, will be found pierced with one or more holes, and each hole will have burs on both surfaces of each card. The hole, or holes, are larger or smaller, according as the card, &c., is more damp or more dry. It is remarkable, that if the nostrils are presented to it, they will be affected with a sulphureous, or rather a phosphorescent smell, just like that produced by an excited electric.

If, instead of paper, a very thin plate of glass, resin, sealing wax, or the like, be interposed between the knob of the discharging rod and the outside coating of the jar, on making the discharge, this will be broken in several pieces.

If the explosion is sent over the surface of a piece of glass, this will be marked with an indelible track, which generally reaches from the extremity of one of the wires to the extremity of the other. In this manner, the piece of glass is very seldom broken by the explosion. But Mr. Henley discovered a very remarkable method to increase the effect of the explosion upon the glass; which is by pressing with weights that part of the glass which lies between the two wires (i.e., that part over which the shock is to pass). He put first a thick piece of ivory upon the glass, and placed upon that ivory a weight at pleasure, from one quarter of an ounce to five pounds: the glass in this manner is generally broken by the explosion into innumerable fragments, and some of it is absolutely reduced into an impalpable powder. If the glass is very thick, and resists the force of the explosion, so as not to be broken by it, it will be found marked with the most lively prismatic colours, which are thought to be occasioned by very thin laminae of the glass, in part separated from it by the shock. The weight laid upon the glass is always thrown by the explosion, and sometimes it is thrown quite off from the ivory. This experiment may be most conveniently made with the universal discharger.

Exper. 2.—Place the extremities of two wires, one above and the other below a card, so as to be about an inch distance from each other, taking care that the card be kept steady. Then, make the charge of a Leyden phial pass from one wire to the other, and it will be found, that a luminous track will pass from the end of that wire which is connected with the positive surface of the phial, to the extremity of the other wire, where a hole will be perforated through the card.

This experiment, to which we shall have occasion to refer hereafter, is by Mr. Lullin of Geneva.

Mr. Symmer made some experiments on the perforation of paper, which we shall mention here, as on them he grounded a principal argument in favour of that theory which he adopted, and of which we shall give an account hereafter.

Exper. 3.—A piece of paper covered on one side with Dutch gilding, and which had been left accidentally between two leaves in a quire of paper, in which a former experiment had been made, was found to have the impression of two strokes upon it, about a quarter of an inch from each other; the gilding was stripped off, and the paper left bare for a little space in both places. In the centre of one of these places was a little

(b) Instead of the card, a piece of baked wood may be cut in that shape, and painted over with lamp-black and oil; which will answer better than the card, it being much more steady, and not so liable to attract moisture. Chap. VIII.

Principles of the round hole, in the other only an indentation or impression, such as might have been made with the point of a bodkin.

Exper. 4.—In the middle of a paper book, of the thickness of a quire, Mr Symmer put a slip of tinfoil; and in another of the same thickness, he put two slips of the same sort of foil, including the two middle leaves of the book between them. On passing the explosion through the two different books, the following effects were produced. In the first, the leaves on each side of the tinfoil were pierced, while the foil itself remained unpierced; but at the same time, it might be perceived that an impression had been made on each of its surfaces, at a little distance from one another; and such impressions were still more visible on the paper, and might be traced, as pointing different ways. In the second, all the leaves of the book were pierced, excepting the two that were included between the slips of tinfoil; and in these two, instead of holes, the two impressions in contrary directions, were visible.

The following experiment shows how easily so hard a substance as glass, may be pierced by the electric spark. It is thus related by Mr Cavallo.

Exper. 5.—Let a glass tube of any diameter, and about five or six inches in length, be closed hermetically, or by means of sealing-wax, at one end, and fill about half of it with olive oil; then stop the aperture of it with a cork, and let a wire pass through the cork, and come so far within the tube, as to have its extremity below the surface of the oil. This end of the wire must touch the surface of the glass, for which purpose it must be bent nearly at right angles, which may be easily done before the cork is put upon the tube. Things being thus prepared, bend into a ring the other extremity of the wire, and suspend it, with the tube hanging to it, to the wire at the end of the conductor. Then work the machine, and bring the knuckle of a finger or the knob of a wire near the outside of the tube, just opposite to the extremity of the wire; the consequence of which will be, that a spark will happen between the wire and the knuckle, which makes a hole through the glass.—By turning the wire about, or raising and lowering it, many holes may be successively made in the same tube, after the manner above described.

Exper. 6.—Roll up a piece of soft tobacco-pipe clay in a small cylinder, and insert in it two wires, so that their ends without the clay may be about a fifth part of an inch from one another. If a shock be sent through this clay, by connecting one of the wires with the outside of a charged jar, and the other with the inside, it will be inflated by the shock, i.e., by the spark, that passes between the two wires, and, after the explosion, will appear swelled in the middle. If the shock sent through it is too strong, and the clay not very moist, it will be broken by the explosion, and its fragments scattered in every direction. To make this experiment with a little variation, take a piece of the tube of a tobacco-pipe, about one inch long, and fill its bore with moist clay; then insert in it two wires, as in the above rolled clay; and send a shock through it. This tube will not fail to burst by the force of the explosion, and its fragments will be scattered about to a great distance. If, instead of clay, the above-mentioned tube of the tobacco pipe, or a glass tube (which will answer as well), be filled with any other substance, either electric or non-electric, inferior to metal, on making the discharge, it will be broken in pieces with nearly the same force. This experiment is the invention of Mr Lane, F.R.S.

Exper. 7.—Place within a common drinking-glass, nearly full of water, two knotted wires, bent in such a manner, as that their knobs may be within a little distance of each other in the water. Connect one of these wires with the outside coating of a pretty large jar, and touch the other wire with the knob of it; on making the discharge, the explosion which must pass through the water between the two knobs, will disperse the water, and break the glass with a surprising violence. This experiment requires great caution.

Sig. Becarria contrived a small mortar, into which a drop of water was put, between the extremities of two wires which went through the sides of the mortar, and a wooden ball was applied over the drop of water; then a charged jar being discharged through the wires of the mortar, and consequently through the drop of water, rarefied the latter, and drove the ball out with considerable force. Mr Lullin produced a greater effect by making the discharge through oil instead of water.

Chap. IX. Of the Methods of estimating the Degree of Accumulated Electricity in Jars and Batteries.

The only method of ascertaining the charge of a Leyden phial or of a battery, which we have hitherto mentioned, is that of observing the repulsive force of the charge on the ball of Henry's quadrant electrometer. But it was found (Vtae 122) that this was not always a just criterion of the amount of the charge; as, even when the jar was insulated, and consequently could receive no charge, the index of the electrometer still rose as high as if the jar was fully charged. We shall now proceed to describe two methods, which, particularly the last, are much less liable to error. The first depends on the following principle.

The distance of the ball of a discharging rod from the knob of a charged phial or battery necessary to produce an explosion, will be greater in proportion to the degree of accumulated electricity which the jar or battery has received.

Exper.—Take a Leyden phial, into the knob of which is fixed a quadrant electrometer; communicate to it a small charge, so that the index of the electrometer may point, we shall suppose, at 10°. In making the discharge, it will be found necessary to bring the ball of the discharging rod almost in contact with the knob of the jar. Now charge the jar to 20°, and it will be found that the explosion will take place, when the ball of the discharging rod is at a greater distance from the knob of the jar, than before; and thus, by repeating the experiment with greater charges, it will be observed, that the distance necessary to produce an explosion will increase nearly in proportion to the charge.

On this principle Mr Lane constructed an electro-meter, which has been found extremely useful, when it was required to discharge a jar or battery a number of times successively, with the same charge. This instrument... The principal part of it consisted originally of a brass ball about an inch and a half in diameter, screwed to a graduated brass rod, and adapted to a proper frame, so that it might be set at any required distance from the prime conductor of the knob of a Leyden phial. The chief use of this instrument is to allow a jar to discharge spontaneously through any proper circuit, without employing a discharging rod, or moving any part of the apparatus, and also to produce successive explosions nearly of the same strength, as observed above. If, for example, the brass ball be placed at the distance of about half an inch from the prime conductor, and a Leyden phial be so situated as to have its knob in contact with the prime conductor, while its outside coating communicates with the ball of the electrometer, it is evident that the communication between the outside and inside of the jar, is interrupted only between the prime conductor and the brass ball, which are half an inch asunder; therefore, in charging the jar, when the charge is become so high as to strike through half an inch of air, the jar will discharge spontaneously, and by keeping the brass ball at the same distance from the prime conductor, and charging the jar successively, the shocks will be nearly of the same strength.

An electrometer of this kind, though not exactly like the original one, is now commonly used by the practitioners of medical electricity, and is delineated in fig. 67. of Plate CLXXXIX. It consists of a glass arm D, which proceeds from the wire of the jar F, and to the extremity E of which a spring socket is cemented, through which a wire passes, which is furnished with a knob B, towards the knob A of the jar, and with an open ring C at its other extremity. Now, as this wire may be slid backwards and forwards, the knob B may be put at any required distance from the knob A, as far as the construction of the instrument will allow. The wire BC is generally marked with divisions which show the distance of the two knobs, when the wire is so situated, as that the required division coincides with the edge of the spring socket; as, for instance, one-tenth, or one quarter of an inch, &c. When the jar F is set against the prime conductor G, as represented in the figure, suppose that the ball B is set at the distance of one-tenth of an inch from the ball A, and that a wire be fixed from the electrometer's ring to the outside coating of the jar, as shown by the dotted line CK; then, when the machine is put in motion, the discharge of the jar, as soon as this becomes sufficiently charged, will be made between the knobs AB, and through the wire CK; and it is evident that these discharges will be of the same strength, as long as the distance between AB remains the same.

This instrument is subject to the following inconvenience, viz. that the force of the explosion, after a time, roughens the surface of the brass ball, and thus, for a reason to be explained hereafter, the instrument is useless, unless the polish of the ball be again renewed. It is also found that this instrument is not accurate in shewing the exact charge of a jar.

The charge of a jar or battery may be most accurately determined by the weight which the repulsive force of the accumulated electricity is able to raise.

Upon this principle Mr Brooke of Norwich constructed a very valuable electrometer, of which he has given a long and accurate account in his Miscellaneous Experiments.

Our limits will not permit us to copy this long description, for which we must therefore refer our readers to Mr Brooke's work. We have, however, the less reason to regret this omission, because we shall presently describe an instrument invented by the late Professor Robison, which appears to us superior to Mr Brooke's both in simplicity and utility.

Mr G. Adams has described an electrometer very similar in principle to that of Mr Brooke, and we shall here copy his description.

"Fig. 68. and 69. represent an electrometer, nearly similar to that contrived by Mr Brooke. The two instruments are sometimes combined in one, or used separately, as in these figures. The arms FH, fig. 69., when in use, are to be placed as much as possible out of the atmosphere of a jar, battery, prime conductor, &c. The arm FH and the ball K are made of copper, and as light as possible. The divisions on the arm FH are each of them exactly a grain. They are ascertained at first by placing grain weights on a brass ball which is within the ball L, (this ball is an exact counterbalance to the arm FH and the ball K when the small slide r is at the first division) and then removing the slide r, till it, together with the ball K, counterbalances the ball L and the weight laid on it.

A, fig. 69., is a dial-plate, divided into 90 equal parts. The index of this plate is carried once round, when the arm BC has moved through 90 degrees, or a quarter of a circle. That motion is given to the index by the repulsive power of the charge acting between the ball D and the ball B.

The arm BC being repelled, shows when the charge is increasing, and the arm FH shows what this repulsive power is between two balls of this size in grains, according to the number the weight rests at when lifted up by the repulsive power of the charge: at the same time the arm BC points out the number of degrees to which the ball B is repelled; so that by repeated trials, the number of degrees answering to a given number of grains, may be ascertained, and a table formed from these experiments, by which means the electrometer, fig. 69., may be used without that of fig. 68.

Mr Brooke thinks that no glass, charged (as we call it) with electricity, will bear a greater force, than that whose repulsive power, between two balls of the size he used, is equal to sixty grains; that in very few instances it will stand sixty grains weight; and he thinks it hazardous to go more than 45 grains.

Hence, by knowing the quantity of coated surface, and the diameter of the balls, we may be enabled to say, to much coated surface, with a repulsion between balls of so many grains, will melt a wire of such a size, or kill such an animal, &c.

Mr Brooke thinks, that he is not acquainted with all the advantages of his electrometer; but that it is clear, it speaks a language which may be universally understood, which no other will do; for though other electrometers will show whether a charge is greater or less, by an index being repelled to greater or smaller distances, or by the charge exploding at different distances, yet the power of the charge is by no means ascertained; but this electrometer shows the force of the With his electrometer, Mr Brooke made a set of experiments, with a view to determine exactly the force of batteries of an inferior power, in melting fine metallic wires of different kinds. The following is the substance of these experiments.

1. With a battery of nine bottles, containing about 16 square feet of coated surface, and charged to 32 grains of repulsion, a shock was eleven times felt through a piece of steel wire twelve inches long and \( \frac{7}{8} \)th of an inch thick; the wire was shortened an inch and a half, being then about ten inches and a half long; by a twelfth shock, the wire was melted to pieces.

2. A shock from the same nine bottles charged to the same degree of repulsion, being sent through a piece of steel wire, 12 inches long and \( \frac{7}{8} \)th of an inch thick, the first time melted the whole of it into small globules.

3. A shock from the same nine bottles charged to the same degree, being sent through a piece of brass wire twelve inches long and \( \frac{7}{8} \)th of an inch thick, melted the whole of it, with much smoke, resembling that from gunpowder; but the metallic part formed itself, in cooling, chiefly into concave hemispherical figures of various sizes.

4. A shock from only eight of the bottles charged to the same degree, did but just melt twelve inches of steel wire \( \frac{7}{8} \)th of an inch thick, so as to fall into several pieces; these pieces in cooling formed themselves into oblong lumps, joining themselves to each other by a very small part of the wire between each lump which was not melted enough to separate, but appeared like oblong beads on a thread at different distances.

5. A shock from the same eight bottles, charged to the same degree, so perfectly heated twelve inches of brass wire about \( \frac{7}{8} \)th of an inch in diameter, as to melt it, or at least soften it so far as to make it fall down by its own weight, from the forceps by which it was held at each end, upon a sheet of paper placed below to catch it, and when it fell down it was so perfectly flexible that, by falling it formed itself into a vermicular shape, and remained entire its whole length, which when it was put into the forceps was about 12 inches; but after the shock was passed through it, it flagged to much as to be stretched by its own weight to almost fifteen inches, and by falling on the paper it was flattened throughout its whole length so much, that when it was examined by a magnifier of half an inch focus, it appeared five or six times as broad as it was thick.

6. A shock from nine bottles charged only twenty grains, was sent through a piece of steel wire, of the same length and diameter as in the former experiments, and heated it sufficiently to melt it, so that it separated in several places; and the pieces were formed into beads strung as in experiment 4.

7. A shock from the same nine bottles charged to twenty grains was sent through ten inches of brass wire \( \frac{7}{8} \)th of an inch diameter; the wire was heated red hot so as to render it very flexible, but it did not separate.

It was shortened, however, nearly three-eighths of an inch.

8. A shock from the same nine bottles charged to the same degree, being sent a second time through the last piece of wire, melted it further in several places.

9. A shock from nine bottles charged to 30 grains, sent through twelve inches of brass wire \( \frac{7}{8} \)th of an inch in diameter, acted on it nearly as in experiment 5, except that it was separated in two places, and the pieces when joined measured about fifteen inches and a half long; it was perfectly flattened by its fall on the paper as before.

10. A shock from nine bottles charged to 30 grains, being sent through eight inches and a half of brass wire of the same diameter, wholly dispersed it in smoke, and left nothing remaining to fall on the sheet of paper placed below it.

11. A shock from twelve bottles charged to 20 grains sent through ten inches of steel wire, \( \frac{7}{8} \)th of an inch in diameter, made it red hot, but did not melt it.

12. A second charge, the same as the last, being sent through the same piece of wire, heated it red hot as before, but did not cause it to separate; the wire was now, however, shortened five-sixteenths of an inch.

13. A shock from the same twelve bottles charged to 25 grains, being sent through the same piece of wire, partly melted it into several pieces, and produced many globules of oxidated metal.

14. With 15 bottles charged to 25 grains, a shock was sent through ten inches of steel wire \( \frac{7}{8} \)th of an inch in diameter, which melted it at the first time, and dispersed a great part of it about the room.

15. A shock from the same 15 bottles charged to 20 grains, just melted ten inches of steel wire of the same diameter as before, so as to cause it to run into several beautiful globules, nearly as in experiment 13.

16. A shock of 15 bottles charged to 15 grains, being sent through ten inches of steel wire of the same diameter as the last, made the wire barely red-hot; but shortened it one-tenth of an inch.

17. The last piece of wire having received a shock from 15 bottles charged to 12 and a half grains, was not made red-hot.

18. A shock from the same 15 bottles, charged to 25 grains, was sent through the same piece of wire, and seemingly tore the wire into splinters.

19. Four bottles charged to 30 grains, just melted three inches of steel wire \( \frac{7}{8} \)th of an inch in diameter, so as to make it fall into pieces.

20. Five bottles charged to 25 grains, melted three inches of such wire as the last into large beautiful globules.

21. With eight bottles charged to 15 grains, three inches of steel wire, \( \frac{7}{8} \)th of an inch in diameter, were melted as in the last experiment; indeed the appearance and effect were so nearly alike in both cases, that the metal after both experiments might have been said to be the same.

22. The force of ten bottles charged twelve grains and a half rather exceeded experiment 19, but scarcely came up to experiments 20 and 21.

23. Suspecting something wrong in experiment 19, Mr Brooke found, that though his bottles hitherto were as nearly of the same size as he could procure them, Principles of them, yet some of them were a little larger than others, Electricity and which was the case in experiment 19, one of the four was smaller than the other three; so that he repeated the experiment with four bottles more equal in size, charging them to 30 grains, and the fusion was as perfect as in any.

24. A charge to 30 grains, with the last eight bottles, beautifully melted six inches of steel wire \( \frac{1}{7} \) th of an inch in diameter.

25. A shock from two bottles charged to 45 grains, was sent through one inch of steel wire, of the same diameter as the last, but only changed its colour.

26. With three bottles charged to 40 grains, a shock sent through one inch and a half of steel wire of the above diameter, dispersed it all about the room.

27. Mr Brooke considering that a steel wire of \( \frac{1}{7} \) th of an inch in diameter, contains nearly twice the quantity of metal which is contained in the same length of wire of \( \frac{1}{7} \) th of an inch in diameter, took three inches of the former, and sent through it a shock from ten bottles, charged to 25 grains. This shock melted it just as the shock from five bottles did in experiment 20.

28. With 20 bottles charged to 12 grains and a half, he melted three inches of steel wire of \( \frac{1}{7} \) th of an inch in diameter, exactly as in the foregoing experiment.

29. As a steel wire of \( \frac{1}{7} \) th of an inch diameter contains nearly twice the quantity of metal in the same length, as is contained in a steel wire of \( \frac{1}{7} \) th, or four times the quantity contained in a steel wire of \( \frac{1}{7} \) th of an inch diameter; it might from the foregoing experiments be expected, that 20 bottles charged to 25 grains would melt three inches of steel wire of \( \frac{1}{7} \) th of an inch in diameter: but on a great many trials he could not procure 20 bottles which would bear the discharge when charged to 25 grains; for at the discharge, there was always one or more bottles broken or perforated. He was now reduced to the necessity of being content with bottles of any size, that would bear the required charge of from one to three gallons each, or that contained from 150 to 300 or more square inches of coated surface each, but all in vain. The only resource left him, as he was not near a glasshouse, was to increase the quantity of surface and not to charge so high and to proportion the one to the other: it was therefore resolved to adopt a third expedient, i.e., instead of employing about 36 square feet of coating, he added a third, or twelve feet, which made it in all 48 feet; and instead of charging to 25 grains, or rather 24 for the sake of a more easy division by three, he annulled one-third of the charge, leaving fifteen grains, and thus he succeeded perfectly well; for by 48 feet of coated surface charged to 16 grains, three inches of steel wire \( \frac{1}{7} \) th of an inch in diameter were as curiously melted as in any of the former experiments.

These bottles, thus broken in large discharges, seem always to break or to be perforated nearly in the thinness, but never in the thickest place, which shows the necessity of the glass being of a considerable thickness.

30. As in experiments 19 and 20, where the coated surface in the former is but half the quantity of that in the latter, and the former is charged to 30, and the latter to 15 grains, to know how high 48 feet of coating must be charged to produce the same effect exactly: and as the coating in four bottles, consisting of a little more than six feet and a half, is contained in 48 feet a little more than seven times; so Mr Brooke tried, by charging 48 feet only to a little more than four grains, or only about one-seventh part to high, as four times seven is 28; that is, but two less than 30: and this had exactly the same effect on the wire, which was \( \frac{1}{7} \) th of an inch in diameter, and three inches long, as it had upon the former.

31. As the last experiment agreed so exactly with experiments 19 and 20, the next thing tried was to see the effect of 48 feet of coated surface charged to a little more than four grains upon six inches of steel wire, the size of the last; this was made very faintly red.

32. By a repetition of the last experiment, with the same length of the same wire, to see how often the same charge might be sent through it without melting it, and to observe the appearance of the wire after each shock, he found that by the eighth shock it was melted into several pieces. After the first shock, the redness produced became less every time, even the last time, when it was separated. By the first shock, though made little more than fairly red, the wire became flexible, that by a small addition to its own weight, it seemed to become almost perfectly straight when cooled: at about the third or fourth shock it began to assume a zig-zag appearance; after the fifth shock the surface of it appeared rough; after the seventh shock the surface was very roughly scorched or scaly; and some of the scales had fallen upon a piece of white paper placed at about half an inch distance below it. The eighth shock melted it in three places; and at these places where the angles appeared the sharpest or most acute, a great number of the scales were driven off about the paper, and appeared as in experiment 18; some of them were almost one-tenth of an inch long, and some of them about a third or fourth part of the diameter of the wire in breadth, and very thin; after the seventh shock it was shortened seven-sixteenths of an inch; the wire was \( \frac{1}{7} \) th of an inch in diameter.

33. Repeating experiment 31. again with the same length of wire of the same diameter, and the same battery charged to the same degree, in order to observe the method of the wire shortening, having fixed an insulated gage parallel to it and at the distance of about a quarter of an inch from it: after the first shock, which made the wire fairly red, (holding it fixed at one end, that the shortening might appear all at the other, which was held so that it might either contract or dilate,) he observed, that it shortened considerably as it cooled; repeating the shock, it did the same, and so on till it was melted, which was by the eighth shock, as before. At the instant when the shock passed through the wire, it appeared to dilate a little; and after it was at the hottest, it gradually contracted after every stroke, as it cooled, about one-sixteenth of an inch each time; the dilatation was so very trifling, as to bear but a very small proportion to its contraction, and sometimes it was doubtful whether or not it dilated at all; but after all the observations it appeared oftenest to dilate, than not.

34. The same 48 feet, negatively charged to a little more than four grains, melted three inches of steel wire \( \frac{1}{7} \) th of an inch in diameter, the same as the positive charge did in experiment 30.

35. The same battery of 48 feet of coated surface, charged Principles of charged to a little more than eight grains, melted three inches of steel wire, \(\frac{1}{6}\)th of an inch in diameter. This is very nearly in proportion to experiment 27, but here the charge was negative, and Mr Brooke says the fusion was the most pleasing he had hitherto had; which he attributes to the charge having been probably so well adjusted as to be exactly sufficient to melt the wire and no more: the heat remained for the longest time, and the fused metal ran into the largest globules; probably the long continuance of the heat, was owing to the charge being just sufficient, and to the size of the lumps into which the fused metal was formed.

36. This was a repetition of experiment 1, with twelve inches of steel wire \(\frac{1}{6}\)th of an inch in diameter, but with this difference, that as then only nine bottles were employed, containing about fifteen feet of coated surface, charged to 32 grains, he here used 18 bottles containing about 32 square feet of coating charged to only 16 grains. This was done to observe the progress of the destruction of the wire, as in experiment 32, as well as to prove the similarity of the effect. The wire being the same size, fort of metal, and length, as recited just above; the first shock made it red-hot throughout its whole length attended with smoke and smell, changed its colour to a kind of copperish hue, and shortened it considerably; the second shock made it of a fine blue, but it did not appear red, and shortened it more; at the third shock it assumed a zig-zag appearance, many radii were very visible at the bendings, and the wire continued to shorten till the eleventh shock, when one of the bottles in the second row of the battery was struck through: the fracture was covered over with common cement, and its place supplied by changing place with one in the third row, supposing the mended one to be the weakest; and with the battery in this state he made the twelfth shock, which separated the wire as in experiment 1, but shortened it only one inch.

37. A shock from 48 feet of coated surface, charged to eight grains, sent through three inches of copper wire \(\frac{1}{6}\)th of an inch in diameter, seven times, gave it the zig-zag appearance, but did not make it much shorter; the eighth shock separated it at one end close to the forceps which held it, but it did not appear to be made at all sensibly red-hot, notwithstanding it must have been often so at the place where it was melted; which space was so very small as barely to be perceptible, like as when a point is set upon any flat surface of iron, and a shock from a pound phial sent through, both the point and flat surface where the point rested, if examined with a magnifying glass, will be found to have been melted, and a speck may be seen; but the redness of the metal will scarcely be visible.

38. A shock from 48 feet, charged to 16 grains, was sent through six inches of lead wire \(\frac{1}{6}\)th of an inch in diameter, and melted it into many pieces.

39. A shock from 48 feet, charged to 15 grains, was sent through six inches of wire like the last, which did not separate it, but made it smoke.

40. A shock like the last, was sent through the same piece of wire a second time, and melted it into several pieces.

The law by which wires resist destruction, in proportion to the diameter of the wire, does not seem to be nearly so equable, in the lead as in the steel wire. For a charge of four grains, in experiment 34, melted three inches of lead wire \(\frac{1}{6}\)th of an inch in diameter; but it took a charge of about three times that power, to destroy three inches of lead wire \(\frac{1}{6}\)th of an inch in diameter; which is about double the quantity of metal in the same length as in that of \(\frac{1}{6}\)th of an inch in diameter. Thus, it is easy to find what different resistance a wire of any of the preceding metals, of equal size and length, will make to the electrical stroke.

The length of the electric circuit, in which the different wires were placed, in the foregoing experiments, from the nearest part of the inside to the nearest part of the outside of the battery, exclusive of the length of the laid wires, was about eight feet.

41. Two gentlemen coming to see a piece of wire melted by electricity, Mr Brooke proceeded to show it them, by fixing twelve inches of steel wire \(\frac{1}{6}\)th of an inch in diameter, and then (supposing the electrometer, and all other things ready placed), to charge the battery, but the electrometer did not move: nevertheless, he continued charging as he supposed; but still the electrometer remained as it was, although he had been charging much longer than would have been necessary, contrary to his design, which was to take a small wire, that a smaller charge might be sufficient. Having been charging a long time, Mr Brooke left off to look about the apparatus, in order to see if all was right: as he was looking he found there was no communication between the battery and the electrometer, and he heard a slight crackling in the battery which convinced him that it was charged. Accordingly he made the discharge, expecting nothing unusual; but the wire was dispersed seemingly in a very violent manner. The report was so very loud that their ears were stunned, and the flash of light so very great, that Mr Brooke's sight was quite confused for a few seconds.

Mr Cuthbertson has lately contrived an electrometer, which possesses all the advantages of Mr Brooke's, added to those of Lane and Henly, with which he has ingeniously combined it.

This valuable instrument is thus described by the inventor:

The electrometer is represented in Plate CLXXXIX. No. 2, fig. 70. GH is a long square piece of wood, about 18 inches long, and six inches broad, in which are fixed three glass supports, DEF, mounted with brass balls, \(a\), \(b\), \(c\). Under the brass ball \(a\) is a long brass hook; the ball \(c\) is made of two hemispheres, the under one being fixed to the brass mounting, and the upper turned with a groove to fit upon it, so that it can be taken off at pleasure. The ball \(b\) has a brass tube fixed to it, about three inches long, cemented on to the top of F; and the same ball has a hole at the top, of about one-half inch diameter, corresponding with the inside of the tube. AB is a straight brass wire, with a knife-edged centre in the middle, placed a little below the centre of gravity, and equally balanced with a hollow brass ball at each end, the centre, or axis, resting upon a proper shaped piece of brass fixed in the inside of the ball \(c\); that side of the hemisphere towards \(c\) is cut open, to permit the end \(c\) A of the balance to descend till it touches the ball \(a\), and the upper hemisphere C. Principles of Electricity also cut open to permit the end c B to ascend; i is a weight, weighing a certain number of grains, and made in the form of a pin with a broad head; the ball B has two holes, one at the top, and the other at the bottom; the upper hole is so wide, as to let the head of the pin pass through it, but to stop at the under one, with its shank hanging freely in b; a number of such pins are commonly made to each electrometer of different weights; (c) k is a common Henly's quadrant electrometer, and when in use, it is screwed upon the top of c.

It is evident, from the construction, that if the foot stand horizontal, and the ball B be made to touch b, it will remain in that position without the help of the weight i; and if it should by any means receive a very low charge of electric fluid, the two balls b, B, will repel each other; B will begin to ascend, and, on account of the centre of gravity being above the centre of motion, the ascension will continue till A rest upon a. If the balance be set again horizontal, and a pin i, of any small weight, be put into its place in B, it will cause B to rest upon b, with a pressure equal to that weight, so that more electric fluid must be communicated than before, before the balls will separate; and as the weight in B is increased or diminished, a greater or less quantity of electric fluid will be required to effect a separation.

When this instrument is to be applied to a jar, or battery, for which purpose it was invented, one end of a wire, L, must be inserted into a hole in b, and the other end into a hole of any ball proceeding from the inside of a battery, as M (d); k must be screwed upon c, with its index towards A; the reason of this instrument being added, is to show, by the index continuing to rise, that the charge of the battery is increasing, because the other part of the instrument does not act till the battery has received its required charge.

If this instrument be examined with attention, it will be found to consist of three electrometers; and answers three different purposes, namely, a Henly's electrometer, Lane's discharging electrometer, and Brooke's steel-yard electrometer; the first not improved, but the two last, which were very defective when first invented, I flatter myself are here brought to perfection. As the only use of Henly's electrometer to this instrument is, as I have said before, to show, by its continuing to increase in divergency, that the battery continues to receive a still stronger charge, it required no improvement; but Lane's electrometer, in its primitive state, could by no means answer the required purpose for batteries, because the ball intended to discharge the battery, was necessarily placed so near to the ball of the battery, that dull and fibrous particles were always attracted by and adhered between the two balls, so as to retard the charging, and often render a high charge impossible: whereas, in this, they are placed at four inches asunder; and when the desired height of charge is obtained, and not before, the ball of the electrometer moves of itself nearer to the ball which is connected with the outside of the battery, and causes a discharge. The defects in Brooke's steel-yard electrometer were, first, that it could not cause a discharge; and secondly, the difficulty of observing the first separation of the balls caused great error. If it were not placed in an advantageous light (which the nature of the experiments could not always permit), it would not be seen, without the attention of an assistant, which is sometimes unpleasant, and cannot always be commanded. But the instrument which I have described, requires no attention or assistance; for as soon as the separation takes place between B and b, the ball A descends, and discharges the battery of itself.

By this combination and improvements, we possess in the present instrument all that can ever be required of an electrometer; namely, by k, we see the progress of the charge; by the separation of B, b, we have the Nielsboer's repulsive power in weight; and by the ball A, the dilution of charge is caused, when the charge has acquired the proper strength propolled.

With this electrometer Lieutenant Colonel Haldane Col. Hall has made some very ingenious experiments to determine the mode of the exact charge of a battery required to produce certain changes in wires of the same kind. His method of estimating the force of the charge is by the number of explosions that it is capable of producing in a jar containing the outside coating of the battery. Thus, if the battery while charging produces three explosions of the jar, he says, it has received three measures of electricity.

Mr Cuthbertson having observed that when he breathed into a jar, it was thus rendered capable of receiving a higher charge, made the following experiments to ascertain the effect of such increased charge.

Exper. 1.—Prepare the electrometer in the manner Breathing shewn in the plate, with the jar M annexed, which contains about 168 square inches of coating (e): put into bear a B the pin, marked 15; take two inches of watch-pan higher, duline wire, fix to each end a pair of spring tongs, as is represented at G m, hook one end to m, and the other to the wire N, communicating with the outside of the jar; let the uncoated part of the jar be made very clean and dry; and let the prime conductor of an electrical machine, or a wire proceeding from it, touch the wire L; then, if the machine be put in motion, the jar and electrometer will charge, as will be seen by the rising of the index of k, and when charged high enough, B will be repelled by b, and A will descend and discharge the jar through the wire, which was confined in the tongs, and the wire will be fused and run into balls.

(c) Instead of the pins, which were found inconvenient, Mr Cuthbertson has lately constructed his electrometer with a sliding piece of brass, so adapted to the arm of the balanced wire, as by sliding nearer to, or farther from the centre of gravity to denote proportional weights.

(d) A chain, or wire, or any body through which the charge is to pass, must be hung to the hook at m, and carried from thence to the outside of the battery, as is represented by the line N.

(e) Take out the pin in B, and observe whether the ball B will remain at rest upon b; if not turn the adjusting screw at C till it just remains upon A. Chap. IX.

Principles of Exper. 2.—Put into the tongs eight inches of the Electricity same fort of wire as before, hang one pair of tongs to the illustrated hook m, and apply the other to the wire which forms the outside communication: take out the pin in B, and put in its stead one marked 30; all the other part of the apparatus remaining as before, and the uncoated part of the jar being previously cleaned and dried: the machine will charge, as is shown by the rising of the index as before; but as soon as the jar has received a greater quantity of electric power than before, a spontaneous explosion will happen without affecting the balls B b, because the discharge will have passed along the uncoated part of the jar from the inside coating to the outside: whence it follows, that, while the jar remains in that clean state, it is incapable of receiving a charge high enough to affect the balls, or even a higher charge than it had received in the first experiment. Let the uncoated part of the jar be therefore rendered, in a slight degree, damp; which is easily done, by breathing into the inside, through a glass tube; put the machine in motion, and no spontaneous explosion will happen, but the balls B b will repel, as in the first experiment, and the discharge will happen from A to a, and pass through the wire placed in the circuit; and though it was eight inches, it will be fused in the same degree as two inches in the last experiment, namely, the wire seen red hot the whole length, and then fall into balls.

Very different degrees of fusion are caused by electric discharges, which may cause great mistakes, if not well attended to. It is proper to adhere to the degree above-mentioned, and particular care ought to be taken to lay the wire, intended for fusion, straight, without any bendings or angles in it. The wire used in the two last experiments, was that which is commonly called watch-pendulum wire, which is flattened; and as it approaches very near to such a sharp edge as might be supposed to affect the experiment, by permitting a diffusion of the electric fluid in its passage, round wires were tried, and the result was the same.

The late Dr Robison contrived an electrometer on similar principles with that of Mr Brooke, but much superior to it in simplicity of construction, and not inferior to any which have been invented in point of accuracy.

Plate CLXXXIX. Fig. 71. exhibits a front view of this instrument, which is thus constructed. A polished brass ball No. 2. A, a quarter of an inch in diameter, is fixed on the point of a common needle about three inches long, and as slender as can be procured of that length. On the other end of the needle is fixed a ball of amber, glass, or other solid non-conducting substance, of about half an inch or three-quarters of an inch in diameter. This ball is fixed in such a way as that the needle does not quite reach to its surface, though the ball F must be completely perforated. From the electric ball there passes a slender glass rod, F, E, L, bent at right angles at E, so that the part FE is about three inches long, and the other extremity L is immediately opposite to the centre of the ball A. A piece of amber C, cut so as to have two parallel cheeks, is fixed on the extremity L of the glass rod. For the principal part of the instrument, a strong dry silk thread is to be prepared by holding it perpendicular in melted sealing-wax, till it shall be fully penetrated by the wax, so as to retain a thin coating of it. The thread, thus coated, must be kept extended, so that it may be quite straight, and it must be made perfectly smooth by holding it before a clear fire and rolling it on a smooth plane. It is then to be passed through a small cube of amber, that has two holes drilled in two of its opposite faces, perpendicularly about half way to the stalk. By these holes the cube is suspended, so as to move readily, on two fine brass pins, between the cheeks of the piece of amber at L. The waxed thread is about six inches long, and is equally divided by the amber cube. To one end of it, B, is fixed a ball of some conducting substance, as of very thin polished metal, or gilt and burnished cork, a quarter of an inch in diameter. The other extremity, D, passes through a cork ball, so as to move with some friction.

This part of the instrument is so constructed, that when FE is perpendicular to the horizon, and the stalk BD, with its balls, is allowed to hang freely, the ball B just touches the ball A. This position is represented in fig. 72.

The ball F is fixed to one end of a glass rod FI made to pass perpendicularly through the centre of a graduated circle GHO, and furnished at its other extremity I with a knopped handle of box wood. HK is the stand of the electrometer, in the head of which is a hole in which the rod FI moves smoothly but not easily. Farther, there is adapted to the glass rod FI an index NH that turns round it. This index is so placed as to be parallel to a line LA drawn through the centre of the ball A. Now, as the circle is divided into 360 degrees, O being marked above and 90 on the right hand; the index will point out the angle which the line LA makes with the vertical line. It is convenient to have another index on the rod FI turning stiffly round it, and extending considerably beyond the circle.

The method of using this instrument will be shown when we speak of the law of electric action in the next part.

Chap. X. Of the Electrophorus.

The electrophorus is an instrument invented by Signior Volta of Como. It generally consists of two of the electrophorus parts; a round plate of metal, or of wood, made perfectly free from points and edges, and covered with tin foil; as A fig. 73; and another circular plate of any conducting substance covered with a coating of some resinous electric, generally of lac dissolved in alcohol, melted sealing-wax, pitch, or of sulphur; as B. The first plate is furnished with a glass handle, or with silk strings, so that it may be occasionally inflated: to this plate Volta has given the name of Scudo.

Sometimes the apparatus is made in three parts, i.e., the resinous electric is formed into a cake independent of the plate B, and this is the most convenient method for experiment. To these three parts Dr Robison has given the following names; viz. the resinous electric he calls the cake, the plate B, the sole, and the plate A the Cover; and these names we shall adopt for the sake of convenience. For the purpose of exhibiting the appearances which we are about to describe in the most brilliant manner, the several parts may be made very thin in proportion to their circumference; but for illustrating the theory The general appearances which have been observed with this apparatus may be reduced to the following heads.

1. If the cake, after being just formed, be suffered to remain on the sole, till it be perfectly cool and hard, while the sole is insulated; on examination the whole will be found negatively electric, and on applying the finger to any part of it, especially the sole, a spark is produced. If the apparatus be now suffered to remain at rest, its electricity gradually becomes weaker, and at length entirely disappears. It may, however, be again produced by rubbing the cake with a piece of new flannel, or, what is better, a piece of hare or mole skin with the fur on, made dry and warm. If after the cake has been thus excited, the cover be placed on it, by means of its insulating handle, and if it be again lifted off, without being touched, no electricity whatever can be observed in the cover.

2. If, however, the cover while in contact with the cake, be touched with the finger, a smart pungent spark will be obtained from the cover; and if, while the finger touches the cover, the thumb is placed upon the sole, a sensible shock will be felt between the finger and thumb.

3. After the above spark or shock has been obtained, the electricity of the electrophorus disappears, and the apparatus is said to be dead; no signs of electricity appearing in either sole or cover, so long as the latter remains in contact with the cake.

4. But if the cover be raised to some distance from the cake, and in a direction parallel to it, and if the cover be touched while held in this position, a smart spark will appear between it and the finger, and will even strike to some distance. This spark will be more remarkable, if obtained from the upper surface of the cover, especially from its edge, which, if it has not been well rounded, will even throw off sparks into the air. The spark received from the cover under these circumstances, is however, not so pungent as that mentioned in No. 2, resembling a spark from any electrified conductor.

5. When the cover is thus raised from the cake, the former is found positively electrified, and the latter, as before, negatively.

6. But the electricity of both cover and cake, while in contact, is negative.

7. The appearances above described may be repeated for a considerable time, with apparently undiminished vivacity, without re-exciting the cake by friction; the apparatus has been observed to retain its electric power, even for several months. Hence it serves as a kind of electrical magazine, and may be repeatedly employed for charging jars, either positively, by imparting to the jar the electric spark from the cover while raised from the cake; or negatively, by receiving the sparks from the cover in contact with the cake. From this property of retaining the electric power for so long a time, Signor Volta denominated the apparatus electrophorus, or electroforo perpetuo.

8. If, before placing the cover on the cake, the sole has been insulated, the same spark may be obtained from the cover, and the same shock may be felt on touching both cake and cover at the same time; but the spark, in this case, is by no means so pungent as that obtained when the sole has not been insulated.

9. If, when the sole has been insulated, the cover be again lifted to a considerable distance from the cake, the sole will be found electrical, and its electricity will be the same as that of the cake, or negative.

10. If, after touching both sole and cover, the cover be raised from the cake, by its insulating handle, and again replaced upon the cake without being touched while separate, the whole apparatus is found to possess no electricity.

11. If both sole and cover be inactive after being joined, they will, when separated, show opposite electricities; the cover being electrified positively and the sole negatively.

12. If both cover and sole be rendered inactive while separate, they will, when placed in contact, be found to possess the electricity opposite to that of the cake, i.e., they will together be in a state of positive electricity.

It is of little consequence what substance forms the glass plate to which the electric coating is applied; formerly Mr. Cavallo used a glass plate, and this was coated with various resinous electrics. Mr. Cavallo, who made several experiments on the construction and phenomena of the electrophorus, found that the most convenient electric was made with the second sort of sealing-wax spread upon a thick glass plate. A plate made by him after this manner, the diameter of which was no more than six inches, was, when once excited, capable of charging a coated phial strongly, that by the explosion, a card could be perforated; this phial might be charged several times successively, without again exciting the plate. Sometimes the cover, when separated from the plate, was so strongly electrified, that it darted strong flashes towards the table on which the electric plate was laid, and even into the air. "The power of some of my plates," says Mr. Cavallo, "is so strong, that sometimes the electric plate adheres to the metal when this is lifted up; nor will they separate even when the metal plate is touched with the finger, or other conductor."

"If, after having excited the sealing-wax," continues he, "I lay the plate with the wax upon the table, and the glass uppermost, i.e., contrary to the common method, then, on making the usual experiment of putting the metal plate on it, and taking the spark &c., I observe it to be attended with the contrary electricity; that is, if I lay the metal plate upon the electric one, and while in that situation touch it with an insulated body, that body acquires the positive electricity, and the metallic, removed from the electric plate, appears to be negative; whereas it would become positive if laid upon the excited wax. This experiment, I find, answers in the same manner, if an electric plate is used which has the sealing-wax coating on both sides, or one of Mr. Adams's, which has no glass plate.

"If the brass plate, after being separated from, be presented..." Chap. X.

Principles of electricity illustrated by experiment.

Electrolysis while the fluid is in contact with its excited surface. If the negative side of the phial be applied, and a spark be taken from the positive, the pith-balls immediately separate negatively; but on taking up the fluid, they immediately close, and as rapidly separate again positively.

If after the phial is removed, the hand be applied to the fluid before it is raised, a small spark strikes into the hand; but on raising the fluid, the balls close and separate instantaneously, and give signs of positive electricity. If the fluid and the brass plate be connected, either by an insulated or uninsulated discharging rod, the balls close and separate again, and the fluid, upon being raised, receives a vigorous negative spark.

It is obvious that in all the preceding experiments, the brass plate continues unchangeably adherent to the lower surface, while the fluid only, or the conducting substance in connection with the upper surface, is moveable. It is of importance that we should know the consequences of making both the metallic surfaces moveable.

But this is not an easy matter; it is very difficult to get a resinous substance thin enough, and at the same time firm enough, for the purpose. The perfect laminae of tallow, which I have been able to procure, are too small to be used with any satisfaction; I have therefore had recourse to glass for the purpose. The result of my repeated trials is the following.

Having substituted a glass plate, about twelve inches in diameter, and one fourth of an inch thick, in the room of the resinous substance, and having reeled it on a ground metallic plate, five inches in diameter, and well connected with the pith-balls g and h, I exposed it to the sparks of a conductor charged positively, and kept my hand at the same time in connexion with the wire a b. The plate took a considerable charge; its upper side was unequivocally positive, and its lower side negative. I placed the fluid on the glass thus charged, and approaching it with my hand, I received a spark. I then approached a b with my hand, and received another. By alternate approaches of this kind, four or five times repeated, the sparks became weaker and weaker till they disappeared; the fluid was then raised, and was strongly negative; but the pith-ball, on the removal of the fluid, closed and separated positively.

I then made the lower the upper surface; and placing the fluid upon it, formed the communication, as in the preceding part of the experiment; but upon being raised, the fluid was strongly positive, and the balls negative.

But if, previous to the placing the fluid on the glass, the pith-balls be carefully discharged of all adherent electricity, both the upper and lower sides of the glass will be charged with positive electricity, or will give signs of their being in the same state at the same time.

It is observable that the succession of electricities, in the preceding experiments seems to vary according to the priority of contact given to the wire or the fluid. But though this happens most frequently, yet such anomalies take place as not to justify us in considering this singular connection of diversities as by any means certain.*

* Cavalli's Electricity, vol. ii.

Mr Morgan's experiments.

Mr Morgan has given us some valuable experimental observations on the Insulated Electrolysis.

His apparatus consists of a rounded piece of wood, A B fig. 74., with smooth edges and covered with tin-foil, placed on an insulating stand C D. On this board or foil is placed the electric plate or cake; a b is a wire with a brass ball from which are hung the electrometer balls g h. G represents the fluid or cover. After relating the usual appearances produced by friction, he proceeds to describe those which arise from connecting the cake with opposite sides of a Leyden phial.

"When the negative surface of a charged phial is placed on the excited surface, by bringing the hand into contact with the opposite side of the phial, a spark is instantly communicated, and the pith-balls g and h, separate negatively.

If the phial be taken off, and the fluid placed in its room, no change is observable on the subsequent removal of the fluid, provided, that no communication has been formed between it and the ground. When such a communication is formed, a charge is communicated, and the fluid and the balls are in opposite states of electricity.

If the positive side of a Leyden phial be placed on the excited surface, the pith-balls separate positively. It must be observed that these experiments are made with a resinous substance.

The appearances of the pith-balls and fluid are materially varied, if the Leyden phial be applied to the Mr Nicholson published in the Phil. Trans. for 1789, some valuable observations on the best means of excitation, which we shall here extract.

1. A glass cylinder was mounted, and a cushion applied with a silk flap, proceeding from the edge of the cushion over its surface, and thence half round the cylinder. The cylinder was then excited by applying an amalgamated leather in the usual manner. The electricity was received by a conductor, and passed off in sparks to Lane's electrometer. By the frequency of these sparks, or by the number of turns required to cause spontaneous explosion of a jar, the strength of the excitation was ascertained.

2. The cushion was withdrawn about one inch from the cylinder, and the excitation performed by the silk only. A stream of fire was seen between the cushion and the silk; and much fewer sparks passed between the balls of the electrometer.

3. A roll of dry silk was interposed, to prevent the stream from passing between the cushion and the silk. Very few sparks then appeared at the electrometer.

4. A metallic rod, not insulated, was then interposed instead of the roll of silk, so as not to touch any part of the apparatus. A dense stream of electricity appeared between the rod and the silk, and the conductor gave very many sparks.

5. The knob of a jar being substituted in the place of the metallic rod, it became charged negatively.

6. The silk alone, with a piece of tin-foil applied behind it, afforded much electricity, though less than when the cushion was applied with a light pressure. The hand being applied to the silk as a cushion, produced a degree of excitation seldom equalled by any other cushion.

7. The edge of the hand answered as well as the palm.

8. When the excitation by a cushion was weak, a line of light appeared at the anterior part of the cushion, and the silk was strongly disposed to receive electricity from any uninsulated conductor. These appearances did not obtain when the excitation was by any means made very strong.

9. A thick silk, or two or more folds of silk, excited worse than a single very thin flap. I use the silk which the milliners call Perian.

10. When the silk was separated from the cylinder, sparks passed between them; the silk was found to be a weak negative, and the cylinder in a positive state.

The foregoing experiments show that the office of the silk is not merely to prevent the return of electricity from the cylinder to the cushion, but that it is the chief agent in the excitation; while the cushion serves only to supply the electricity, and perhaps increase the pressure at the entering part. There likewise seems to be little reason to doubt but that the disposition of the electricity to escape from the surface of the cylinder is not prevented by the interposition of the silk, but by a compensation after the manner of a charge; the silk being then as strongly negative as the cylinder is positive; and, lastly, that the line of light between the silk and cushion in weak excitations does not consist of returning electricity, but of electricity which passes to the cylinder, in consequence of its not having been sufficiently supplied during its contact with the rubbing surface.

11. When the excitation was very strong in a cylinder newly mounted, flashes of light were seen to fly across its inside, from the receiving surface to the surface in contact with the cushion, as indicated by the brush figure. These made the cylinder ring as if struck with a bundle of small twigs. They seem to have arisen from part of the electricity of the cylinder taking the form of a charge. This appearance was observed in a 9-inch and a 12-inch cylinder, and the property went off in a few weeks. Whence it appears to have been chiefly occasioned by the rarity of the internal air produced by handling, and probably restored by gradual leaking of the cement.

12. With a view to determine what happens in the state of the inside of the cylinder, recourse was had to a plate machine. One cushion was applied with its filken flap, cylinder 4 inch thick. During the excitation, the surface opposite to the cushion strongly attracted electricity, which it gave out when it arrived opposite to the extremity of the flap; so that a continual stream of electricity passed through an insulated metallic bow terminating in balls, which were opposed, the one to the surface opposite the extremity of the silk, and the other opposite to the cushion; the former ball showing positive and the latter negative signs. The knobs of two jars being substituted in the place of these balls, the jar applied to the surface opposite to the cushion was charged negatively, and the other positively. This disposition of the back surface seemed, by a few trials, to be weaker, the stronger the action of the cushion, as judged by the electricity on the cushion side.

Hence it follows, that the internal surface of a cylinder is so far from being disposed to give out electricity during the friction by which the external surface acquires it, that it even greedily attracts it.

13. A plate of glass was applied to the revolving plate, and thrust under the cushion in such a manner as to supply the place of the silk flap. It rendered the electricity stronger, and appears to be an improvement of the plate machine.

14. Two cushions were then applied on the opposite surfaces with their silk flaps, so as to clap the plate between them. The electricity was received from both by applying the finger and thumb to the opposite surfaces of the plate. When the finger was advanced a little towards its correspondent cushion, so that its distance was less than between the thumb and its cushion, the finger received strong electricity, and the thumb none; and, contrariwise, if the thumb were advanced beyond the finger, it received all the electricity, and none passed to the finger. This electricity was not stronger than was produced by the good action of one cushion applied singly.

15. The cushion in experiment 12, gave most electricity when the back surface was supplied, provided that surface was suffered to retain its electricity till the rubbed surface had given out its electricity. From the two last paragraphs it appears, that no advantage is gained by rubbing both surfaces; but that a well managed friction on one surface will accumulate as much electricity as the present methods of excitation seem capable of collecting; but that, when the excitation is weak, on account of the electric matter not paffing with sufficient facility to the rubbed surface, the friction enables the opposite surface to attract or receive it, and if it be supplied, both surfaces will pass off in the positive state; and either surface will give out more electricity than is really induced upon it, because the electricity of the opposite surface forms a charge. It may be necessary to observe, that I am speaking of the facts or effects produced by friction; but how the rubbing surfaces act upon each other to produce them, whether by attraction or otherwise, we do not here inquire.

16. When a cylinder is weakly excited, the appearances mentioned (par. 8.) are more evident the more rapid the turning. In this case, the avidity of the surface of the cylinder beneath the silk is partly supplied from the edge of the silk, which throws back a broad cascade of fire, sometimes to the distance of above 12 inches. From these causes it is that there is a determinate velocity of turning required to produce the maximum of intensity in the conductor. The stronger the excitation, the quicker may be the velocity; but it rarely exceeds five feet of the glass to pass the cushion in a second.

17. If a piece of silk be applied to a cylinder, by drawing down the ends so that it may touch half the circumference, and the cylinder be then turned and excited by applying the amalgamated leather, it will become very greedy of electricity during the time it passes under the silk. And if the entering surface of the glass be supplied with electricity, it will give it out at the other extremity of contact; that is to say, if insulated conductors be applied at the touching ends of the silk, the one will give, and the other receive, electricity, until the intensities of their opposite states are as high as the power of the apparatus can bring them; and these states will be instantly reversed by turning the cylinder in the opposite direction.

As this discovery promises to be of the greatest use in electrical experiments, because it affords the means of producing either the plus or minus states in one and the same conductor, and of instantly repeating experiments with either power, and without any change of position or adjustment of the apparatus, it evidently deserves the most minute examination.

18. There was little hope (par. 6.) that cushions could be dispensed with. They were therefore added; and it was then seen, that the electrified conductors were supplied by the difference between the action of the cushion which had the advantage of the silk, and that which had not; so that the naked face of the cylinder was always in a strong electric state. Methods were used for taking off the prelude of the receiving cushion; but the extremity of the silk, by the construction, not being immediately under that cushion, gave out large flashes of electricity with the power that was used. Neither did it appear practicable to prevent a row of points or other apparatus to intercept the electricity which flew round the cylinder; because such an addition would have materially diminished the intensity of the conductor, which in the usual way was such as to flash into the air from rounded extremities of four inches diameter, and made an inch and half ball become luminous and blow like a point. But the greatest inconvenience was, that the two states with the backward and forward turn were seldom equal; because the deposition of the amalgam on the silk, produced by applying the leather to the cylinder in one direction of turning, was the reverse of what must take place when the contrary operation was performed.

Notwithstanding all this, as the intensity with the two cushions was such as most operators would have called strong, the method may be of use, and I still mean to make more experiments when I get possession of a very large machine which is now in hand.

19. The more immediate advantage of this discovery is, that it suggested the idea of two fixed cushions method of with a moveable silk flap and rubber. Upon this principle, which is so simple and obvious, that it is wonderful it should have been so long overlooked, I have constructed a machine with one conductor, in which the two opposite and equal states are produced by the simple process of loosening the leather rubber, and letting it pass round with the cylinder (to which it adheres) until it arrives at the opposite side, where it is again fastened. A wish to avoid prolixity prevents my describing the mechanism by which it is let go and fastened in an instant, at the same time that the cushion is made either to press or is withdrawn, as occasion requires.

20. Although the foregoing series of experiments naturally lead us to consider the silk as the chief agent in excitation; yet as this business was originally performed by a cushion only, it becomes an object of inquiry to determine what happens in this case.

21. The great Beccaria inferred, that in a simple cushion, the line of fire, which is seen at the extremity manner of contact from which the surface of the glass recedes, performed consists of returning electricity; and Dr Nooth grounded his happy invention of the silk flap upon the same rubber supposition. The former asserts, that the lines of light without both at the entering and departing parts of the surface a silk flap, are absolutely similar; and thence infers, that the cushion receives on the one side, as it certainly does on the other. I find, however, that the fact is directly contrary to this assertion; and that the opposite inference ought to be made, as far as this indication can be reckoned conclusive: for the entering surface exhibits many luminous perpendiculars to the cushion, and the departing surface exhibits a neat uniform line of light. This circumstance, together with the consideration that the line of light behind the silk in par. 8. could not consist of returning electricity, showed the necessity of farther examination. I therefore applied the edge of the hand as a rubber, and by occasionally bringing forward the palm, I varied the quantity of electricity which passed near the departing surface. When this was the greatest, the sparks at the electrometer were the most numerous. But as the experiment was liable to the objection that the rubbing surface was variable, I passed a piece of leather upon a thin flat piece of wood, then amalgamated its whole surface, and cut its extremity off in a neat right line close to the wood. This being applied by the constant action of a spring against the cylinder, produced a weak excitation; and the line where the contact of the cylinder and leather ceased (as abruptly Principles of Electricity (as possible) exhibited a very narrow fringe of light.

Another piece of wood was prepared of the same width as the rubber, but one quarter of an inch thick, with its edges rounded, and its whole surface covered with tin-foil. This was laid on the back of the rubber, and was there held by a small spring, in such a manner as that it could be slid onward, so as occasionally to project beyond the rubber, and cover the departing and excited surface of the cylinder without touching it. The sparks at the electrometer were four times as numerous when this metallic piece was thus projected; but no electricity was observed to pass between it and the cylinder. The metallic piece was then held in the hand to regulate its distance from the glass; and it was found, that the sparks at the electrometer increased in number as it was brought nearer, until light appeared between the metal and the cylinder; at which time they became fewer the nearer it was brought, and at last ceased when it was in contact.

The following conclusions appear to be deducible from these experiments. 1. The line of light on a cylinder departing from a simple cushion consists of returning electricity; 2. The projecting part of the cushion compensates the electricity upon the cylinder, and by diminishing its intensity prevents it striking back in such large quantities as it would otherwise do; 3. That if there was no such compensation, very little of the excited electricity would be carried off; And, 4. That the compensation is diminished, or the intensity increased, in a higher ratio than that of the distance of the compensating substance; because, if it were not, the electricity which has been carried off from an indefinitely small distance, would never fly back from a greater distance and form the edge of light.

I hope the considerable intensity I shall speak of will be an apology for describing the manner in which I produce it. I wish the theory of this very obscure process were better known; but no conjecture of mine is worth mentioning. The method is as follows:

Clean the cylinder, and wipe the silk.

Grease the cylinder by turning it against a greased leather till it is uniformly obscured. Use the tallow of a candle.

Turn the cylinder till the silk flap has wiped off so much of the grease as to render it semitransparent.

Put some amalgam on a piece of leather, and spread it well, so that it may be uniformly bright. Apply this against the turning cylinder. The friction will immediately increase, and the leather must not be removed until it ceases to become greater.

Remove the leather, and the action of the machine will be very strong.

My rubber, as before observed, consists of the silk flap pasted to a leather, and the cushion is pressed against the silk by a slender spiral spring in the middle of its back. The cushion is loosely retained in a groove, and rests against the spring only, in such a manner that by a sort of libration upon it as a fulcrum, it adapts itself to all the irregularities of the cylinder, and never fails to touch it in its whole length. There is no adjustment to vary the pressure, because the pressure cannot be too small when the excitation is properly made. Indeed the actual withdrawing of the cushion to the distance of \( \frac{1}{10} \)th of an inch from the silk, as in par. 2, Principles of Electricity Illustrated by Experiment, will not materially affect a good excitation.

The amalgam is that of Dr Higgins, composed of zinc and mercury. If a little mercury be added to melted zinc, it renders it easily pulverizable, and more mercury may be added to the powder to make a very soft amalgam. It is apt to crystallize by repose, which seems in some measure to be prevented by triturating it with a small proportion of grease; and it is always of advantage to triturate it before using.

A very strong excitation may be produced by applying the amalgamated leather to a clean cylinder with a clean silk; but it soon goes off, and is not so strong as the foregoing, which lasts several days.

To give some distinctive criterions by which other electricians may determine whether the intensity they produce exceeds or falls short of that which this method affords, I shall mention a few facts.

With a cylinder 7 inches diameter and cushion 8 mer. inches long, three brushes at a time constantly flew out of a 3-inch ball in succession too quick to be counted, and a ball of \( \frac{1}{2} \) inch diameter was rendered luminous, and produced a strong wind like a point. A 9-inch cylinder with an 8-inch cushion occasioned frequent flashes from the round end of a conductor 4 inches diameter; with a ball of \( \frac{1}{2} \) inches in diameter the flashes ceased now and then, and it began to appear luminous: a ball of \( \frac{1}{2} \) inch diameter first gave the usual flashes; then, by quicker turning, it became luminous with a bright spark moving about on its surface, while a constant stream of air rushed from it; and, lastly, when the intensity was greatest, brushes of a different kind from the former appeared. These were less luminous, but better defined in the branches; many started out at once with a hoarse sound. They were reddish at the item, sooner divided, and were greenish at the point next the ball, which was bright. A ball of \( \frac{1}{2} \)ths of an inch in diameter was surrounded by a steady faint light, enveloping its exterior hemisphere, and sometimes a flash struck out at top. When the excitation was strongest, a few flashes struck out sidewise. The horizontal diameter of the light was longest, and might measure one inch, the item of the ball being vertical.

With a 12-inch cylinder and rubber of \( \frac{7}{2} \) inches, a 5-inch ball gave frequent flashes, upwards of 14 inches long, and sometimes a 6-inch ball would flash. I do not mention the long spark, because I was not provided with a favourable apparatus for the two larger cylinders. The 7-inch cylinder affords a spark of 10\( \frac{1}{2} \) inches at best. The 9-inch cylinder, not having its conductor insulated on a support sufficiently high, afforded flashes to the table which was 14 inches distant. And the 12-inch cylinder, being mounted only as a model or trial for constructing a larger apparatus, is defective in several respects, which I have not thought fit to alter. When the five-inch ball gives flashes, the cylinder is enveloped on all sides with fire which rushes from the receiving part of the conductor.

It is of consequence that electricians should employ usual methods common method of estimating the power of their kinds of machines, so as to admit of comparing those of different sizes or constructions. This is usually done by describing the length and appearance of the simple spark drawn from the prime conductor; or the distance to electrical machines. Nicholson has been led to alter his opinion; and he now prefers a plate to a cylinder.

From considering the defects of the usual methods of estimating the power of machines, Mr Cuthbertson was led to propose the explosion of steel wire as a proper measure; and he has made several experiments to show that this method is the least liable to error. Mr Nicholson has given an account of these experiments in his Journal for August 1798; but they seem to require measuring a repetition and farther extension before they can be received as conclusive.

As glass, though preferable to all electrics that can be employed for the purpose of excitation, from its substitutes durability and unchangeable nature, is from its brittleness attended with considerable expense, various experiments have been thought of to substitute in its place some other electric in the construction of electrical machines.

Dr Ingenhouz, the inventor of the plate machine, made a variety of experiments for this purpose. Pasteboard thoroughly dried and heated, and then soaked and varnished with a solution of amber in linseed oil, formed plates which were strongly electrified when rubbed with a cat's skin or hare's skin. He tried baked wood boiled in linseed oil, but with less success. A cylinder of strong silk velvet, formed by stretching that substance upon two circular wooden disks, was found to afford considerable electrical force when caused to revolve against a cushion covered with hare's skin.* Phil.

And lastly, the same philosopher contrived a portable Transf. for apparatus for charging a jar by means of a varnished silk ribbon, exposed to the friction of a rubber attached to the external coating, while the opposite electricity of the silk was taken off by a metallic part communicating with the inside.

It was at the beginning of 1784, that M. Walckiers de St Amand undertook to construct a machine, in which a piece of silk was made to revolve incessantly, and pass between two pair of rubbers. He made one of small dimensions, and afterwards a larger one, in which the silk was twenty-five feet in length, and five feet broad. In the following year M. Rouland, professor of natural philosophy in the university of Paris, constructed a machine of the same kind.† As the advantages and effects of these machines appear to be considerable, we shall here insert the description of the latter from Nicholson's Journal for December 1798.

A, B, fig. 80, is a wooden table four feet and a half long, two feet nine inches wide, and somewhat more of silk than an inch and a half thick; its feet are 18 inches long. Upon this table are fastened by strong wooden screws, a b c d, two crofs pieces, each nine inches broad, which carry the uprights C, D, E, F, which last are 27 inches in height. At about two-thirds or more of the height of these uprights, there are cut notches of an inch square each, in which the axes of the two cylinders G and H turn freely. These axes are parallel to the table and to each other, and are kept in their place by clamps of wood screwed over them. The cylinders G and H are formed of light wood glued together, and covered at the ends by a circular piece, whose rounded edges arise half an inch above the surface of the cylinders themselves. Their diameter is eight inches; the axes are of box-wood, and are less than an inch in diameter, having a shoulder which prevents the ends of the cylinders from...

Principles of Electricity lastly, the cylinders are covered with serge.

The handle is copper, its radius being six inches long.

K, L, is a piece of taffety covered with oily and resinous matter, of the same kind as used in France in the construction of air-balloon, which, M. Rouland says, renders the silk very electrical; the breadth of the silk is nearly one inch less than the length of the cylinders, and it is wrapped round them with its ends sewed together.

The whole breadth of the silk is taken hold of or pinched between two flattened tin tubes opposite each other at M, and two of the same kind at N: these are the rubbers, and may be made to press against each other, more or less strongly, by means of screws. They are retained by strings of silk fastened to the four uprights of the machine. v v are two brass chains hooked upon the rubbers, and communicating with the earth; o p and q r are four pieces of taffety, prepared in the same manner as the principal piece, sewed in the direction of their length to the rubbers, and fastened to each other by their corresponding corners by means of threads of silk. The metallic tubes or rubbers are covered with cat's skin.

S represents the conductor. It is a cylinder of brass three inches in diameter, 36 inches in length, including the balls at the end, whose diameters are four inches: one of these balls has a ring, t above it, which serves to form a communication between the conductor S and any other conductor.

The upper and lower parts of this cylindrical prime conductor are armed with two plates of brass y y, whose length is equal and correspondent to the breadth of the taffety, which is 26 inches, and 13 1/2 inches or 11 feet long: the edges of the plates are about half an inch distant from the silk, and serve instead of the metallic points that were used by M. Walckiers, but rejected by M. Rouland, because they were apt to stick into the silk and damage it.

The conductor S is suspended by silk strings, fastened to the uprights of the machine by the hooks and rings i i: its situation is parallel to the cylinders G, H, and equidistant from each. The action of this machine is as follows: The cylinder H is moved rapidly on its axis by means of the handle, and the cylinder G moves of course in the same direction on the two extremities of its axis, provided the taffety K, L, be properly stretched. This tension is easily obtained; because the cross pieces to which the uprights C, D, and E, F, are fixed, may be moved nearer or further from each other, and fastened by means of the screws a b and c d, which pass through holes cut in the direction of the table.

The rotation of the cylinders necessarily producing a circulation of the taffety, it must consequently be rubbed in its passage between the tin tubes covered with cat's skin at M and N; and by this friction it obtains what is called the negative electricity, which is communicated from both parts of the silk to the common conductor S. But it may be made to electrify positively, by removing the rubbers to the middle of the silk, so that the prime conductor may communicate with them: or, if the two cushions be removed to half the distance between the revolving cylinders and the prime conductor, positive and negative electricity may be had at the same time, the rubbers being in a negative state, and the prime conductor in a positive state.

The advantages of a machine of this construction beyond those of glass are stated by the inventor to be, 1. It is not brittle in any part. 2. Its excitation is more steady, because it requires no amalgam. 3. Its dimensions have no limit.

The power of excitation in this way appears to have been very considerable. The facts are not related with so much detail as could be wished in the report of the academy; but it appears that the negative sparks from the conductor of Walckiers, which was five feet long, were from 15 to 17 inches in length, very loud and dense, and very painful to the hand; that pointed bodies emitted very sensible sparks to the conductor; and that a battery of 50 square feet was charged by 30 turns of the machine, which gives 19 feet of silk rubbed to charge one foot of glass*. In another instance,* see Phil. however, it is said, that a square foot was charged by Journal, one turn of the machine, which answered to 31 1/2 square feet of silk. It is not said whether the labour of turning was considerable or not.

M. Rouland made several trials to substitute plain silk instead of that which was varnished; and he also tried woollens and mixed cloth containing goat's hair; but none of these answered to his satisfaction.

CHAP. XII. Of the Electric Properties of Air.

We have ranked air among the electrics, but it will be seen by the table of electric substances given in page 646, that it is but an imperfect electric. We have observed at the beginning of this part, that it may even become a conductor by being impregnated with moisture. It is also found that when air is heated to a considerable degree, it becomes a conductor; this according to Cavallo, may be shewn by the following experiment. Electrify a common ball electrometer, or the prime conductor with Henly's quadrant electrometer placed upon it; the balls will, of course, separate from each other, or the index of the quadrant will denote the degree of electricity communicated to the prime conductor. Now bring a red-hot iron within a sufficient distance of the electrometer or the prime conductor, and it will be found that they soon lose their electricity, it being conducted away by the heated air that surrounds the iron; that the heated iron is the cause of the loss of electricity may be proved, by repeating the experiment with the same iron when cold, as in this case it will be found that the electrometer of the conductor, will not lose its electricity so soon, unless the iron be brought very near.

Mr Read made the following experiment to prove that hot air is not a conductor.

"It has been," says Mr Read, "commonly said, that hot air conducts electricity. With a view to affirm certain this matter, the following experiments were made. To one end of a long piece of wood (which served as a handle,) was fixed a glass rod fifteen inches long; to the remote end of the glass was fixed a pith-ball electrometer. Having electrified the balls, I held them by the wood handle, and projected them into a large oven, immediately after the fire was drawn out of it; the consequence was, that when I performed the operation slowly, the balls lost their electricity; but Chap. XII.

Principles of Electricity.

Illustrated electricity, in the first case, was found to have escaped along the glass into the wooden handle, and so to the earth, owing to the great heat the glass rod had acquired, by which it became a conductor of the fluid; for until it had cooled a little, the balls could not be charged again.

"I shall lay before the reader one circumstance more, because it may tend to throw light on what degree of heat the oven was in at the time the observations were made. The baker having pointed out to me the hottest part of the oven; with a quick motion in and out, I plunged the electrified balls into that part of it, by which one thread ball was burned off, but the remaining ball showed that it still retained its electric charge, because it was strongly attracted on the approach of my finger."

Air, as an electric, may be electrified either by excitation or by communication.

Air may be excited by any circumstance which tends to produce motion among its particles; as by friction, evaporation, heat and cold, expansion and contraction, and by any chemical processes in which these circumstances are produced.

1. That air may be excited by friction was sufficiently shown by the experiments related to demonstrate the sensibility of Bennet's electrometer; by these experiments it appeared, that whenever a cloud of dust or powder was raised in the air surrounding the electrometer, the slips of gold leaf, by their separation, manifested signs of electricity, which must doubtless have been produced by the attraction of the particles of dust or powder against the particles of air.

2. Air may be rendered electrical by the vapour or smoke which rises into it from evaporating or burning substances. At the end of the first part of this article, we noticed, in a general way, Sig. Volta's experiments on the electricity produced by evaporation; we must now consider this subject rather more at large. The production of electricity by evaporation, may be thrown by the following experiments.

Exper. 1. Upon the cap of Bennet's electrometer c, fig. 81., place a metallic cup containing a little water; drop into the water a red-hot coal, or a red-hot piece of clean iron; a vapour will arise from the water, and the strips of gold leaf n n will diverge with strong negative electricity. If at the same time, an iron wire p, fixed to a rod of glass or sealing-wax, with a common ball electrometer hanging from its extremity, be held by the glass or sealing-wax in the air, at a little distance above the cup, the balls d will be found to diverge with positive electricity.

Exper. 2. Let there be two of the above electrometers, as A, B, fig. 82.; upon the cap of the electrometer B, place a metallic cup d, as in the last experiment, and into the cap of the electrometer A, let there be screwed a bent wire m, with a piece of tin soldered to its other extremity. If now, the electrometer B with its metallic cup be placed immediately below the tin r, and a cullender c, containing a few live coals, be held over the cup, and if water be poured from the jug upon the coals in the cullender, so as to fall into the metal cup, the slips of gold leaf in both electrometers will diverge; those of the electrometer B, with negative electricity, and those of the electrometer A, with positive electricity.

The experiments on the electricity produced by evaporation, may be very conveniently made by heating the final end of a pretty long tobacco-pipe, and pouring water into the bowl of it; the water running down to the heated part, which should be held over the cap of Bennet's electrometer, is suddenly expanded into vapour, and the slips of gold leaf will separate with negative electricity.

In the above experiment it has been seen, that the electrometer from which the vapour arose, was always electrified negatively; from having observed this to be bodies not always the case in his experiments, Sig. M. Volta always considered it as a general law. Mr Cavallo, however, mentions some experiments made by a professor at Mantua, and by himself, which seem to contradict this supposition.

All the experiments, (says Mr Cavallo), made on evaporation for some years after this discovery, were attended with results conformable to the above-mentioned general law; but two remarkable exceptions have of late been discovered, which, besides their contradicting the said law, seem to point out a more intimate connection between the electric fluid and other bodies. The first of these exceptions was discovered and published three years ago, by a learned professor of the academy of Mantua; the second was very lately discovered by myself.

The Mantuan professor observed, that when water was evaporated by being put in contact with a red hot-piece of rusty iron, it would leave the iron electrified positively; whereas when the experiment was tried with a clean piece of iron, the electricity acquired by the metal would be of the negative kind.

When I first attempted to repeat this curious experiment, the result did by no means answer my expectations; the electricity, which was produced being of the negative, and not of the positive kind; but observing that sometimes no sensible degree of electricity was produced, though the evaporation was very quick and copious, I began to suspect that the iron, which I had employed, was not sufficiently covered with rust, in consequence of which two opposite states of electricity might possibly be produced, viz. the negative from the clean, and the positive from the rusty part of the iron; which two opposite states, by counteracting each other, would leave the iron un-electrified. After various repetitions of this experiment, in which the red-hot iron was thrown into the inflated water, or the water was poured upon the red-hot and inflated iron, I found that this was actually the case.

I procured some old iron nails, which had remained exposed to the atmosphere for several years, and of course had contracted a very thick coat of rust; and on performing the experiment with them, I obtained positive electricity, agreeably to the assertion of the above-mentioned gentleman. The same nail very seldom would answer for more than one experiment; for the action of the fire and of the water generally removed a great deal of the rust, and exhibited the naked metal, which would afterwards acquire the negative electricity. Here follows the manner of performing this remarkable experiment.

Insulate a metallic or earthen plate, and pour a small Principles of small quantity of water in it, and let a sensitive electrometer be connected with the water; then drop a red-hot piece of iron into the plate, and it will be found, that, if very ruddy iron be used, the electrometer will be opened with positive electricity; if the iron be clean, or free from rust, the electrometer will acquire the negative electricity; and lastly, if the iron be partially ruddy, the electrometer will acquire little or no electricity, though in every case the evaporation may be equally quick and copious.

The other exception of the above-mentioned general law is shown by means of red-hot glass, which I chanced to discover very lately. The various degrees of electric power that are produced by the evaporation of water from different substances induced me to diversify the experiments as much as I could, in order to discover, if possible, the reason why those different effects took place when the evaporation seemed to be equally quick and copious. Amongst other substances, I tried glass, and found that it generally produced little or no electricity. The water was sometimes poured upon the hot glass, but in general the hot glass was dropped into the inflated water, which was contained in a tin cup. However, the difference of effect was found not to be occasioned by those two different modes of proceeding. Having repeated this experiment a great many times, I at last found, that the effect depended on the different nature of the glass. If white and clean flint glass be made red-hot, and in that state be dropped into the vessel of water, a quick evaporation will ensue, and the vessel is electrified positively. If the flint glass be not very clear, there will not be any electricity generated by the evaporation, &c. And lastly, if the experiment be tried with more impure glass, as the glass of which wine bottles are made, the negative electricity will be produced.

In performing this experiment, it is necessary to take care that no pieces of coal adhere to the glass, which will frequently happen when a piece of glass is heated in a common fire; for in that case negative electricity will be produced by the evaporation, though the best flint glass be used.

It has frequently happened, in the course of my experiments, that no electricity whatever has been produced by the evaporation of water from certain substances; however, as in those cases the evaporation was not very copious, I attributed the deficiency of electricity to the weakness of the evaporation. But a very remarkable instance of this sort is mentioned in the dissertation of the above-mentioned ingenious professor. He flaked 25 pounds weight of quicklime with a sufficient quantity of water, and though a very copious evaporation took place, yet it was not attended with any electricity. Should any person suspect, that the deficiency of electricity in this experiment was owing to the want of burning coals or actual fire, he should consider, that in other similar processes electricity is produced without any actual fire; thus the evaporation, which is occasioned by the effervescence of iron filings in diluted vitriolic acid, produces negative electricity.

After a careful examination of the above-mentioned experiments, the origin of the electricity, which is observed in the evaporation of water and other evaporable substances, whether solid or fluid, seems not to be reconcileable to the general law already noticed, nor can I form any plausible theory that can be sufficient to account for all the phenomena. If the production of Electricity electricity in those experiments depended upon the increased or diminished capacity of water for holding the electric fluid, it should seem to be immaterial whether the water be evaporated in one way or in another, provided the evaporation be made with equal quickness and in equal quantities. Were it not known that glass or iron made red-hot produces no electricity in cooling, we might suspect, that the electricity, which is produced by the evaporation of water, may be counteracted by the contrary electricity, which is produced by the cooling of glass or iron; but it has been observed by several ingenious persons, that red-hot glass and red-hot iron produce no electricity whatever when suffered to cool upon insulated stands.

It has been found that electricity promotes evaporation. This may be proved by the following

Exper. Upon the prime conductor of an electrical Evaporation machine, place a shallow metallic dish; as a pewter or iron plate, containing a small quantity of water; and let a red-hot similar dish, containing such a quantity of water as that the two dishes may exactly counterpoise each other, be placed on a table at a distance from the machine. Now let the machine in motion, and after a certain time has elapsed, place the two dishes again in the scales, and it will be found that the dish which stood on the prime conductor is lighter than the other; evidently showing that more of the water has been evaporated.

This experiment might with more propriety have been given when describing the chemical effects of the electric power.

We shall return to this subject, under Atmospheric Electricity, to which the consideration of the other circumstances effecting the electricity of air by excitation, more properly belongs.

Air may be electrified by communication in two methods of ways; by simple electrification, as it is called, or by electrifying charging a stratum of it situated between two conductors, the air of a room.

Exper. 1.—Fix two or three pointed needles into the prime conductor of an electrical machine, and set the glass in motion so as to keep the prime conductor electrified for several minutes. If now, an electrometer be brought within the air that is contiguous to the prime conductor, it will exhibit signs of electricity, and this air will continue electrified for some time, even after the machine has been removed into another room. The air, in this case, is electrified positively; it may be negatively electrified by fixing the needles in the negative conductor while inflated, and making a communication between the prime conductor and the table, by means of a chain or other conducting substance.

The air of a room may be electrified in another way. Charge a large jar, and inflate it; then connect two or more sharp pointed wires or needles, with the knob of the jar, and connect the outside coating of the jar with the table. If the jar be charged positively, the air of the room will soon become positively electrified likewise; but if the jar be charged negatively, the electricity communicated by it to the air, will become also negative. A charged jar being held in one hand, and the flame of an inflated candle, held in the other, being brought near the knob of the jar, will also produce the same effect. A stratum of air may be charged in the same manner as a plate of glass, when its opposite surfaces are placed in contact with metallic plates which serve as a coating to the plate of air.

To perform this experiment, take two circular boards, each three or four feet in diameter, made perfectly smooth, and their edges rounded; coat one side of each board with tinfoil, so that it may be turned up over the edge of the board, and let it be burnished so as to render it as smooth as possible. These boards must be placed, with their coated sides parallel to each other, horizontally, and so that they may be set at a greater or smaller distance, and they must both be inflated. For this purpose, it is most convenient to fix one of the boards on a strong support of glass or baked wood, and to suspend the other by silk strings from the ceiling of the room, from which it may be raised or lowered by a proper pulley, so as to be placed at the required distance from the lower board.

The boards being thus placed in their situation, at the distance of about an inch from each other, on their being electrified, the stratum of air, interposed between them, will present phenomena similar to those of a plate of glass under the same circumstances. On connecting one of the boards with the prime conductor, while the other is inflated, the air will receive no charge agreeably to what was remarked of an inflated Leyden phial. But if, while one of the boards is electrified from the prime conductor, the other be made to communicate with the earth or other conducting bodies, the plate of air will receive a charge, and when the communication between the boards is completed by conductors, an explosion will take place. The explosion in this case, however, is by no means so remarkable as that which is produced from an equal surface of coated glass, for reasons which will be explained hereafter.

The experiment of charging a plate of air was first made by M. Æpinus and M. Wilcke, who being at Berlin together, jointly made several experiments.

They made several experiments to give the electric shock by means of air, and at length succeeded by suspending large boards of wood covered with tin with the flat sides parallel to one another, and at some inches asunder; for they found, that upon electrifying one of the boards positively, the other was always negative. But the discovery was made complete and indubitable by a person touching one of the plates with one hand, and bringing his other hand to the other plate; for he then received a shock through his body, exactly like that of the Leyden experiment.

With this plate of air, they made a variety of curious experiments. The two metal plates, being in opposite states, strongly attracted each other, and would have rushed together, if they had not been kept asunder by strings. Sometimes the electricity of both would be discharged by a strong spark between them, as when a pane of glass bursts with too great a charge. A finger put between them promoted the discharge, and felt the shock. If an eminence was made on either of the plates, the self discharge would always be made through it, and a pointed body fixed upon either of them prevented their being charged at all.*

At the end of the table of conductors given in page 646, it was observed that a Torricellian vacuum was a non-conductor of electricity. Some experiments were made by Mr Walib, which proved the perfect impermeability of a vacuum by the electric light. But the most complete experiments on this subject are those of Mr W. Morgan and Mr Cavallo. The following are Mr Morgan's experiments.

A mercurial gage B, fig. 83, about 15 inches long, Mr Morgan carefully and accurately boiled, till every particle of air was expelled from the inside, was coated with tin-foil, five inches down from its sealed end (A), and being inverted into mercury through a perforation D, in the power of a brass cap E, which covered the mouth of the cistern perfect vacuum H; the whole was cemented together, and the air was exhausted from the inside of the cistern through a valve C, in the brass cap E just mentioned; which producing a perfect vacuum in the gage B, afforded an instrument peculiarly well adapted for experiments of this kind. Things being thus adjusted, a small wire, F, having been previously fixed on the inside of the cistern, to form a communication between the brass cap E, and the mercury G, into which the gage was inverted; the coated end A was applied to the conductor of an electrical machine; and, notwithstanding every effort, neither the smallest ray of light, nor the slightest charge, could ever be procured in this exhausted gage. It is well known, that if a glass tube be exhausted by an air-pump, and coated on the outside, both light and charge may very readily be procured. If the mercury in the gage be imperfectly boiled, the experiment will not succeed; but the colour of the electric light, which, in air rarefied by an exhausteur, is always violet or purple, appears in this case of a beautiful green; and what is very curious, the degree of the air's rarefaction may be nearly determined by this means. There have been instances known, in a course of experiments, where a small particle of air having found its way into the tube B, the electric light became visible, and as usual of a green colour; but the charge being often repeated, the gage has at length cracked at its sealed end, and in consequence the external air, by being admitted into the inside, has gradually produced a change in the electric light, from green to blue, from blue to indigo, and so on to violet and purple, till the medium has at last become so dense, as no longer to be a conductor of electricity. There can be little doubt, from the above experiments, of the non-conducting power of a perfect vacuum; and this fact is still more strongly confirmed by the phenomena which appear upon the admission of a very minute particle of air into the inside of the gage. In this case, the whole becomes immediately luminous, upon the slightest application of electricity, and a charge takes place, which continues to grow more and more powerful, in proportion as fresh air is admitted, till the density of the conducting medium arrives at its maximum, which it always does when the colour of the electric light is indigo or violet. Under these circumstances, the charge may be so far increased, as frequently to break the glass. In some tubes, which have not been completely boiled, they will not conduct the electric fluid, when the mercury is fallen very low in them; yet upon letting in air into the cistern H, so that the mercury shall rise in the gage B, the electricity, which was before latent in the inside, shall now become visible, and as the mercury continues to rise, and of consequence the medium is rendered less rare, the light shall grow... Electricity grows more and more visible, and the gage shall at last be charged, notwithstanding it has not been near an electrical machine for two or three days. This seems to prove, that there is a limit, even in the rarefaction of air, which sets bounds to its conducting power; or, in other words, that the particles of air may be so far separated from each other, as no longer to be able to transmit the electricity; that if they are brought within a certain distance of each other, their conducting power begins, and continually increases, till their approach also arrives at its limit, when the particles again become so near, as to resist the passage of the electricity entirely, without employing violence, which is the case in common and condensed air, but more particularly in the latter.

It is surprising to observe, how readily an exhausted tube is charged with electricity. By placing it at ten or twelve inches from the conductor, the light may be seen pervading its inside, and as strong a charge may sometimes be procured, as if it were in contact with the conductor. Nor does it signify how narrow the bore of the glass may be; for even a thermometer tube, having the minutest perforation possible, will charge with the utmost facility; and in this experiment, the phenomena are peculiarly beautiful.

Let one end of a thermometer tube be sealed hermetically; let the other end be cemented into a brass cap with a valve, or into a brass cock, so that it may be fitted to the plate of an air-pump. When it is exhausted, let the sealed end be applied to the conductor of an electrical machine, while the other end is either held in the hand, or connected to the floor. Upon the slightest excitation, the electricity will accumulate at the sealed end, and be discharged through the inside in the form of a spark; and this accumulation and discharge may be incessantly repeated, till the tube is broken. By this means, a spark 42 inches long may be procured; and if a proper tube could be found, we might have a spark three or four times that length: if, instead of the sealed end, a bulb be blown at that extremity of the tube, the electric light will fill the whole of that bulb, and then pass through the tube in the form of a brilliant spark, as in the foregoing experiment; though in this case, the charge, after a few trials, will make a small perforation in the bulb. If, again, a thermometer, filled with mercury, be inverted into a cistern, and the air exhausted in the manner before described for making the experiment with the gage, a Torricellian vacuum will be produced; and now the electric light in the bulb, as well as the spark in the tube, will be of a vivid green; but the bulb will not bear a frequent repetition of charges, before it is perforated in like manner as when it has been exhausted by an air-pump. It can hardly be necessary to observe, that in these cases the electricity assumes the appearance of a spark, (v) from the narrowness of the passage through which it forces its way. If a tube, 40 inches long, be fixed into a globe eight or nine inches in diameter, and the whole be exhausted, the electricity, after passing in the form of a brilliant spark throughout the length of the tube, will, when it gets into the inside of the globe, expand itself in all directions, entirely filling it with a violet and purple light, and exhibiting a striking instance of the vast elasticity of the electric power.

Mr Morgan concludes his remarks with acknowledging his obligations to Mr Brooke of Norwich for communicating to him his method of making mercurial gages.

Mr Brooke's method of making mercurial gages is nearly as follows: Let a glass tube L (fig. 84.), hermetically sealed at one end, be bent into a right angle with two or three inches of the other end. At the distance of about an inch or less from the angle, let a bulb K, of about three-fourths of an inch in diameter, be blown in the curved end, and let the remainder of this part of the tube be drawn out I, so as to be sufficiently long to take hold of when the mercury is boiling. The bulb K is designed as a receptacle for the mercury, to prevent its boiling over; and the bent figure of the tube is adapted for its insertion into the cistern: for by breaking off the tube at M within an eighth or a fourth part of an inch of the angle, the open end of the gage may be held perpendicular to the horizon when it is dipped into the mercury in the cistern, without obliging us to bring our finger or any other substance into contact with the mercury in the gage, which never fails to render the instrument imperfect. It is necessary to observe, that if the tube be 14 or 15 inches long, it will be impossible to boil it effectually for the experiments mentioned above in less than three or four hours, although Mr Brooke seems to prefer a much shorter time for the purpose; nor will it even then succeed, unless the greatest attention be paid that no bubbles of air lurk behind, which is frequently the case: but experience has taught how to guard pretty well against this disappointment, particularly by taking care that the tube be completely dry before the mercury is put into it; for if this caution be not observed, the instrument can never be made perfect. There is, however, one evil which it is sufficient to remedy; and that is, the introduction of air into the gage, owing to the unboiled mercury in the cistern: for when the gage has been a few times exhausted, the mercury which originally filled it becomes mixed with that into which it is inverted, and in consequence the vacuum is rendered less and less perfect, till at last the instrument is entirely spoiled.

Mr Cavallo's experiments were made with an excellent air-pump, which is described in the 73rd volume of the Philosophical Transactions.

The following is the result of Mr Cavallo's experiments as given by himself.

"From these experiments it appears, first, that in the same air-pump, which amounts to about one thousandth part of the same subject, both the electric light and the electric attraction, though very weak, are still observable; but, secondly, that the attraction and repulsion of electricity become weaker in proportion as the air is more rarefied, and in the same manner the intensity of the light is gradually diminished. Now, by reasoning on this analogy, we may conclude

(v) By cementing the string of a guitar into one end of a thermometer tube, a spark may be obtained, as well as if the tube had been sealed hermetically. When an insulated conductor is thus presented to any other conductor, Signor Volta calls it a Conjugate Conductor.

In order to show by experiment the above-mentioned property or increase of capacity in a conductor, take the metal plate of an electrophorus, and holding it by its insulating handle in the air, electrify it so high, as that the index of an electrometer annexed to it might be elevated to $60^\circ$; then lowering this metal plate by degrees towards a table or other conducting plain surface, you will observe that the index of the electrometer will fall gradually from $60^\circ$ to $50^\circ$, $40^\circ$, $30^\circ$, &c. Notwithstanding this appearance, the quantity of electricity in the plate remains the same, except the said plate be brought so near the table as to occasion a transmigration of the electricity from the former to the latter; at least the quantity of electricity will remain as much the same as the dampness of the air, &c. will permit. The decrease, therefore, of intensity is owing to the increased capacity of the plate, which is now conjugate, viz. opposed to another conducting surface. In proof of which, remove gradually the metal plate from the table, and it will be found that the electrometer rises again to its former elevation, namely to $60^\circ$, excepting the loss of that quantity of electricity, which during the experiment must have been imparted to the air.

The two following experiments will throw more light upon the reciprocal action of the electric atmospheres. First, suppose two flat conductors, electrified both positively or both negatively, to be presented towards, and to be gradually brought near, each other; it will appear by two annexed electrometers, that the nearer those two conductors come to each other, the more their intensities will increase; which shows, that either of the two conjugate conductors has a much less capacity now, than when it was singly insulated, and out of the influence of the other.

Secondly, let the preceding experiment be repeated, with this variation only, viz. that one of the flat conductors be electrified positively, and the other negatively; the effects then will be just the reverse of the preceding; viz. the intensities of their electricities will be diminished, because their capacities are increased, the nearer the conductors come to each other.

Let us now apply the explanation of this last experiment to that of bringing an electrified metal plate towards an uninflated conducting plane; for as this plane acquires a contrary electricity by the vicinity of the electrified plate, it follows that the intensity of the electricity of the metal plate must be diminished, and in the same proportion its capacity is increased; consequently the metal plate in that case may receive a greater quantity of electricity.

This property may be rendered still more evident, by inflating the conducting plane whilst the electrified plate is very near it, and afterwards separating them; for then both the metal plate and the conducting plane (which may be called the inferior plane) will be found electrified, but poffelled of contrary electricities, as may be ascertained by electrometers.

If the inferior plane be inflated first, and then the electrified plate be brought over it, then the latter will cause an endeavour in the former to acquire a contrary electricity, which however the inflation prevents from taking place. Principles of taking place; hence the intensity of the electricity of Electricity the plate is not diminished, at least the electrometer will shew a very little and almost imperceptible depression, which is owing to the imperfection of the inflation of the inferior plane, and to the small rarefaction and condensation of the electric fluid, which may take place in different parts of the said inferior plane. But if in this situation the inferior plane be touched, so as to cut off the inflation for a moment, then it will immediately acquire the contrary electricity, and the intensity in the metal plate will be diminished.

If the inferior plane, instead of being insulated, were itself a non-conducting substance, then the same phenomena would happen, viz. the intensity of the electrified metal plate laid upon it would not be diminished. This, however, is not always the case; for if the said inferior non-conducting plane be very thin, and be laid upon a conductor, then the intensity of the electrified metal plate will be diminished, and its capacity will be increased by being laid upon the thin insulating stratum, because, in that case, the conducting substance which stands under the non-conducting stratum acquiring an electricity contrary to that of the metal plate, will diminish its intensity, &c., and the insulating stratum will only diminish the mutual action of the two atmospheres, more or less, according as it keeps them more or less alund.

The intensity or electric action of the metal plate, which diminishes gradually as it is brought nearer and nearer to a conducting plane not insulated, becomes almost nothing when the plate is nearly in contact with the plane, the compensation or accidental balance being then almost perfect; hence if the inferior plane only opposes a small resistance to the passage of the electricity (whether such resistance be occasioned by a thin electric stratum, or by the plane's imperfect conducting nature, as is the case with dry wood, marble, &c.), that resistance, and the interval, however small, that is between the two planes, cannot be overcome by the weak intensity of the electricity of the metal plate, which on that account will not dart any spark to the inferior plane (except its electricity were very powerful, or its edges not well rounded) and will rather retain its electricity; so that, being removed from the inferior plane, its electrometer will nearly recover its former height. Besides, the electrified plate may even come to touch the imperfectly conducting plane, and may remain in that situation for some time; in which case the intensity being reduced almost to nothing, the electricity will pass to the inferior plane exceedingly slowly.

But the case will not be the same, if, in performing the experiment, the electrified metal plate be made to touch the inferior plane edgewise; for then its intensity being greater than when laid flat, as it appears by the electrometer, the electricity easily overcomes the small resistance, and passes to the inferior plane, even across a thin electric stratum; because the electricity of one plane is balanced by that of the other, only in proportion to the quantity of surface which they oppose to each other within a given distance; whereby, when the metal plate touches the other plane in flat and ample contact, its electricity is not dissipated.

Hitherto we have considered in what manner the action of electric atmospheres must modify the electricity of the metal plate in various situations. We must now consider the effects which take place when the electricity is communicated to the metal plate whilst standing upon the imperfectly conducting plane; however, the explanation of this easily follows from what has been said above. Suppose, for instance, that a Leyden phial or a conductor were so weakly electrified, that the intensity of its electricity were only half a degree, or even less; if the metal plate, when standing upon the proper plane, were touched with that phial or conductor, it is evident that either of them would impart to it a quantity of its electricity, proportional to the plate's capacity, viz. so much of it as would make the intensity of the electricity of the plate equal to that of the electricity in the conductor or phial, supposed of half a degree; but the plate's capacity, now that it lies upon the proper plane, is above 100 times greater than if it stood insulated in the air; or, which is the same thing, it requires 100 times more electricity in order to shew the same intensity; therefore, in this case, it must acquire upwards of a hundred times more electricity from the phial or conductor. It naturally follows, that when the metal plate is afterwards removed from the proper plane, its capacity being lessened so as to remain equal to the hundredth part of what it was before, the intensity of its electricity must become of 50°; since, agreeably to the supposition, the intensity of the electricity in the phial or conductor was of half a degree.

Having premised thus much respecting the capacity of conductors, we shall now proceed to describe Signor Volta's method of rendering sensible minute degrees of electricity.

His method, in short, is to communicate the other-Description wife unobservable quantity of electricity to the metallic plate of an electrophorus, while standing on an imperfectly insulating plane; for the capacity of the metallic plate being thus augmented, it will acquire a much greater quantity of electricity than if it stood completely insulated in the air, and when it is again separated from the plane its capacity will be diminished; consequently, its electricity increasing at the same time, the intensity of this will be rendered manifest either by sparks or by means of a delicate electrometer.

The particulars necessary to be kept in view in this method, are the following. The metal plate must be at least five inches in diameter, with the edge well rounded, and having a varnished glass handle, or instead of the glass, three silk strings. The inferior plane must be of a very imperfect conducting nature, as dry marble, very dry and slightly varnished wood, a common piece of wood covered with oiled silk, or such like substance; but let the substance be what it will, its surface must be very smooth, and such as to coincide as well as possible with the surface of the metal plate; on which account, if a marble slab be chosen for the inferior plane, it will be proper to fit the metal plate to that of the iron, by grinding one against the other. What Mr Cavallo found to be very fit for this purpose was a paper drum, consisting of a common wooden hoop, such as are used for barrels, over which a piece of thick writing paper was pasted, and on the back of which he pasted a piece of tin-foil. The upper surface of the paper was varnished only once with shell-lac dissolved Principles of dissolved in alcohol or spirit of wine. This sort of plane has many advantages, viz., it is easily made, and from its lightness is very portable; its surface is perfectly flat, excepting when the hoop is not very strong, for then the contraction of the paper has power sufficient to warp it; and lastly, as the thickness of the paper and of the varnish may be varied at pleasure, and very easily, the plane may be rendered of any required degree of conducting power.

Having such a semi-conducting plane and metallic plate properly constructed, the former is to be laid upon a table, and the latter is to be placed upon it, taking care that the inferior plane be not excited by any degree of friction. If the surface of the inferior plane should have acquired any electricity by accidentally rubbing it, &c., the best way of freeing it of that electricity is to pass it two or three times over the flame of a candle. Now the metallic plate is to be struck five or six times with the corner of a dry handkerchief, a piece of dry flannel or paper, &c.; then it is to be raised from the inferior plane by means of its insulating handle, and presented to an electrometer, when it will be found sensibly electrified. If the metallic plate be struck while it is not in contact with the semi-conducting plane, it will be found either to possess no electricity or an incomparably smaller degree than it acquires in the other mode.

By this means electricity may be obtained from substances which could hardly be supposed electrified, and that not only in sufficient quantity to ascertain its quality, but even sufficient to afford sparks. Signor Volta has given to this apparatus the name of condensing apparatus.

Mr Cavallo, observing that in stroking the metallic plate, in order to obtain electricity from various substances, and especially from the hand, the plate was often moved so as to occasion some friction on the inferior plane, whereby this was excited, and consequently the result of the experiment rendered precarious, thought of the following method of preventing such motion.

Upon a varnished glass handle he cemented a brass tube about six inches long and three-fourths of an inch in diameter, from the extremity of which proceeded a fine flexible wire about 14 inches long. Now, when the metallic plate was situated upon the inferior plane, he held the glass handle of the brass tube with his left hand, in such a manner that the end of the wire might touch the plate, the rest remaining in the air. Sometimes, in order to make a better contact, the end of the above-mentioned wire was put into a hole purposely made in the edge of the plate. In this disposition of the apparatus, the substances to be tried are stroked upon the brass tube, and the electricity produced by them is conveyed to the metallic plate by the wire, which being fine and flexible, communicates no motion to the plate.

Another improvement of Mr Cavallo's consists in rendering sensible degrees of electricity still more minute than those which may be discovered by the condensing apparatus.

Notwithstanding the great sensibility of Volta's condenser, yet sometimes the electricity acquired by the metallic plate from some substances was so small as not to affect an electrometer sufficiently to ascertain its quality, or even its existence; hence it naturally occurred to Mr Cavallo, that for the same reason for which the principles of metallic plate of the condensing apparatus manifested such minute degrees of electricity as could not be otherwise observed, another smaller plate, or small condensing apparatus, might be employed to render the weak electricity of the large metallic plate sensible. Accordingly, he constructed a small plate of about the size of a shilling, having a glass handle covered with sealing-wax; and when the large metallic plate seemed to be so weakly electrified as not to affect an electrometer sensibly, he placed the small plate upon the inferior plane, and touched it with the edge of the large plate; then, after removing the small plate, he took up the small one from the plane, holding it by the extremity of the glass handle, and presented it to the electrometer, which was generally so much affected by it as to diverge to its utmost limits.

In this manner Mr Cavallo often obtained electricity more than sufficient for ascertaining its quality, from a single stroke of the corner of a handkerchief; viz., the large plate being placed upon the proper plane, was stroked once; then being removed and presented to an electrometer, it appeared not electrified; but by touching the small plate with the edge of it, that small plate acquired thereby electricity sufficient to make an electrometer diverge.

When this secondary condensing apparatus is used, care must be taken to hold the large plate almost vertically while the small plate is touched by it. There is no need of having another inferior plane for the small plate, the large one being sufficient for both; for immediately after taking up the large plate, weakly electrified, with one hand, you lay down the small plate, &c.

The final quantity of electricity that can be discovered by this means is really surprising, and there is hardly any substance, excepting the metals, or those which cannot be subjected to trial, as water and other fluids, which will not produce some electricity when rubbed or stroked against the large plate of the condensing apparatus, and that electricity is afterwards condensed by being communicated to the small plate.

The discovery of Volta's condenser led to a discovery of the doubler, for which we were first indebted to the Reverend Abraham Bennet of Wirksworth, though the instrument has been much improved by Mr Nicholson and Mr Read.

The doubler in its first and simplest form consisted of three parts, which are represented at fig. 85, Plate CXCI. viz., a polished brass plate A, with an insulating handle fixed in its centre; a similar plate B with an insulating handle fixed in its periphery, and the cap of Bennet's gold-leaf electrometer C, which serves as a third plate. The two plates A and B are varnished on the under side, and the handles are made of mahogany fixed to the plates by means of glass nuts covered with sealing-wax.

The method of demonstrating the presence of manipulative electricity by means of this apparatus is as follows:

1. Place B upon C, and communicate some electricity to the latter, while the plate B is touched with the finger. The consequence will be that C will receive a greater degree of electricity than it would have been capable of acquiring if B had not been present.

2. Remove 2. Remove the communication from C, and take the finger from off B, then raise this latter by its insulating handle, and B and C will exhibit the opposite states of electricity more strongly than when they are in contact.

3. Place A upon B, and touch A with the finger. The consequence will be that A will receive a portion of electricity of a state opposite to that of B, or A will be in the same state of electricity with C.

4. Place B upon C, and touch B with the finger as before, and at the same time apply A edgeways to C. In this situation, A will communicate the greatest part of its electricity to C.

5. Remove A, take the finger from B, and raise B from C. The opposite states of electricity in B and C, will now be stronger than before, on account of the additional electricity afforded by A.

6. Place A upon B again, as in the third stage of the process, and repeat the subsequent manipulations. In each of them the intensity of the electricity is supposed to be doubled, and by proceeding in this manner for a certain time, the electricity originally communicated to C, though at first too small to affect the strips of gold leaf, will at last become sufficiently sensible to produce a considerable divergence of them.

Though the above process is sufficiently simple and evident, yet it requires to be learned, and takes up a certain time for its performance. It was therefore desirable that an instrument should be formed which might complete this series of operations by a very simple mechanical movement. The first instrument constructed with this view was contrived by Dr Darwin, and was shown to Mr Nicholson in the month of December 1787. This instrument consisted of four metallic plates, two of which were moveable by wheel-work into positions which required them to be touched by the hand in order to produce the effect. It appeared to Mr Nicholson that the whole operation, including the touching, might be done by a simple combination without wheel-work by the direct rotation of a winch. This was soon afterwards effected, and communicated by him to the Royal Society in 1788. Mr Nicholson's description of his revolving doubler, was first printed in the 7th volume of the Philosophical Transactions, and has been reprinted by Mr Nicholson in his Philosophical Journal for May 1800, from which we have copied it.

Fig. 86 represents the apparatus of the doubler supported on a glass pillar 6½ inches long. It consists of the following parts. Two fixed plates of brass, A and C, are separately insulated and disposed in the same plane, so that a revolving plate B may pass very near them, without touching. Each of these plates is two inches in diameter; and they have adjusting pieces behind, which serve to place them accurately in the required position. D is a brass ball, likewise of two inches diameter, fixed on the extremity of an axis that carries the plate B. Besides the more essential purpose this ball is intended to answer, it is loaded within on one side, that it serves as a counterpoise to the revolving plate, and enables the axis to remain at rest in any position. The other parts may be distinctly seen in fig. 87. The shaded parts represent metal, and the white represent varnished glass. ON is a brass axis, passing through the piece M, which last sustains the plates A and C. At one extremity is the ball D already mentioned; and the other is prolonged by the addition of a glass stick, which sustains the handle L and the piece GH separately insulated. E, F, are pins rising out of the fixed plates A and C, at unequal distances from the axis. The cross-piece GH, and the piece K, lie in one plane, and have their ends armed with small pieces of harpsichord-wire, that they may perfectly touch the pins EF in certain points of the revolution. There is likewise a pin I, in the piece M, which intercepts a small wire proceeding from the revolving plate B.

The touching wires are so adjusted, by bending, that when the revolving plate B is immediately opposite the fixed plate A, the cross-piece GH connects the two fixed plates, at the same time that the wire and pin at I form a communication between the revolving plate and the ball. On the other hand, when the revolving plate is immediately opposite the fixed plate C, the ball becomes connected with this last plate, by the touching of the piece K against F; the two plates, A and B, have then no connection with any part of the apparatus. In every other position the three plates and the ball will be perfectly unconnected with each other.

Mr Bennet and Mr Cavallo observed, soon after the discovery of the doubler, that it never fails to exhibit an electric state by the mere operation, without any communication of electricity being previously made. Mr Bennet endeavoured to find out a method of depriving the doubler of this inherent electricity, and after a number of trials, he considered the following as the best mode of answering this purpose.

He connected the plates A and C together by a wire hooked at each end upon two small knobs on the backs of the plates, the middle of the same wire touching the pillar which supports the doubler. Another wire was hooked at one end upon the back of the plate B, and at the other end, to the brass ball which counterbalances this plate. Thus all the plates were connected with the earth, and by turning the handle of the doubler, it might be discharged of electricity in every part of its revolution.

After often trying this method of depriving the doubler, Mr Bennet observed that its spontaneous discharge was almost always negative. He then touched A and C with a positively charged bottle, and turned the doubler till it produced sparks for a long time together; and after this strong positive charge he hooked on the wires as above, and revolved the plate B about a hundred times, which so deprived the doubler of its positive electricity, that when the wires were taken off, it produced a negative charge at about the same number of revolutions which it required before.

The positively charged bottle was again applied, and the wires being hooked upon the plates as before, B was revolved only fifty times, yet this was found sufficient to deprive it of its positive charge, and in many experiments five or six revolutions were sufficient; but he never thought it safe to stop at so few, and therefore he generally turned the handle 40 or 50 times between every experiment.

Left electricity adhering to the electrometer should obstruct the above experiments, Mr Bennet did not let it stand in contact with the doubler during its revolutions, Principles of solutions, but touched the plate A with the cap of the Electricity electrometer, after he supposed its electricity was become sufficiently sensible; but left even this contact by experiment should communicate any electricity, he made a cap of shell lac for his electrometer, having a small thin tube in the centre, to which the gold leaf was suspended within the glass, and a bent wire was fixed to the top which might easily be joined to the plate A of the doubler, and thus the gold leaf was more perfectly inflated, and the electricity could not be diffused over so large a surface. The glass which inflates the plates and crofs piece of the doubler was also covered with shell lac.

Dr Robison conceived that Mr Bennet's original proposal for doubler might be freed from error as far as possible, by employing a thin stratum of air as the intermediate between the three plates. The method which he proposes for effecting this is very ingenious. Stick on one of the plates three very small spherules, made from a capillary tube of glass or from a thread of sealing-wax. The other plate being laid on them, rests on mere points, and can scarcely receive any friction, which may disturb the experiment.

Mr Cavallo, finding that Mr Bennet's mode of obviating the inconveniences of the doubler did not succeed with him, constructed a new instrument, which he calls a collector of electricity, and a description of which was inserted in the 78th volume of the Philosophical Transactions. It consists of a plate of tin, supported by two upright sticks of glass; on each side of which plate are two frames of wood covered with gilt paper, which do not touch the tin-plate, but stand parallel to it at a little distance. These frames are fastened to the platform of the instrument by hinges; so that if electricity be communicated to the plate, it will receive a large quantity without any considerable intensity, because its capacity is much augmented by the vicinity of the plane of gilt paper on each side. But if these planes be thrown back into the horizontal position, which is easily done by means of their hinges, the electricity, which before was compensated in the plate, will have its intensity greatly increased. An electrometer connected with this plate will therefore show signs of electricity by means of a communication made between a large stock of electricity, and the tin-plate in its first position, though the intensity of that stock may have been too small to have affected the electrometer without this contrivance.

It does not appear, in the author's description of this instrument, that it removes the equivocal effect of the doubler; for it is evident that it does not in its simple process enter the province of the doubler in which this effect takes place. The doubler requires six or seven turns before it will exhibit spontaneous electricity; at which period the first charge is magnified above twelve thousand times; but his simple instrument will scarcely exceed one hundred times, and therefore requires the electricity to be one hundred and twenty times as strong as that which causes the uncertainty of the doubler. Whence it may be inferred, that the doubler would have acted unequivocally with all such electricities as this instrument is capable of exhibiting.

Mr Cavallo has since constructed another instrument, which he calls a multiplier of electricity, and which he considers as quite free from equivocal results.

"The figs. 88. and 89. represent this new instrument, and they are about two-thirds of the real size. QRS is the bottom board, upon which are steadily fixed by experiment the glas sticks H, G, two flat brass plates A and C.—B is a similar brass plate supported by a glass stick I, which is cemented in a hole made in the wooden lever KL, which moves round a fleady pin K, that is screwed tight in the bottom board. By moving this lever backwards and forwards, the plate B may be alternately put in the two situations represented by the figures. N is a thick brass wire fixed tight into the bottom board. There is a fourth brass plate D, similar to the other three, which is supported not by glass, but a wire; and this wire is screwed fast to an oblong piece of brass FP, that slides in a groove made for the purpose in the bottom board QRS; so that by applying a finger's nail to the notch on the end F, the sliding piece FP may be drawn out either entirely or to a certain length, and of course the plate D will be removed to any required distance from the fixed plate C. I need not say anything particular respecting the sockets of those brass plates, they being clearly indicated in the figures, excepting only that the socket of the plate A reaches as high as the top of it, and serves to receive a wire, or other apparatus, on certain occasions.

The parts of this instrument are so adjusted, as that when the lever is in the situation of fig. 88, viz. is pushed as far toward Q as it can go, the plate B comes parallel to the plate A, and about one-twentieth of an inch distant. At the same time the extremity of the wire OM just touches the fixed wire N, and of course renders the plate B uninflated. But as soon as the lever begins to be moved towards S, the communication of the plate B with the wire N, or with the ground, becomes interrupted, and B remains uninflated. And when the lever has been moved as far as it can go towards S, the wire M comes in contact with the plate C, as shown in fig. 89. Then the two plates B and C communicate with each other, though they are otherwise uninflated. The fourth plate D being supported by a wire, communicates with the ground; and when the sliding piece PF is pushed home, it stands parallel to, and at about one-twentieth of an inch from the plate C.

When the instrument is situated as in fig. 88, if an electrified body be brought into contact with the plate A, this plate will imbibe a great deal more of that electricity than it would otherwise, because its capacity is increased by the vicinity of the uninflated plate B, and therefore, if after the communication of that electricity, the plate B, by moving the lever, be removed from that situation, and A be made to touch an electrometer, this will be electrified more feebly by it, than it would have been by the contact of the original electrified body itself. So far the plate A acts like a condenser, or collector of electricity. But let us now consider the instrument as a multiplier.

When the plate A has received a small quantity of electricity by the contact of any electrified body whatever, and that body is removed, the plate B being uninflated and opposed to the electrified plate A, will, like the metal plate of an electrophorus, acquire the contrary electricity, by either receiving from, or giving to, the ground some electric fluid, according as... Principles of the plate A happens to be electrified. Thus, suppose that A has been electrified positively, B will become negative, and vice versa. If now the lever be pulled towards S, the plate B will remain electrified negatively, the communication with the ground being cut off; and when B comes into the situation represented by fig. 4th, at which time the wire M touches the plate C, the negative electricity of B will go to C, because the capacity of C for holding electricity is considerably augmented by the vicinity of the uninflated plate D.

If after this the lever be moved back again to its first situation, B will be made negative a second time in the same manner as before: and by pushing the lever again towards S, that second charge of negative electricity will be communicated from B to C; and thus, by repeating the operation, which consists in merely moving the lever backwards and forwards, a considerable quantity of negative electricity will be accumulated upon C.

In fact, the action of this instrument resembles very much that of an electrophorus; for the plate A may represent the excited resinous plate, B may represent the metal plate of the electrophorus, and C is a kind of reservoir, into which the successive charges of the plate B are collected.—When a number of those charges or portions of electricity has been communicated to C, if the sliding piece FP be drawn out about an inch, and of course the plate D be removed to the like distance from the plate C, the capacity of the plate C will thereby be much diminished: and therefore if an electrometer be brought into contact with it, the electricity will be manifested: whereas the electricity originally communicated to the plate A, could not have affected an electrometer in any sensible degree.

In using this instrument, 30 or 40 additions of electricity are the utmost number practicable; for after that number the augmentation of the charge upon C will not go any farther; the limit of which is, when the charge of C is increased to such a degree, as to leave a portion of electricity upon B, equal to that portion which B can receive from the action of A.

In this case, let C touch an electrometer as mentioned above, and if the electrometer does not diverge, proceed to a second process; for though its pendulums do not diverge, yet some electricity remains in them, which must not be disturbed, as it will help the effect of the second operations, which is as follows: Push in the slider FP, and go on moving the lever backwards and forwards as before, by which means, after a certain number of additions, the plate C will acquire a second charge, about as high as the former: and if then the slider FP be pulled out, and C brought into contact with the same electrometer, the divergency of the pendulums, which before was either not at all or hardly perceptible, will thereby be rendered more conspicuous: and thus it may be increased still farther by a third and a fourth operation. But if, notwithstanding those repeated operations, the electrometer should be found not to diverge, the quantity of electricity may still be augmented by another method, which is, by communicating that little electricity of C to the plate A of another instrument of the same sort, and proceeding with that in the manner already described.*

* Cavalli's Electricity, vol. iii.

In Nicholson's Journal for September 1804, is a paper by Mr W. Wilson, containing a description of an instrument which Mr Wilson calls a compound condenser of electricity, and which he considers as an improvement on Mr Cavalli's multiplier, answering the purpose of a condenser, a single and double multiplier, and a doubler. The instrument is very complicated, containing no less than five plates. Like all complicated instruments of this kind, it is of course subject to error from its own spontaneous electricity.

Mr Nicholson has constructed an instrument for Mr Nicholson's certaining final degrees of electricity, without, as he says, a possibility of equivocal result. This instrument he calls the spinning condenser, and it is thus described in his Journal for April 1797.

"Fig. 95 represents a vertical section of the instrument. A is a metallic vase, having a long steel axis which passes through a hole in the stand H at K, and rests on its pointed end in an adjustable socket at C. The use of the vase is, by its weight, to preserve, for a considerable time, the motion of spinning which is given by the finger and thumb applied to the nob at the top of the instrument. The flared parts D and E represent two circular plates of glass nearly 1½ inch in diameter. The upper plate is fixed to the vase, and revolves with it; the lower is fixed to the stand. In the lower plate are inserted two metallic hooks, diametrically opposite each other, at F and G. They are cemented into holes drilled in the edge of the glass, which is near two-tenths of an inch thick. In the upper plate are inserted in the same manner two small tails of the fine flattened wire used in making silver lace. These tails are bended down so as to strike the hooks in the revolution, but in all other positions they remain freely in the air without touching any part of the apparatus. At C is a screw, which by raising or lowering the vase keeps the faces of the glass plates from each other at whatever distance may be required. The faces of the glass plates which are opposed to each other are coated with segments of tin-foil, as represented, fig. 91 and 92, the latter of which represents the upper plate. Each of the tails communicates with the tin-foil coating to which it is contiguous, as does also the hook F with that coating of the lower plate nearest to it. But the hook G is entirely insulated from the whole apparatus, and is intended to communicate only with the electrified body or atmospheric conductor L. The lower coating nearest to G is made to communicate permanently with the stand H, and consequently with the earth.

In this situation, suppose the motion of spinning to be given to the apparatus, and the effects will be these: One of the tails will strike the hook G, by which means the upper coating annexed to that tail will assume the electric state of L by communication. But this state, on account of the proximity of the lower uninflated plate to which it is, at that instant, directly opposed, will be as much stronger than that of L, as a charge exceeds simple electrization. The tail G with its plate or coating proceeds onward, and after half a revolution arrives at the situation to touch the hook F. The upper coating, the lower on the side of F, the hook F itself, and the tail V, must then constitute one jointly inflated metallic mass, in which no charge subsists, but which is simply electrified by the whole charge received..." And of this mass the surfaces of the plates themselves, constituting the electric well of Franklin, will throw out all their electricity to the hook and tail. But the coating and its tail instantly pals round, leaving F electrified, and proceed to bring another charge from G and deposit it as before. The balls at F are therefore very speedily made to diverge.

It is scarcely necessary to remark, that the two upper coatings do nothing more than double the speed of the operation; one of the tails being employed in collecting, while the other is depositing; and that the gold-leaf electrometer may be advantageously substituted for the cork-balls.

The instrument I caused to be made was five inches high. The receiving side G was connected with a coated jar of four square feet coating, and the giving side F was connected with Bennet's gold-leaf electrometer. The electrometer was rendered as strongly positive as it was capable of being, and the jar was rendered negative, by giving it as much of that power as was produced by drawing a common stick of sealing-wax once through the hand. In this state the jar was incapable of attracting the finest thread. The vase was then made to spin; and the effect was, that the leaves of the electrometer first gradually collapsed, and then in the same manner gradually opened, and struck the sides of the glass of the electrometer with negative electricity. The experiment was renewed and repeated with every requisite variation."

To conclude, the methods of ascertaining minute degrees of electricity may be reduced to three.

1. If the absolute quantity of electricity be small and pretty much condensed, as that produced by a small tourmaline when heated, or by a hair when rubbed, the only effectual method of manifesting its presence, and ascertaining its quality, is to communicate it to a very delicate electrometer, i.e., one that is very light and has no great extent of conducting surface.

2. When we wish to ascertain the presence of a considerable quantity of electricity, which is dispersed, or expanded into a great space, and is little condensed, such as the constant electricity of the atmosphere in clear weather, or such as the electricity which remains in a large Leyden phial after the first or second discharge; this may be best ascertained by means of Cavallo's collector or multiplier, or by the condenser with Cavallo's improvement of the small plate.

3. When the electricity to be ascertained is neither very considerable in quantity nor much condensed, such as the electricity of the hair of certain animals, of the surface of chocolate when cooling, &c. In this case the best method is to apply a metallic plate furnished with an inflating handle, such as one of the plates of the doubler, to the electrified body, and to touch the plate with a finger while it remains for some time in this situation; which done, the plate is to be removed and brought near a sensible electrometer; or its electricity may be communicated to the plate of a small condenser, by which it will be rendered more conspicuous. In this operation care must be taken not to bring the plate too near the body whose electricity is to be examined, lest the friction, likely to happen between the plate and the body, should produce some electricity, the origin of which might be attributed to some other cause.

Mr Nicholson, in his Journal for September 1797, proposes what appears to be a valuable improvement in Bennet's electrometer.

"There are, (says he) two particulars in which this excellent instrument appears capable of improvement: the first, to render it portable, without danger to the Bennet's gold-leaf, and the second to express its various degrees of electrization by a scale of divisions.

I have reflected much on the probable means of securing the gold-leaf from fracture by carriage, but hitherto with little prospect of success. There was some hope that a single flip of this gold might be preserved in a fleath or box, with its sides very nearly in contact; but when I placed such a flip upon a gilded piece of wood of the same superficial dimensions, to which it was fastened at one end, its flexibility was such that the leaf very readily slid along the surface of the wood, and became full of folds, by inclining the fastened end a very few degrees lower than the other extremity. There was still left immediate expectation that the flips could be actually and repeatedly confined between two leaves or cushions, as in the book of the gold-beaters, without their being broke by continual agitation. To this, however, my attention will probably be directed when I may again resume this object. In the mean time, I recommend it to other philosophers, as a very desirable improvement in the mineralogical apparatus, and should rejoice to be anticipated by their successful researches.

The weight of one flip of gold-leaf, in the electrometer of Bennet, is about 1/600th part of a grain; but this, as well as the sensibility of the instrument, must vary, not only from the figure and dimensions of the piece, but the nature and thickness of the gold itself. It seemed, therefore, unnecessary to endeavour to render two of these instruments comparable with each other.

All that could be done was, to distinguish the different intensities as thrown by the divergencies of the leaf; or, as I have taken it, the distances at which they strike a pair of uninflated metallic bars. In Plate CXLIII, fig. 93. A represents the inflated metallic cap, from which, at C, depend the two narrow pointed flips of gold-leaf. BB is the glass shade, which serves to support the cap, and defend the leaves from the motion of the surrounding air. DD are two flat radii of braids, which open and shut by means of one common axis, like a pair of compasses. By a contrivance of springs, they are disposed to open when left at liberty; but the micrometer screw E serves to draw a nut, which has two steel bars, with a claw at the end of each, that enters into a correspondent slit, in two small cylindrical pieces, to which the radii are fixed respectively. This apparatus is seen in another position in fig. 94. KL represents a piece of braids, which serves as the frame for the work, and fits the lower socket of the electrometer, FF, fig. 3. In this the letters IH indicate the cylindrical pieces which carry the radii, and are seen from beneath. On the side of the nut G, one of the steel drawing pieces is seen; the other being on the opposite side, and consequently not visible. Towards L appear the two reaction springs. The other parts require no verbal description." In the common construction of the gold-leaf electrometer, there are two pieces of tin-foil pasted on opposite parts of the internal surface of BB; against which the gold-leaf strikes when its electricity is at the maximum. If the radii DD be left at the greatest opening, our instrument does not then differ from that in common use. But if the divergence produced by the contact of an atmospheric conductor, or any other source of electricity, be so small as to render it doubtful whether the leaves be electrified or not, the radii may then be brought very gradually together by means of the screw, until the increased divergency from their attractive force be sufficient to ascertain the kind of electricity possessed by the leaves. In this and all other cases, the division on the micrometer head, which stands opposite the fixed index, at the time the leaves strike the radii, will shew the greater or less degree of intensity.

In his Journal for January 1799, he has the following remarks on the glass case of this instrument.

"Under all the uncertainties concerning the place occupied by the electric charge of coated glasses, though it may seem unfair to make any inference respecting glasses which is uncoated, yet, upon the whole, there appears to be a probability that the interposition of naked glasses may impede the action of electrified bodies. This question more immediately points at the tube in which the gold-leaf electrometer of Bennet is inclosed. To determine whether the tube of the electrometer does affect the electric state of the included leaf, either by compensation or otherwise, I took a piece of window-glass eighteen inches long, two inches wide and one-twentieth of an inch thick, which I cleaned very well, and then passed it several times through the hot air over the flame of a candle. In this state one end of the glass was laid gently upon the electrified plate of Bennet's electrometer, and then suddenly raised by a turn of the wrist. It was scarcely possible to discern that the leaves were at all affected; but when the electrometer was in the plus state a very slight collapson was produced by raising the glass, and the contrary effect was produced when the electrometer was negative. Some days afterwards the experiment was repeated, after the gold-leaf had been changed for other pieces, which were very pointed and delicate in their movements. The result was, that the glass was always shewn by the electrometer to be in a weak positive state; and, when the electricity of the electrometer was made plus, the collapson was equal to the divergence when it was minus.

In making these experiments I had previously supposed that the influence of the metallic state of the electrometer would produce somewhat of the nature of a charge upon the glass; and consequently that the intensity of the leaves would have been diminished during the existence of that charge; and also, that in such a case the action of the metal through the glass would be subject to the same diminution as in the series of jars. But as the glass did not appear to act in this manner, it seems proper to conclude that clean glass does not affect the electric state of bodies by its vicinity, and that the divergence of the balls or the gold-leaf in the electrometers of Cavallo and Bennet is not diminished by the tube which surrounds them.

From a variety of experiments it was clearly ascertained that the metallic coatings, though by their vicinity they may diminish the intensity of the electric state in the leaves, do nevertheless increase the angle of divergence by their attraction.

When the gold-leaf electrometer is made with a very small tube, its sensibility is somewhat increased by the nearness of the coatings; but the chance of rendering it unserviceable from casual friction, which excites the glass, and causes the gold-leaf to stick to it, together with the less perfect view of the divergence through a tube of small curvature, afford reasons why a diameter of less than an inch should be rejected. Other reasons of convenience indicate that the diameter of the glass should not much exceed this quantity.

I was once induced to think that the considerable magnitude of the cap of Bennet's electrometer might render it less capable of being acted upon by small quantities of electricity. Experiment did not however give much countenance to this supposition. By trials with heads of different size, the smallest were found to be rather more sensible to extremely minute electricities, and less so to such as were greater. The influence of very weak electricity may produce the opposite state in the whole of a small head, but only in part of a larger; the remaining part of this last assuming the opposite state, and robbing the leaves of part of their intensity. But in higher electricities the whole of the large head may be urged to give electricity to the leaves, in a quantity which the smaller head could not give without acquiring a higher degree of intensity, and consequently more strongly resisting the desired process. It appears therefore that the maximum of effect with a given electricity, acting without communication, will not be obtained but by a head of a definite figure and magnitude."

In No. 82, experiment 5, we described a method of imitating the planetary motions by the motion communicated by the current of air from electrified points; this imitation may be done in various other ways, of which we shall only add the following.

1. From the prime conductor of an electric machine suspend six concentric hoops of metals at different distances from one another, in such a manner as to represent in some measure the proportional distances of the planets. Under these, and at the distance of about half an inch, place a metallic plate, and upon this plate, within each of the hoops, a glass bubble blown very thin and light. On electrifying the hoops, the bubbles will be immediately attracted by them, and will continue to move round the hoops as long as the electrification continues. If the electricity is very strong, the bubbles will frequently be driven off, run hither and thither on the plate, making a variety of surprising motions round their axis; after which they will return to the hoop, and circulate as before; and if the room is darkened, they will all appear beautifully illuminated with electric light.

2. Provide a ball of cork about three quarters of an inch in diameter, hollowed out in the internal part by cutting it in two hemispheres, scooping out the inside, and then joining them together with paste. Having attached this to a silk thread between three and four feet in length, suspend it in such a manner that it may Principles of may just touch the knob of an electric jar, the outside of which communicates with the ground. On the first contact it will be repelled to a considerable distance, and after making several vibrations will remain stationary; but if a candle is placed at some distance behind it, so that the ball may be between it and the bottle, the ball will instantly begin to move, and will turn round the knob of the jar, moving in a kind of ellipse as long as there is any electricity in the bottle. This experiment is very striking, though the motions are far from being regular; but it is remarkable that they always affect the elliptical rather than the circular form.

In the table of conductors we have placed flame, smoke, and the vapour of hot water. That these vapours are conductors may be shewn by the following experiments.

**Exper. 1.**—Bring the knobs of two metallic discharging rods, communicating the one with the outside, and the other with the inside of a charged phial, opposite each other, each within an inch of the flame of a candle, so that the flame may be in the middle between them. The flame will be seen to spread on each side towards the knobs, and will produce the discharge of the jar.

Mr Cuthbertson has proposed a method of distinguishing positive from negative electricity by the flame of a candle. He places the flame of a candle exactly in the middle between two metallic balls at the distance of four inches from each other, so that the centre of the flame is in a line with that of the balls. The balls are about three-fourths of an inch in diameter, and communicate by insulated wires, the one with the positive and the other with the negative conductor. If the machine be then put in motion, the flame will waver very much, but will seem to incline rather to the negative than the positive ball. After turning the machine for about 50 revolutions (if the glass be a plate of two feet diameter), the negative ball will begin to grow warm, while the positive still remains cold. After 200 revolutions, the negative ball will become too hot to be touched, while the positive will continue as cold as at first.*

* Nicholson's Journal, Nov. 1802.

A charged phial may be gradually discharged by passing it for some time backwards and forwards through the flame of a large candle, so that the flame may act alternately on the knob and the outside coating.

**Exper. 2.**—Suspend a cork-ball electrometer about four or five feet above the prime conductor of an electrical machine; then turn the winch very gently, and it will be found that the balls do not diverge. Now place a green wax taper just blown out in the prime conductor, so that its smoke may ascend towards the balls, and these will diverge a little with the same degree of motion communicated to the machine.

The same effect, but in a less degree, will be produced if, instead of the taper, a vessel of hot water is placed below the balls, thus shewing that steam is a conductor, though inferior to smoke in its conducting power.

These experiments are by Mr Henly, and are among several others related by him in the 64th volume of the Philosophical Transactions. His reason for employing a green taper, was, that on account of the verdigris which it contained, it occasioned much smoke with little heat.

It has been remarked in the Introduction, that glass, though one of the most perfect electrics when cold, becomes a conductor when heated red hot. This is proved by the following experiment, which also shews that other electrics change their nature when heated.

Take a small glass tube of about one-twentieth of an inch in diameter, and above a foot long; close it at one end, and introduce a wire into it, so that it may be extended through its whole length: let two or three inches of this wire project above the open end of the tube, and there fasten it with a bit of cork; tie round the closed end of the tube another wire, which will be separated from the wire within the tube only by the glass interposed between them. In these circumstances endeavour to send a shock through the two wires; i.e. the wire inserted in the glass tube, and that tied on its outside, by connecting one of them with the outside, and touching the other with the knob of a charged jar, and you will find that the discharge cannot be made, unless the tube be broken; because the circuit is interrupted by the glass at the end of the tube, which is interposed between the two wires. But put that end of the tube to which the wire is tied into the fire, so that it may become just red hot, then endeavour to discharge the jar again through the wires, and you will find that the explosion will be easily transmitted from wire to wire, through the substance of the glass, which, by being made red hot, is become a conductor.

In order to ascertain the conducting quality of hot resinous substances, oils, &c. bend a glass tube in the form of an arch CEF, fig. 95, and tie a silk string GCD to it, which serves to hold it by when it is to be set near the fire; fill the middle part of this tube with rosin, sealing-wax, &c. then introduce two wires, AE, BF, through its ends, so that they may touch the rosin, or penetrate a little way in it. This done, let a person hold the tube over a clear fire, so as to melt the rosin within it; at the same time, by connecting one of the wires, A or B, with the outside of a charged jar, and touching the other with the knob of the jar, endeavour to make the discharge through the rosin, and you will observe that, while the rosin is cold, no shocks can be transmitted through it; but it becomes a conductor according as it melts, and when totally melted, the shocks will pass through it very freely.

The electric power of glass may also be destroyed by reducing the glass to powder. This was ascertained by other electrics when powdered become conductors.

M. Willeke, and Dr Priestley; but it has been most satisfactorily proved by M. Van Swinden, in the following experiments.

**Exper. 1.**—He covered a cake of white iron with powdered glass, so as to form a cake about an inch thick, a foot long, and eight inches broad, and he placed above this cake, another plate of iron so as to form a coating. He then attempted to charge this coated plate, but without success; he could produce no shock.

**Exper. 2.**—Supposing that the conducting power of the glass in the above experiment might arise from some humidity which it had contracted, he dried it in a crucible, and repeated the experiment. In this case, it appeared slightly electric, so long as the machine was worked,

Principles of Electricity illustrated by experiment.

Exper. 3.—Into a jar, coated on the outside, he put a quantity of powdered glass, and having furnished it in other respects like a Leyden phial, he proceeded to examine whether it would receive a charge. He found that it could be completely charged, a proof that the powdered glass acted the part of a conductor.

By similar experiments M. Van Swinden found that flowers of sulphur acted as a conductor though more imperfectly than powdered glass.

Soon after the discovery of the Leyden phial and shock produced by it, it became a desirable object with electricians to ascertain how far the shock might be conveyed, and how long a time would be required to convey it to any considerable distance.

The French philosophers were the first to appear in this field, but they did little more than excite the English to go far beyond them in these great undertakings. A circuit was made by the former of 900 toises, consisting of men holding iron wires betwixt each two, through which the electric shock was sensibly felt. At another time they made the shock pass through a wire two thousand toises in length, that is, near a Paris league, or about two English miles and a half; though part of the wires dragged upon wet grass, went over chasms, hedges, or palisades and over ground newly ploughed up. Into another chain they took the water of the basin in the Tuileries, the surface of which was about an acre, and the phial was discharged through it.

Mr Monnier the younger, endeavoured to determine the velocity of the electric power; and for this purpose made the shock pass through an iron wire of 950 toises in length, but he could not observe that it spent a quarter of a second in passing it. He also found, that when a wire of 1319 feet, with its extremities brought near together, was electrified, that the electricity ceased at one end the moment it was taken off at the other.

But all these attempts of the French would scarcely have deserved to be mentioned, but that they preceded the greater, the more numerous, and more accurate experiments of the English. The names of the English gentlemen, animated with a truly philosophical spirit, and who were indefatigable in this business, deserve to be transmitted to posterity.

The principal agent in this scene was Dr Watson. He planned and directed all the operations, and never failed to be present at every experiment. His chief assistants were Martin Folkes, Esq., president of the Royal Society, Lord Charles Cavendish, Dr Bevis, Mr Graham, Dr Birch, Mr Peter Daval, Mr Trembley, Mr Elliot, Mr Robins, and Mr Short. Many other persons, and some of distinction, gave their attendance occasionally.

Dr Watson, who wrote the history of their proceedings, in order to lay them before the Royal Society, begins by observing (what was verified in all their experiments) that the electric shock is not, strictly speaking, conducted in the shortest manner possible, unless the bodies through which it passes, conduct equally well; for that, if they conduct unequally, the circuit is always formed through the best conductors, though the length of it be ever so great.

The first attempt these gentlemen made, was to convey the electric shock across the river Thames, making use of the water of the river for one part of the chain of communication. This they accomplished on the 14th and 18th of July of 1747, by fastening a wire all along Westminster bridge, at a considerable height above the water. One end of this wire communicated with the coating of a charged phial, the other being held by an observer, who in his other hand held an iron rod, which he dipped into the river. On the opposite side of the river stood a gentleman who likewise dipped an iron rod in the river with one hand, and in the other held a wire, the extremity of which might be brought into contact with the wire of the phial.

Upon making the discharge, the shock was felt by the observers on both sides of the river, but more sensibly by those who were stationed on the same side with the machine; part of the electric fire having gone from the wire down the moist stones of the bridge, thereby making several shorter circuits to the phial, but still all passing through the gentlemen who were stationed on the same side with the machine. This was, in a manner demonstrated by some persons feeling a sensible shock in their arms and feet, who only happened to touch the wire at the time of one of the discharges, when they were standing upon wet steps which led to the river.

Upon this and the subsequent occasions, the gentlemen made use of wires, in preference to chains, for this reason among other reasons, that the electricity which was conducted by chains, was not so strong as that conducted by wires. This, as they well observed, was occasioned by the junctures of the links not being sufficiently close, as appeared by the flashing and snapping at every juncture, where there was the least separation. These lesser snappings being numerous in the whole length of a chain, very sensibly lessened the great discharge at the prime conductor.

Their next attempt was to force the electrical shock to make a circuit of two miles, at the New-river at Stoke Newington. This they performed on the 24th of July 1747, at two places; at one of which, the distance by land was 800 feet, and by water 2000; in the other the distance by land was 2800 feet, and by water 8000. The disposition of the apparatus was similar to what they before used at Westminster bridge, and the effect answered their utmost expectations. But, as in both cases, the observers at both extremities of the chain, which terminated in the water, felt the shock, as well when they stood with their rods fixed into the earth 20 feet from the water, as when they were put into the river; it occasioned a doubt, whether the shock was formed through the windings of the river, or a much shorter way by the ground of the meadow; for the experiment plainly showed, that the meadow ground, with the grass on it, conducted the electricity very well.

By subsequent experiments, they were fully convinced, that the electricity had not in this case been conveyed by the water of the river, which was two miles in length, but by land, where the distance was only one mile; in which space, however, the electric power must necessarily have passed over the New-river twice, have gone through several gravel pits, and a large stubble field.

† p. 360. On the 28th of July they repeated the experiment at the same place, with the following variation of circumstances. The iron wire was, in its whole length, supported by dry sticks, and the observers stood upon original electrics; the effect of which was, that they felt the shock much more sensibly than when the conducting wire had lain upon the ground, and when the observers had stood likewise upon the ground, as in the former experiment.

Afterwards, everything remaining as before, the observers were directed, instead of dipping their rods into the water, to put them into the ground, each 150 feet from the water. They were both smartly struck, though they were distant from each other above 500 feet.

The same gentlemen, pleased with the success of their former experiments, undertook another, the object of which was to determine, whether the electric power could be conveyed through dry ground; and at the same time to carry it through water to a greater distance than they had done before. For this purpose they pitched upon Highbury-barn, beyond Illington, where they carried it into execution on the 5th of August 1747. They chose a station for their machine almost equally distant from two other stations for observers, upon the New-river, which were somewhat more than a mile asunder by land, and two miles by water. They had found the streets of London, when dry, to conduct very strongly, for about 40 yards; and the dry road at Newington about the same distance. The event of this trial answered their expectations. The electric fire made the circuit of the water when both the wires and the observers were supported on original electrics, and the rods dipped into the river. They also both felt the shock, when one of the observers was placed in a dry gravelly pit, about 300 yards nearer the machine than the former station, and 100 yards distant from the river; from which the gentlemen were satisfied, that the dry gravelly ground had conducted the electricity as strongly as water.

The last attempt of this kind which these gentlemen made, and which required all their sagacity and address in the conduct of it, was to try whether the electric shock was perceptible at twice the distance to which they had before carried it, in ground perfectly dry, and where no water was near, and also to distinguish, if possible, the comparative velocity of electricity, and of found.

For this purpose they fixed upon Shooter's-hill, and made their first experiment on the 14th of August 1747, a time, when, as it happened, but one shower of rain had fallen during five preceding weeks. The wire communicating with the iron rod, which made the discharge, was 6732 feet in length, and was supported all the way upon baked sticks; as was also the wire which communicated with the coating of the phial, which was 3868 feet long, and the observers were distant from each other two miles. The result of the explosion demonstrated, to the satisfaction of the gentlemen present, that the circuit performed by the electricity was four miles, viz., two miles of wire, and two of dry ground, the space between the extremities of the wires, a distance, which, without trial, as they justly observed, was too great to be credited. A gun was discharged at the instant of the explosion, and the observers had stop watches in their hands, to note the moment when they felt the shock; but as far as they could distinguish, the time in which the electric power performed that vast circuit was instantaneous.

In all the explosions where the circuit was made of any considerable length, it was observed, that though the phial was very well charged, yet that the snap at the gun-barrel made by the explosion was not near so loud as when the circuit was formed in a room; so that a bystander, says Dr Watson, would not imagine, from seeing the flash and hearing the report, that the stroke, at the extremity of the conducting wire, would have been considerable, the contrary of which, when the wires were properly managed, he says, always happened.

Still the gentlemen, unrestrained in these pursuits, were desirous of ascertaining, if possible, the absolute velocity of electricity though a certain space; because, though in the last experiment, the time of its progress was certainly very small, they were desirous of knowing, small as that time might be, whether it was measurable, and Dr Watson had contrived an excellent method for that purpose.

Accordingly, on the 5th of August 1748, the gentlemen met for the last time, at Shooter's-hill; when it was agreed to make an electric circuit of two miles, by several turnings of the wire, in the same field. The middle of this circuit they contrived to be in the same room with the machine, where an observer took in each hand one of the extremities of the wires, each of which was a mile in length. In this excellent disposition of the apparatus, in which the time between the explosion and the shock could be observed with the greatest exactness, the phial was discharged several times; but the observer always felt himself shocked at the very instant of making the explosion. Upon this the gentlemen were fully satisfied, that through the whole length of this wire, which was 12,276 feet in length, the velocity of the electric power was instantaneous.

We have noticed the increased evaporation from liquids by means of electricity. The following experiment, which is commonly exhibited by lecturers on electricity, is usually considered of the same kind.

Stick a small piece of sealing-wax on the end of a wire, and set fire to it. Then put an electrical machine in motion, and present the wax just blown out at the distance of some inches from the prime conductor. A number of extremely fine filaments will immediately dart from the sealing wax to the conductor, on which they will be condensed into a kind of net-work, resembling wool.

If the wire with the sealing-wax be stuck into one of the holes of the conductor, and a piece of paper be presented at a moderate distance to the wax, just after it has been ignited, on setting the machine in motion, a network of wax will be formed on the paper. The same effect, but in a slighter degree, will be produced, if the paper be briskly rubbed with a piece of elastic gum, and the melting sealing-wax be held pretty near the paper immediately after rubbing.

If the paper thus painted, as it were, with sealing-wax, be gently warmed by holding the back of it to the fire, the wax will adhere to it, and the result of the experiment will thus be rendered permanent.

A beautiful experiment of the same nature is made Electricity illustrated by experiment.

To make camphor flow into ramifications.

Curious experiment of Professor Lichtenberg.

The electrophorus, that is, a plate of some resinous substance, as sulphur, rosin, gum-lac, &c., is first excited, either by rubbing or otherwise; then a piece of metal of any shape, at pleasure, as for instance, a three-legged compass, a piece of brass tube, or the like, is set upon the electrophorus, and to this piece of metal, so placed, a spark is given, of the electricity contrary to that of the plate; this done, the piece of metal is removed, by means of a stick of sealing-wax or other electric, and some powder of rosin, kept in a linen bag, is shaken upon the electrophorus; this powder will be found to fall about those points upon the plate, which the piece of metal touched, forming some radiated appearances, much like the common representations of flares; at the same time, upon the greatest part of the plate, that is, besides those flares, there is hardly any powder at all. Now, it is to be remarked, that if the plate be excited negatively, and the spark given to the metal set upon it is positive, the appearance will be as above described; but if, on the contrary, the plate is positive and the spark is negative, then the powder of rosin will be found to fall upon those parts of the plate which in the other case is left uncovered, and to leave the flares clean; in short, it will do just the reverse of what it did in the other case; or, in other words, the powder of rosin will be attracted by those parts only of the electrophorus which are electrified positively.

The configurations produced in the above experiment of M. Lichtenberg appeared to curious that they were soon imitated by various electricians, particularly by Mr Cavallo and the Reverend Abraham Benney, inventor of the doubler. The directions given by this last gentleman are as follows.

To make red figures, take a pound of rasped Brazil wood; put it into a kettle with as much water as will cover it, or rather more; also put in about an ounce of gum arabic and a lump of alum about as big as a large nut; let it boil about two hours, or till the water is strongly coloured; strain off the extract into a broad dish, and set it in an iron oven, where it is to remain till all the water be evaporated, which with me was effected in about twelve hours; but this depends on the heat of the oven, which should not be so hot as to endanger its burning. Sometimes I have boiled the strained extract till it was considerably infusified before it was placed in the oven, that it might be sooner dry.

When it is quite dry but not burnt, scrape it out of the dish, and grind it in a mortar till it be finely pulverized. In doing this, it is proper to cover the mortar with a cloth, having a hole through to prevent the powder from flying away and offending the nose, and also to do it out of doors if the weather be dry and calm, that the air may carry away the powder necessarily escaping, and which otherwise is very disagreeable.

When ground fine, let it be sifted through muslin or a fine hair-sieve, returning the coarser part into the mortar to be ground again. When the grinding and sifting are finished, the powder is ready for use. The resinous plate I have mostly used was composed of five pounds of rosin, half a pound of bees-wax, and two ounces of lamp-black, melted together, and poured upon a board fifteen inches square, with ribs upon the edges at least half an inch high, to confine the composition whilst fluid; thus the resinous plate was half an inch thick, which is better than a thinner plate, the figures being more distinct. After the composition is cold, it will be found covered with small blisters, which may be taken out by holding the plate before the fire, till the surface be melted, then let it cool again, and upon holding it a second time to the fire, more blisters will appear; but by thus repeatedly heating and cooling the surface, it will at last become perfectly smooth. Some plates were made smaller, and the resinous composition confined to the form of an ellipse, a circle, or escutcheon, by a rim of tin half an inch broad, and fixed upon a board.

The next thing to be done is to prepare the paper, which is to be softened in water, either by laying the pieces upon each other in a vessel of cold water, or first pouring a little hot water upon the bottom of a large dish, then laying upon it a piece of paper, so that one edge of the paper may lie over the edge of the dish, to remain dry; that it may afterwards be more conveniently taken up. Then pour more hot water upon its upper surface. Upon this place another piece in the same manner, again pouring on more water, and thus proceed till all the pieces are laid in. By using hot water, the paper will be more softened in a few minutes than if it remains in cold water a whole day.

When the figures are to be made, the resinous plate must lie horizontally, whilst the electricity is communicated, if the experiment requires anything to be placed upon the plate; but it is convenient afterwards to hang it up in a vertical position whilst the powder is projected, lest too much powder should fall where it is not required.

A little of the powder may be taken between a finger and thumb, and projected by drawing it over a brush; or, which is better, a quantity of powder may be put into the bellows and blown towards the plate. When the figure is sufficiently covered with powder, let the plate be again laid horizontally upon a table; then take one of the softened papers out of the water by its dry edge, and lay it carefully between the leaves of a book, pressing the book together, and let it lie in this situation about half a minute. Then remove the paper to a dry place in the book, and press it again about the same time, which will generally be sufficient to take off the superfluous moisture. Then take up the paper by the two corners of its dry edge, and place the wet edge a little beyond the figure on the resinous plate, lowering the rest of the piece gradually till it covers the figure without sliding; then lay over it a piece of clean dry paper, and press it gently; let it remain a short time, and then rub it closer to the plate with a cloth, or, which is better, press it down by means of a wooden roller covered with cloth, taking care that the paper be not moved from its first position. When the paper is sufficiently pressed, let it be taken up by its dry Principles of dry edge, and laid upon the surface of a vessel of water Electricity with the printed side downwards; by this means the illustrated superfluous powder will sink in the water, and the by experiment figure will not be so liable afterwards to spread in the paper. After the paper has remained on the water during a few minutes, take it up and place it between the leaves of a book, removing it frequently to a dry place. If it be desired that the paper should be speedily dry, let the book-leaves in which it is to be placed be previously warmed, and by removing it to several places it will be dry much sooner than by holding it near a fire, and without drawing the paper crooked.

By the above process, it is obvious, that leather, calico, or linen, as well as paper, may be printed with these figures, and the effects of the diffusion of electricity upon a refrinous plate be exhibited to those who have not leisure or inclination to perform the experiments.

The figures represented in Plate CXCII. were formed much after Mr Bennet's method.

The apparatus used for making them consisted only of a common Leyden phial, and a plate of glass 15 inches square, covered on one side with a varnish of gum-lac dissolved in spirit of wine, and several times laid over. The other side is covered with tin-foil laid on with common paste. When it is to be used, the glass plate is put upon a metallic stand, with the tin-foil side laid undermost; the phial is to be charged, and the knob drawn over the varnished side. Thus any kind of figure may be drawn, or letters made, as represented in the plate; and from every figure beautiful ramifications will proceed, longer or shorter according to the strength of the charge. On some occasions, however, the charge may be too strong, particularly where we wish to represent letters, so that the whole will be blended into one confused mass. The round figures are formed by placing metallic rings or plates upon the electrical plate; and then giving them a spark from the electrified bottle, or sending a shock through them. The figures may be rendered permanent by blowing off the loose chalk, and clapping on a piece of black-sized paper upon them; or if they are wanted of another colour, they may easily be obtained by means of lake, vermilion, rose-pink, or any of the ordinary colours ground very fine. The easiest way of applying them seems to be by a barber's puff bellows.

We shall conclude this part of our article with noticing the effects produced by electricity on magnetic needles.

These may be stated in the following proposition.

An electric shock communicates a magnetic power to needles, and frequently reverses or destroys that polarity.

By electricity Dr Franklin frequently gave polarity to needles and reversed them at pleasure. A shock from four large jars, sent through a fine sewing needle, he says, gave it polarity, so that it would traverse when laid on water. What is most remarkable in these electrical experiments upon magnets is, that if the needle, when it was struck, lay east and west, the end which was entered by the electric blast pointed north; but that if it lay north and south, the end which lay to wards the north would continue to point north, whether the fire entered at that end or the contrary; though he imagined that a stronger stroke would have reversed the poles even in that situation, an effect which had been known to have been produced by lightning. He also observed, that the polarity was strongest when the needle was struck lying north and south, and weakest when it lay east and west. He takes notice that, in these experiments the needle, in some cases, would be finely blued like the spring of a watch, by the electric flame; in which case, the colour given by a flash from two jars only might be wiped off, but that a flash from four jars fixed it, and frequently melted the needles.

The jars which the doctor used held seven or eight gallons, and were coated and lined with tin-foil.

Dr Van Marum made several experiments on communicating polarity to needles with his very powerful machine. He and his coadjutor tried to give polarity to needles made of watch springs from three to five inches in length, and likewise to steel bars nine inches long, from a quarter of an inch to half an inch broad, and about a line in thickness. The result was, that when the bar or needle was placed horizontally in the magnetic meridian, whichever way the shock entered, the end of the bar that stood toward the north acquired the north polarity, or the power of turning towards the north when freely suspended, and the opposite end acquired the south. If the bar, before it received the shock, had some polarity, and was placed with its poles contrary to the usual direction, then its natural polarity was always diminished, and often reversed; so that the extremity of it, which in receiving the shock looked towards the north, became the north pole, &c.

When the bar or needle was struck standing perpendicularly, its lowest end became the north pole in any case, even when the bar had some magnetism before, and was placed with the south pole downwards. Ceteris paribus, the bars seemed to acquire an equal degree of magnetic power, whether they were struck whilst standing horizontally in the magnetic meridian, or perpendicular to the horizon.

When the bar or needle was placed in the magnetic equator, whichever way the shock entered, it never gave it any magnetism; but if the shock was given through its width, then the needle acquired a considerable degree of magnetism, and the end which lay towards the west became the north pole, and the other end the south pole.

If a needle or bar, already magnetic, or a real magnet, was struck in any direction, its power was always diminished. For this experiment, they tried considerably large bars, one being 7.08 inches long, 0.26 broad, and 0.05 thick.

When the shock was so strong in proportion to the size of the needle, as to render it hot, then the needle generally acquired no magnetism at all, or very little.

These experiments were made with the extraordinary power of a battery composed of 135 phials, containing among them about 130 square feet of coated surface. PART IV.

THEORY OF ELECTRICITY.

CHAP. I. A Concise View of the Principal Theories of Electricity.

SECT. I. Of the Theories of Electricity before the Time of Franklin.

The first electricians supposed, that electrical attraction was performed by means of unctuous effluvia emitted by the excited electric. These were supposed to attach themselves to all bodies, and to carry back with them those which were not too heavy. For in that age of philosophy all effluvia were supposed to return to the body from which they had been emitted, since no person could otherwise account for the substance not being sensibly wasted by the constant emission. When these light bodies, on which the unctuous effluvia had fastened, had arrived again at the excited electric, a fresh emission of the effluvia was supposed to carry them back again. But this effect of the effluvia was not thought of till electrical repulsion had been sufficiently observed.

When the Newtonian philosophy had made some progress, and the extreme subtilty of light, and other effluvia of bodies, was demonstrated, so that philosophers were under no apprehension of bodies being wasted by continual emission, the doctrine of the return of effluvia was universally given up, as being no longer necessary; and they were obliged to acquiesce in the unknown doctrines of attraction and repulsion, as natural properties of certain bodies, the unknown cause of which they scarcely attempted to explain.

Early in the 18th century, M. du Faye discovered that there were two states of electricity, or, as he supposed, two different kinds of electricity, produced when different electrics were excited. "Chance (says he) has thrown in my way a principle, which calls a new light upon the subject of electricity. The principle is, that there are two distinct kinds of electricity, very different from one another; one of which I call vitreous, the other resinous electricity. The first is that of glass, rock crystal, precious stones, hairs of animals, wool, and many other bodies. The second is that of amber, copal, gum-lac, silk, thread, paper, and a vast number of other substances. The characteristics of these two electricities are, that they repel themselves, and attract each other. Thus a body possessed of the vitreous electricity, repels all other bodies possessed of the vitreous, and on the contrary, attracts all those possessed of the resinous electricity. The resinous also repels the resinous, and attracts the vitreous. From this principle one may easily deduce the explanation of a great number of other phenomena; and it is probable that this truth will lead us to the discovery of many other things."

This discovery of M. du Faye was the origin of a theory of electricity, which is commonly called the theory of two fluids, and which we shall presently consider more at length.

Hitherto attraction and repulsion were the only electrical phenomena which had been observed; and to the explanation of these, the above general theories appeared sufficiently competent. But when electricity began to show itself in a greater variety of appearances, and to make itself sensible to the smell, the sight, the touch and the hearing; when bodies were not only attracted and repelled, but made to emit strong sparks of fire, attended with a considerable noise, a painful sensation, and a strong phosphorical smell, electricians were obliged to make their systems more complex, in proportion as the facts accumulated. It was then generally supposed that the electric power, which now began to assume the name of the electric fluid, was the same with the chemical principle of fire; though some thought it was a fluid sui generis, which very much resembled that of fire; and others, with M. Boulanger at their head, thought that the electric fluid was nothing more than the finer parts of the atmosphere, which crowded upon the surfaces of electric bodies, when the grosser parts had been driven away by the friction of the rubber.

During this time, it was imagined, that the electric matter was produced from the electric body by friction; but by a discovery of Dr Watson's, it became evident to universally believed, that the glass globes and tubes, came from fevered only to set the fluid in motion, and by no means the earth, to produce it. He was led to this discovery by observing, that, upon rubbing the glass tube, while he was standing upon cakes of wax or rosin (in order, as he expected, to prevent any discharge of the electric matter upon the floor), the power was, contrary to his expectation, so much lessened, that no snapping could be observed upon another person's touching any part of his body; but that, if a person not electrified held his hand near the tube while it was rubbed, the snapping was very sensible. The event was the same when the globe was whirled in similar circumstances. For, if the man who turned the wheel, and who, together with the machine, was suspended upon silk, touched the floor with one foot, the electric fire appeared upon the conductor; but if he kept himself free from any communication with the floor, little or no fire was produced.—He observed, that only a spark or two would appear between his hand and the insulated machine, unless he at the same time formed a communication between the conductor and the floor; but that then there was a constant and copious flow of the electric matter observed between them. From these, and some other experiments of a similar kind, the Doctor discovered what he called the complete circulation of the electric matter. When he found, that, by cutting off the communication of the glass globe with the floor, all electric operations were stopped, he concluded, that the electric fluid was conveyed from the floor to the rubber, Theory of rubber, and from thence to the globe. For the same reason, feeling the rubber, or the man who had a communication with it, gave no sparks but when the conductor was connected with the floor, he as naturally concluded, that the globe was supplied from the conductor, as he had before concluded that it was supplied from the rubber. From all this he was at last led to form a new theory of electricity, namely, that, in electric operations, there was both an afflux of electric matter to the globe and the conductor, and likewise an efflux of the same electric matter from them. Finding that a piece of leaf silver was suspended between a plate electrified by the conductor, and another communicating with the floor, he reasons from it in the following manner: "No body can be suspended in equilibrium but by the joint action of two different directions of power; so here the blast of electric ether from the floor setting through it, drives the silver towards the plate electrified. We find from hence, likewise, that the draught of electric ether from the floor is always in proportion to the quantity thrown by the globe over the gun barrel (the prime conductor at that time made use of), or the equilibrium by which the silver is suspended could not be maintained." Some time after, however, the Doctor retracted this opinion concerning the afflux and efflux, and supposed that all the electric phenomena might be accounted for from the excess or diminution of the quantity of electric matter contained in different bodies. This is the theory that was more fully explained by Franklin. It has been disputed whether Dr Watson or Franklin were the original contriver of this theory. It is possible that Watson may have formed the idea independently of Franklin; but certainly to this latter able and acute philosopher is due the merit of having framed and applied the hypothesis of positive and negative electricity, which, with some modification has been since almost universally adopted.

One great difficulty with which the first electricians were embarrassed, was to ascertain the direction of the fluid. At first, all electric powers, as we have already observed, were supposed to reside in the excited globe or glass tube. The electric spark therefore was imagined to proceed from the electrified body towards any conductor that was presented to it. It was never imagined that there could be any difference in this respect, whether it was amber, glass, sealing wax, or anything else that was excited. This progress of the electric matter was thought to be quite evident to the senses; and therefore the observation of electric appearances at an insulated rubber occasioned the greatest astonishment.—In this case, the current could not be supposed to flow both from the rubber and the conductor, and yet the first appearances were the same. To provide a supply of the electric matter, therefore, philosophers were obliged to suppose, that notwithstanding appearances were in both cases much the same, the electric fluid was really emitted in one case by the electrified body, and received by it in the other. But now being obliged to give up the evidence from sight for the manner of its progress, they were at a loss, whether, in the usual method of electrifying by excited glass, the fluid proceeded from the rubber to the conductor, or from the conductor to the rubber. It was, however, soon found, that the electricity at the rubber was the reverse of that at the conductor, and in all respects the same with that which had before been produced by the friction of sealing wax, sulphur, rosin, &c. Seeing, therefore, that both the electricities were produced at the same time, by one and the same electric, and by the same friction, all philosophers were naturally led to conclude, that both were modifications of one fluid; though in what manner that fluid was modified throughout the immense variety of electric phenomena, was a matter not easy to be determined.

On this subject the Abbé Nollet adopted the doctrine of afflux and efflux already mentioned. He supposed, that, in all electrical operations, the fluid is thrown into two opposite motions; that the afflux of this matter drives all light bodies before it by impulse upon the electrified body, and its efflux carries them back again. He was, however, very much embarrassed in accounting for facts where both these currents must be considered; as in the quick alternate attraction and repulsion of light bodies by an excited glass tube, or other excited electric. To obviate this difficulty, he supposes that every excited electric, and likewise every body to which electricity is communicated, has two orders of pores, one for the emission of the effluvia, and another for the reception of them. M. de Tour improved upon Nollet's hypothesis, and supposed that there is a difference between the affluent and effluent current; and that the particles of the fluid are thrown into vibrations of different qualities, which makes one of these currents more copious than the other, according as sulphur or glass is used. It is impossible, however, that suppositions so very arbitrary could be at all satisfactory, or received as proper explanations of the electric phenomena.

About this time the Leyden phial was discovered; and the extraordinary effects of it rendered the inquiries into the nature of the electric fluid much more general than before. It would be tedious, and indeed impossible, to give an account of all the theories which were now invented. One of the most remarkable was that of Mr Wilson. According to this gentleman, the Mr Wilson's chief agent in all the operations of electricity, is Sir Isaac Newton's ether; which is more or less dense in all bodies in proportion to the smallness of their pores, except that it is much denser in sulphureous and unctuous bodies. To this ether are ascribed the principal phenomena of attraction and repulsion: the light, the sulphureous or rather phosphorescent smell with which violent electricity is always attended, and other sensible qualities, are ascribed to the groser particles of bodies driven from them by the forcible action of this ether. He also endeavours to explain many electrical phenomena by means of a subtile medium at the surface of all bodies; which is the cause of the refraction and reflection of the rays of light, and also refits the entrance and exit of this ether. This medium, he says, extends to a small distance from the body, and is of the same nature with what is called the electric fluid. On the surface of conductors this medium is rare, and easily admits the passage of the electric fluid; whereas, on the surface of electrics, it is dense and resists it. The same medium is rarefied by heat, which thus changes conductors into non-conductors. By far the greater number of philosophers, however, rejected the opinion of Mr Wilson; and as they neither chose to allow the electric fluid to be fire nor ether, they were obliged to own... According to this theory, all the operations of electricity depend upon one fluid *sui generis*, extremely subtle and elastic. Between the particles of this fluid there subsists a very strong repulsion with regard to each other, and as strong an attraction with regard to other matter. Thus, according to Dr Franklin's hypothesis, one quantity of electric matter will repel another quantity of the same, but will attract and be attracted by any terrestrial matter that happens to be near it. The pores of all bodies are supposed to be full of this subtle fluid; and when its equilibrium is not disturbed, that is, when there is in any body neither more nor less than its natural share, or than that quantity which it is capable of retaining by its own attraction, the fluid does not manifest itself to our senses. The action of the rubber upon an electric disturbs this equilibrium, occasioning a deficiency of the fluid in one place, and a redundancy of it in another. This equilibrium being forcibly disturbed, the mutual repulsion of the particles of the fluid is necessarily exerted to restore it.

If two bodies be both of them overcharged, the electric atmospheres repel each other, and both the bodies recede from one another to places where the fluid is less dense. For as there is supposed to be a mutual attraction between all bodies and the electric fluid, such bodies as are electrified must go along with their atmospheres. If both the bodies are exhausted of their natural share of this fluid, they are both attracted by the denser fluid existing either in the atmosphere contiguous to them, or in other neighbouring bodies; which occasions them still to recede from one another as if they were overcharged.

This is the Franklinian doctrine concerning the cause of electric attraction and repulsion; but it is evident, that the reason just now given why bodies negatively attracted ought to repel one another, is by no means satisfactory. Dr Franklin himself had framed his hypothesis before he knew that bodies negatively electrified would repel one another; and when he came afterwards to learn it, he was surprised, and acknowledged that he could not satisfactorily account for it. Other philosophers therefore invented different solutions of this difficulty, of which that above mentioned is one. But by some this was rejected. They said, that as the denser electric fluid, surrounding two bodies negatively electrified, acts equally on all sides of those bodies, it cannot occasion their repulsion. The repulsion, according to them, is owing rather to an accumulation of the electric on the surfaces of the two bodies; which accumulation is produced by the attraction, and the difficulty the fluid finds in entering them. This difficulty is supposed chiefly to be owing to the air on the surface of bodies, which Dr Priestley says is probably a little condensed there. This he deduces from an experiment of Mr Wilson, corrected by Mr Canton. The experiment was made in order to observe the course of the electric light through a Torricellian tube. A singular appearance of light was observed upon the surface of the quicksilver, at which the fluid was supposed to enter. Mr Wilson supposed that this was owing to a subtile medium spread over the surface of the quicksilver, and which prevented the easy entrance of the electric fluid. But this was afterwards discovered by Mr Canton to be owing to a small quantity of air which had been left in the tube. It is plain, however, that as the attraction is equal all round, and likewise the difficulty with which the fluid penetrates the air, bodies negatively electrified ought not to repel one another on this supposition more than the former. Nay, they ought to attract each other; because, in the place of contact, the resistance of the air would be taken off, and the electric fluid could come from all other quarters by the attraction of the bodies.

This theory is evidently no solution of the difficulty; seeing it is only explaining one fact by another, which requires explanation at least as much as the first. We shall see hereafter how this difficulty may be explained.

What gave the greatest reputation to Dr Franklin's theory, was the easy solution which it afforded of the phenomena of the Leyden phial. The fluid is supposed to move with the greatest ease in bodies which are conductors, but with extreme difficulty in *electric per se* Leyden phials, which glass is absolutely impermeable to it. It is moreover supposed, that all electrics, and particularly glass, on account of the smallness of their pores, do at all times contain an exceeding great, and always an equal quantity of this fluid; so that no more can be thrown into any one part of any electric substance, except the same quantity go out at another, and the gain be exactly equal to the loss. These things being previously supposed, the phenomena of charging and discharging a plate of glass admit of an easy solution. In the usual manner of electrifying by a smooth glass globe, all the electric matter is supplied by the rubber from all the bodies which communicate with it. If it be made to communicate with nothing but one of the coatings of a plate of glass, while the conductor communicates with the other, that side of the glass which communicates with the rubber must necessarily be exhausted in order to supply the conductor, which must convey the whole of it to the side with which it communicates. By this operation, therefore, the electric fluid becomes almost entirely exhausted on one side of the plate, while it is as much accumulated on the other; and the discharge is made by the electric fluid rushing, as soon as an opportunity is given it by means of proper conductors, from the side which was overloaded to that which is exhausted.

It is not, however, necessary to this theory, that the very same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the space of an equal portion of the natural quantity of fluid belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue till the exhausted side of the glass is supplied by the fluid naturally. Theory of rally existing in the conductors contiguous to it. In Electricity, this case, the motion of the electric fluid, in an explosion, will rather resemble the vibration of the air in founds, than a current of it in winds.

It will soon be acknowledged (says Dr Priestley), that while the substance of the glass is supposed to contain as much as it can possibly hold of the electric fluid, no part of it can be forced into one of the sides, without obliging an equal quantity to quit the other side; but it may be thought a difficulty upon this hypothesis, that one of the sides of a glass plate cannot be exhausted, without the other receiving more than its natural share; particularly, as the particles of this fluid are supposed to be repulsive of one another. But it must be considered, that the attraction of the glass is sufficient to retain even the large quantity of electric fluid which is natural to it, against all attempts to withdraw it, unless that eager attraction can be satisfied by the admission of an equal quantity from some other quarter. When this opportunity of a supply is given, by connecting one of the coatings with the rubber, and the other with the conductor, the two attempts to introduce more of the fluids into one of the sides are made, in a manner, at the same instant. The action of the rubber tends to disturb the equilibrium of the fluid in the glass; and no sooner has a spark quitted one of the sides, to go to the rubber, than it is supplied by the conductor on the other; and the difficulty with which these additional particles move in the substance of the glass, effectually prevents its reaching the opposite exhausted side. It is not said, however, but that either side of the glass may give or receive a small quantity of the electric fluid, without altering the quantity of the opposite side. It is only a very considerable part of the charge that is meant, when one side is said to be filled while the other is exhausted.

It is a little remarkable, adds Dr Priestley, that the electric fluid, in this and in every other hypothesis, should so much resemble the ether of Sir Isaac Newton in some respects, and yet differ from it so essentially in others. The electric fluid is supposed to be, like ether, extremely subtle and elastic, that is, repulsive of itself; but instead of being, like the ether, repelled by all other matter, it is strongly attracted by it; so that, far from being, like the ether, rarer in the small than in the large pores of bodies, rarer within the bodies than at their surfaces, and rarer at their surfaces than at any distance from them; it must be denser in small than in large pores, denser within the substance of bodies than at their surfaces, and denser at their surfaces than at a distance from them.

To account for the attraction of light bodies, and other electrical appearances, in air of the same density with the common atmosphere, when glass (which is supposed to be impermeable to electricity) is interposed; it is conceived, that the addition or subtraction of the electric fluid, by the action of the excited electric on one side of the glass, occasions, as in the experiment of the Leyden phial, a subtraction or addition of the fluid on the opposite side. The state of the fluid, therefore, on the opposite side being altered, all light bodies within the sphere of its action must be affected in the very same manner as if the effluvia of the excited electric had actually penetrated the glass, according to the opinions of all electricians before Dr Franklin.

This hypothesis has been greatly improved by M. Æpinus of St Peterburgh, and by the Hon. Henry Cavendish; and we shall now proceed to an illustration of the theory as given by these gentlemen.

Theory of Æpinus.

Electrical phenomena are produced by a fluid of a peculiar nature, which we call the Electric Fluid, which has the following properties.

1. Its particles repel each other with a force increasing as the distances decrease.

2. Its particles attract the particles of all other matter with a force increasing as the distances decrease, and this attraction is mutual.

3. The Electric Fluid by reason of its extreme subtlety is capable of penetrating other bodies, but all bodies are not penetrated by it with equal facility. In those bodies which we call non-electrics, such as metals and water, it moves very readily; but in those bodies which have been called electrics per se, such as glass, &c., it either does not move at all, or moves with great difficulty.

4. Every body has a certain quantity of electric fluid which is proper to it, and may therefore be called its natural quantity: this quantity is proportional to the mass.

5. We say that a body is electrified positively when the quantity of electric fluid which it has in any way received is greater than its natural quantity; and when that quantity is less than its natural quantity, we say that the body is electrified negatively.

6. The phenomena which depend on the action of the electric fluid may be reduced to two classes; the first comprehending the cases in which the fluid removes from one body into another which has less of it; the other those in which the bodies containing the fluid are in motion, so as to approach or recede from each other, or so as to attract and repel each other.

Such is the hypothesis of M. Æpinus; let us now inquire what consequences may be drawn from it.

Let us suppose a body to contain a certain quantity of the electric fluid, and let us examine the state of a particle of the fluid, as P, near the surface of the body. There is a mutual attraction between the particle P, and the particles of matter in the body; and there is a mutual repulsion between it and the other particles of electric fluid in the body. The whole attracting force may be equal or unequal to the whole repulsive force. If they be equal, P is in equilibrium, and has no tendency to motion.

Now let us suppose the body to have received a quantity of fluid over and above its natural quantity; i.e., let the body be electrified positively. As, while the body was in its natural state, the attractive and repulsive forces were in equilibrium, the increase of fluid will augment the repulsive force, which will now exceed the attractive force, and the particle P will be repelled towards that surface to which it is nearest, till it at length quits the body. The repulsive power will continue to act upon other particles, which will be successively pushed nearer the surface, so as to produce a constant efflux of the fluid till the equilibrium is re-established, Let us now conceive that the body has lost a quantity of electric fluid, or that it is electrified negatively. The repulsive force of the fluid upon the particle P will then be less than the attractive force of the matter contained in the body or the same particle, this attraction will begin to act, and the particle will move nearer the centre. The attraction continuing to act, particles near the surface, and those of contiguous bodies will successively move towards the centre of the body; or a continual influx of fluid will take place till the equilibrium is restored.

**Definition.**—When a body contains its natural quantity of electric fluid, we shall say that it is saturated.

It will be convenient for us to have general expressions for these several states of a body, in order to estimate the forces.

Let Q represent the natural quantity of fluid,

\(a\) the attractive force of the other matter in the body, which we shall hereafter call simply the matter,

\(r\) the repulsive force of the fluid; and

\(f\) the redundant or deficient fluid.

Then in the case in which a body is saturated, \(a = r\) will represent the degree of force with which the particle P is attracted; and \(r - a\) the force with which it is repelled. But here \(a = r\); consequently \(a = r\) and \(r - a = 0\).

But let the quantity \(f\) be added to Q, and uniformly distributed through the body; the fluid will now be \(Q + f\). As we must admit the repulsive force to be proportional to the quantity of fluid, we shall have

\[ \frac{Q}{Q} : \frac{Q + f}{Q} = \frac{r}{Q} : \frac{fr}{Q}, \quad \text{or} \quad r + \frac{fr}{Q}. \]

This quantity will represent the force with which P is repelled by the whole fluid of the body. But it is also attracted by the matter of the body, with the force \(a\); the whole force exerted on P will therefore be \(a - r - \frac{fr}{Q}\); but \(a - r = 0\): the whole action exerted on P is therefore \(\frac{fr}{Q}\), or the force with which the particle P is repelled, is \(\frac{fr}{Q}\).

To conceive this more readily, we are to remember that when the quantity of fluid \(= Q\), P is in equilibrium; it will therefore be necessary only to consider the action of the superabundant fluid \(f\). Then to find the repulsive force of this, we say \(Q : f = r : \frac{fr}{Q}\) as before; but to this we must affix the sign —, as we must consider repulsive forces as negative, and attractive as positive. The particle P then being repelled with this force \(\frac{fr}{Q}\), it will quit the body unless it be opposed by some obstacle, and the repelling force continuing to act on other particles, an efflux of fluid will be produced.

The force \(\frac{fr}{Q}\) will however be continually diminishing, but will not entirely cease till \(f = 0\).

Now let the quantity of fluid \(f\) be subtracted from Theory of Electricity. There will now be a continual efflux of fluid from the conductor towards the cushion, and the conductor will, in its turn, be electrified negatively. In this case, if we present a fine metallic point to the conductor, there will issue from the point a luminous pencil, which is produced by the efflux of fluid from the point to the conductor, in order to restore the equilibrium.

We have hitherto considered the fluid as uniformly diffused through the body. But it will often happen that there will be a redundancy of fluid in one part of the body, while there is at the same time a deficiency in another part. In order to simplify our formula, we shall suppose the body BC (fig. 96) divided into two equal parts, AB, AC, and that the fluid in AB exceeds its natural quantity, while that in AC is less than the same quantity, the proportion of the fluid acquired on one side to that lost on the other being variable at pleasure. Let us examine the situation of two particles P, p, placed towards the two extremities.

Let Q represent the quantity of fluid necessary for the saturation of AB or AC, a = the attraction of the whole matter in AB for the particle P or p, r = the repulsion of the whole fluid uniformly distributed in AB on the same particle, r' = the repulsion of an equal quantity of fluid in AC on the same particle, f = the quantity of redundant fluid in AB, g = the deficient quantity in AC.

Now the force by which the particle P or p is attracted by the matter of BC when saturated, will be \( \frac{a-r-r'}{Q} \), which when the body is in its natural state will be equal to \( \frac{a}{Q} \). But AB contains the redundant fluid \( f \), and AC the deficient fluid \( g \). The whole action exerted must therefore be \( \frac{(Q-f)\times r}{Q} - \frac{(Q-g)\times r'}{Q} \). But \( a-r-r'=0 \); therefore the whole action is \( \frac{gr'-fr}{Q} \), or rather, since \( r \) is greater than \( r' \), \( \frac{fr-gr'}{Q} \), which will represent the force by which the particle P is repelled. In the same manner, \( \frac{gr-fr'}{Q} \) will represent the force by which \( p \) is attracted.

Now, let us suppose a particle \( p' \) in the middle of the body BC; while the body is saturated, it will be in equilibrium; but as the one half of the body AB contains the redundant fluid \( f \), and the half AC the deficient fluid \( g \), the particle \( p' \) will be repelled in the direction AC by the force \( \frac{fr}{Q} \). But it is repelled in the direction AB by the force \( \frac{gr}{Q} \); therefore the whole repulsive force by which it is impelled in the direction AC will be \( \frac{fr+gr}{Q} \), or \( \frac{f+g\times r}{Q} \).

From what we have said above, it appears that so will be prolonged as there is a redundancy of fluid in AB, and a deficiency in AC, the redundant fluid has a tendency there to flow from A to C; and if the body be a perfect conductor, or such as is permeable to the fluid, its state cannot be permanent till the fluid is uniformly distributed between the two halves, unless it is acted on by some external force. But in a non-conductor, or perfect electric, this state may subsist, and it will be continued for a longer or shorter time, in proportion as the electric be more or less perfect.

If we had supposed the part AC to be overcharged, instead of AB, P would have been repelled with a stronger force, which would be represented by \( \frac{f\times r+r'}{Q} \), which is evidently greater than \( \frac{fr-gr'}{Q} \), the repulsive force in the first case. The particle \( p \) is also less attracted than before, when AB is undercharged instead of AC.

The above remarks will equally apply to the case of two conducting bodies AB and CD, fig. 98, separated by an electric, Z.

It is proper to observe that the quantities \( f \) and \( g \), were indefinite in the above reasoning. Their value may be such that the tendency to influx or efflux may cease, or may be reversed; for supposing \( gr'-fr=0 \), or \( g : f = r : r' \); and we shall have \( \frac{fr}{r'} \). In this case the attraction of the redundant matter balances the repulsion of the redundant fluid, and P is neither attracted nor repelled. Hence we have this important fact, that a body may be neutral, even where it is redundant or deficient.

When one extremity of the body is thus rendered inactive, the state of the other extremity is changed.

To find this state we must put \( \frac{fr}{r'} \) in place of its equal \( g \), in the formula \( \frac{gr-fr'}{Q} \); and we shall have \( \frac{f\times(r^2-r'^2)}{Qr} \).

Again the forces may be so balanced, that there shall be no tendency to influx at C, fig. 96. Make \( g = \frac{fr}{r'} \), which expresses the action at C. The action at B, the other end, will be obtained by putting \( \frac{fr}{r'} \) in place of \( g \) in the formula \( \frac{fr-gr'}{Q} \) as before, and the result \( \frac{f\times(r^2-r'^2)}{Qr} \), will express the repelling force at B.

In order, the better to conceive the relative effects in each of the above cases, we must observe that the repulsion of the part AB on the particle P must increase in proportion as the quantity of additional fluid acquired by AB is greater. On the other hand the attraction of the part AC for the same particle will increase according as the quantity of fluid subtracted from AC is greater. Now, as we have supposed the quantities of fluid in the two parts variable, we may suppose a case to happen, in which, for instance, the quantity lost by AC may be such that the excess of its attraction on P thence resulting, may exactly counterbalance the diminished attraction arising from its great distance, compared to the repulsion of the part AB on the same Theory of same particle. In this case, P will remain immovable.

If, on the contrary, the quantity of fluid lost by AC be not sufficient to compensate for the greater distance, the repulsion of AB will prevail over the attraction of AC, and the particle P will quit the body.

The particle ρ will also undergo certain changes in these different cases. If the particle P remain immovable, for instance, the particle ρ will have a progressive motion towards the body A, since this is near the part AC of which the attractive force in this case exceeds the repulsive force of AB. If the particle P has already a tendency towards the body A, the particle ρ will for a still stronger reason be attracted towards A.

In general, according to the different degrees of force exerted by the two parts of the body, it will happen that the fluid will be attracted and repelled on both sides by turns, or it will be attracted on one side, while it is repelled on the other, and v. v., or lastly, it may remain immovable on one side, while it is attracted or repelled on the other.

If we suppose that the redundancy of fluid in AB is exactly equal to the deficiency in AC, then the particle ρ will have a tendency to penetrate the body A, while the particle P will be repelled by it.

To prove this, let us suppose that the parts AB, AC act by turns on the particle ρ placed at a determinate distance; and let us conceive the repulsive force of the part AB to be concentrated in a determinate point, while the attractive force of the part AC must be supposed concentrated in a corresponding point on the other side. For, whatever be the law, in proportion to the distance which the repulsion of the particles of the electric fluid follows; the attraction of the particles of matter in the electrified body ought to follow the same law: since, without this, there could be no counterpoise between the attraction and repulsion of the particles in the natural state of the body. It follows then, that the attraction exerted by AC upon the particle ρ must be equal, in the present case, to the repulsion of AB on the same particle. Since, on one side, the particle is repelled by AB by reason of the excess of fluid in that part, and on the other it is attracted by AC by reason of the quantity of matter in that part, and which is proportional to the quantity of fluid which is supposed to have passed into AB. In the present case, therefore, where the particle ρ is nearer to AC than to AB, the attraction will prevail over the repulsion, and the particle will penetrate to AB, and pass through it to the body A.

In the same manner we might prove that the particle P would be repelled from A.

The equilibrium between the forces of the parts AB, AC being disturbed, it is clear that there will be an attempt to restore it, so that a portion of the redundant fluid in AB will pass into AC, till the body be brought back to its natural state. The return to this state will be more or less slow, according as the body is a more or less perfect electric; but if it is a conductor the fluid will pervade it in an instant, and an equal distribution will immediately take place.

It has been stated that the fluid does not move with equal facility through all bodies, but that in moving through electrics it meets with more or less resistance. Theory of Electricity. It will be proper, before we proceed farther, to consider the nature of this resistance. It may either arise solely from the inertia of the particles of the fluid, which is the case in a perfect fluid; or it may resemble the resistance opposed by a parcel of grain to the descent of small shot through it, or the resistance of a plastic or ductile body, such as clay or lead, to the motion of a body through its pores. In the first case, any inequality of force, however small, is capable of producing a uniform distribution of the fluid, or at least such a distribution as will make the excess of the mutual attractions and repulsions equal to the degree of external force by which an unequal distribution may be kept up. But in the two last cases, before a particle of fluid can change its place, it must overcome the tenacity of the adjoining particles of the body, and consequently, when an unequal distribution has been produced by an external force, it will not be rendered equable by a removal or alteration of that force, but there will remain such an inequality of distribution, as will cause the want of equilibrium between the attractions and repulsions to be counterbalanced by the tenacity of the body.

From the different states of the particles P, ρ, as described in the above cases, we may conclude, that, during the return of a body to its natural state, the readiness with which the fluid flows from AB into AC must depend much on the nature of the surrounding bodies, and the greater or less facility with which these are pervaded by the electric fluid.

If the fluid is not uniformly distributed throughout every part of the body, or if, though there be a uniform distribution, the two parts of the body are unequal, we shall always obtain results analogous to those which have been given. There is an infinity of cases supposable, relative to the different states of AB and AC; but as each of these cases has a determinate relation to the most simple case, which we have been considering, it may always be reduced to this.

Let us suppose, for example, that the part AB is double, triple, &c. the part AC, and that the portion of fluid, which is superabundant in AB, is equal to that which is deficient in AC: If we conceive the particle ρ situated between these two parts, the point in which we must suppose the repulsive force of AB to be concentrated, will not be the same as that given in (315); but the point in which ρ must be placed that it may be attracted by AC and repelled by AB, will be between the centres of action of AB and AC, though not at an equal distance from these parts. Then, in the case where ρ is nearer to the centre of action of AC than to that of AB, this particle will tend to penetrate into AC, while the particle P will be equally repelled from it.

Having thus examined the action between the particles of fluid moving in a body, and the particles of electrical matter in the same body, we shall proceed to consider bodies on the action of electrified bodies on each other.

Let there be two bodies, A and B, in their natural state.

Let M represent the common matter in A. m, the common matter in B. F₁, the fluid required to saturate A. F₂, the fluid required to saturate B. x, the mutual action between a particle of fluid and the correspondent matter. This action is represented Theory of ed by an unknown quantity, because it is indeterminate: varying with every change of distance.

As the actions of these bodies on each other are reciprocal, it will be sufficient here to consider how the body A is affected. There are three circumstances to be taken into consideration.

1. The particles of fluid in A attract the particles of matter in B with the force \( \propto \); so that the whole attraction of A on B will be the product of F' and m multiplied by \( \propto \) or \( \Gamma m \propto \); or

1st. F tends towards m with the force \( +Fm\propto \).

2. The particles of fluid in A repel the particles of fluid in B with the same force \( \propto \); so that the whole repulsion of A on B will be \( Ff\propto \); or

2nd. F tends to separate from f with the force \( -Ff\propto \).

3. The particles of matter in A are attracted by the particles of fluid in B with the same force \( \propto \), so that the whole attraction of B on A will be \( Mf\propto \); or,

3rd. M tends to approach f with the force \( +Mf\propto \).

The whole tendency of A to approach or to separate from B may therefore be represented by the symbol \( \propto \times Fm + Mf - Ff \). But as, from the hypothesis, the attraction of the particles of fluid in A for the particles of matter in B is equal to the repulsion between the particles of fluid in A and the particles of fluid in B, which are competent to the matter attracted by the fluid in A, the attraction \( Fm\propto \) is balanced by the repulsion \( Ff\propto \). We have, therefore, only to consider the remaining attraction, or the attraction of the matter in A for the fluid in B, or \( Mf\propto \). On the whole, therefore, A will move towards B, and, as all action is equal and contrary, B will move towards A with an equal force.

This would be the necessary consequence of the hypothesis, as it stands; but as we see no attraction between bodies in their natural state, there must be some defect in the hypothesis. To remedy this, Æpinus brings another repulsive force into play, and supposes that every particle of matter in A repels every particle of matter in B, as much as it is attracted by so much of the fluid in B as is necessary for its saturation. Now, therefore, the whole action exerted by B on A will be \( \propto \times Fm - Ff - Mm + Mf \), so that as \( Fm\propto \) is balanced by \( Ff\propto \), and \( Mm\propto \) by \( Mf\propto \), there will remain no excess on either side, and consequently the bodies will have no tendency to motion.

Great objection has been made to this additional part of M. Æpinus's hypothesis, and indeed Æpinus himself acknowledges, that this circumstance appeared to him hardly admissible; it seeming inconceivable that a particle in A shall repel a particle in B, or recede from it electrically, while it tends toward it by planetary gravitation. But more attentive consideration showed him, that there was nothing in it contrary to the observed analogy of natural operations. We see innumerable instances of inherent forces of attraction and repulsion; and nothing hinders us from referring this lately discovered power to the clas of primitive and fundamental powers of nature. Nor is it difficult to reconcile this repulsion with universal gravitation; for when bodies are in their natural state, the electric attractions and repulsions balance each other, and there is nothing to disturb the phenomena of planetary gravitation; and when they are not in their natural electrical state, it is Theory of fact that their gravitation is disturbed. Although we cannot conceive a body to have a tendency to another body, and at the same time a tendency from it, when we derive our notion of these tendencies entirely from our own consciousness of effort, nothing is more certain than that bodies exhibit at once the appearances which we endeavour to express by these words. We bring the north poles of two magnets near each other, and they recede from each other; if this be prevented by some obstacle, they press on this obstacle, and seem to endeavour to separate. If while they are in this state, we electrify one of them, we find that they will now approach each other; and so we have a distinct proof that both tendencies are in actual exertion by varying their distances, so that one or other force may prevail; or by placing a third body, which shall be affected by one but not the other, &c. We do not understand, nor can we conceive, how either force, or how gravity resides in a body. It must be granted, therefore, that this additional circumstance of Æpinus's hypothesis has nothing in it that is repugnant to the observed phenomena of nature.

In order to simplify the algebraic expressions which we employ in considering the actions of these bodies, we may remark, that, as in the natural state of the bodies they do not affect each other, we need only, in examining the actions of bodies not in their natural state, consider the action of the redundant fluid or the redundant matter in them, that is, the fluid or matter which is unsaturated: for we may consider an overcharged body as one which contains a quantity of saturated fluid, and a quantity of unsaturated fluid additional; and an undercharged body as one containing a quantity of saturated matter, and a quantity of unsaturated matter in addition.

Suppose two bodies A and B overcharged, or containing each a quantity of unsaturated fluid, which we shall call \( F' \) and \( f' \). Their mutual action on each other will be \( F'\times f' + \propto \), and it is evident from what was said before that this is a repulsion. Hence we have the following general proposition.

1. Two overcharged bodies repel each other with the force \( F'\times f' + \propto \).

Now let these bodies be undercharged, or contain each a quantity of unsaturated matter, \( M' \) and \( m' \). Their mutual action will now be \( M'\times m' + \propto \). This action is also repulsive, and hence

2. Two undercharged bodies repel each other with the force \( M'\times m' + \propto \).

Again, let one of the bodies A be overcharged or contain the unsaturated fluid \( F' \), and the other B undercharged, or contain the unsaturated matter \( m' \). Their mutual action will now be expressed by the symbol \( F'\times m' + \propto \), and will be attractive; or

3. Two bodies which are, one overcharged, and the other undercharged, attract each other with the force \( F'\times m' + \propto \).

Lastly, let one of the bodies be overcharged or undercharged, and the other in its natural state. We infer from the above formulæ, that they will neither attract nor repel each other, or that they will be neutral; for here either \( F' \) or \( f' \), or \( M' \) or \( m' \), one of the factors which made part of the above products, is wanting. This may be inferred also, independently of the formulæ, by con- Theory of Electricity.

Theorem of fidering that the redundant fluid or redundant matter in one body, is as much repelled or attracted by the fluid or matter in the other, as it is attracted or repelled by the matter or fluid in this other. Hence,

4. If of two bodies, one be in its natural state, they will neither attract nor repel each other.

The truth of the three first propositions will be evident from the experiments related in the last Part, Chap. I., where we found that bodies which were electrified both positively or both negatively, repelled each other, and that when one body was electrified positively and the other negatively, they attracted each other. But the last proposition seems contrary to the phenomena; and it certainly contradicts a part of the Franklinian doctrine, which maintains that there is an attraction between an electrified and a non-electrified body, we shall presently, however, demonstrate the truth of the proposition, but must now proceed in our explanation of Aepinus's theory.

Suppose the body BC, fig. 97, to be overcharged in the half AC, and undercharged in the half AB, and let us now represent the redundant fluid in the part AC by the symbol \( f' \), and the redundant matter in AB by \( m' \); let the body D near BC be overcharged with the redundant fluid \( F' \); let \( z \) and \( z' \) denote the force of action exerted on D at the distances of this body from the overcharged or undercharged parts of BC. Now D is repelled by AC, with the force \( F'f'z' \), but it is attracted by AB with the force \( F'm'z' \); on the whole, therefore, D will be attracted or repelled by BC, according as \( F'f'z' \) is greater or less than \( F'm'z' \), or (because F is common to both) as \( m'z' \) is greater or less than \( f'z' \). But this will depend on the proportion that \( f' \) bears to \( m' \), or \( z \) to \( z' \). Now, the former of these is regulated by many external circumstances which may tend to produce a greater or less redundancy or deficiency of fluid; and the latter depends on the law of electric action. Without inquiring at present into this law, it is sufficient to recollect that the action decreases with every increase of distance, and that the attraction and repulsion at the same distance are equal. Both, therefore, vary according to the same law, and \( z \) is always greater than \( z' \).

But the sensible action of BC on D, and (as action and reaction are equal and contrary) of D on BC, may vary with every new position of BC, and even in the same position.

1. Let us suppose that BC contains on the whole its natural quantity of fluid, but that part of it is taken from AB, and crowded into AC. This, which is a very common case in electricity, may be expressed in our symbolic manner by making \( f' = m' \). Now, in this case, \( F'f'z' \) is greater than \( F'm'z' \), as \( z \) is greater than \( z' \). A mutual repulsion will therefore take place between BC and D, and this may be expressed by \( F'f' \times (z - z') \).

2. If D were placed on the redundant side of BC, it is evident that the action would be reversed, and the above symbol will express the attraction between BC and D.

Again, if instead of supposing D to be overcharged, we make it undercharged, the actions will again be changed: in its present situation it will be repelled; on the opposite side of BC it will be attracted.

3. No action may be exerted between them; for the redundancy and deficiency in BC may be inversely proportional to the forces, or we may have \( f'm': z': z \). Now, multiplying extreme and mean terms, we have \( f'z = m'z' \), and again \( m' = \frac{f'z}{z'} \). In this case the actions counterbalance each other, and when D is at the present distance from the overcharged part AC, it is neither attracted nor repelled. D, and that part of BC that is contiguous to it, may both be overcharged, and yet BC may exert no action on D, or may be neutral with respect to it.

Now suppose D on the opposite side of BC; the effects will be different; for as \( m' = \frac{f'z}{z'} \), and \( m'z' \) is now become \( m'z \), and \( f'z \) is changed into \( f'z' \), the action on D will be expressed by \( F' \times \left( \frac{f'z}{z'} - \frac{f'z}{z} \right) = F'f' \times \frac{z - z'}{z} \); of course D will be attracted.

Again, we may have \( f' \) and \( m' \) so proportioned as that when D, which we suppose overcharged, is placed at the undercharged end of BC, it shall be neither attracted nor repelled, or that at this exact distance BC shall be neutral. In this case, \( m' = \frac{f'z}{z} \). But if D be on the opposite side of BC, it will be strongly repelled with the force \( F'f' \times \left( \frac{z - z'}{z} \right) \).

Hence we see that when the overcharged end of an electrified body becomes neutral with respect to another body that is also overcharged, the undercharged end strongly attracts that body; and when the undercharged end becomes neutral to the body, this is strongly repelled by the overcharged end, as we may deduce from this reasoning the following general conclusion.

When an electrified body is neutral at one end, it is rendered more active at the other.

One circumstance merits particular attention. In the above paragraphs, the neutrality of BC has been confined to a particular distance of the body D, it being required that \( m' \) should \( = \frac{f'z}{z'} \); let D be placed nearer to BC and both \( z \) and \( z' \) are increased. Their increase may be in the same proportion; or one may increase faster than another: in the former case, the value of \( \frac{z}{z'} \), remains the same, and the neutrality continues; in the latter, if \( z \) increases faster than \( z' \), \( f'z \) becomes greater than \( m'z' \), and D will be repelled: on the other hand, if \( z' \) increases faster than \( z \), D will be attracted. Let D be carried farther from the overcharged end of BC, and the effects will be reversed.

We have been supposing that D is overcharged throughout, but let us take two bodies AB, and CD, unequal fig. 98. AB being overcharged in u B, and undercharged in u A; and CD being overcharged in v D, and undercharged in v C.

In the first place, let us have the overcharged end of AB opposite the undercharged end of CD as in the figure. Let F and \( f' \) be the fluid natural to each, \( F' \) and \( f' \) the redundant fluid in u B, and v D, and \( M' \) and \( m' \) the deficient fluid in u A and v C. Let Z and Z' Theory of Electricity

Let \( Z \) denote the intensity of action exerted by a particle in \( u \) on a particle in \( v \); and let \( x \) and \( x' \) in like manner express the intensity of action of a particle in \( u \) on a particle in \( v \).

It will easily appear from the former examples that the action of \( CD \) on \( AB \) will be

\[ F'f'Z - Ff'Z' = M'm'x + M'f'x' \]

in which formula the attractions are denoted by \( + \), and the repulsions by \( - \).

The attractive or repulsive power will prevail according as the sum of the first and last terms in the numerator of the above fraction is greater or less than the sum of the two middle terms. Again the value of each term will vary with the quantity of redundant fluid or of redundant matter, and with the intensity of the electric action. As it would lead us into too long a discussion were we to notice the numerous varieties of effect, we shall only state the most simple case, as being the most frequent and most useful.

Let us suppose that the overcharged part of each body is as much redundant in fluid as the undercharged part is deficient; in which case we have \( F' = M' \) and \( f' = m' \). The action will now be expressed by the formula

\[ F'f'(Z - Z') - x + x' \]

It is evident that the external effect produced on \( AB \) must depend on the law of action; if \( Z + x' \) be greater than \( Z' + x \), \( AB \) will be attracted, but if \( Z + x' \) be less than \( Z' + x \), it will be repelled.

It will be a considerable relief to the imagination to express these abstract values by some sensible quantities, such as lines, and this may conveniently be done in the following manner. From a fixed point in a straight line, measure off portions respectively equal to \( BC \), \( BD \), \( AC \), and \( AD \), between those points of the bodies \( AB \), \( CD \), fig. 98, in which we suppose the forces of the redundant fluid and matter to be concentrated, and at the extremities of these portions erect ordinates proportional to these forces. Though the law of action be but imperfectly known, it will readily be seen of what kind the movements of the bodies will be. Thus in fig. 100, from \( C \) in the line \( CZ \), make \( CP = BC \); \( CQ = BD \); \( CR = AC \), and \( CT = AD \); and erect the ordinates \( Pp \), \( Qq \), \( Rr \), and \( Tt \). If the action of electricity be like other attractive and repulsive forces with which we are acquainted, that is, decreasing with an increase of distance, and more slowly as that distance becomes greater, the ordinates will be bounded by such a curve as \( PQRSTZ \), that will have its convexity towards the axis \( Cz \).

In our construction, the pair of ordinates \( Pp \), \( Qq \) are evidently equidistant with the pair \( Rr \), \( Tt \); as are \( Pp \), \( Rr \), with \( Qq \), \( Tt \). It is also clear that the sum of \( Pp \) and \( Tt \) is greater than the sum of \( Qq \) and \( Rr \). Bisect \( Cz \) in \( v \), and draw \( Vv \) perpendicular to it, cutting \( PT \) and \( QR \) in \( x \) and \( y \). Then \( xv \) is the half of \( Pp + Tt \), and \( yv \) is the half of \( Qq + Rr \). Again, \( Qm \) and \( Tn \) being drawn parallel to \( Cz \), it is evident that \( Pm \) is greater than \( Rr \), and in general, if any pair of ordinates be brought nearer to \( C \), their difference increases; and if two pairs be brought nearer to \( C \), the difference of the nearer pair will increase faster than that of the more remote.

To apply what has been stated,

1. When the overcharged end of \( AB \) is towards the undercharged end of \( CD \), \( AB \) is attracted, as \( Pp + Tt \) is greater than \( Qq + Rr \).

2. The nearer the bodies are brought, the more the attraction will increase, as the difference between \( Pm \) and \( Rr \) is thus made greater.

3. The greater the length of \( AB \) or \( CD \), the distance \( BC \) being the same, the more the attraction will increase: for \( pr \) or \( qt \) (which represent the length of \( AB \)) being increased, \( Rr \) is diminished more than \( Tt \).

But if the overcharged end of \( CD \) be opposite to the overcharged end of \( AB \), their mutual action will be represented by \( F'f' \left( \frac{Pp + Qq + Rr - Tt}{Ff} \right) \)

and \( AB \) will be repelled; the repulsion becoming greater or less, as the attractions, by every change of distance.

Having thus examined at some length the results of a redundancy or deficiency of fluid, supposing it to be immoveable, we must now proceed to consider the consequences of its mobility.

Let \( D \), fig. 97, contain redundant fluid while \( BC \) is fixed in its natural state, and let the fluid in \( D \) be movable. The redundant fluid in \( D \) will exert its repulsive power, and will drive the fluid of \( BC \) from the proximate end \( B \) towards the remote end \( C \), so that the fluid will be rarefied in \( AB \), and constricted in \( AC \). Without examining here the mutual actions of the redundant fluid and matter, it is clear that we have a case similar to that described in No. 309, and as \( f' = m' \) and \( x \) is greater than \( x' \), \( D \) will be attracted by \( BC \), with the force \( F'f' \times (x - x') \).

We may now solve the difficulty mentioned in No. 327, and perceive that the hypothesis agrees with the fact even in the case in which it appeared so opposite. Had the fluid been immoveable, no attraction would indeed have taken place; but as it is supposed moveable, the redundant matter in the vicinity of \( D \) prevails, and a mutual attraction ensues.

For the sake of greater simplicity, we have supposed the fluid in \( D \) immoveable, but let us suppose it moveable. In that case, as soon as the uniform distribution on \( BC \) is disturbed, and it becomes overcharged in \( AC \), and undercharged in \( AB \), certain forces begin to act on \( D \), tending to disturb its uniformity. The redundant matter towards \( B \) attracts the fluid in \( D \), more than the redundant fluid toward \( C \), which is more remote, repels it; \( x' \) being less than \( x \). By this attraction the fluid of \( D \) tends to constrict in the proximate extremity, and thus again \( AB \) is more undercharged, and \( AC \) more overcharged than before. Thus the mutual action between the bodies is still more increased. But it is still of the same kind; for however small the redundancy in \( D \) may be, it can never be made deficient in its remote extremity by the irregular disposition of the fluid in \( BC \), unless \( BC \) contain more or less than its natural quantity. By the change in the disposition of fluid in \( D \), it is clear that the similar change in \( BC \) must be increased; the fluid will be still more rarefied at \( B \) and condensed at \( C \), and this will go on till all is in equilibrium. There are several forces combining to hold in equilibrium a particle in \( BC \). The redundant fluid in \( D \) impels it towards \( C \); but the redundant fluid here again impels it towards \( B \), while the redundant... Theory of redundant matter at B attracts it the same way; and Electricity, these two forces of BC must be supposed to balance the action of D.

We may here conclude that the density of the fluid in BC increases gradually from B to C; at B it must be less, and at C greater than the natural density, and there will consequently be some point between B and C where it is of the natural density. This point may be called a neutral point; though we do not mean to imply by this term that a particle situated at this point is neither attracted nor repelled.

We have supposed the fluid in D redundant; but let it be deficient. Then the attraction of the redundant matter in D will change the distribution of the moveable fluid in BC, and will constitute it in B, and rarely it in C. Again, the redundant fluid at B will act more strongly on the moveable fluid in D, and tend to impel it towards the remote extremity; and D will thus become undercharged in its proximate extremity, and less undercharged at its remote end than if BC were away. The unequal distribution of fluid in BC will thus be increased; but though both BC and D will be farther from their natural state, the remote end of D can never be overcharged.

It is clear, that when things are in the state which we have described, D and BC will attract each other with the same force as when D was equally undercharged.

Let a body, A, (fig. 101.) that is overcharged, be placed near the extremities of two oblong parallel conductors, B and C, that are in their natural state. By the action of A, the fluid in B and C will be repelled towards their remote ends N and n, where it will be condensed, while at their proximate ends, S and s, it will be rarefied. Both B will attract and be attracted by A. Now the redundant fluid in NB repels the redundant fluid in nC, and in like manner the redundant matter in SB repels the redundant matter in sC; the bodies B and C therefore repel each other, and will separate; but they ought to approach each other, for SB attracts nC, and NB attracts sC; but the repelling parts being nearer each other than the attracting parts, the forces of the former will prevail. If the body A were undercharged, it is clear that the same sensible appearances would take place, though the internal motions of the bodies would be the reverse of the former.

If another body in the same state with A be placed near the opposite ends of B and C, their internal motions will be diminished or prevented, and of course the sensible appearances should diminish also.

If another conductor, as E, be placed near s, opposite to A, it will be affected in the same manner with C, and its proximate extremity f will repel s; but if it be placed at the remote end, or in the position of F, this remote end will be attracted. As the body A, when redundant or deficient, affects every other body in its vicinity, while these do not by themselves affect each other, A is called the electrified body, and the others are said to be electrified by it. The electricity of these bodies is called Induced Electricity.

We have hitherto supposed the fluid moveable, except at first in A; but let us suppose that there is some obstruction to its mobility, and let us examine what will be the consequences. We may state the obstruction as uniform, and as being such that some small force is required to enable a particle of fluid to pass between two particles of matter.

When an overcharged body is placed near an imperfect conductor, it is clear that the fluid cannot be propelled to the remote extremity of the conductor in so great a quantity. We may conceive the distribution of the fluid, by taking a constant quantity from the intensity of the force of the overcharged body at every point of the conductor. This shows that the distribution will not be so unequal between imperfect, as between perfect conductors, and hence that the attraction between the former will not be so strong as between the latter. It will also be much longer before an equilibrium can be brought about. This leads us to an important consequence; viz. that the neutral point will not be so far from the other body when the fluid is of its natural density, as it would be, were there no obstructions. The advance of this point along the imperfect conductor will also be very slow; and it is clear, that the final accumulation at the remote extremity of an imperfect conductor will be less than if the conductor were perfect, and the neutral point will be nearer to the other extremity.

The obstruction we are considering will be attended with another remarkable effect. The contipation of the fluid at the commencement of the action will always be greatest at a place much nearer to the disturbing cause than the remote end of the conductor, and beyond that point it will diminish. In the time that elapses during the progress of this change, the condensed fluid tends to repel the fluid beyond it, and thus some of this remote fluid may be displaced, and a part of the imperfect conductor made deficient, while there is a small condensation beyond it. By this again a rarefaction and condensation may be produced in another part, thus causing a very irregular distribution of the fluid.

The effect of such a mode of action will be that there may be several neutral points in an imperfect conductor, and several overcharged and undercharged portions, and hence its action on distant bodies may be extremely various. The formula

\[ \frac{f}{r} - \frac{g}{r'} + \frac{h}{r''} - \frac{i}{r'''}, \]

where \( f, g, h, i \) express the different portions in opposite states, and \( r, r', r'', r''' \), the repulsion at different distances, may be conveniently employed to denote the action in such circumstances. Hence, if another body be placed in the direction of the axis, it will be attracted at one distance, repelled at a greater, again attracted at a still greater distance, and so alternately.

The obstruction may not be considerable, and then the action of the neighbouring overcharged body will produce a deficiency in the proximate part of the conductor, a redundancy farther on, then a deficiency, and so on. Presently these will shift, and successively disappear at the farther end, and the body will remain with only one neutral point. A greater obstruction will leave the body with more than one neutral point, and the number of these will be in proportion to the obstruction.

The removal of an overcharged body from the vicinity of conductors will have different results according as the conductors are perfect or imperfect, that is, according as there is obstruction or not. In the former case, imperfect conductors... Theory of case, the electricity induced by the vicinity of the overcharged body will be instantly destroyed on the removal of the body. But where there is an obstruction acting, though, on the removal of the body, the forces that tend to restore the equilibrium in the conductor begin to act, and restore it in part, they can never do this completely; for when the force by which a particle is propelled from an overcharged part to one undercharged is just sufficient to balance the obstruction, it will remain in that state of distribution at which it had arrived. We may expect then, that imperfect conductors will retain a part of their induced electricity.

On the removal of the electrifying body, the electric appearances induced by it in the conductor will disappear in a contrary order to that in which they were produced, and they will be left in a state of unequal distribution, or with a degree of electric power, proportioned to their imperfection as conductors.

We have now given an account of the principal consequences of the theory of Æpinus, a theory which till of late was little known in Britain, owing probably to the very lame and imperfect account given of it by Dr Priestley in his popular work on electricity. More justice has been done to this theory by Mr Cavendish, who before he saw M. Æpinus's work had framed an hypothesis of his own upon very similar principles. Mr Cavendish's paper, in which he has treated this subject in a very able and learned manner, appeared in the 61st vol. of the Phil. Trans.

To this paper we shall be much indebted presently; but in the mean time we shall only extract from it the hypothesis, which is as follows.

There is a substance which we call the electric fluid, the particles of which repel each other, and attract the particles of all other matter, with a force inversely as some less power of the distance than the cube; the particles of all other matter also repel each other, and attract those of the electric fluid, with a force varying according to the same power of the distances. Or, to express it more concisely, if you look upon the electric fluid as a matter of a contrary kind to other matter, the particles of all matter, both those of the electric fluid and of other matter, repel particles of the same kind, and attract those of a contrary kind, with a force inversely as some less power of the distance than the cube.

For the future, he would be understood never to comprehend the electric fluid under the word matter, but only some other sort of matter.

It is indifferent whether we suppose all sorts of matter to be ended in an equal degree with the foregoing attraction and repulsion, or whether you suppose some sorts to be ended with it in a greater degree than others; but it is likely that the electric fluid is ended with this property in a much greater degree than other matter; for in all probability, the weight of the electric fluid in any body bears but a very small proportion to the weight of the matter; but yet the force with which the electric fluid therein attracts any particle of matter must be equal to the force which the matter therein repels that particle; otherwise the body would appear electrical, as will be shown hereafter.

To explain this hypothesis more fully, suppose that one grain of electric fluid attracts a particle of matter at a given distance with as much force as n grains of any matter, lead for instance, repel it: then will one grain of electric fluid repel a particle of electric fluid with as much force as n grains of lead attract it; and one grain of electric fluid will repel one grain of electric fluid with as much force as n grains of lead repel n grains of lead.

All bodies, in their natural state with regard to electricity, contain such a quantity of electric fluid interspersed between their particles, that the attraction of the electric fluid in any small part of the body in a given particle of matter, shall be equal to the repulsion of the matter in the same small part, in the same particle.

A body in this state is said to be saturated with electric fluid; if the body contains more than this quantity of electric fluid, he calls it overcharged; if less, he calls it undercharged.

Sect. III. Of the Theory of two Fluids.

This theory originated, as we have said, in M. du Faye's discovery of the different electricities produced by rubbing glass and sealing-wax.

Let us suppose that there are two electric fluids, which have a strong affinity for each other, while, at the same time, the particles of each are strongly repulsive of each other. Let us suppose these two fluids in some measure equally attracted by all bodies, and existing in intimate union in their pores; and while they continue in this manner to exhibit no mark of their existence, let us suppose that the friction of an electric produces a separation of these two fluids, causing (in the usual method of electrifying) the vitreous electricity of the rubber to be conveyed to the conductor, and the resinous electricity of the conductor to be conveyed to the rubber. The rubber will then have a double share of the resinous electricity, and the conductor a double share of the vitreous; so that, upon this hypothesis, no substance whatever can have a greater or less quantity of electric fluid at different times; the quality of it only can be changed.

The two electric fluids being thus separated, will begin to show their respective powers, and their eagerness to rush into re-union with each other. With whichever of these fluids a number of bodies are charged, they will repel one another: they will be attracted by all bodies, which have a less share of that particular fluid with which they are loaded; but will be much more strongly attracted by bodies which are wholly destitute of it, and loaded with the other. In this case, they will rush together with great violence.

On this theory, the electric spark consists of both the fluids rushing in contrary directions, and making a double current. When, for instance, the finger is presented to a conductor loaded with vitreous electricity, it discharges it of part of the vitreous, and returns so much of the resinous, which is supplied to the body from the earth. Thus both the bodies are unelectrified, the balance of the two powers being restored.

When the Leyden phial is presented to be charged, and consequently the coating of one of its sides is connected with the rubber, and that of the other with the conductor; the vitreous electricity of that side which is connected with the conductor is transmitted to that which Theory of which is connected with the rubber, which returns an equal quantity of its refrinous electricity; so that all the vitreous electricity is conveyed to one of the sides, and all the refrinous to the other. These two fluids being thus separated, attract each other very strongly through the thin substance of the intervening glass, and rush together with great violence, whenever an opportunity is presented, by means of proper conductors. Sometimes they will force a passage through the substance of the glass itself; and in the mean time, their mutual attraction is stronger than any force that can be applied to take away either of the fluids separately.

Dr Priestley gives the following view of the comparative merits of this theory and that of Dr Franklin.

"In the first place, (says he,) the supposition itself of two fluids, is not quite so easy as that of one, though it is far from being disagreeable to the analogy of nature, which abounds with affinities, and in which we see innumerable instances of substances formed, as it were, to unite and counteract one another.

The two fluids being supposed, the double current from the rubber to the conductor, and from the conductor to the rubber, is an easy and necessary consequence. For if, on the common supposition, the action of the rubber puts a single fluid into motion in one direction, we might expect, that if there were two fluids, which counteracted each other, they would, by the same operation, be made to move in contrary directions. And a person that has been used to conceive that a single fluid may be made to move either way, viz. from the rubber to the conductor, or from the conductor to the rubber at pleasure, according as a rough or a smooth globe is used, can have much less objection to this part of the hypothesis.

Admitting then this different action of the rubber and the electric upon the two different fluids, the manner of conveying electric atmospheres, or powers, to bodies is the same on this as on any other theory; and it is apprehended that the phenomena of negative electricity are more easily conceived by the help of a real fluid, than by no fluid at all. Indeed Dr Franklin himself ingenuously acknowledges, that he was a long time puzzled to account for bodies that were negatively electrified, repelling one another; whereas M. du Faye, who observed the same fact, had no difficulty about it, supposing that he had discovered another electricity, similar, with respect to the properties of elasticity and repulsion, to the former.

By this double action of the rubber, the method of charging a plate of glass is exceeding easy to conceive. Upon this hypothesis, all the vitreous electricity quits its union with the refrinous on the side communicating with the conductor, and is brought over to the side communicating with the rubber; which, by the same operation had been made to part with its refrinous electricity in return.

All the vitreous electricity being thus brought to one side of the plate of glass, and all the refrinous to the other, the phenomena of the plate while standing charged, or discharged, are perhaps more free from all difficulty than upon any other hypothesis. When one of the sides of the glass is conceived to be loaded with one kind of electricity, and the other with the other kind; the strong affinity between them, whereby they attract each other, with a force proportioned to their nearness, immediately supplies a satisfactory reason, why Theory of so little of either of the fluids can be drawn from one Electricity of the sides without communicating as much to the other. Upon this supposition, that consequence is perhaps more obvious, than upon the supposition of one half of the glass being crowded with the electric matter, and the other half exhausted. In the former case, every attempt to withdraw the fluid from one of the sides, is opposed by the more powerful attraction of the other fluid on the opposite side. On the other hypothesis, it is only opposed by the attraction of the empty pores of the glass.

Lastly, The explosion upon the discharge of the glass has as much the appearance of two fluids rushing into union, in two opposite directions, as of one fluid proceeding only in one direction. The same may be said of the appearance of every electric spark, in which, upon this hypothesis, there is always supposed to be two currents, one from the electric, or the electrified body, and the other to it.

I do not say, continues Dr Priestley, that the burr which is usually seen on both sides of a quire of paper pierced by an electric explosion, and the current of air flowing from the points of all bodies electrified negatively as well as positively; are material objections to the doctrine of a single fluid. But upon the supposition of two fluids and two currents, the difficulty for accounting for these facts would hardly have occurred.

It is almost needless to observe, that the influence of points is attended with exactly the same difficulty upon this theory, as upon the other. It is equally easy, or equally difficult, to suppose one fluid to enter and go out at the point of an electrified conductor at different times, as to suppose, that, of two fluids, one goes out, and the other goes in, at the same time.

That bodies immersed in electric atmospheres must acquire the contrary electricity, is quite as easy to suppose upon this, as upon any other hypothesis. For, in this case, suppose the electrified body to be possessed of the vitreous electricity, all the vitreous electricity of the body which is brought near it will be driven backwards to the more distant parts, and all the refrinous electricity will be drawn forwards. And, when the attraction between the two electricities in these different bodies is so great as to overcome the opposition to their union occasioned by the attraction of the bodies that contained them, the form of their surfaces, and the resistance of the interposing medium, they will rush together; an electric spark will be visible between them; and the electricity of both will appear to be discharged; the prevailing electricity of each being saturated with an equal quantity of the opposite kind, from the other body.

This hypothesis will likewise easily account for the difficulty of charging a very thick plate of glass, and the impossibility of charging it beyond a certain thickness; for these fluids, at a greater distance, will attract one another less forcibly, and at a certain still greater distance will not attract at all."

Dr Priestley makes the following answer to the principal objection that may be urged against this theory.

"If it be asked (says he,) why the two fluids meeting on the surface of the globe, or in the electric explosion, do not unite by means of their strong affinity, and Theory of and make no further progress; it may be answered, Electricity—that the attraction between all other bodies and the particles of both these fluids, may be supposed to be at least as strong as the affinity between the fluids themselves; so that the moment any body is dislodged from one, it may recruit itself to its usual point of saturation, from the other.

Besides, in whatever manner it be that one of the electric fluids is dislodged from any body (since upon every theory the two electricities are produced at the same time) the opposite electricity will, by the same action, be dislodged from the other substance. And whatever it be that dislodges the fluid from any substance, it will be sufficient to prevent its return; consequently, supposing both the substances necessarily to have a certain proportion of electric matter, each may be immediately supplied from that which was dislodged from the other.

The rubber, therefore, at the time of excitation, gives its vitreous electricity to that part of the smooth glass against which it has been pressed, and takes an equal quantity of the resinous in return. The glass being a non-conductor, does not allow this additional quantity of electricity to enter its substance. It is therefore diffused upon the surface, and, in the revolution of the globe is carried to the prime conductor. There it repels the vitreous, and violently attracts the resinous electricity; and (the points of the conductor favouring the mutual transition), the vitreous, which abounds upon the globe, passes to the conductor; and the resinous, which abounds upon the nearest parts of the conductor, rushes upon the globe. There it mixes with, and saturates what remained of the vitreous electricity on the part on which it flows, and thereby reduces it to the same state in which it was before it was excited. Every part of the surface of the globe performs the same office, first exchanging electricities with the rubber and then with the conductor.

The solution of this difficulty will also solve that of the electric explosion, in which there is a collision, as it were, of the two fluids, while yet they completely pass one another. For still each surface of the glass may be supposed to require its certain portion of electric matter, and therefore cannot part with one part without receiving an equal quantity of the other. It must be considered also, that the air through which the fluids pass, has already its natural quantity of electricity, so that being fully saturated, it can contain no more, and that the two fluids only rush to the places from which they had been forcibly dislodged, and where the greater body of the opposite fluid waits to embrace them."

Although, in our explanation of electrical phenomena, we shall adopt the theory of Epinus and Cavendish, it is proper to observe that this theory does not universally prevail among the electricians of the present day. The hypothesis of Du Fay, or the theory of two fluids, is still maintained by several, especially on the continent. This theory has lately found two strenuous advocates in France, M. M. Haug and Tremery.

Their principal objection to the theory of Epinus seems to be founded on that part of his hypothesis with which Epinus himself was not perfectly satisfied, but which (in No. 321.) we have attempted to defend, viz. his introduction of a repulsive force among the particles of matter in a body.

Vol. VII. Part II.

"In fact, (say they,) the supposition of a single fluid Theory of which the particles mutually repel each other, and are attracted by the particles of matter in all known bodies, gives rise to many distinct forces, which cannot be in equilibrium, and which, by their mode of acting, are such, that two bodies which are in their natural state, and which are not attracted by any other force besides that of electricity, must tend towards each other.

"The supposition of a repulsive force among the particles of matter in solid bodies becomes unnecessary if we conceive the electric fluid as composed of two fluids, of which one possesses the property which Epinus attributes to the particles of matter in the body. It is much better to admit a repulsion at a distance among the particles of two peculiar fluids, which, like all others, repel each other, even in contact, than to conceive such a repulsion to exist among the particles of bodies that are in their nature solid. Those philosophers who endeavour to explain all the phenomena on the principle of a single fluid, believed themselves that its particles repelled each other at a distance, as from one surface of the Leyden phial to the other; and as what we call action at a distance, is properly no more than a fact on which we ground a theory, without inquiring what is the cause which furnishes the point of difference, it is sufficient that the manner in which we conceive this fact enables us to adapt it to our theory.

"Epinus, who does not conceal his reluctance to admit that such a force as that which we have mentioned can take place, would doubtless, (say these gentlemen,) have adopted the hypothesis of two fluids, if in his time the nature of the electrical phenomena had been better understood. But, at that period, the means of observation not being so perfect, experiments had not been made with that precision which characterize those which we owe to M. Coulomb, and which have formed the foundation of those important discoveries, by means of which this celebrated philosopher, far exceeding the point at which Epinus rested, has carried the science to a high degree of perfection, in that beautiful series of memoirs, in which we must admire the address with which he has availed himself both of experiment and calculation.

"Almost all the phenomena of electricity, then, seem to depend on the action of two peculiar fluids, which act in such a manner, that the particles of each mutually repel each other at a distance, with a force which is inversely as the square of this distance, and attract the particles of the other fluid with the same force.

"It is of consequence not to confound these two fluids with the two currents, the one of influent and the other of affluent matter, by which Nollet attempted to explain the phenomena. These two currents belong to the same matter, and proceed, one from the conductor towards surrounding bodies, the other from these towards the conductor.

"We shall now endeavour to apply the hypothesis of two fluids to the explanation of same phenomena which do not appear to agree with it, and which, by the manner in which we are accustomed to view them, seem to indicate that vitreous and resinous electricity are only modifications of the same fluid.

"The experiments which seem to militate against our theory are very few, and may be reduced to the following." Theory of Electricity.

"Exper. 1.—If upon a cake of rosin we trace various designs with the point of a conducting substance, which is at one time electrified positively, or by vitreous electricity, and at another negatively, or by resinous electricity; and if on this surface, thus electrified, we let fall a powder (g) properly disposed; the designs thus rendered visible will present characters peculiar to each species of electricity; thus shewing, according to the followers of Franklin and Aepinus, a superabundance of electric fluid on one side and a deficiency on the other.

"Exper. 2. When a conducting body terminating in a point, is electrified positively or by vitreous electricity, we perceive at the point a luminous brush. And if, all other things being equal, we substitute negative or resinous electricity, the point is illuminated with a star or luminous point.

"According to the theory of positive and negative electricity, the brush indicates the transmission of electric fluid from the body which is electrified positively, and the star its entrance into the body which is negatively electrified.

"Exper. 3.—When an electric explosion takes place, all the electric fluid appears constantly to pass from the body electrified positively to that which is electrified negatively."

Here they cite the method of proving this, by piercing a card placed between the conducting balls of the universal discharger. (Vid. No. 106. Exp. 2.)

These experiments, to which the theory of positive and negative electricity is happily applied, seem at first sight inexplicable, according to the hypothesis of two fluids. In fact, the particles of these two fluids being subject to the same laws, it seems,

1. That the designs traced on a cake of rosin, or other ideo-electric substance, with the point of a conductor, electrified at one time positively and at another negatively, should on the whole be similar.

2. That the luminous appearance observed at the summit of a pointed conductor, ought always to be the same, whatever be the electrical state of the body.

3. That when an electric discharge has taken place, the vitreous and resinous electricities, which mutually attract each other, ought to form a luminous train on each surface of the card, and the card ought to be perforated in a point equally distant from the two extremities of the balls of the discharger.

The following is the manner in which M. Tremery undertakes to explain these appearances.

"The matter, (says he,) to the action of which we attribute the electrical phenomena, being considered as compounded of two peculiar fluids, we may conclude that all bodies, considered in the relation which they bear to these fluids, do not possess the same properties; it is possible that vitreous and resinous electricity may be of such a nature, that, on the one hand, certain bodies, whether electrics or conductors, may have with respect to them different conducting powers; and on the other hand, that the coercive power (u) of ideo-electrics may vary according as they are opposed to the motion of Theory of particles proper to vitreous electricity, or to the motion of particles proper to resinous electricity.

"If, for instance, the air of the atmosphere, in which electrical phenomena usually take place, has an incomparably greater coercive power with respect to the resinous electricity than it has to the vitreous, it would be very easy to explain the experiments that we have quoted. In this case, the resinous electricity, because of the almost infinite resistance that the air would oppose to the motion of its particles, might be regarded as inherent in the surface of the bodies; whence it follows, that the same circumstances would take place, as if the body electrified regularly had the property of exercising by itself an attraction for the vitreous or positive electricity; a property which bodies in the negative state are supposed to have, according to the theory of Franklin.

"If now, the coercive power that we have supposed the air to have with respect to the resinous electricity, could diminish so as to become equal to that which it has with respect to the vitreous, it would happen, that the signs which induce us to regard the vitreous electricity as positive, and the resinous as negative, would disappear, so that all the phenomena would seem to depend equally on the action of the two fluids that would be subject to the same law. In this new circumstance, we should observe a luminous pencil at the summit of a pointed conductor electrified regularly or negatively, and when an electric discharge took place, the vitreous and resinous electricities would appear to approach each other.

"If, under these circumstances, the coercive power of the air with respect to the vitreous electricity, should increase, so as in its turn to become incomparably greater than what it had with respect to the resinous electricity, it is evident, that the electric matter, acting in the midst of such a substance, would produce phenomena exactly similar to those with which we are acquainted; but, in this case, the vitreous or positive electricity would perform the office of the resinous or negative, and vice versa, and they would mutually exchange signs. A luminous pencil would appear at a point electrified negatively or resinously, and a luminous star at a positive or vitreously electrified point; and when two conducting bodies, electrified differently, were placed at a convenient distance, all the electric matter would appear to move from the negative body towards the positive *."

Chap. II. A theoretical Explanation of the Phenomena of Electricity.

Sect. I. Of the Nature and Distribution of the Electric Fluid.

Before we enter on a theoretical explanation of the Nature of phenomena of electricity, it will not be improper to inquire fluid.

(g) This powder should be composed of two substances, which, by their mutual friction against each other, are capable of receiving opposite electricities.

(h) By coercive power our author understands that which ideo-electrics or conductors oppose to the motion of the particles that are proper to each of the two fluids, that, according to this hypothesis, are supposed to form by their union the electric fluid. Theory of electricity somewhat more at large into the nature of that subtle agent which we have distinguished by the name of the electric fluid, and to notice some of the more plausible opinions that have been hazarded on the subject.

One of the first questions that naturally arises from the very name of fluid is, What proofs have we of the materiality of this power?

Besides the properties of attraction and repulsion, which are properties of matter, we have many other evidences that are very persuasive, as being more distinctly the objects of our senses.

1. The spark that appears when the electric power passes suddenly through the air or any other resisting medium, and the spark, by which it is accompanied, are strong evidences in favour of the materiality of the power, by which they are produced. The noise of the spark is occasioned by the sudden impression made on the air, or some other elastic fluid, through which the spark passes. When the air is confined in close vessels, as in a tube above water, no very durable effect is indeed produced on the water in the tube. But this is owing to the rapidity with which the expansion and subsequent condensation take place. Again, it is objected, that it is impossible to communicate motion to a very delicate lever, nicely balanced, by throwing on it any quantity of electricity. Some pretend to have done this; but, however, the impossibility of doing it is no argument against the materiality of the electric fluid; and we might just as well say, that a musket ball is not material, because it may be fired through a paper or thin board delicately suspended, without imparting to them any part of its motion.

2. The light and heat accompanying the spark, are proofs of the materiality of the electric power. These are chemical phenomena; and whether we consider them as effects of the fluid as a simple, or as resulting from its decomposition, we conceive that they prove the materiality of the electric power, as completely as the materiality of caloric and light have been proved.

We are aware that this reasoning will not satisfy those philosophers who deny the materiality of caloric and light; we know that much stress is laid on the experiments of Count Rumford, as completely subversive of the materiality of heat, experiments that could even stagger the opinion of a Robison. Without defining in the least to detract from the merit of that ingenious and able experimentalist, for whom we entertain a very high esteem, we must confess, that we do not consider his experiments as warranting the conclusions that have been drawn from them, and we are still disposed to think the materiality of caloric and light as fully proved as can be expected, with respect to matter that is not absolutely tangible.

From the similarity of the chemical effects of the supposed to electric fluid with those of elementary fire or caloric, it was long ago (as we have shewn in the beginning of this Part) supposed, that they were the same, and this is still the opinion of some electricians. We cannot here pretend to enter on a full discussion of this question, but we shall briefly state the arguments in favour of the identity of caloric, and the objections that we have to make to them.

Electricity is the same with caloric (say the advocates for their identity) because,

1. Both produce the same chemical effects, expansion, fluidity, inflammation, oxidation, &c.

2. Those bodies that are the best conductors of caloric, as the metals, are also the best conductors of electricity; and glass, which is a very bad conductor of caloric, is one of the most perfect non-conductors of electricity.

To the first argument for their identity, we shall reply in the words of M. Berthollet, who once considered against this them as the same, but from experiments was satisfied that supposition, their effects were different.

"A wire of platina was submitted to shocks which were nearly strong enough to effect its combustion; and to be satisfied of this, a shock was excited, by which a great part of the wire was melted and dispersed; afterwards the shocks employed were a little weaker, and immediately after each, the wire was touched to judge of the temperature it had acquired: a heat was felt, which was dissipated in a few minutes, and which, at the utmost, was estimated to resemble that of the boiling point of water. If electricity liquefied metals, and brought them into combustion by the heat it excites, the platina wire must after a shock, which differed but little from that which would have produced its dispersion and its combustion, have approached the degree of temperature which occasions its liquefaction: Now this degree, which is the most elevated that can be obtained, would, according to the valuation, more or less accurate, of Wedgwood, be 3227° of Fahrenheit.

"When the shock is sufficiently strong to destroy the aggregation of the platina wire, it begins by detaching molecules from its surface, which exhale like smoke; if it is strong enough to produce combustion, the remains of the wire appear to be torn into filaments.

"A thermoscope blackened with ink, and placed in the steam of a strong electric spark, only experienced a dilatation which was nearly equal to one degree of Reaumur's thermometer, and this slight effect might depend on the oxidation of the iron of the ink; placed beside the current, it did not show any dilatation, although the air was necessarily affected by the electric action: it was the same when it was placed in contact with a metallic conductor, which received a stream less powerful than in the preceding experiments.

"A cylinder of glass filled with air, with an exciter at each of its extremities, to one of which was fixed a tube communicating with another cylinder filled with water, produced an impulse at each shock, which raised the water more than a diameter above its level, but its effect was instantaneous.

"These experiments seem to me to prove that electricity does not act on substances, and on their combinations, by an elevation of temperature, but by a dilatation which separates the molecule of bodies. The flight heat observed in the platina wire, is only the effect of the compression produced by the molecules which first experience the electric action, or which experience it in a greater degree; it must, therefore, be compared to that excited by percussion or compression.

"If the dilatation was the effect of heat, that experienced by a gas, in the experiment related above, would not have been instantaneous; it would only have experienced a progressive diminution by cooling, as when its expansion is owing to heat.

"In the experiment by which ammoniacal gas is decomposed, Theory of composed, the gas undoubtedly receives the electric action, and nevertheless it is not heated; and as soon as the decomposition is finished, its volume remains unchanged, because the electric action which is employed in this experiment, is not sufficiently energetic to cause a perceptible dilatation. No sensible dilatation is produced by a gas in a shock which is not very strong, because the impulse not being gradual like the expansion arising from caloric, and being excited instantaneously, the resistance of the liquid becomes too great, and cannot be overcome unless the dilatation has great energy.

An experiment of Dieman and his learned associates confirms this explanation: they caused a shock to pass through lead placed in a vessel filled with azotic gas, which could not oxidate it; it was reduced into powder retaining all its metallic properties: If it had experienced a liquefaction similar to the action of heat, it would have cooled gradually, and would have congealed into one, or at least into several masses.

When a metal is submitted to the electric action, the effects produced immediately by the electricity must be distinguished from those which are owing to its oxidation: the first are limited to the diminution or destruction of the effects of the force of cohesion, to removing and dispersing the molecules (if by this a little heat is disengaged, it is only owing to the compression sustained by some of the parts); but those which are occasioned by the oxidation, produce a high degree of heat, and then the effects allude all the appearances of an ordinary combustion: hence it arises, that the most oxidable metals are those which become red with the greatest facility, and which must shew the properties of a metal liquefied by heat.

Electricity favours this oxidation in as much as it diminishes the force of cohesion; it is thus that an alkali renders the action of sulphur on oxygen much more powerful, by destroying the force of cohesion opposed to it, and that a metal dissolved in an amalgam, is oxidated much more easily than when it is in a solid state. It is only by destroying the effects of the force of cohesion that heat itself produces the oxidation of metals; but the expansive action of electricity will have a great advantage over that of caloric, because its action is confined to the solid which it encounters in its course, so that the gas itself will not experience a dilatation in opposition to the condensation which accompanies the combination: To this circumstance may be applied what is observed in the action of hydrogen gas, which is capable of completely reducing an oxide of iron placed in the focus of a burning glass, although water, whose two elements receive the heat equally, is decomposed by this metal.*

To the second argument we shall answer, that though in the instance of metals it is correct, in so far as that these bodies are the best conductors, both of caloric and electricity, there are however, bodies that conduct caloric very well, but either do not conduct electricity, or do it very imperfectly.

Even in the case of metallic bodies, so far as can be inferred from the imperfect experiments that have been made on their comparative conducting power, it should appear that the order of their conducting power, with respect to caloric, is not the same as that with respect to electricity.

Farther, caloric takes some time to pass through the best conductors, while the electric fluid pervades the longest with inconceivable velocity.

Again, if electricity were the same with caloric, they should mutually produce the same effects, and should exist simultaneously. But this is by no means the case; a body may be strongly electrified without being sensibly increased in temperature, and so far is heat from producing electricity (except in a few instances), that where the former is present in any considerable degree, the latter is destroyed.

Lastly, the mode in which electricity and caloric pass along conductors is, we think different. Caloric seems undoubtedly to penetrate their substance, while electricity appears not to extend beyond the surface, except it meet with some resistance. The following experiment is usually adduced to prove that electricity pervades the substance of conductors.

Take a wire of any kind of metal, and cover part of it with some electric substance, as rosin, sealing-wax, &c., then discharge a jar through it, and it will be found that it conducts as well as without the electric coating. This, says Mr Cavallo, proves that the electric matter passes through the substance of the metal, and not over its surface. A wire, adds he, continued through a vacuum, is also a convincing proof of the truth of this assertion. Even here, however, the proof, if impartially considered, will be found very defective. It is a fact agreed upon by all philosophers, that bodies which to us are apparently in contact, do nevertheless require a very considerable degree of force to make them actually touch one another. Dr Priestley found that a weight of five pounds was necessary to press 20 shillings into close contact, when lying upon one another on a table. A much greater weight was necessary to bring the links of a chain into contact with each other. It cannot be at all incredible, therefore, that a wire, though covered with sealing-wax or rosin, should still remain at some little distance from the substance which covers it.

M. Coulomb proves that in an overcharged conducting body, the fluid does not penetrate into its substance, but diffuses itself merely over the surface.

By means of a very delicate electroscope, he examined pits made in a conducting body of various depths, and found that in the shallowest of them there was no sensible electricity; whence he naturally draws the conclusion, that the electricity in such bodies does not extend beyond the surface. The reader may see a description of the electroscope employed, and a detail of the experiments in the Memoirs of the French Academy for 1786, p. 72, or the Journal de Physique, vol. ii. (of the series by Delametherie), p. 236.

Dr Robison repeated Coulomb's experiments with the same results.

Another opinion that has been maintained with respect to electricity, is that it is the same with light. The differs from principal argument for the identity of electricity and light. Light seems to be that bodies are impregnated with the latter by means of the former, and indeed that light commonly appears when the electric fluid passes in any quantity from one body to another.

Another reason given for their identity, is, that both move with inconceivable velocity.

A strong argument against the identity of light and electricity, Theory of electricity, is that the former passes through glass and other transparent electrics, which seem to be impermeable to the electric fluid.

As to the impregnation of opaque bodies with light by means of electricity, this is the effect of chemical decomposition, as will presently appear, and is really produced by light itself.

What has been now said is, we think, sufficient to prove, that the electric fluid is neither calorific nor light. But the appearance of calorific and light, in many cases, shows that there is an intimate connection between them and the electric fluid. In short, they seem to form part of its composition; and we are inclined to consider it as a compound, containing calorific and light, and probably some peculiar constituent, to which we give the name of electricity. This opinion is not new; it was the hypothesis of Mr. James Ruffel, who filled the natural philosophy chair at Edinburgh, above thirty years ago.

Mr. Ruffel considered the electric fluid as a compound of several others, containing particularly elementary fire, from which it derived its great elasticity or power of repulsion. The elasticity of the electric fluid he supposed to differ from that of air, in acting at a distance; whereas the action of the air is only on adjoining particles. Hence bodies that contain more electric fluid than the spaces around them, have a tendency to repel each other.

Mr. Ruffel considered the characteristic ingredient of the compound, i.e., the electricity, as united to the other constituents by chemical affinity, or, as it was then called, Electric Attraction. This attraction acts at all distances, but not exactly according to the same law, as the repulsive power of the elastic fluid; and in general, while in this state of composition, counteracts the repulsion of the electric particles. Again, the electricity attracts the particles of other bodies, but with different degrees of affinity. Non-electrics or conductors are attracted by it at all distances, but electrics only at very small and imperceptible distances, and at such distances only its own particles attract each other.

Hence this compound fluid repels its own particles at all considerable distances, but attracts them when very near. It also attracts conductors at all distances, but electrics only when very near. The appearances of light and heat were considered by Mr. Ruffel as proofs of a partial decomposition, and as evincing the presence of elementary fire: the peculiar odour of the electric spark, and the effect produced in certain influences on the organ of taste, were also regarded as proofs of chemical decomposition, and of the compound nature of the electric fluid.

Again, conducting bodies containing electric fluid, if forced very near, attract each other; otherwise they repel each other. Electrics contain the electric fluid in consequence of the electricity existing in the compound: a part of this must be attached to the surface of the electric, but not in its elastic state, since when brought so near as to be attracted, its particles are subjected to their own mutual action, and hence the repulsion occasioned by its combination with the other ingredient of the fluid is overcome by the redoubled attraction; the electric fluid is thus partially decomposed, and the electricity attaches itself to the surface of the Theory of Electric. Thus the electric fluid may appear in two states; elastic when entire, and unelastic when partially decomposed.

The electricity may be rendered unelastic in several ways, as by friction, by which the electric fluid contained in the air is forced into closer contact, thus producing a decomposition of the fluid, and causing its electricity to unite with the surface of the rubbed body. This operation may be compared to the forcible wetting of a dry sponge, or of some powder, as that of the puff hall, which, when dry, do not easily imbibe moisture; but when wetted by mechanical compression, retain it very forcibly. The electricity unites with bodies in this way during several operations of nature, as in the melting and cooling of some substances in contact with electrics; and it may be thus forcibly united to the surface of electrics by means of metallic coatings, into which the fluid is forced by the skilful management of its mutual repulsions. This operation, again, was compared by Mr. Ruffel to the condensation of the moisture of humid air on a cold pane of glass; and the evacuation of fluids from the other side of the coated pane he compared to the evaporation of the moisture from the other side of the cold pane, in consequence of the heat that was extricated from the condensed vapour.

The analogy that exists between electricity and calorific, has induced some to apply to the former the doctrine of capacity, so ingeniously applied to calorific by Dr. Crawford. This doctrine seems to be one of the fundamental principles of Mr. Wilkinson's theory of electricity; the substance of which is contained in the following extract.

"From some experiments, I am induced to suppose, Mr. Wilkinson, that electricity is universally diffused, but not equally; for, hypothesizing that those bodies are the best conductors which contain the greatest quantity, and those the best non-conductors which contain the least.—Thus metallic bodies are the best conductors; all fluids, except air and oil, are also conductors. The disposition in the body to retain electricity may be termed its capacity.

When conducting bodies undergo any change, if by such change their capacities become altered, then signs of electricity are evinced.

If the change should be of such a nature, that their capacity for electricity becomes increased, the substance will be in a state of abstracting it from surrounding bodies, and therefore will evince negative signs; the same as frigorific mixtures produce negative signs of heat.

If, in the change it undergoes, the capacity of the substance for electricity is diminished, it gives out a portion of its natural quantity, and evinces positive signs, or a state of superabundance.

When any substance, in the change it undergoes, gives out electricity, it becomes proportionally diminished in its conducting powers; so, on the contrary, when it acquires an increase, it increases also its powers as a conductor.

Thus a metallic substance, which is a good conductor, when oxidated is a very imperfect one. In the change from its reguline state to a calx, electricity is given out." This capacity for electricity is not regulated by any known laws, such as the densities or the specific gravities of the bodies.

In many substances, the conducting power seems to depend on the addition of other principles; thus wood, when a conductor, is so in consequence of the moisture it contains; when deprived of it by drying, it resists the passage of electricity.

What this peculiar change may be, is difficult to conceive; but when electric bodies become partial conductors, it seems to be effected by the agency of heat.

When the pressing action is very considerable, as in the case of metallic bodies, great quantities of heat are extricated. Thus a nail, when struck violently, soon exhibits signs of considerable warmth; the caloric infused in its interstices is exuded on the surface, in consequence of the approximation of the constituent particles of the iron.

Whether the caloric diffused in the interstices, or combined with the body, is given out by pressure, is a fact difficult to determine. Those substances which are non-conductors, and consequently capable, from excitation, of giving out signs of electricity, do not all of them lose their power, when freed from the rubbing action. Those bodies which are usually termed refractory, continue for a certain space of time in their conducting state, until they are equalized with the surrounding air; and, continuing in a disposition to abstract electricity from surrounding bodies, will therefore evince negative signs (1).

The doctrine of bodies having different capacities for electricity was ingeniously employed by Mr G. Morgan to account for the effects produced on electrics by friction.

"If (says he) we admit the corporeal nature of that which is hence with accuracy called the electric fluid, let us attend to the necessary consequences of what we admit:—1st. That the electric fluid, like all other corporeal substances, is capable of attracting, and of being attracted. 2d. That in consequence of this capacity, it enters into an union with other bodies, and that as the nature of the substances to which it is united may vary, so the degree of force by which it is united may show an equal variety. 3d. That when the electric fluid is separated from any body, this separation must be the effect of lessening the force by which it was united to that body, and thus giving the attractive force of another body the superiority; or it must be the effect of very much increasing the force of the third body, and thus destroying the equilibrium.

Suppose that any body, A, should be capable of uniting to itself, or suppose the law of its constitution were such as to admit of its attaching, fifty particles of the electric fluid to itself, when near or in contact with another body, B, which likewise has an attraction to those particles; now, in case any such change should take place as would add twenty particles to B, and leave thirty only in A, this change, it is evident, must proceed either from a diminution of A's attracting force, or from an adequate increase of force in B. Having deduced, from the corporeal nature of the electric fluid, such consequences as show that when it is separated from a body, it must proceed from a diminution of attractive force in the body that yields, or an increase of the same force in the body that takes; let us now examine how friction is likely to be the cause of such changes.

By attending to the nature of friction, we shall find it to be nothing more than a succession of prelure or contacts of the different parts of different substances against each other; and the question in the present case is this:—whether contact is necessarily attended with a change of attractive force in the different substances which are brought together? or whether the close union of a particle of silk, hair, leather, &c., to a particle of glass, may be attended with a change of capacity in those bodies to retain the electric fluid?—If this question be admitted, I think the particular mode in which friction operates is easily discovered.

Briefly my idea of the manner in which friction operates, is this: when two electrics are pressed closely together, while they continue together, they become capable of taking more, or retaining less; and if this be allowed, I think the various appearances of bodies in a state of excitation are easily accounted for.

However, it may be asked, if the change produced in the surfaces of two bodies be the effect merely of bringing the bodies nearer together; why does not contact alone produce the same effect? I must answer, that the several instances of spontaneous electricity enumerated by Wilcke, Äpinus, and others, appear to me to be so many evidences of the preceding theory. In these instances we see the excitation of surfaces take place in such circumstances as will not rationally admit of any other cause than simple contact.

It is evident, I think, that contact alone is adequate to the production of electricity. I would add, that in the only case where contact may be applied most completely, electricity is produced in a most remarkable degree.—By Bennet's new electroscope, we find that the slightest evaporation (which is certainly the union of watery with ærial particles) produces immediate signs of electricity. How rationally all the electrical appearances of our atmosphere may be ascribed to the same source, will be shown more fully hereafter.

Before I quit this subject, I would explain to you the reasons why, in many cases, agreeably to the preceding hypothesis, friction is necessarily much more powerful in its effects than prelure.

Suppose

(1) Mr Coulomb endeavours to prove that the electric fluid is not distributed among conducting bodies in contact by chemical affinity, but merely by its repulsive motion.

When two bodies, equal and similar, placed in contact, are tolerably perfect conductors, such as the metals, the electricity communicated from one to the other is in an instant divided equally between them; but when one of the bodies is an imperfect conductor, as a plain of paper, it will take some time before the paper receives the half of the electricity of the metal. In all cases, however, the electricity is equally divided. Vid. Mem. de l'Acad. Roy. de Paris, pour 1786. p. 69. Suppose A to be a particle of silk, brought into contact with a particle of glass, which I call B; by the increase of attraction consequent upon the union, the combined bodies become capable of attracting a portion of the fluid, which I say, is equal to five. Now A is no sooner separated from B, than another particle of silk comes in contact, and produces a similar effect. The portion accumulated is now ten. A third comes into successive contact with B, and adds to the accumulation; and while the rubbing goes on, a series of successive effects is produced by a series of successive unions and separations; for A is no sooner separated from B, than it is brought into that state in which it was before the union, and consequently disposed to part with what it gained by the union. Now if you suppose A and B, instead of being single particles, to be surfaces, all of whose parts operate at the same time, you may easily perceive how the effect would be increased.

In the preceding case, I described the capacity of A and B to be enlarged by their union. If it had been lessened, the subsequent effects would have been sufficient; for, in such a case, after the dissolution of their contact, they would be disposed to receive or retake what they had lost by their union. But I will speculate no longer on the consequences of friction, as elucidated from the supposed corporeal nature of the electric fluid, and from the changes supposed to take place on the attractive force of different bodies when brought into very close contact with each other.*

* Morgan's Lectures, vol. i.

Sig. Brugnatelli, from the chemical properties of the supposed electric fluid, and, from several experiments which he has made upon the subject, concludes that it should be ranked among the acids. This fluid, says he, reddens the tincture of turpentine, which as the fluid diffuses returns again to a blue colour; it penetrates the metals, oxidizes them, and produces hydrogen gas. In fine, it possesses all the properties of an acid. He therefore denominates it the electric or oxi-electric acid, and of course the salts which are formed by its combination with fusible bases, are called electrats. On some of these he makes the following observations.

1. The electrat of gold is formed of small, brilliant, and transparent points. 2. The electrat of silver consists of small prismatic crystals, terminated by fixed pyramids, which are limpid and transparent, and strongly reflect the light. They are tasteless and insoluble in water. 3. The electrat of copper consists of cubical transparent crystals, which dissolve in the acid with effervescence. The crystals are of a beautiful green colour. 4. The electrat of iron is of a reddish yellow colour, and opaque. 5. The electrat of zinc is opaque, and of a grayish colour.

The electric acid, according to this author, is not decomposed, when it oxidizes the metals, but the oxygen required for their oxidation, is derived from the water employed in his experiments.

Having thus considered pretty fully the chemical nature of the electric fluid, we shall return to its mechanical properties, and endeavour to ascertain the law by which its particles act on each other, and how it is distributed in bodies of various figures, and in various relations.

It was long a desideratum among electricians to discover the law of action according to which the particles of the electric fluid attract and repel each other. AE. Electricity. Principia, we have seen, states no other law than that the action decreases according to the distance increases. Mr Cavendish suspected, but did not prove either by demonstration or experiment, that the action of electricity was, like that of gravitation, inversely as the square of the distance.

Lord Stanhope attempted to prove that this was the law of electric action, both experimentally and mathematically, and concluded from the result of both his experiments and reasoning, that the supposition was just. But Dr Robison did not consider the experiment of Lord Stanhope, as sufficiently accurate, or sufficiently detailed, to warrant the conclusions that his Lordship had drawn.

That eminent philosopher, nearly 40 years ago, made of Electricity, Parts iv. v. and vi. were attended with results similar to those of Lord Stanhope.

Dr Robison's experiments were made with the assistance of his excellent electrometer, which he describes in No. 266. The mode of using this instrument is as follows.

The body whose electricity is to be examined is connected with the electrometer by a wire, the end of which is inserted into the hole at F, and made to touch the end of the needle. Now the index is to be turned to the right by the handle I, till it come to 90. In this position LA, and consequently CB, is horizontal; and the moveable ball B rests on A and moves with it. The balls being now electrified, the handle is turned back till the index arrive at 90, from which it set out. If during this motion the balls be noticed, it will be found that in some position of the index they will separate. Bring them again together, and again separate them, till the exact point of separation be ascertained. This will give their repulsion when in contact, or at the distance of their centres. Then turn the index still more to the vertical position, and the balls will separate still more. Let an afflant now move the long index till it become parallel to the stalk of the electrometer, which will be known by its hiding the latter from his view. If the stalk be poised, by laying a weight of some grains on the cork ball D, till the stalk become horizontal and nicely balanced, we know exactly the weight that denotes the degree of repulsion that will cause the balls to separate when in the horizontal position, by computing for the proportional lengths of BC and DC. Then, by a very simple computation, we shall find the weight denoting the degree of repulsion with which they separate in any oblique position of the stalk, and again, by the revolution of forces, we find the degree of repulsion with which the balls separate when AL is oblique, and BC makes with it any given angle.

The intention of Dr Robison's experiments was to ascertain the law of repulsion of two small spheres, as whatever was the law of distribution of the particles in a sphere, which we shall consider presently, the general action of its particles on those of another sphere will not differ materially from the law of action between two particles, if the spheres are very small in proportion to their distance.

The result of the experiments was that the mutual repulsion... Theory of repulsion of two small spheres, electrified either positively or negatively, was very nearly inversely as the square of the distance of their centres, or a little greater. Thus, if we express the distance by \( x \), the law of repulsion was as nearly as possible \( \frac{1}{x^2} \). One of the balls being much larger than the other appeared to cause no difference in the results.

Repeating the experiment with balls electrified oppositely, and which of course attracted each other, the results obtained were not quite so regular; but the general result was a deviation from the above law rather less than in the preceding case, this being in defect, while that was in excess.

Sir Isaac Newton has demonstrated, (Princip. lib. i. pr. 74,) that if particles of matter act on each other with a force in the inverse duplicate ratio of the distances, spheres composed of such particles and of equal density at equal distances, will act on each other according to the same law. He has demonstrated that the same holds in the case of hollow spherical shells, and that these act on each other in the same manner as if all their matter were crowded into their centres; and he has farther demonstrated, that if the law of action between the particles be different from what has been stated, the action of spheres or spherical shells will also be different.

M. Coulomb of the French academy made a number of most valuable experiments for the purpose of ascertaining this point, and obtained the same results.

This distinguished academician has published in the memoirs of the Royal Academy at Paris for 1784, 1785, 1786, and 1787, papers which rank him very high among those who have contributed to advance the science of electricity.

In the Memoirs for 1785 appeared the papers that contain the experiments by which he proved the law of electric action. These we cannot here pretend to detail, but the result is highly satisfactory. They were made with the assistance of a very delicate electrometer, the construction of which we shall describe under the article Electrometer.

The reader may satisfy himself very nearly of the truth of this law by the following simple experiment.

A, fig. 102, is the convex extremity of an excited surface. BC is a metallic rod, delicately suspended on the point E. CF is designed to contain any weight which may be applied to the extremity of the rod. The apparatus should be as light as possible, and is best made of reed and cork covered with tinfoil.

While the surface A is in an excited state, B is brought within a certain distance of it, and the weight moved by its influence is carefully observed. A similar observation is then made at a second, a third, and a fourth distance.

Varieties will be discovered in the result of these observations, proceeding from the impossibility of keeping the surface for any considerable time in the same state of excitation. These varieties, however, are trifling; and in a vast number of experiments, the weight will diminish very nearly in the duplicate ratio of the increased distance.

We may now safely conclude that the law of electric action is like that of gravitation, so that electrified bodies attract or repel each other with a force that is inversely as the square of the distance. The affirming of this important law is of infinite consequence. It affords us a full conviction of the truth of the propositions respecting the action of bodies that are overcharged at one end, and undercharged at the other. It renders certain what we could formerly infer only from a reasonable probability. We now see that the curve described in No. 338, must really have its convexity turned towards the axis, and that \( Z + x' \) will always be greater than \( Z' + x \).

We now proceed to consider the manner in which the electric fluid is distributed, when it is redundant or deficient in bodies; and for this purpose we cannot do better than lay before the reader the following series of propositions, chiefly taken from Mr Cavendish's paper, but accommodated to the true law of action above laid down.

**Lemma I.**—Let the whole space comprehended between two parallel planes, infinitely extended each way, be filled with uniform matter, the repulsion of whose particles is inversely as the square of the distance; the plate of matter formed thereby will repel a particle of matter with exactly the same force, at whatever distance from it it be placed.

For, suppose that there are two such plates, of equal thickness, placed parallel to each other, let \( A \), fig. 103, be any point not placed in or between the two plates; let \( BCD \) represent any part of the nearest plate; draw the lines \( AB, AC, \) and \( AD \), cutting the furthest plate in \( b, c, \) and \( d \); for it is plain that if they cut one plate, they must, if produced, cut the other: the triangle \( BCD \), is to the triangle \( bcd \), as \( AB^2 \) to \( Ab^2 \); therefore a particle of matter at \( A \) will be repelled with the same force by the matter in the triangle \( BCD \), as by that in \( bcd \). Whence it appears, that a particle at \( A \) will be repelled with as much force by the nearest plate, as by the more distant; and consequently will be impelled with the same force by either plate, at whatever distance from it it be placed.

**Cor. 1.**—The same will be true of the action of plates of equal thickness and equal density, or of such thickness and density as to contain quantities of matter or fluid proportional to their areas.

**Cor. 2.**—The action of all such sections made by parallel planes, or by planes equally inclined to their axis, is equal.

**Cor. 3.**—The tendency of a particle to a plane, or plate of uniform thickness and density, and infinitely extended, is the same, at whatever distance it be placed from the plate, and it is always perpendicular to it.

**Cor. 4.**—This tendency is proportional to the density and thickness of the plate or plates jointly.

**Problem 1.**—In fig. 104, let the parallel lines \( Aa, Bb, \) &c., represent parallel planes infinitely extended each way; let the spaces \( AD \) and \( EH \) be filled with uniform solid matter: let the electric fluid in each of these spaces be moveable and unable to escape: and let all the rest of the matter in the universe be saturated with immoveable fluid. It is required to determine in what manner the fluid will be disposed in the spaces \( AD \) and \( EH \), according as one or both of them are over or undercharged.

Let \( AD \) be that space which contains the greatest quantity Theory of quantity of redundant fluid, if both spaces are overcharged, or which contains the least redundant matter, if both are undercharged; or if one is overcharged, and the other undercharged, let AD be the overcharged one. Then, first, There will be two spaces, AB and GH, which will either be entirely deprived of fluid, or in which the particles will be pressed close together; namely, if the whole quantity of fluid in AD and EH together, is less than sufficient to saturate the matter therein, they will be entirely deprived of fluid; the quantity of redundant matter in each being half the whole redundant matter in AD and EH together; but if the fluid in AD and EH together is more than sufficient to saturate the matter, the fluid in AB and GH will be pressed close together; the quantity of redundant fluid in each being half the whole redundant fluid in both spaces. 2dly, In the space CD the fluid will be pressed close together; the quantity of fluid therein being such as to leave just enough fluid in BC to saturate the matter therein. 3dly, The space EF will be entirely deprived of fluid; the quantity of matter therein being such, that the fluid in FG shall be sufficient to saturate the matter therein; consequently, the redundant fluid in CD will be just sufficient to saturate the redundant matter in EF. And 4thly, The spaces BC and FG will be saturated in all parts.

Cor. 1.—If the two plates be equally overcharged, all the redundant fluid will be crowded on the remote surfaces, and the adjacent surfaces will be in their natural state.

Cor. 2.—If the redundant fluid in the one be just sufficient to saturate the redundant matter in the other, the two remote surfaces will be in their natural state, all the redundant fluid being crowded in the stratum CcD, and all the redundant matter being in EeF.

Lemma II.—Let BDE be a sphere, whose center is D; if the space BB is filled with uniform matter, whose particles repel with a force inversely as the square of the distance, a particle placed anywhere within the space BB, as at P, will be repelled with as much force in one direction as another, or it will not be impelled in any direction. This is demonstrated in Newt. Princip. lib. i. prop. 70. It follows also from his demonstration, that if the repulsion is inversely as some higher power of the distance than the square, the particle P will be impelled towards the centre; and if the repulsion is inversely as some lower power than the square, it will be impelled from the centre.

Problem 2.—Let the sphere BDE be filled with uniform solid matter, overcharged with electric fluid; let the fluid therein be moveable, but not able to escape from it; let the fluid in the rest of infinite space be moveable, and sufficient to saturate the matter therein; and let the matter in the whole of infinite space, or at least in the space BB, whose dimensions will be given below, be uniform and solid: it is required to determine in what manner the fluid will be disposed both within and without the globe.

Take the space BB, such that the interfaces between the particles of matter therein shall be just sufficient to hold a quantity of electric fluid, whose particles are pressed close together so as to touch each other, equal to the whole redundant fluid in the globe, besides the quantity requisite to saturate the matter in BB; and take the space BB, such that the matter therein shall be just able to saturate the redundant fluid in the globe: then in all parts of the space BB, the fluid will be pressed close together, so that its particles shall touch each other: the space BB will be entirely deprived of fluid; and in the space CB, and all the rest of infinite space, the matter will be exactly saturated.

Cor. 1.—If the globe BDE is undercharged, everything else being the same as before, there will be a space BB, in which the matter will be entirely deprived of fluid, and a space BB in which the fluid will be pressed close together; the matter in BB being equal to the whole redundant matter in the globe, and the redundant fluid in BB being just sufficient to saturate the matter in BB; and in all the rest of space the matter will be exactly saturated, exactly similar to the foregoing.

Cor. 2.—The fluid in the globe BDE will be disposed in exactly the same manner, whether the fluid without is immovable, and disposed in such manner, that the matter shall be everywhere saturated, or whether it is disposed as above described; and the fluid without the globe will be disposed in just the same manner, whether the fluid within is disposed uniformly, or whether it is disposed as above described.

Let BC, fig. 106, be a cylindrical conducting body, and A an overcharged body. Draw bc parallel to BC, and draw BB, CC, PP, &c., perpendicular to BC, to represent the uniform density of the fluid, when BC is in its natural state; and let BD, CR, PS, &c., represent the unequal densities at different points, while it is opposed to the overcharged body A. Now these ordinates will be bounded by a line dnr, cutting the line bc in n, a point in the line nN drawn perpendicular to N, the neutral point of the conductor. The whole quantity of fluid in BC will be represented by the parallelogram bce, CB; but this must be equal to the space BC nd; again, the redundant fluid in any portion, as PC or PN, may be represented by the spaces pirc, or tpe, and the deficient fluid in any portion BQ, may be represented by the space bdoq. Now, the action of BC on any body placed near it, will entirely depend on the space contained between the curve line and the axis bc. With respect to this curve, the only circumstance that we can ascertain, is that variations of curvature at every point are proportional to the forces exerted by the spherical body A; and are, therefore, inversely as the squares of the distances from A, as will be shown presently. The exact place of the point n, and the length of the ordinates, will vary according to the diameter of the conductor. We shall at present consider only the simplest case, or that where the conductor is of no sensible diameter, like a very fine wire.

Let such a slender conducting canal be represented by AE, fig. 107, and let BB, CC, EE, &c., represent the density of the contained fluid, this being kept in a canal near the state of unequal density by its repulsion for some overlying uniform charged body. Now, a particle at C is impelled in the direction CE by all the fluid that is on the side of A; and it is impelled in the direction CA by all the fluid on the side of E. The moving force will arise from the difference of these repelling forces. When the diameter of the canal continues the same, this will arise from the difference of density only. Therefore, the force of the element at E may be expressed by the excess of D above Cc + the action at the distance CD. Draw \( \beta e \) parallel to \( AE \); then the force of the element \( E \) may be expressed by the formula

\[ \frac{d^2}{c^2} x, \]

and this is the force repelling the particle in the direction \( CA \).

Take \( CF = CD \); the force at \( F \) will be expressed by

\[ \frac{f \phi}{c^2} x, \quad \text{or} \quad \frac{f \phi}{c^2} x, \]

and this force also impels the particle in the direction \( CA \). The joint action of the two is

\[ \frac{d^2 + f \phi}{c^2} x. \]

If \( b c e \) were a straight line, \( d^2 + f \phi \) would always be proportional to \( c^2 \), and might be expressed by \( m \times c^2 \), \( m \) denoting some number that expresses what part of \( c^2 \) the sum of \( d^2 \) and \( f \phi \) is equal to, suppose \( \frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \ldots \), &c. But in the present case \( d^2 + f \phi \) is not always proportional to \( c^2 \), for \( d^2 \) does not increase so fast as \( c^2 \), while \( f \phi \) increases faster.

We may, however, without any sensible error, express the accelerating force tending towards \( A \), in the neighbourhood of any point \( C \), by

\[ \frac{m c^2}{c^2} x, \]

that is, by

\[ \frac{m}{x}, \]

which is the fluxion of the area of a hyperbola \( HDG \), of which \( CC' \) and \( CK \) are asymptotes. The whole action of the fluid between \( F \) and \( D \) may be expressed by the area \( C'CDD'H \). Hence, the action of the smaller conceivable portion of the canal that joins to \( C \) on either side, or the difference of the actions of the two adjacent elements, is equal to the action of all beyond it. The state of compression is therefore scarcely affected by anything at a sensible distance from \( C \), and the density of the fluid in an indefinitely small canal is uniform.

Having thus found that the fluid in very small canals is very nearly of an uniform density, we may now proceed to examine the communication of electricity by means of conducting canals; which forms one of the most important parts of the theory.

Let us suppose that the body \( A \) communicates by the canal \( EF \), with another body \( D \), placed on the contrary side of it from \( B \), as in fig. 108, and let these two bodies be either saturated, or over or undercharged; and let the fluid within them be in equilibrium. Let now the body \( B \) be overcharged: it is plain that some fluid will be driven from the nearer part \( MN \) to the further part \( RS \); and also some fluid will be driven from \( RS \), through the canal, to the body \( D \); so that the quantity of fluid in \( D \) will be increased thereby, and the quantity in \( A \), taking the whole body together, will be diminished; the quantity in the part near \( MN \) will also be diminished; but whether the quantity in the part near \( RS \) will be diminished or not, does not appear for certain; but probably it will be not much altered.

Cor.—In like manner, if \( B \) is made undercharged, some fluid will flow from \( D \) to \( A \), and also from that part of \( A \) near \( RS \), to the part near \( MN \).

Suppose now that the bodies \( A \) and \( D \) communicate by the bent canal \( MPNPM \) (fig. 109.) instead of the straight one \( EF \): let the bodies be either saturated or over or undercharged as before; and let the fluid be at rest; then, if the body \( B \) is made overcharged, some fluid will still run out of \( A \) into \( D \); provided the repulsion of \( B \) on the fluid in the canal is not too great.

The repulsion of \( B \) on the fluid in the canal, will at first drive some fluid out of the leg \( MP \) into \( A \), and out of \( NP \) into \( D \), till the quantity of fluid in that part of the canal which is nearest to \( B \) is so much diminished, and its repulsion on the rest of the fluid in the canal is so much diminished also, as to compensate the repulsion of \( B \); but as the leg \( NP \) is longer than the other, the repulsion of \( B \) on the fluid in it will be greater; consequently some fluid will run out of \( A \) into \( D \), on the same principle that water is drawn out of a vessel through a syphon; but if the repulsion of \( B \) on the fluid in the canal is so great as to drive all the fluid out of the space \( GPH \) \( G \), so that the fluid in the leg \( MGPM \) does not join to that in \( NHPN \); then it is plain that no fluid can run out of \( A \) into \( D \); any more than water will run out of a vessel through a syphon, if the height of the bend of the syphon above the water in the vessel, is greater than that to which water will rise in vacuo.

This is Mr Cavendish's reasoning; but Dr Robison objects to it, that in these cases the fluid does not move on the principle of a syphon, and that there is nothing to prevent the fluid from expanding in \( GPH \). He was therefore of opinion, that it would always move from \( A \) to \( D \) over the bend.

Cor.—If \( AB \) is made undercharged, some fluid will run out of \( D \) into \( A \); and that though the attraction of \( B \) on the fluid in the canal is ever so great.

We shall now consider the action of electrified bodies on the canal of communication, in some of the most important cases. But, as we are confined in our limits, and have much important matter yet to treat of, we must content ourselves, with enumerating facts without proving them by rigid demonstration.

Let \( ACa \), fig. 110, represent a thin conducting plate, seen edgewise, to the centre of which the slender canal \( CP \) is perpendicular. It is required to determine the action exerted by the fluid, or matter, uniformly disposed over the plate, on the fluid moveable in \( PC \).

1. To find the action of a particle at \( C \) on the fluid in the whole canal. Join \( AP \), and let \( CP \) be denoted by \( x \), \( AP \) by \( y \), and \( AC \) by \( r \). Also, let \( f \) represent the intensity of action at the distance \( r \) of the scale from which the lines are measured.

The action of \( AmP \) is \( \frac{f}{y^2} \), and it may be demonstrated that the action of \( A \) on the whole of \( CP \) is

\[ f \left( \frac{1}{r} - \frac{1}{y} \right) = f \left( \frac{y - r}{ry} \right). \]

2. To find the action of the plate whose diameter is \( Aa \) on a particle at \( P \).

Let \( a \) denote the area of a circle whose diameter is \( = 1 \). The action required will be expressed by the fluent \( 2fa \left( 1 - \frac{x}{y} \right) \).

Cor.—If \( PC \) be very small in comparison of \( AC \), the action will be nearly the same as if the plate was infinite.

3. To find the action of the plate on the whole column. This will be expressed by the fluent \( 2fa \left( x + r - y \right) \). Our mathematical readers, who are familiar with the method of fluxions, (and to no others will these theorems be intelligible), will readily see the meaning of these expressions.

The following geometrical construction will render the action of the plate for the whole column, or its parts, more familiar, and more easily remembered.

Produce PC till CK is = CA, and with the centre P, describe the arch AI, crossing CK in I. Then the electrical action will be expressed by \(2fa \times IK\); and this expression represents a cylinder whose radius is r of the scale, and whose height is \(= 2IK\).

Again, about the centre p, with the distance \(rA\), describe the arch Ai, cutting CH in i. Then we have \(2fa \times IK\), expressing the action of the plate on the column C, and \(fa \times 1i\), expressing its action on Pp.

By the formula \(2fa \times IK\), is meant, that the action exerted by the whole plate on PC is the same as if all the fluid in the cylinder expressed by \(a \times 2IK\) were placed at the distance from the acting particle denoted by i.

Cor. 1.—If PC is very great compared with AC, the action is nearly the same as it would be if the column were infinitely extended.

Cor. 2.—If besides, another column \(pC\) is very small when compared with AC, the action on PC will be to that on \(pC\), as \(pC\) to AC nearly.

The redundant fluid cannot be uniformly diffused over the whole plate, as we have hitherto supposed, since the mutual repulsion of its particles will render it denser at the circumference. As it is difficult to determine the variation of density, we shall only state the result of the extreme case, where the whole redundant fluid is crowded into the circumference of the plate.

The action of the fluid in the canal is now \(fa(r - \frac{r^2}{y})\), and the whole action of the fluid crowded into the circumference will be \(fa(r^2) \times \left(\frac{y - r}{ry}\right)\)

\(= fa(r^2) \times \left(\frac{y - r}{y}\right)\). This may be thus represented geometrically. Describe the quadrant CbBE, crossing AP in B, and Ap in b. Draw BD and bd parallel to PC. Now, PB is \(= y - r\), and DC \(= r(y - r)\). The expression \(fa(r^2) \times \left(\frac{y - r}{y}\right)\) will therefore denote a cylinder whose radius is r, and height DC, multiplied by f.

Again, \(DC\) will be the height of the cylinder expressing the action on \(pC\), and \(Dd\) that of the cylinder expressing the action on Pp.

Cor. 1.—If CP is very great compared with CA, D is very near to A, and I to C, and CD has to IK very nearly the ratio of equality.

Cor. 2.—But if the column \(pC\) is very short, the action of the fluid uniformly diffused over the plate, is to the action of the fluid crowded into the circumference nearly as \(4AC\) to \(pC\).

From this corollary we see that the recess of the fluid towards the circumference, has a much less effect on short columns than on long ones, i.e. the action in the former case will be much less diminished. Any external force that tends to impel fluid along the canal, and from thence to diffuse it over the plate, will impel a greater quantity to the plate when the fluid of the Theory of plate is crowded into the circumference, than if it were uniformly diffused over the plate, and this difference will be greater when the canal is short.

Lastly, When KL is equal to AP, or PL to KI, the repulsion exerted by the whole fluid of the plate, collected in K, on the fluid in the canal CL, is equal to the repulsion of the same fluid, when crowded into the circumference, on the column CP.

Cor. 1.—When CP is very long in comparison with AC or KC, the actions of the two fluids in both the above situations is nearly equal.

Cor. 2.—The action exerted by the whole fluid on the column CP, when uniformly diffused, is to its action when collected in K, as \(2IK\) to CD.

Cor. 3.—If CNO be a spherical surface, or a spherical shell, of the same diameter and thickness with the plate Aa, and containing redundant fluid of uniform density, the action exerted by this fluid on the column CL is equal to twice the action of the fluid on the column CP, when the fluid is uniformly diffused over the plate, and to four times its action on the same column, when it is crowded into the circumference.

Let there be two circular plates, represented edgewise at DE, de, fig. 111., or two spherical shells ABO, two plates abo, of the same diameters and thicknesses with the connected plates, containing redundant fluid of uniform density, infinitely extended, perpendicular to their surfaces and passing through their centres, and let the fluid in these canals be of uniform density and equally diffused.

It may be demonstrated that the repulsions exerted by the fluid in the plates or spheres on the canals are as the diameters of the plates or spheres.

Cor. 1.—When the canals are very long compared to the diameters of the spheres or plates, the repulsions are nearly in the same proportion.

Cor. 2.—The more the length of the canals diminishes when compared with the diameters of the plates or spheres, the more the repulsions approach to equality.

Cor. 3.—When the density of the fluid in two spherical shells is inversely as their diameters, the repulsions of the contained fluid on a column of fluid infinitely extended, will be equal.

Cor. 4.—When the quantities of redundant fluid in two spheres are proportional to their diameters, the repulsions exerted by them on a canal infinitely extended are equal.

Cor. 5.—If there be two overcharged spheres, or spherical shells, as ABO, ab, fig. 112., that communicate by a conducting canal infinitely extended, the quantities of redundant fluid they contain are proportional to their diameters; and they will be nearly so if the canals be very long.

Cor. 6.—When the spheres of conducting matter are in equilibria, the pressures exerted by the fluid on their surfaces are nearly proportional to their diameters.

It follows from this corollary that the tendency of fluid to escape from such spheres is, ceteris paribus, inversely as the diameters.

Let there be four circular plates, as HK, AB, DF, LM, fig. 113., equal and parallel to each other, and let two of them, AB and HK, communicate by an indefinite... Theory of definite canal GC perpendicular to their planes and palling through their centres; let DF and LM communicate in like manner by the canal EN, both canals being in the same straight line: let HK be overcharged, and LM just saturated. It is required to determine the disposition of the fluid, and its proportion in the plates, so that the above condition may be possible and permanent, while all is in equilibrio?

As HK and AB communicate and are equal, as HK is overcharged, AB will be so also, and in the same degree, and the fluid will be similarly disposed in both. HK and AB being in this situation, if DF and LM be brought near them to within the distance CE, as in the figure, the redundant fluid in AB will act on the moveable fluid in DF, and force some of it along the canal EN into LM, rendering this latter overcharged. Now, if this redundant fluid in LM be taken off, the repulsion which LM was beginning to exert on the canal NE, will be diminished or destroyed. Hence, more fluid will move from DF into LM, and this will again be overcharged. The redundant fluid in LM may again be taken off, but in less quantity than before, and so on repeatedly, till no more can be taken off. DF will thus be rendered undercharged, or will contain redundant matter. This will act on the fluid in GC, and attract it from G, and consequently the fluid will now move from AK into AB, by which HK will be rendered less overcharged, and AB more so than at first. The thus increased redundancy of fluid in AB will act more strongly on the moveable fluid in DF, and repel a part of it into LM as before. DF will thus be again rendered deficient, and by its redundant matter will again act on the canal GC. Thus, by repeatedly touching LM to take off the fluid driven into it from DF, or by allowing LM to communicate with conducting bodies, an equilibrium will be produced; and when this is the case, HK contains a certain quantity of redundant fluid, AB contains redundant fluid in a greater degree, DF contains redundant matter, and LM is in its natural state. The problem may now be reduced to this. To find what proportion the redundant fluid in HK bears to that in AB, and what proportion this latter bears to the deficient fluid in DF?

To determine these proportions it is necessary that,

1st, The repulsion exerted by the redundant fluid in AB on the fluid in EN be precisely equal to the attraction exerted by the redundant matter of DF on the same canal.

2ndly, The repulsion exerted by the redundant fluid in HK on the whole fluid of the canal GC, balances the excess of the repulsion of the redundant fluid in AB on GC above the attraction of the redundant matter of DF on the same canal.

If we call the redundant fluid in AB, \( f \); the redundant matter in DF, \( m \); and the redundant fluid in HK, \( f' \); as the fluid in HK and AB is similarly disposed, (they being equal), and as it is probable that the redundant fluid in AB, and the redundant matter in DF, are similarly disposed, it follows, that their actions on the fluid in the canals will be similar, and proportional to their quantities nearly.

Let \( n \) be to \( n \), as the repulsion exerted by the fluid in AB on the fluid that would occupy CE, to the repulsion exerted by the fluid in AB on the fluid in EN or CG.

AB acts on EN with the force \( f \times (n-1) \); and DF acts on EN with the force \( m \); but these actions must balance each other, as LM is inactive. Therefore

\[ f \times (n-1) = mn, \quad \text{and} \quad m = f \times \frac{(n-1)^2}{n}. \]

If \( f \) repels the fluid in CG with the force \( fn \), \( m \) attracts the fluid in CG with the force \( m \times (n-1) \); but as \( m = f \times \frac{(n-1)^2}{n} \), the attractive force of \( m \) for CG will be \( f \times \frac{(n-1)^3}{n} \times (n-1) \): Therefore the repulsion of \( f \) is to the attraction of \( m \), as \( fn \) to \( f \times \frac{(n-1)^3}{n} \)

\[ = f^n : f \times (n-1)^3 = n : n-1^3. \]

Let \( r \) denote the repulsion of \( f \), and \( a \) the attraction of \( m \); then \( r : a = n : (n-1)^3 \); and \( r : (r-a) = n : n-1^3 = n : (2n-1) \).

But the repulsion of \( f' = r - a \); therefore \( n^3 : (2n-1) = f' : f' \), and \( f' = f \times \left( \frac{2n-1}{n^3} \right) \); or \( f = f' \times \left( \frac{n^3}{2n-1} \right) \).

If we suppose \( n^3 \) much greater than \( 2n-1 \), we shall have the quantity of redundant fluid in AB much greater than that in HK.

When EC is very small in proportion to AC, it will appear, on referring back to No. 382., that \( I \) is to \( n \) nearly as \( CE : CA \); and consequently \( n = AC \) nearly, and there is so little difference between \( \frac{n^3}{2n-1} \) and \( \frac{n^3}{2n-1} \), that we may take the former for the latter without any material error. Now we have \( f = f' \times \frac{n}{2} \) very nearly.

Suppose AC to represent 6 inches, and CE \( \frac{1}{5} \)th of an inch, we shall have \( n = 12a \) and \( f = 60f' \), or more exactly \( f' = \left( \frac{n^3}{2n-1} \right) = \frac{14400}{239} = 60 \frac{4}{5} \).

This, it will be remembered, represents the redundant fluid in HK; hence it will appear how great must be the redundancy in HK.

Again, when AB and DF are very near, \( n \) is a large number, and the deficiency in DF is nearly equal to the redundancy in AB. In the above example \( m \) is \( \frac{4}{5} \)ths of \( f' \), as \( m = f \times (n-1) \).

But though there is this great deficiency in DF, and redundancy in AB, DF is not electrical on the side next LM, nor is AB more electrical than HK; in short, this case affords another example of bodies being neutral while redundant or deficient, in addition to what was advanced in No. 313, 314.

It will readily occur to the reader, that cases exactly such as we have now stated never happen in any sensible course of experiment; but when the canals are very long in comparison of the diameters of the plate, and effect, when AB is very near DF, the proportions will not greatly vary.

We have been very particular in the examination of Mode of rethis case, because it is of great importance, and will afford us in explaining some of the principal phenomena, by degrees. If \( AB \) be touched by any body, this body will receive from it a part of its redundant fluid, but only a part; for only so much fluid will quit \( AB \) as is sufficient to render it neutral, while the touching body communicates with the ground. This will happen till the redundant matter in \( DF \) attracts fluid on the remote side of \( AB \) as much as the redundant fluid in \( AB \) repels it. The repulsion of \( AB \) on \( EN \) is now diminished, the attraction of \( DF \) will therefore prevail, and this will be no longer neutral. If now \( DF \) be touched, it may again be made neutral with respect to \( EN \); but \( AB \) will again repel the fluid in \( CG \), and being redundant on that side will again become electric. \( AB \) being touched again, loses more fluid, and \( DF \) becomes electric by deficiency. Thus by alternately touching \( AB \) and \( DF \), the redundancy in \( AB \) may be exhausted, and the deficiency in \( DF \) supplied.

But the equilibrium that is thus gradually produced may be effected at once. If we suppose a slender conducting canal \( abd \), brought very near the plates on the outside, so that the end \( a \) is near to \( A \), and \( d \) to \( D \); the first effect of the vicinity of \( a \) to \( A \), will be to cause the fluid in \( ab \) to recede a little from \( a \), by reason of the repulsion of the redundant fluid in \( AB \). Thus, redundant matter will be left at \( a \), and this will strongly attract redundant fluid from \( A \), and \( a \) may receive a spark. Should the fluid approach still nearer the surface at \( A \), the corresponding part of \( DF \) will be rendered more attractive, and by the fluid retiring from \( a \) along \( ab \), some of the natural fluid of this canal will be pulled towards \( d \); this increases the deposition of \( A \) to part with fluid, and of \( d \) to receive it, while \( a \) is disposed to give out and \( D \) to receive. Thus all contributes to favour the passage of almost the whole of the redundant fluid in \( AB \) to rush from \( AB \), by \( A \), along \( abd \) into \( DF \).

It is also clear that, without the canal \( abd \), there is a strong tendency of the fluid in \( AB \) for the matter in \( DF \), and that, of course, these plates will strongly attract each other.

The theorems we have now given respecting the deposition of the electric fluid are the result of mathematical reasoning, founded on the hypothetical nature of the fluid, and its assumed law of action. We shall conclude this section with relating the result of M. Coulomb's experiments on this subject, given in the Memoirs of the Academy for 1786 and 1787. M. Coulomb gives the following general theorem.

In a body of any form, \( AFBde \), fig. 114., which is supposed filled with fluid whose particles act on each other with a force that is inversely as the square of the distance, let there be raised a perpendicular \( ab \) infinitely small, and let a plane, perpendicular to \( ab \) at the point \( b \), divide the body into two parts; one \( dacb \), infinitely small, the other \( bAFBc \), of any determinate dimensions. Then the action of the particles composing the thin slice, estimated in the direction \( ab \), on the particle \( b \), must be equal to the action of the whole fluid in the rest of the body, if \( b \) be supposed at rest. Now, as whatever be the deposition of the fluid, the law of continuity will be the same, it is evident that if we take \( ab \) sufficiently small, the difference of the density at \( a \) and at \( c \) may be infinitely small; and that the action of \( dcbe \) will be infinitely near to an equilibrium with that of \( daeb \). Hence the action of the fluid in the rest of the body will be reduced to nothing, or will be infinitely small. But this cannot take place when the action of the mass at a finite distance on a particle of fluid, is infinitely small with respect to that of a particle in contact on the same particle, unless we suppose the quantity of fluid at a finite distance nearly nothing. It follows that the whole redundant fluid must be concentrated on the surface, and the interior parts be merely saturated.

M. Coulomb then proceeds to examine the density of the electric fluid in different bodies that are in contact. He first examines the density of two globes of different diameters in contact.

After a number of experiments, he gives the result in the following table, representing the manner in which the fluid is distributed between the two globes. The first column shows the proportion of the radii of the globes, the second the proportion of their surfaces, and the third the corresponding proportion of their densities. It must be remarked that this table shows only the proportional density of the globes, when after being separated, the fluid is uniformly diffused over their surfaces.

| 1 | 1 | |---|---| | 2 | 4 | | 4 | 16 | | 8 | 64 | | infinite | infinite |

Thus it appears, that the greater the proportion of the surfaces of the globes, the nearer the proportion of their densities approached to 2, but never attained this.

This is very different from the proportions between two spheres that communicate by a very long slender canal, which, as was shown in No. 390, contained quantities of fluid proportional to their diameters, and that the densities were inversely as the diameters; and this M. Coulomb found to agree very exactly with experiment.

M. Coulomb next proceeds to examine the density of the fluid in various parts of the surface of the globes in contact, in order to ascertain the distribution.

His method of proceeding was this. He hung a small circle of gilt paper to a thread of lac, fixed to a cylinder of glass or baked wood; the paper was varnished with some electric substance. The body to be examined was first touched with the paper circle, the electricity of which was then examined by means of his electrometer, and an elimination of the density of the spheres made on the supposition that the circle brought off one half of the electricity of the touched point.

The result of numerous experiments made with two globes in contact was as follows. The more unequal the globes were, the more the density of the small globe varied from the point of contact to the distance of 180°, and the nearer it approached to uniformity in the large globe, increasing rapidly from the point of contact, where it was 0, to 7° or 8° from that point. Thus, when he placed a sphere of 8 inches in contact with one of two inches, he found the density of the small globe indefinite till about 30° from the point of contact; that at 45° it was nearly the one-fourth of what it was at 90°, and hence it increased in the proportion of... Theory of electricity, where it was uniform. In the larger globe, on the other hand, the density was still about 4° or 5°; hence it increased rapidly, and from 3° to 18° it was nearly uniform.

From these results we may conclude that Mr Cavendish's mathematical demonstration of the uniform distribution of the fluid in a globe that communicates with another by a slender canal, is conformable to the fact.

A small globe between two equal larger globes, was found to possess the same electricity as the other two, when the proportion of their radii was not more than 5 to 1; when it was greater, the small globe showed no electricity.

Three equal globes being placed in contact, the density in the middle one was $\frac{1}{1.34}$ of that in the other two. When a small globe, after having been in contact with a larger one that was overcharged, was removed to a very small distance, the electricity of the small globe in the fronting point was opposite to that of the large one, at a little greater distance the small globe was neutral, and still farther off, it was redundant.

When the diameters of the globes were 11 and 8 respectively, the small globe at the fronting point was negative, till it was at the distance 1, when it was neutral, and beyond this it was positive. When the diameters were 11 and 4, the neutral distance was 2, and when they were 11 and 2, the distance at which the small globe was neutral was 2½.

It is indifferent whether the globes be solid, or consist merely of a thin shell. This circumstance is an additional proof of the justness of the theoretical investigation, on the supposition of the fluid being diffused over the surface, leaving the interior parts in a neutral state.

**Sect. II. An Application of the Theory of Aepinus and Cavendish to the principal Phenomena of Electricity.**

On an attentive consideration of the phenomena that have already passed under our review, and a careful comparison of these with the theory of positive and negative electricity, as improved by Aepinus and Cavendish, it will, we think, appear, that this theory is adequate to the explanation of the facts.

The comparison of the theory with the experiments may readily be made, and we have already hinted at it in several cases. We cannot, however, pursue this to any extent, and must restrict ourselves in the remainder of this chapter to the more important and interesting phenomena, leaving the rest to be supplied by the reader, for which purpose we have furnished him with ample materials.

We have already, in our illustration of the theory of Aepinus, so fully considered the phenomena of electric attraction and repulsion in a general view, that little more needs to be done, than to explain a few of the more remarkable cases.

The phenomena of attraction and repulsion may be reduced to the following simple propositions.

**Prop. I.**—If any body be electrified by any means, and if another body be brought near it, this latter becomes electrified by position.

We shall illustrate this proposition by the following simple experiment.

Let there be provided three metallic conductors, each supported on an insulating stand, such as A, B, C, fig. 115. Set these in a row, with their extremities touching each other, and at one end of the row, as at c, place a stand, to which is hung a ball electrometer with silk threads. On bringing an excited electric near a, the opposite end of the conductor, the pith ball will approach the end c. Care must, however, be taken not to bring the electric too near a, as to make the ball strike the opposite extremity; as in that case the experiment would come under our second proposition. When the excited electric is removed, the ball retires to its perpendicular situation. The same effect will be produced if the electrometer be placed at the side of the conductor, instead of its extremity, clearly shewing that it is affected by the conductor, and not immediately by the excited electric.

This is an instance of induced electricity, and is easily explained on the principles mentioned in No. 344. The approach of the excited electric to the end a of the compound conductor, renders this end deficient, if the electric be overcharged, or redundant if it be undercharged; and the opposite extremity is in the contrary state, and hence attracts the ball of the electrometer.

Although the opposite extremities of the conductor are in opposite states, the fluid is variously disposed in various parts of the conductor; as may be proved in the following manner. While the excited electric remains near a, take away the two extreme conductors, A and C, or, if only two have been employed, take away the remote one; remove the excited electric, and examine the parts of the conductor separately. The part A will be found entirely negative; if the electric were overcharged, C will be entirely positive; and if three pieces have been employed, the middle piece B will be faintly positive. If the pieces be again united, they will be found devoid of electricity. The same appearances will be more completely seen by forming a conductor of a series of metallic balls, lapsed by silk threads, one of which will be found scarcely electrical.

**Prop. II.**—When an insulated body is brought very near an electrified body, a spark passes between them, and the insulated body becomes electrified permanently by communication, while the electricity of the electrified body is diminished.

In this case the electricity imparted is of the same kind as that of the electrified body, positive if this were positive, and vice versa. The proposition may be illustrated by the same apparatus of the conductors and electrometers, and scarcely requires an explanation.

When the electricity is in a small degree, the spark is either very small or scarcely perceptible, but there is no doubt, that it takes place in all cases. The spark is owing to the sudden transference of a portion of the fluid from the electrified body to the unelectrified body.

**Prop. III.**—When an electrified body has communicated part of its electricity to another body, this latter is repelled, unless it has communicated its acquired store to other bodies.

The flying feather, the cork balls, and many other experiments related in the first chapter of Part III. Theory of amply illustrate this proposition, which expresses one of the most general facts in electricity.

Before the electrified body has communicated part of its electricity to the body presented to it, this latter is in its natural state; but after the communication, both are either redundant or deficient, and consequently repel each other, as appears from No. 323, 324.

From these general propositions we may deduce the following corollaries, an application of which will serve still further to illustrate and explain the phenomena of electric attraction and repulsion.

Cor. 1.—The vivacity of the appearances produced by a transference of fluid will be proportional to the quantity of fluid transferred.

Cor. 2.—The phenomena of communicated electricity will be more remarkable, the greater the conducting power of the bodies to which it is communicated.

It will have appeared from numerous experiments related in Part III., especially that of the dancing balls in No. 94, that an imperfect conductor, such as glass, permits the communication of electricity only in the point presented to an electrified body; whereas, when electricity is communicated to one point of a tolerably perfect conductor, such as the prime conductor of a machine, the whole conductor is instantly pervaded, and becomes electrical in every part.

Cor. 3.—When an electrified body has a free communication with a perfect conductor, its electricity cannot apparently be communicated to a body touched by it.

For the mass of the earth, with which the body communicates, bears so great a proportion to the body itself, that when the electricity of the latter is communicated to the former, it becomes imperceptible in both.

Cor. 4.—When an unelectrified body is presented to an electrified body, the former is first attracted, comes into contact with the electrified body, and is then repelled.

This corollary has been illustrated by numerous experiments; we may instance the dancing figures, &c., and the appearances are easily explained. The unelectrified body becomes electrical by induction; in consequence of this, it is attracted to the electrified body, from which it receives a spark, becomes electrified by communication, and being now in the same state with the electrified body, is repelled by it.

It will probably have been observed, in making the experiment of presenting a feather, or a pith ball, suspended by a string to the prime conductor, that they cling to the conductor, and are not repelled for some time. The reason of this is, that these bodies are imperfect conductors, especially when very dry, and hence their surface is not easily pervaded by the fluid; when this becomes equally diffused, they are repelled. The same circumstance explains why the balls of the common electrometer sometimes adhere together, and then separate with a jerk.

Cor. 5.—Electrical attraction and repulsion are not prevented by the interposition of unelectrified non-conducting substances.

A thin plate of glass may be interposed between the conductor and the pith-ball in the experiment of No. 399, and still, though the plate be very extensive, the electrometer will be affected.

Nay, an insulated electrified body may be covered with a glass bell, and it will yet attract a ball presented to it.

As this single circumstance affords one of the best arguments against the hypothesis of material electric atmospheres, which has been maintained, and is still of electric maintained, by some of our most eminent electricians; we shall take this opportunity of giving a brief account of this hypothesis, and stating the reasons which induce us to reject it.

It has been supposed, that the electric fluid is collected around the surface of an electrified body, forming a kind of atmosphere; and that on these atmospheres depended the action of these electrified bodies. If the reader will examine the plates of Lord Stanhope's Principles of Electricity, he will see the figures of conductors surrounded with a thinning margin, like the line of coasts and islands in a map.

This idea of electric atmospheres was first held at a very early period of the science by Otto Guericke, and afterwards by the academicians del Cimento, who contrived to render the electric atmosphere visible, by means of smoke attracted by, and uniting itself to a piece of amber, and gently rising from it, and vanishing as the amber cooled. But Dr Franklin exhibited this electric atmosphere with great advantage, by dropping rosin on hot iron plates held under bodies electrified, from which the smoke rose and encompassed the bodies, giving them a very beautiful appearance. He made other observations on those atmospheres: he took notice that they and the air did not seem to exclude one another; that they were immovably retained by the bodies from which they issued; and that the same body, in different circumstances of dilatation and contraction, is capable of receiving and retaining more or less of the electric fluid on its surface. However, the theory of electrical atmospheres was not sufficiently explained and understood for a considerable time; and the investigation led to many very curious experiments and observations. Mr Canton took the lead, and was followed by Dr Franklin. Messrs Wilcke and Epinus prosecuted the inquiry, and completed the discovery. The experiments of the two former gentlemen prepared the way for the conclusion that was afterwards drawn from them by the latter, though they retained the common opinion of electric atmospheres, and endeavoured to explain the phenomena by it. The conclusion was, that the electric fluid, when there is a redundancy of it in any body, repels the electric fluid in any other body, when they are brought within the sphere of each other's influence, and drives it into the remote parts of the body, or quite out of it, if there be any outlet for that purpose.

By atmosphere M. Epinus says, no more is to be understood than the sphere of action belonging to any body, or the neighbouring air electrified by it. Sig. Becaria concurs in the same opinion, that the electrified bodies have no other atmosphere than the electricity communicated to the neighbouring air, and not with the electrified bodies. And Mr Canton likewise, having relinquished the opinion that electrical atmospheres were composed of effluvia from excited or electrified bodies, maintained that they only result from an alteration in the state of the electric fluid contained in, or belonging to the air surrounding these bodies to a certain distance; for instance, that excited glass repels the electric fluid from it, and consequently beyond that distance makes makes it more dense; whereas excited wax attracts the electric fluid existing in the air nearer to it, making it rarer than it was before.

Among the supporters of this doctrine is Dr Peart of Gainborough, who has distinguished himself as a zealous opponent of the chemical theory of Lavoisier, the fallacy of which he has, in his own opinion, fully demonstrated. But Dr Peart's atmospheres are not those of most electricians; they consist of chemical elements, of ether and phlogiston, by the union and reciprocal action of which all the phenomena of electricity are effected. We are afraid of doing more than stating this leading principle of Dr Peart's hypothesis, lest we should share the fate of Mr Read, with whom the Doctor is very angry for only partially agreeing with him.

We must therefore refer such of our readers, as wish for more satisfaction on this head to the Doctor's pamphlets on electricity and magnetism, and on electric atmospheres.

It is perhaps a sufficient refutation of this doctrine of material atmospheres, that electrical attraction and repulsion may take place, where these atmospheres cannot, according to the general opinion, be formed. Thus, in the instance given above, it is scarcely conceivable, that the excited electric on one side of the glass pane, or bell, should so speedily extend its atmosphere to the other side of the pane, or, in the case of the bell, that it should extend it at all, so as instantaneously to affect an electrometer presented to the other side. Nay, it is well known, that an electrified body will affect a conducting wire, so as to render it positive at one end, and negative at the other, though the wire be completely enveloped in sealing-wax, or some other electric substance. It therefore becomes a question, how, if the interposed body be impermeable to the electric fluid, (and we see no reason to think that glass and other perfect electricities are not so), the electric atmosphere can be produced? The one atmosphere can, in this instance, produce the other only by acting at a distance on the particles of which this latter is to be formed. Even supposing that the one atmosphere could produce the other in this way, we should gain nothing by the supposition. It only supposes innumerable attractions and repulsions in place of one.

Dr Franklin whirled an electrified ball, suspended by a silk thread, many times about his head with great rapidity, and found that its electricity was not sensibly diminished by the motion. Now it is scarcely conceivable, that the electric atmosphere could remain attached to the ball under these circumstances, or that it could be so instantaneously formed, or renewed in every point of its revolution, as to be capable of acting the moment the motions were ended; for the electricity of the ball must in this way have been greatly lessened, or nearly exhausted; whereas Dr Franklin found that, when the air was very dry, the electricity of the revolving ball was, when the ball was stopped, not less than that of a similar ball that had remained for the same time in a state of rest.

We have said that we see no reason to think, that glass is permeable to the electric fluid. We are aware, that this permeability is supported by some electricians, and that experiments have been related in proof of their opinion. Among the most plausible of these, are the experiments of Mr Lyons of Dover, which may all be reduced to the following. A wire is brought from the outside of a phial charged by the knob, and terminates in a sharp point at a small distance from a thin glass plate; it is commonly introduced into a glass tube, having a ball at the end, and the point of the wire reaches to the centre of the ball; and another wire is connected with the discharging rod, and also comes very near, and frequently close to the other end of the glass, opposite to the pointed wire. With this apparatus he obtains a discharge, and therefore says that the glass is permeable to the electric fluid.

Dr Robison repeated most of Mr Lyons's experiments, and found that, in the above way, he did indeed procure discharges, but that these were very incomplete, and very unlike the full and audible discharge usually obtained; they were always very faint, except when the glass was perforated.

To terminate this long digression, it must be remarked, that the impermeability of electric fluids supposed in our only fixed theory, shews that the redundancy or deficiency induced in an overcharged or undercharged electric, does not extend beyond the surface; for, when the surface is rendered electrical by excitation in any way, the impermeability of the body prevents the redundant fluid from penetrating to any depth, or from expanding to supply the deficiency on the surface. Hence we find, that an excited electric, when plunged into water, quickly loses its electricity by communication with this conducting medium.

We must now return to our corollaries, of which we shall deduce one more.

Cor. 6.—As non-electrics are conductors, and as some electrics are excited by rubbing them with non-fated electrics, it will follow, that if the non-electrics be insulated and separated from the electric, the former will show signs of electricity as well as the latter, but that, while they remain together, no signs of electricity can be exhibited by either.

This corollary may be illustrated by numerous facts that have been related in the preceding parts of this article.

The sheets of paper in No. 19, showed no signs of electricity while in contact with the table; the sulphur in the experiments of Wilcke and Æpinus, was not electrical while within the metallic cups, &c.

When cases of this kind occur, in which two bodies, that would, when separated after mutual contact, show signs of opposite electricities, are, when united, said to compensate each other, the circumstance is easily explained.

In whatever way excitation is produced by friction or other means, which we do not pretend to explain, it must happen that the adjoining surfaces of two bodies rubbed together, must be in opposite states, and the one overcharged in the same degree as the other is undercharged. When the bodies, which we shall suppose to be two plates, are joined, so that the one exactly covers the other, they must be inactive; because a particle of moveable fluid in any part of one surface of the overcharged plate, will be as much attracted by the undercharged surface of the farther plate, as it is repelled by the overcharged surface of the nearer plate. As the surfaces are supposed equal, coincident, and equally electrical, their actions must balance each other. The action Theory of action of the united bodies will be expressed by $Fm'x$ Electricity. ($z-z'$) or $Fm'; z-z'$ being here $=0$.

But now again, if the plates be separated, a considerable part of the redundant fluid will fly back from the one surface to the other, being propelled thither by the repulsion of its own particles, and drawn by the attraction of the redundant matter in the other surface. But, as the electric is a non-conductor, it will retain a portion of fluid, or will remain deprived of a portion, in a stratum a little way within the surface, the two plates must, after separation, be in opposite states, and the non-electric plate, if it has been inflated before separation, will, after separation, appear electrified.

We shall close our consideration of electrical attraction and repulsion, by explaining two very beautiful experiments of Dr Franklin; one of which, the electrical well, has been described in No 79; the other shall be described presently.

It appears from Mr Cavendish's account of the deposition of fluid in a sphere, given in No 372, that when the sphere is overcharged, all the redundant fluid is crowded into the surface, leaving the internal parts in a neutral state. Now the vessel that represents the electrical well is exactly in this condition; the electrometer, therefore, when let down within the cavity of the vessel, cannot be affected, because all that space is neutral; but when the balls are raised above the brim of the vessel, they are affected, because they come within the sphere of action of the redundant surface.

The other experiment to which we allude, is that of the electrified can and chain, which is thus made.

Inflate a metallic can, or any other concave piece of metal, and place within it a pretty long metallic chain, having a silk thread tied to one of its ends. At the handle of the can, or to a wire proceeding from it, suspend a cork ball electrometer; then electrify the can, by giving it a spark with the knob of a charged phial, and the balls of the electrometer will immediately diverge.

If, in this situation, one end of the chain be gradually raised up above the top of the can, by the silk thread, while the lower end of the chain remains in it, the balls of the electrometer will converge a little, and more or less in proportion to the greater or less elevation of the chain above the top of the vessel. A similar experiment was made by Mr Ronayne, which is as follows:—He excited a long slip of white flannel, or a silk ribbon, by rubbing it with his fingers; then, by applying his hand to it, took off as many sparks as the excited electric would give; but when the flannel, &c. had lost the power of giving any more sparks in this manner, he doubled, or rolled it up; by which operation the contracted flannel, &c. appeared to strongly electrical, that it not only afforded sparks to the hand, brought near, but it threw out spontaneous brushes of light, which appeared very beautiful in the dark.

To explain this experiment, we must have recourse to an inference, that is easily deducible from the same theorem of Mr Cavendish: namely that in overcharged bodies of all shapes, the redundant fluid will be much more dense near the surface than in the more internal parts; and that it will be also denser in all elevated or protuberant parts of these bodies, as also near the extremity of oblong bodies; and in general, that the redundant fluid, or redundant matter, will bear a much nearer proportion to the surfaces of bodies, than to their quantities of matter. Hence we may perceive, that when the chain, in the above experiment, is lifted up, it will attract to itself a part of the denser fluid, leaving that of the surface of the vessel, to which the electrometer is attached, more rare; and consequently, the divergence of the balls will decrease, in proportion as the chain is more elevated above the rim of the cup. Mr Ronayne's experiment admits of a similar explanation.

The well known effects of points, in causing a quick discharge of electricity, seem to agree very well with points explained.

It appears from 391, that, if two similar bodies of different sizes are placed at a very great distance from each other, and connected by a slender canal, and overcharged, the force with which a particle of fluid placed close to corresponding parts of their surface is repelled from them, is inversely as the corresponding diameters of the bodies. If the distance of the bodies is small, there is not so much difference in the force with which the particle is repelled by the two bodies; but still, if the diameters of the two bodies are very different, the particle will be repelled with much more force from the smaller body than from the larger. It is indeed true, that a particle placed at a certain distance from the smaller body, will be repelled with less force, than if it be placed at the same distance from the greater body; but this distance is in most cases pretty considerable. If the bodies are spherical, and the repulsion inversely as the square of the distance, a particle placed at any distance from the surface of the smaller body, less than a mean proportion between the radii of the two bodies, will be repelled from it with more force than if it be placed at the same distance from the larger body.

We may probably, therefore, be well assured, that if two similar bodies are connected together by a slender canal, and are overcharged, the fluid must escape faster from the smaller body than from an equal surface of the larger; but as the surface of the larger body is greatest, it is not certain which body ought to lose most electricity in the same time; and indeed it seems impossible to determine positively from this theory which should, as it depends in a great measure on the manner in which the air opposes the entrance of the electric fluid into it. Perhaps, in some degrees of electrification, the smaller body may lose most, and in others the larger.

Let now ACB (fig. 116.) be a conical point, standing on any body DAB, C being the vertex of the cone; and let DAB be overcharged: Mr Cavendish supposes, that a particle of fluid placed close to the surface of the cone anywhere between b and C, must be repelled with at least as much, if not more, force, than it would, if the part A a b B of the cone was taken away, and the part a C b connected to DAB by a slender canal; and consequently, from what has been said before, it seems reasonable to suppose that the waste of electricity from the end of the cone must be very great in proportion to its surface; though it does not appear from this reasoning, whether the waste of electricity from the whole cone should be greater or less than from a cylinder of the same base and altitude.

All that has been here said relating to the flowing out Theory of electricity from overcharged bodies, holds equally true with regard to the flowing in of electricity into undercharged bodies.

But a circumstance which probably contributes as much as any thing to the quick discharge of electricity from points, is the swift current of air caused by them, as taken notice of in No 8 et seq. and which is produced in this manner.

If a globular body ABD is overcharged, the air close to it, all round its surface, is rendered overcharged by the electric fluid, which flows into it from the body; it will therefore be repelled by the body; but as the air all round the body is repelled with the same force, it is in equilibrium, and has no tendency to fly off from it. If now the conical point ACB be made to stand out from the globe, as the fluid will escape much faster in proportion to the surface from the end of the point than from the rest of the body, the air close to it will be much more overcharged than that close to the rest of the body: it will therefore be repelled with much more force; and consequently a current of air will flow along the sides of the cone from B towards C; by which means there is a continual supply of fresh air, not much overcharged, that the electricity would have but little disposition to flow from the point into it.

The same current of air is produced in a less degree, without the help of the point, if the body, instead of being globular, is oblong or flat, or has knobs on it, or is otherwise formed in such a manner as to make the electricity escape faster from some parts of it than the rest.

In like manner, if the body ABD be undercharged, the air adjoining to it will also be undercharged, and will therefore be repelled by it; but as the air close to the end of the point will be more undercharged than that close to the rest of the body, it will be repelled with much more force; which will cause exactly the same current of air, flowing the same way, as if the body was overcharged; and consequently the velocity with which the electric fluid flows into the body, will be very much increased. We believe, indeed, that it may be laid down as a constant rule, that the faster the electric fluid escapes from any body when overcharged, the faster will it run into that body when undercharged.

Points are not the only bodies which cause a quick discharge of electricity; in particular, it escapes very fast from the ends of long slender cylinders; and a swift current of air is caused to flow from the middle of the cylinder towards the end: this will easily appear by considering, that the redundant fluid is collected in much greater quantity near the ends of the cylinders than near the middle. The same thing may be said, but we believe in a less degree, of the edges of thin plates.

What has been just said concerning the current of air, serves to explain the reason of the revolving motion of Dr Hamilton's and Mr Kinnersley's bent pointed wires, (No 81.) for the same repulsion which impels the air from the thick part of the wire towards the point, tends to impel the wire in the contrary direction.

It is well known, that if a body B is positively electrified, and another body A, communicating with the ground, be then brought near it, the electric fluid will escape faster from B, at that part of it which is turned toward A, than before. This is plainly conformable to theory; for as A is thereby rendered undercharged, B will in its turn be made more overcharged, in that part of it which is turned towards A, than it was before. But it is also well known, that the fluid will escape faster from B, if A be pointed, than if it be blunt, though B will be less overcharged in this case than in the other; for the broader the surface of A, which is turned towards B, the more effect will it have in increasing the overcharge of B. The cause of this phenomenon is as follows.

If A is pointed, and the pointed end turned towards B, the air close to the point will be very much undercharged, and therefore will be strongly repelled by A, and attracted by B, which will cause a swift current of air to flow from it towards B; by which means a constant supply of undercharged air will be brought in contact with B, which will accelerate the discharge of electricity from it in a very great degree; and moreover, the more pointed A is, the swifter will be this current. If, on the other hand, that end of A which is turned towards B is so blunt, that the electricity is not disposed to run into A faster than it is to run out of B, the air adjoining to B may be as much overcharged as that adjoining to A is undercharged; and, therefore may, by the joint repulsion of B and attraction of A, be impelled from B to A, with as much, or more force, than the adjoining air to A is impelled in the contrary direction; so that what little current of air there is, may flow in the contrary direction.

We might here give an account of Coulomb's experiments on the dissipation of electricity into the air, and along imperfect conductors. But we must defer this to the article Electrometer, under which we shall describe the instrument with which they were made. We must now proceed to the theory of the Leyden phial.

In the 4th, 6th, and 7th chapters of the third part, we have related a considerable number of experiments, illustrating the phenomena of charged electrics. Before we examine the theory of the Leyden phial, it will therefore be necessary to consider the phenomena only in a simple case, and for this purpose we shall give an experiment, by which the late Dr Robison used to illustrate the theory of charged glaas.

Fig. 117 represents the professor's apparatus. G is the extremity of a prime conductor, on which is fixed a quadrant electrometer H. AB represents a round plate of tin-foil, pasted on a plate of glaas, the edges of which extend beyond the tin-foil about two inches. The plate of glaas is fixed to a wooden stand, so that it may be placed upright, and at any required distance from the conductor. DF is another plate of equal dimensions with AB, having a wire EN fixed in its centre, with its extremity N, terminating in a small ball, from which is hung a common ball electrometer. The wire also passes through a wooden ball O, which is fastened to the insulating stand P. It is necessary that the glaas plates be very clean and dry, and a little warm.

The conductor G is to be connected with the plate AB, by a wire reaching to the centre C. Now move the electrical machine slowly, till the index of the quadrant rise to 30° or 40°, and mark the number of turns required to produce this effect. Take off the electricity, Theory of and having removed the connecting wire GC, turn the machine again slowly, till the index be in the same situation. The difference in the number of turns in this latter case, from the former, will shew pretty nearly the expenditure of fluid necessary to electrify only the plate of tin-foil. This difference will be found very trifling, when a low degree of electricity is employed; and to this it is necessary to confine the electrification, to prevent too great a diffusion from the edges of the plate. Now replace the wire, and cause the index of the electrometer to point again at 30°; bring forward the plate DF, taking care to keep it just opposite and parallel to AB without touching it. No sensible change will be produced on the index, till the plate DF come within four or five inches of AB, and it may even be brought much nearer, without making the index sink more than two or three degrees, unless a spark pass between AB and DF. Remove DF again to the distance of two or three feet, and fasten to the ball N a piece of chain, or metallic thread, so that it may lie on the table. Now raise the electrometer again to 30°, and advance DF gradually towards AB. The index will gradually fall as DF advances, but will rise again to its former height, if DF be carried back to its original situation.

These appearances are easily explained in the principles laid down in No 392, 393. For as DF advances towards AB, the redundant fluid in the latter repels a part of the fluid in DF towards the remote end of the wire EN, as is shewn by the separation of the balls at N; hence an accumulation commences in AB, and the index of the electrometer HG falls just as if part of the fluid in the prime conductor were communicated to AB. When DF is made to communicate with the floor, much more electricity is repelled from DF, according as it approaches nearer to AB; but, by reason of the communication, the electrometer at N gives no signs of electricity.

If, instead of connecting AB with the prime conductor, we adapt to the wire GC, at the extremity G, a metallic plate of the same dimensions as AB, with an electrometer attached to it next AB, and if this apparatus be any how electrified, and the separation of the balls at H be noted, before DF, which communicates with the floor, be approached, on attending to the charges, it will be seen, that the divergency of the balls corresponds very nearly to the distance of DF, as is required by the theory.

Now, while the plates are near each other, especially if DF communicates with the floor, if we suspend a pith ball by a silk thread between AB and DF, the ball will be strongly attracted by either of these plates that is nearest to it, suppose DF; and, having touched this, it will be immediately repelled, and drawn towards AB, by which it will be again repelled to DF, and it will thus be driven backwards and forwards like the electrified spider described in No 126, as long as any electricity remains in either of the plates. In the mean time, the index of the electrometer at H will gradually descend, till the motion of the pith ball ceases.

All these appearances are more remarkable, according as the plates are nearer to each other, and when they come in contact, the phenomena are the most complete.

If, when the plates are charged, we approach one end of a bended wire, (having a downy feather at each end,) to the plate DF, and bring the other to AB, we shall observe the feathers spread out their fibres to the plates, and then the equilibrium will be restored, or the plates will be discharged.

Having, by means of this experiment, brought again into view the phenomena of charging and discharging a coated electric, we are prepared to explain the theory of the Leyden phial, which can easily be done by recurring to the important theorem of the disposition and actions of four parallel plates, so fully detailed in No 392.

The following observations will also afford some idea of the manner in which the fluid is disposed in the Leyden phial.

It fully appears from what has been said in No 499, that the electric fluid is not able to penetrate a plate of glass without breaking it; and yet it seems able to penetrate to a very small depth, we might almost say, an imperceptible depth, within the surface of the glass.

Let ACGM, fig. 118, represent a flat plate of glass, or any other substance which will not suffer the electric fluid to pass through it, seen edgeways; and let BbDd, and Eeff, or B'd' and E'f', as we shall call them for shortness, be two plates of conducting matter of the same size, placed in contact with the glass, opposite to each other; and let B'd' be positively electrified; and let E'f' communicate with the ground; and let the fluid be supposed either able to enter a little way into the glass, but not to pass through it, or unable to enter it at all; and if it is able to enter a little way into it, let B'b'd', or B'd' as we shall call it, represent that part of the glass, into which the fluid can enter from the plate B'b, and e'f' that which the fluid from E'f' can enter. By the above mentioned proposition, No 134, it appears that if b'e', the thickness of the glass, is very small in respect of b'd', the diameter of the plates, the quantity of redundant fluid forced into the space B'd' or B'b', (that is, into the plate B'd', if the fluid is unable to penetrate at all into the glass, or into the plate B'd', and the space b', together if the fluid is able to penetrate into the glass) will be many times greater than what would be forced into it by the same degree of electrification, if it had been placed by itself; and the quantity of fluid driven out of E'f' will be nearly equal to the redundant fluid in B'b'.

If a communication be now made between B'b and E'f', the canal NRS, the redundant fluid will run from B'b to E'f'; and if in its way it passes through the body of any animal, it will, by the rapidity of its motion produce in it that sensation, called a shock.

It appears from No 392, that, if a body of any size was electrified in the same degree as the plate B'd', and a communication was made between that body and the ground, by a canal of the same length, breadth, and thickness as NRS; that then the fluid in that canal would be impelled with the same force as that in NRS, supposing the fluid in both canals to be incomprehensible; and consequently, as the quantity of fluid to be moved, and the resistance to its motion, is the same in both canals, the fluid should move with the same rapidity in both; and there seems no reason to think that the case will be different, if the communication is made by canals of real fluid.

Therefore Therefore, in the opinion of Mr Cavendish, as great a shock would be produced by making a communication between the conductor and the ground, as between the two sides of the Leyden phial, by canals of the same length, and same kind. This seems a necessary consequence of this theory; as the quantity of fluid which passes through the canal is, by the supposition, the same in both; and there is the greatest reason to think, that the rapidity with which it passes, will be nearly, if not quite, the same in both.

It may be worth observing, that the longer the canal NRS is, by which the communication is made, the less will be the rapidity with which the fluid moves along it; for the longer the canal is, the greater is the resistance to the motion of the fluid in it; whereas the force with which the whole quantity of fluid in it is impelled, is the same whatever be the length of the canal. Accordingly it is found in melting small wires, by directing a shock through them, that the longer the wire, the greater charge it requires to melt it.

As the fluid in B is attracted with great force by the redundant matter in E, it is plain that if the fluid is able to penetrate at all into the glaas, great part of the redundant fluid will be lodged in b d, and in like manner there will be a great deficiency of fluid in e. But in order to form some estimate of the proportion of the redundant fluid, which will be lodged in b d, let the communication between E f and the ground be taken away, as well as that by which B d is electrified; and let so much fluid be taken from B d, as to make the redundant fluid therein equal to the deficient fluid in E. If we suppose that all the redundant fluid is collected in b d, and all the deficient in E f, so as to leave B d and E f saturated; then as the electric repulsion is inversely as the square of the distance, a particle of fluid placed anywhere in the plane b d, except near the extremities b and d, will be attracted with very much as much force by the redundant matter in e, as it is repelled by the redundant fluid in b d. Hence it follows, that if the depth to which the fluid can penetrate is very small in respect of the thickness of the glaas, but yet is such that the quantity of fluid naturally contained in b d or e is considerably more than the redundant fluid in B d; then, as the repulsion is inversely as the square of the distance, almost all the redundant fluid will be collected in b d, leaving the plate B d not very much overcharged; and in like manner E f will be not very much undercharged; if the repulsion were inversely as some lower power than the square, B d will be very much overcharged, and E f very much undercharged; and if the repulsion were inversely as some lower power than the square, B d will be very much undercharged, and E f very much overcharged.

It is a part of Dr Franklin's theory, that no electric fluid can be thrown into one side of a coated plate, unless an equal quantity be at the same time abstracted from the other side; and that consequently the charged plate contains no more fluid than before it was charged. We find, indeed, that one side of the plate will not receive a charge, unless the other side at the same time communicate with the ground. He infers the same consequence from the circumstance, that if a jar be discharged through a person when inflated, the person is not found electrified; the necessary consequence of which is, according to Dr Franklin, that any number of jars may be charged by the same turns of a machine, provided that the outside of one jar communicates with the inside of the next successively, while the outside of the last has a communication with the ground. He found, however, by experiment, that a greater number of turns was necessary, than his theory required; but he attributed this circumstance to the diffusion of the fluid into the air. But we learn from our theory that the redundant matter in the plate that communicates with the ground is less than the redundant fluid in the other plate in the proportion of n-1 to n; and that the proportion of redundant fluid in the next plate or jar is no greater. If we have any number of jars, the charge of the nth jar in the series, will be \( \frac{n-1}{m} \).

If the charge of the first jar or n=60, that of the 10th will =51 nearly.

Though a coated plate will not receive a charge, unless one side communicate with the ground, it may however be rendered electrical, as appears when we attempt to charge it while insulated. For when we attempt to electrify one side, the other gives a spark which proves this to be electrified also. Again, when a charged phial is discharged by means of an insulated discharger, it always remains electrical, positively or negatively, according as the body from which it was charged was positive or negative.

It was supposed by Wilcke, that when a jar is charged by connecting one side of it with the prime conductor, and the other with the rubber, it is neutral on both sides. But if this were the case, it could not be discharged; and in fact, it will be found by experiment to be equally active on both sides.

It is scarcely necessary to remark, that the theory of the Leyden phial, and that of a coated plate, are the verifying facts; and hence we have an easy method of comparing the theory with experiment, by taking two plates of the same kind of glaas, and of an equal thickness, but different in the extent of coated surface. If we charge both plates, by means of very long conducting wires attached to both sides, we are to measure how often the charge of the lesser plate is contained in the greater, which is easily done by the following method of Dr Cavendish.

When a jar is charged, observe the electrometer that Cavendish's is connected with it, and immediately communicate the charge to another equal jar, the perfect equality of measuring which has been previously ascertained by the methods, which will presently appear. Again, note the electrometer. This will give the elevation, that indicates one half, independent of all theory. Now electrify a jar, or a series of equal jars, to the same degree as the first, and communicate the charge to a coated plate of mirror glaas, discharging the plate after each communication, till the electrometer reaches the degree that indicates one half. This shows how often the charge of the plate is contained in that of the jar or row of jars.

Let the charge of the plate be to that of the jars as x to 1. Then by each communication, the electricity is diminished in the proportion of \( \frac{1+x}{1} \) to 1. If m communications have been made, it will be reduced in the proportion of \( \frac{1+x^m}{1} \) to 1. Therefore \( \frac{1+x^m}{2} = 2 \), and \( \frac{1+x}{2} = \sqrt{2} - 1 \). When When \( x \) is small in proportion to 1, we shall come very near the truth, by multiplying the number of communications by 1,444, subtracting 0.5 from the product. The remainder shews how often the charge of the plate is contained in that of the jars or \( \frac{1}{x} \).

The important discovery of Franklin, that the charge of coated glass resides in the glass, and not in the coating, led Beccaria to a no less important discovery; namely, that in a charged plate of glass, and probably of any other electric, there are several strata, inconceivably thin, that are alternately in a positive and negative state, and that the number of these strata increases as the electrification is continued.

This disposition of the surfaces of electric plates explains many phenomena; particularly the experiments with charged plates described in No. 151, and some curious appearances observed by Beccaria, and ranked by him under the head of vindicating electricity.

They are thus described by Mr. Cavallo:

1. \( AB, ab \), fig. 119, represents a plate of glass, vindicating electricity. Coated on both sides with the two metallic coatings, \( CD, ca \), which are not stuck to the plate, but only laid upon it.

From the upper coating \( CD \), three silk threads proceed, which are united at their top \( H \), by which the said coating may be removed from the plate in an insulated manner, and may be presented to an electrified electrometer as represented in fig. 120, in order to examine its electricity. \( FG \) is a glass stand, which insulates and supports the plate, &c.

2. Let the plate \( AB, ab \), be charged in the common manner, by means of an electrical machine, so that its surface \( AB \) may acquire one kind of electricity, (which may be called K) and the opposite surface \( ab \) may acquire the contrary electricity, (which we shall call L). Then, if the coating \( CD \) be removed from the plate, and be presented to an electrified electrometer, as represented in fig. 120, it will be found possessed of the electricity K, viz. of the same kind with that which was communicated to the surface \( AB \) of the glass plate; from whence it is deduced, that the surface \( AB \) has imparted some of its electricity to the coating. Now, this disposition of the charged plate to give part of its electricity to the coating, is what the learned F. Beccaria nominates the negative vindicating electricity.

3. If the coating be again and again alternately laid upon the plate and removed, its electricity K will be found to decrease gradually, till after a number of times (which is greater or less, according as the edges of the plate insulate more or less exactly), the coating will not appear at all electrified. This state is called the limit of the two contrary electricities; for if now the above-mentioned operation of coating and uncoating the plate be continued, the coating will be found possessed of the contrary electricity, viz. the electricity L. This electricity, L, of the coating is weak on its first appearance; but it gradually grows stronger and stronger till a certain degree; then infensibly decreases, and continues decreasing until the glass plate has entirely lost every sign of electricity.

By this change of electricity in the coating, it is deduced, that the surface \( AB \) of the glass plate changes property; and whereas at first it was disposed to part with its electricity, now, (viz. beyond the limit of the two contrary electricities) it seems to vindicate its own property, that is, to take from the coating some electricity of the same kind with that of which it was charged; hence this disposition was by F. Beccaria called the positive vindicating electricity.

4. This positive vindicating electricity never changes, though the coating be touched every time it is removed. It appears stronger, and lasts a very considerable time after the plate has been discharged; which is a very surprising property of glass, and probably of all good and solid electrics.

5. If, soon after the discharge of the plate, the coating be alternately taken from the plate, and replaced, but with the following law, viz., that when the coating is upon the plate, both coatings be touched at the same time, and when the coating is cut off, this be either touched or not: then the surface \( AB \) of the plate, on being uncoated every time, takes a quantity of electricity, which it alternately loses every time it is coated.

6. On removing the coating in a dark room, a flash of light appears between it and the glass, which is still more conspicuous if the coating be removed by the fingers being applied immediately to it, viz. not in an insulated manner, because, when the coating is not insulated, the glass plate can give to, or receive from it, more of the electric fluid, and that more freely, than otherwise.

7. It is observable, that in the negative vindicating electricity, the glass loses a greater or less portion of electricity in an inverse proportion of the charge given to the plate, viz. the part lost is greater when the charge has been the weaker; for in the positive vindicating electricity, the force of receiving electricity is the stronger, when the charge has been stronger, and contrarywise.

8. If, after every time that the coating \( CD \) is removed, the atmospheres \( E, e \), that is, the air contiguous to the surface of the glass plate, be examined, they will be found electrified as in the following table, viz. the threads of an electrometer, brought within one or two inches, or more, of the surfaces \( AB, ab \), will diverge with electricities contrary to those expressed in the table.

| During the time of the negative vindicating Electricity | During the time of the positive vindicating Electricity | |----------------------------------------------------------|--------------------------------------------------------| | If the plate has been charged | The air \( e \) is electrified \( L \) | | The air \( E \) | The air \( e \) |

The theory of coated glass naturally leads us to that of the electrophorus; for though this apparatus is not exactly similar to a charged plate, as has been supposed by some; there is yet a considerable resemblance in the phenomena.

We have given a description of the electrophorus, and of its effects in Chap. X. of Part III. where we also stated, that, for illustrating the theory, it was proper to Theory of to make the several parts of the apparatus of considerable thickness, as the more instructive but minute changes are thus greatly increased, though the showy and brilliant phenomena are not so remarkable. Fig. 121 represents a section of the three parts of the apparatus in contact, where ABCD is the electric cake, CDEF the sole, and ABHG the cover. They are here represented lying horizontally on each other; but for experiment, it will be most convenient to have them fixed vertically to glass supporters, furnished with leaden feet to keep them steady.

We might here give a mathematical explanation of the phenomena of the electrophorus; and the actions of every part of the apparatus might easily be stated by means of the propositions in No. 308 to 314, and the corresponding ones in No. 228—335, taking into consideration the true law of action. But as this would be going over again much of the ground that we have already trodden, where our readers might not be pleased with being obliged to follow us; we shall treat the subject in a manner somewhat more popular, the result, however, of strict mathematical reasoning.

Having related the general phenomena in No. 207, we have now to consider only the disposition of fluid in the various parts of the apparatus in various situations, and the mutual forces that operate between them.

We shall consider the instrument under various states.

1. When the cake is left to cool after being made, it becomes negative by cooling; and if it were by itself, the surface on both sides would be negative to a considerable thickness near the edges; and the fluid would probably grow denser by degrees towards the middle, where it would have its natural density. This disposition may be inferred from No. 371, 372. But as it cools in conjunction with the sole, the attraction of the redundant matter in the cake for the moveable fluid in the sole, must disturb its uniform diffusion in the sole, and cause it to approach the cake. And as this probably happens while the cake is still in a conducting state, the disposition of its fluid will be different from what is described above, and the final disposition of the fluid in the cake and sole will resemble that given in No. 371, where the plates may represent the cake and sole. It will be sufficient at present to consider the cake and sole as divided into only two strata; one containing redundant fluid, and the other deficient, neglecting the neutral stratum interposed between them in each. The cake then consists of a stratum ABbaA, containing redundant matter, and a stratum abCD containing redundant fluid; and the sole of a stratum DCnm containing redundant fluid; that is, all that belongs naturally to the space DCFE, and of a stratum mmFE, containing redundant matter. We may call this the primitive state of the cake and sole; and if this is once changed by communication with unelectrified bodies, it can never be recovered without new excitation.

2. If the sole is touched by a body that communicates with the ground, fluid will enter it, till the repulsion of the redundant fluid in the sole for a superficial particle is equal to the attraction of the redundant matter in the cake for the same particle. What we have said concerning infinitely extended plates rendered neutral on one side, may suffice to give a notion of the present disposition of the fluid in the sole. The inferior surface will be neutral; and the density of the fluid will increase towards the surface DC. The sole contains more than its natural quantity of fluid, but is neutral by the balance of opposite forces. Let it now be inflated. This may be called the common state of the electrophorus.

3. Place the cover GHRA on the cake. A particle Z, at the upper surface of the cover, must be more attracted by the redundant matter in the stratum ABba than it will be repelled by the redundant fluid in the remote strata; for the fluid in the cake is less than when it is in its natural state, and therefore Z is attracted by the cake. The redundant fluid which has entered the remote side of the sole is less than what would be sufficient to saturate the redundant matter of the cake, because it only balances the excess of the remote action of this matter above the nearer action of the compressed fluid in the sole, and this smaller quantity of redundant fluid acts on Z at a greater distance than that of the redundant matter in the cake. Therefore the particle Z, lying immediately within the surface GH, is on the whole attracted; some fluid will move toward the cake, and its natural state of uniform diffusion in the cover will be changed into a violent state, in which the fluid will be compressed on the surface AB, and abstracted from the surface GH. There will now be a stratum GgPH containing redundant matter, and another gBA containing redundant fluid. But this disposition will disturb the arrangement that had taken place in the sole, and had rendered it neutral on the interior surface. The particle Z situated in that surface, will be more repelled by the compressed fluid in the stratum gCA than it will be attracted by the equivalent more remote redundant matter in GHg. Fluid is now therefore disposed to quit the surface EF, and the sole will appear positively electric, but in a small degree only, if the cover be thin. All this may be observed by attaching a small ball electrometer to the lower surface of the sole, or touching the sole, with it, and then trying its electricity by excited glass, or sealing-wax.

4. A particle of fluid Z, placed immediately without the surface GH, is more attracted by the deficient stratum GHgq and by ABba, than it is repelled by the redundant strata beyond them, and hence the cover must be sensibly negative. This is the common state of the whole apparatus after setting on the cover. The lower surface of the sole is slightly positive, and the upper surface of the cover more sensibly negative. A smart spark will be seen between the apparatus and the finger, and fluid will enter, till the attraction of the redundant matter in ABba balances the repulsion of the redundant fluid in DCFE.

5. A spark may now be obtained from the sole; for its neutral it was faintly positive before, and there is now the additional action of the fluid that has entered into the cover. Part of the fluid in the sole is therefore disposed to fly to any body that is presented to it. But when this transference has taken place, the equilibrium at the surface GH is destroyed, and this surface again becomes negative, and will attract fluid, although the cover contains already more than its natural quantity. A small spark will therefore be seen between the cover and any conducting body presented to it. By touching it, the neutrality neutral or equilibrium may be restored at GH; but it will be destroyed again at EF, from which a positive spark may be obtained, leaving GH negative in its turn. This would go on for ever, in a series of communications continually diminishing so as at last to become insensible, if the three parts of the electrophorus be thin. This shews the necessity of making them otherwise, if the instrument be intended for illustrating the theory.

The equilibrium is at length completed at the surfaces GH and EF, both of which are neutral with respect to surrounding bodies, although both the cover and sole contain more than their natural share of electric fluid. This state of the apparatus may be called its neutral state; and it may be produced at once, instead of doing it by these alternate touches of GH and EF. If we touch at once both these surfaces, we shall have a bright, pungent spark, and a small shock. If this be the object of the experiment, the state No. 428, which gives occasion to it may be called the charged state of the electrophorus.

When the apparatus has been thus rendered neutral with respect to surrounding bodies, it may continue in this state for any length of time, without its capability of producing the other phenomena being diminished, provided that no fluid pass from the cover to the cake.

6. Now if the cover be removed to a distance, both parts of the apparatus will exhibit strong marks of electricity. For the cover contains much redundant fluid, and must therefore appear strongly positive; it will give a brisk spark, which may be employed for any purpose, particularly for charging a jar positively by the knob, if we just touch the cover with the knob. Again, the sole will attract fluid, or it will be negative, though it contains more than its natural quantity of fluid; it will therefore take a spark. The sole, therefore, in the absence of the cover, may be employed to charge a jar negatively by the knob. By being touched with the finger, or with the knob of a jar held in the hand, it will be reduced to the common state described in No. 426; and now all the former experiments may be repeated. We may call this the active or the charging state of the electrophorus.

7. If the electrophorus be not insulated, a shock may, however, be obtained, by touching first the sole, and then the cover, without taking off the finger; but will not be so smart as when the negative cover is touched at the same time with the sole. The difference will, however, be scarcely perceptible when the pieces are thin.

8. If the apparatus has not been insulated, the cover when put on will afford a spark, in the manner already mentioned, and this will be rather stronger than when it is insulated; for the fluid being allowed to escape from the sole, does not obstruct the entry of fluid into the cover. If then, without removing the finger from the cover, we touch the sole, we feel nothing; but if we first touch the sole, and then, without removing the finger from it, touch the cover, we shall obtain a shock. By this series of alternate touches, the period of the electrophorus is completed. For it is first charged or rendered neutral, by touching the plates in contact; then, by touching both when separate, the whole is reduced to the common state. When after having been in the neutral state they are separated, they have opposite electricities, the sole having that of the cake. When brought together, each in the common state, they have opposite electricities, the cover having that of the cake.

9. By being long exposed to the air without the Method of cover, the electrophorus gradually loses its activity. Renewing this may however be again restored in several ways. One of the most obvious methods is, to produce a fresh excitation of the resinous cake; and this is best done by rubbing it with a piece of new flannel, of cat or hare's skin, or, what answers still better, a piece of mole skin. This friction renders the cake negative. It may also be electrified negatively, by placing on it a jar charged negatively in the inside, and then touching the knob of the jar with any conducting body that communicates with the ground. By this means it may be very strongly excited, if the jar be large, and if the cake be covered with a piece of tinfoil; that comes closely in contact with its whole surface. But one of the most expeditious and effectual methods of restoring the energy of the cake, will be to electrify it by means of an electrical machine, while the surface of the cake is connected with the rubber.

The only important part of the theory of electricity which we have yet to consider, is that of the condenser, but as this will be greatly elucidated by an application of Coulomb's experiments on inflators, we shall delay till we give an account of these in the article ELECTROMETER.

PART V.

ON ATMOSPHERICAL ELECTRICITY.

THE phenomena of electricity, that we have hitherto described, are sufficiently curious, and many of them extremely interesting; but they are trifling, when compared with those that are now to come under our consideration. In the present part of our article, we are to view the electric fluid as one of the principal agents, employed to produce some of the most remarkable and astonishing phenomena of nature. We are about to prove, by a series of the most satisfactory experiments, that thunder and lightning are merely the effects of a vast explosion of accumulated electricity in the atmosphere.

CHAP. I. Of Thunder.

Sect. I. Of the Identity of Electricity and Lightning.

It is not surprising that the experiments in which the electric spark is made to produce the effects which we have recounted in the third part of this article, should have led philosophers to conceive a similarity between these effects, and those produced by lightning.

Dr Wall and Mr Grey seem to have fancied a resemblance between thunder and the snapping noise produced by applying the fingers to an excited electric; but how such a resemblance should strike them, is not easy to conceive; and indeed it seems to have been merely a bold conjecture.

The abbé Nollet appears to have formed the first rational idea of their similitude, and expresses himself on the subject in the following remarkable manner.

"If any one should take upon him to prove from a well connected comparison of phenomena, that thunder is in the hands of nature what electricity is in ours; that the wonders which we now exhibit at our pleasure, are little imitations of those great effects that frighten us, and that the whole depends upon the same mechanism: if it can be demonstrated that a cloud prepared by the action of the winds, by heat, by a mixture of exhalations, &c., is opposite to a terrestrial object; that this is the electrified body, and at a certain proximity to that which is not: I avow that this idea, if it was well supported, would give me a great deal of pleasure; and in support of it, how many specious reasons present themselves to a man who is well acquainted with electricity! The universality of the electric matter, the readiness of its action, its inflammability, and its activity in giving fire to other bodies, its property of striking externally and internally even to their smallest parts, the remarkable example we have of this effect in the Leyden experiment, the idea which we might truly adopt in supposing a greater degree of electric power, &c., all these points of analogy which I have been some time meditating, begin to make me believe, that by taking electricity for the model, one might form to one's self, in respect to thunder and lightning, more perfect and more probable ideas than have hitherto been offered."

But the first electrician who formed a plan for ascertaining the truth of this hypothesis, was Dr Franklin, who truly realized the fable of Prometheus in bringing down fire from heaven.

Before we relate Dr Franklin's experiments, we shall state the points of resemblance which led him to think of making them.

He begins his account of the similarity of the electric fluid and lightning, by cautioning his readers not to be flattered at the great difference of effects in point of degree; since that was no argument of any disparity in their nature. It is no wonder, says he, if the effects of the one should be so much greater than those of the other. For if two gun-barrels electrified will strike at two inches distance, and make a loud report; at how great a distance will 10,000 acres of electrified cloud strike and give its fire, and how loud must be the crack!

1. Flashes of lightning, are generally seen crooked and waving in the air. The same is the electric spark always, when it is drawn from an irregular body at some distance. He might have added, when it is drawn by an irregular body, and through a space in which the best conductors are disposed in an irregular manner, which is always the case in the heterogeneous effects of atmosphere of our globe.

2. Lightning strikes the highest and most pointed objects in its way, preferably to others; as high hills and trees, towers, spires, masts of ships, points of spears, &c. In like manner, all pointed conductors receive or throw off the electric fluid more readily than those which are terminated by flat surfaces.

3. Lightning is observed to take the readiest and best conductor; so does electricity in the discharge of the Leyden phial.

4. Lightning sets fire to inflammable bodies; so does electricity.

5. Lightning, as well as electricity, fuses metals.

6. Lightning rends some bodies. The same does electricity.

7. Lightning has often been known to strike people blind. And a pigeon, after a violent shock of electricity, by which the Doctor intended to have killed it, was observed to have been struck blind likewise.

8. Lightning destroys animal life. Animals have likewise been killed by the shock of electricity.

9. Magnets have been observed to lose their virtue, or to have their poles reversed by lightning. The same effect has been produced by electricity.

Reasoning on the similarity of these effects, he formed the bold attempt to draw down lightning from the clouds, and examine by experiment whether he could produce effects similar to those of nature. Having observed the effects of pointed conductors in attracting the electric fluid more easily than those of any other form, he conceived that pointed rods of iron, fixed in the air, when the atmosphere was loaded with lightning, might draw from it the matter of thunderbolts, without noise or danger, into the body of the earth. His account of this supposition is given by himself in the following words: "The electric fluid is attracted by points. We do not know whether this property be in lightning; but since they agree in all the particulars in which we can already compare them, it is not improbable that they agree likewise in this. Let the experiment be made."

In the year 1752, while waiting for the erection of a spire in the city of Philadelphia, not imagining that a pointed rod of moderate height could answer the lightning purpose; at last it occurred to him, that, by means of a common kite, he could have readier access to the clouds, higher regions of the atmosphere than any other way whatever. Preparing, therefore, a large silk handkerchief, and two cords sticks of a proper length on which to extend it, he took the opportunity of the first approaching thunder-storm to take a walk into a field where there was a field proper for his purpose. But dreading the ridicule which too often attends unsuccessful attempts in science, he communicated his intention to nobody but his son, who assisted him in raising the kite. A considerable time elapsed before there was any appearance of success. One very promising cloud had passed over the kite without any effect; when, just as he was beginning to despair, he observed some loo loose threads of the hempen string to stand erect, and avoid one another, just as if they had been suspended by the conductor of a common electrical machine. On this he presented his knuckle to a key which was fastened to the string, and thus obtained a very evident electric spark. Others succeeded, even before the string was wet; but when the rain had begun to descend, he collected electric fire pretty copiously. We are told, that when he saw the fibres of the string erect themselves, he uttered a deep sigh, and wished that moment to be his last, feeling that by this discovery his name would be immortalized. He had afterwards an insulated iron rod to draw the lightning into his house; and performed almost every experiment with real lightning, that had before been done with the artificial representations of it by electrical machines. That he might lose no opportunity of making his experiments, he connected two bells with his insulated rod; and these by their ringing, gave him notice whenever his apparatus was electrified by the lightning.

Although we have recounted Dr Franklin's experiments first, he was not however, the first who verified his own hypothesis. This was done in France, about a month before Dr Franklin's experiments with the kite.

The most active persons were two French gentlemen, Mefris Dalibard, and Delor. The former prepared his apparatus at Marly la ville, situated about five or six leagues from Paris; the latter at his own house, which was on some of the highest ground in that capital. Mr Dalibard's apparatus consisted of an iron rod, 40 feet long, the lower extremity of which was brought into a fenny box where the rain could not enter; while on the outside, it was fastened to three wooden poles, by silken strings defended from the rain. This machine was the first that happened to be favoured with a visit from the ethereal fire. Mr Dalibard himself was not at home; but, in his absence he had intrusted the care of his apparatus to one Coifier, a joiner, who had served 14 years among the dragoons, and on whose courage and understanding he could depend. This artificer had all the necessary instructions given him, and was desired to call some of his neighbours, particularly the curate of the parish, whenever there should be any appearance of a thunder-storm. At length the long-expected event arrived. On Wednesday the 10th of May 1752, between two and three in the afternoon, Coifier heard a pretty loud clap of thunder. Immediately he ran to the machine, taking with him a vial furnished with a brafs wire; and presenting the wire to the end of the rod, a small spark issued from it with a snap like that of a spark from an electrified conductor. Stronger sparks were afterwards drawn, in the presence of the curate and a number of other people. The curate's account of them was, that they were of a blue colour, an inch and a half in length, and smelled strongly of sulphur. In taking them, he received a stroke on his arm, a little below the elbow; but he could not tell whether it came from the brafs wire inserted into the phial, or from the bar. He did not attend to it at the time, but the pain continuing, he uncovered his arm when he went home, in the presence of Coifier. A mark was perceived round it, such as might have been made with a blow of the wire on his naked skin.

Eight days after, Mr Delor witnessed the same appearances at his own house, though only a cloud passed over, without either thunder or lightning. His apparatus differed little from that of Mr Dalibard, except that his rod was 99 feet high, and answered rather better than that of the other gentleman. As it was found that only a small quantity of electric fluid could be collected by a single pointed rod, these experimentalists added to this apparatus a number of insulated iron bars, communicating with the pointed iron conductor, constituting what they called a magazine of electricity.

Dr Franklin having proved the identity of lightning and electricity, was desirous of ascertaining whether the electricity produced from the clouds was positive or negative. The first time he succeeded in making an experiment for this purpose, was on the 12th of April 1753, when the lightning appeared to be negative. Having found that the clouds electrified negatively for eight successive thunder-gusts, he concluded that their electricity was always negative, and set about forming a theory to account for this. But he afterwards found he had concluded too soon. For on the sixth of June following, he met with one cloud which was electrified positively; upon which he corrected his former theory, but did not seem able perfectly to satisfy himself with any other. The Doctor sometimes found the clouds would change from positive to negative electricity several times in the course of one thunder-gust, and he once observed the air to be strongly electrified during a fall of snow, when there was no thunder at all.

The experiments of Dr Franklin and M. Dalibard, were soon known over all Europe, and the electricians of every country were eager to participate in the glory and satisfaction to be derived from such grand undertakings. M. M. Mazeas, Monnier, and de Roma in France, Canton and Willon in England, and above all, Beccaria in Italy, made a number of interesting experiments on the electricity to be drawn from the clouds, and soon discovered that signs of electricity might be obtained, not only during thunder-storms, but almost at all times, and in every kind of weather. But before we relate these observations, we must conclude our present subject. We shall only here describe Dr Priestley's method of constructing a thunder-rod for making such observations.

"On the top of any building, which will be the most convenient if it stand upon an eminence, erect a conducting pole as tall as a man can well manage, having on the top of it a solid piece of glass or baked wood a foot in length. Let this be covered with a tin or copper vessel in the form of a funnel, to prevent its ever being wetted. Above this, let there rise a long slender rod, terminating in a pointed wire, and having a small wire twisted round its whole length, to conduct the electricity the better to the funnel. From the funnel make a wire descend along the building, about a foot distance from it, and be conducted through an open sash, into any room which shall be most convenient for making the experiments. In this room let a proper conductor be insulated, and connected with the wire coming in at the window. This wire and conductor, being completely insulated, will be electrified whenever there is a considerable quantity of electricity in the air; and notice will be given when it is properly charged, either..." by Mr Canton's balls hung to it, or by a set of bells.

Sect. II. Of the Phenomena and Effects of Lightning.

A thunder-storm commonly commences in the following manner. At first a low dense cloud begins to form in a part of the atmosphere which was previously clear; this cloud increases fast, but only from its upper part, and spreads into an arched form, appearing like a large heap of cotton wool. Its lower surface is generally level, as if it reeled on a smooth plane. The wind is all this time very gentle, and frequently it is imperceptible.

Numberless small ragged clouds, like teazled flakes of cotton, soon begin to make their appearance, moving about in various directions, and perpetually changing their irregular surface, appearing to increase by gradual accumulation. As they move about, they approach each other, and appear to stretch out their ragged arms towards each other; they do not often come in contact, but after approaching very near each other, they evidently recede, either in whole, or by bending away their ragged arms.

While this irregular motion continues, the whole mass of small clouds gradually approaches towards the large cloud which first appeared, and with which they finally coalesce; frequently, however, uniting with each other into larger masses, before the general coalescence takes place. The upper cloud often increases by accretion of fresh vapour, without any affluence from the smaller masses. When this happens, its lower surface, which was before level and regular, becomes ragged, and stretches out its irregular tatters towards each other, and towards the earth. The clouds now thicken fast, moving about swiftly in all directions, and flashes of lightning are seen to dart from one cloud to another; the wind now rises or increases, generally blowing in squalls. The lightning becomes more frequent, striking between the clouds and the earth, often in two places at once; flashes of various shapes and various brilliancy are produced, and frequently a vast expanse of horizon appears in one blaze of light. The thunder is now heard to roar at a distance, gradually approaching nearer, and soon succeeded by heavy rain.

The circumstances to be noticed as attending a thunder-storm, chiefly respect the form and colours of the lightning, the sound of the thunder, and the deviations produced when an explosion takes place between a cloud and some imperfectly conducting body on the surface of the earth.

The form of the flash is various, but in ordinary lightning it is generally angular, or zig-zag; this zig-zag is sometimes larger than at others, and in some instances the flash is divided into several distinct currents. These diversities might be expected from the heterogeneous nature, and various conducting power, of the several substances which float in our atmosphere. As these substances are placed in no certain order, the electric spark, in passing through the air, and striking successively from one of these bodies to another, as so many stepping stones irregularly placed, can seldom observe the same tract, and hence its zig-zag appearance.

Sometimes the flash appears as one dense ball of fire, especially when it strikes from a cloud to any part of a building, when it is generally described as a globe of fire falling on the building.

The colours of the flash are also various; pale straw colours of colour, vivid yellow, and various shades of blue, are the most prevailing tints. These various colours probably depend on the different density of the air through which the light has passed, or perhaps, on its different nature. We found, when relating the experiments of Mr Morgan on the appearance of the electric light in rarefied air, that its colour varied with the degree of rarity produced in the exhausted receiver; and from the experiments on passing the electric spark through various gases, we found that the colour of the light varied considerably with the nature of the gas through which it passed.

Lightning very often appears without being succeeded by thunder, but we believe there is scarcely an instance where the latter is not preceded by the former; we say, scarcely an instance, for we have on record in the 77th volume of the Philosophical Transactions, an account of a thunder-storm that happened on the banks of the Tweed in 1785, in which an explosion took place that killed a man and two horses, but was not preceded by any flash. The respectable recorder of this account, Mr Brydone, was not himself a witness of the accident, but could not learn from the persons whom he interrogated, (two Scotch peasants), who had seen the accident, that there was any preceding flash of lightning. In such singular circumstances, and with such doubtful authority, we should be disposed to suspend our belief, and, until some similar instance better authenticated shall occur, to take it for granted, that a clap of thunder is always preceded by a flash of lightning.

The sound which attends the explosion of the lightning, varies according to the distance from which it is heard, and the nature of the country where the storm takes place. At a little distance, it is generally a hoarse grumbling noise, which appears to extend through a considerable part of the atmosphere, and gradually dies away. If it be heard very near, the crash is instantaneous, and exactly similar to the explosion of a cannon, when we are very near it at the time of its being fired.

When the explosion begins very near, the snap begins with great sharpness, and for some time resembles the violent tearing of a piece of strong silk; but it becomes more mellow as it proceeds to a greater distance.

If the country where the storm happens be high and irregular, where there are numerous objects capable of reverberating the sound, the explosion consists of a long and broken succession of claps, the loudness of which varies more according to the nature and circumstances of the reverberating objects, than according to the length of time which intervenes between the claps. In a level and low country, where there is no diversity of reverberating objects, and particularly at sea, the series of explosions is regular, and their loudness decreases as the length of time increases.

The explosion of thunder differs from the snap produced by the electric spark, or even the explosion of a jar or a battery, not only in its degree of loudness, but in its nature; it is a long-continued, rumbling, unequalable noise. The long-continued roar of thunder, is certainly owing to the commencement and termination of the explosion reaching our ears at different periods of time; and the unequally loud rumbling noise is owing to the different parts of the explosion striking the ear in a different manner.

It will not be improper here to mention the method by which the distance of a thunder-stroke may be ascertained. By observing the flash, and counting, by means of a watch with a second hand, the number of seconds which elapse between the appearance of the flash and the commencement of the roar, this may be easily effected; for we know that sound travels at the rate of 186,708 feet in a minute; by reducing the time observed between the flash and the report, into seconds, and allowing for each its proper number of feet, we obtain, with sufficient accuracy, the distance of the stroke from the place of observation.

To understand the manner in which the explosion of thunder is produced, we must observe, that the air of the atmosphere is often arranged in strata, and these strata are bounded by clouds. That the clouds are stratified, is very evident. From various causes, to be explained hereafter, these strata, or the opposite surfaces of a particular stratum, are possessed of opposite states of electricity, or the stratum becomes charged as the plate of air between the two coated boards, described in No. 235. Numerous experiments have proved, that during a thunder-form, there is a contemporaneous accumulation and deficiency of the electric fluid, or that there are two parts in the atmosphere, that are in the opposite states of positive and negative electricity. Hence we may easily conceive the nature of the explosion; for when the accumulation and deficiency, on the opposite surfaces of the stratum of air, have attained a certain height, a discharge must take place, similar to the spontaneous discharge of a Leyden phial.

The explosion commonly takes place in the heavens, and is merely the restoration of the equilibrium between opposite clouds; but in some instances, the explosion happens between the clouds and the earth. In this latter case, it is believed by most electricians, that the earth is in the negative state; but Mr G. Morgan is of opinion, that the deficiency is never in the earth, but in some other cloud to which an easier passage is found through so good a conductor as the wet earth, than through the air, which is an imperfect conductor. Mr Morgan brings a great many arguments in support of his opinion, but for these we must refer to his lectures. It is of little consequence to our present purpose, whether the deficiency is in the earth or in some adjacent cloud; it is sufficient to know, that lightning sometimes strikes from the cloud to the earth, or from the earth to the clouds. When this happens, and when the accumulated fluid comes in contact with any body that is an imperfect conductor, such as trees, buildings, &c., it produces those devastations which are sometimes the attendants of severe thunder-storms; these, therefore, we are now to consider.

Lightning, when it strikes a building, for the most part attacks the highest parts of it, as the chimneys, or spires, especially if they are surmounted by any metallic work, which is always the case with spires of churches, and not unfrequently on chimney-tops, where iron machines have been placed to prevent smoking. In most of the cases which have been recorded of spherical houses being damaged by lightning, it has entered by the chimney, down which it seems to be conducted by the smoke and foot. Having entered the house, it commonly proceeds to the bell and nearest conductors in its passage, particularly bell-wires, gilt cornices, frames of pictures, and other gilded furniture; these it commonly destroys, fusing, and very often oxidizing, the metal as it passes along. Some very remarkable instances are related of the power of lightning in fusing metals; we have heard of the fusion of bells, of large chains, and of iron conducting-rods near an inch in diameter; but the authority on which these facts are related does not seem worthy of our implicit confidence. There are instances, however, sufficiently credible, where the pointed end of a conductor has been rounded, parts of leaden spouts melted, and the edge and point of a knife completely fused. But in general the bell wires of a house suffer the most; these are always shortened and very commonly melted in some parts; while in others, they are entirely dissipated in oxide, marks of which are very commonly visible on the walls.

It has been disputed, whether the fusion of metals by lightning be such a chemical fusion as is occasioned by fire, or what is called a cold fusion. Dr Franklin was of the latter opinion; the principal arguments for which, metals by arc, that money has been melted in a person's pocket, and a sword within its scabbard, without the pocket or scabbard being destroyed. We confess ourselves at a loss to conceive what is meant by a cold fusion, as we have no idea of a metallic body being fused at all, i.e., reduced into those globular forms which metals that have been subjected to the action of lightning and electricity usually assume, but by the power of a degree of heat, which would, when applied to bodies sufficiently inflammable, set these on fire.

That the explosion of lightning frequently does this, is sufficiently certain. In the ordinary cases, indeed, of fires to a building's being struck by lightning, inflammation does not ensue, because the parts of the building through which the fluid passes, are either in their nature very little inflammable, or are so hard and dense in their texture, that they are not easily inflamed. But when the building attacked contains matters of a very combustible nature, such as hay, straw, and more especially gun-powder, a fire is very commonly the consequence; and accordingly, we every now and then hear of instances of stables being burned, and powder magazines blown up by lightning.

When the lightning in its course meets with any obstruction, as in passing through a body which is an imperfect conductor, it overcomes this obstruction by forcing its passage through the resisting body; hence, we very commonly find large beams shattered, and stones and bricks either driven from their places, or split and perforated in an unequal manner. Frequently, the lightning will forsake one conducting body, as the handle of a bell-wire, and strike through the wall of the room, attracted by some conductor, either of greater power or larger dimensions, such as a kitchen grate, on the other side. This effect of lightning is exactly similar to the perforation and rending of bodies by electricity, as we related when treating of the mechanical effects of that power; it is undoubtedly owing to the sudden expansion of the air or moisture contained within the pores of the refilling body.

We have seen that animals are destroyed by lightning; but the effects of this power on the animal body come to be explained with more propriety in a future part of this work, where we shall treat of the effects of electricity on vegetable and animal life.

We shall here only relate the unfortunate death of the celebrated Professor Richman of St. Petersburg. This happened on the 6th of August 1753, as he was making experiments on lightning drawn into his own room. He had provided himself with an instrument for measuring the quantity of electricity communicated to his apparatus; and as he stood with his head inclined to it, Mr. Solokow an engraver, who was near him, observed a globe of blue fire, as big as his fist, jump from the instrument, which was about a foot distant, to Mr. Richman's head. The professor was instantly dead, and Mr. Solokow was also much hurt. The latter, however, could give no particular account of the way in which he was affected; for, at the time the professor was struck, there arose a sort of steam or vapour, which entirely benumbed him, and made him sink down to the ground, so that he could not even remember to have heard the clap of thunder, which was a very loud one. The globe of fire was attended with an explosion like that of a pistol; the instrument for measuring the electricity (called by the professor an electrical gnomon) was broken to pieces, and the fragments thrown about the room. Upon examining the effects of the lightning in the professor's chamber, they found the door-case half split through, and the door torn off and thrown into the room. They opened a vein in the body twice, but no blood followed; after which, they endeavoured to recover life by violent friction, but in vain; upon turning the corpse with the face downwards during the rubbing, an inconsiderable quantity of blood ran out of the mouth. There appeared a red spot on the forehead, from which spurted some drops of blood through the pores without wounding the skin. The shoe belonging to the left foot was burst open, and uncovering the foot at that part, they found a blue mark; from whence it was concluded, that the electric matter having entered at the head, made its way out again at that foot. Upon the body, particularly on the left side, were several red and blue spots, resembling leather shrunk by being burnt. Many more also became visible over the whole body, and particularly over the back. That upon the forehead changed to a brownish red, but the hair of the head was not fringed. In the place where the shoe was unstriped, the flocking was entire; as was the coat everywhere, the waistcoat only being fringed on the fore flap where it joined the hinder; but there appeared on the back of Mr. Solokow's coat long narrow streaks, as if red-hot wires had burned off the nap, and which could not well be accounted for.

When the professor's body was opened next day, the cranium was very entire, having neither fissure nor contra-fissure; the brain was sound; but the transparent pellicles of the windpipe were excessively tender, and easily rent. There was some extravasated blood in it, as also in the cavities below the lungs. Those of the breast were quite sound; but those towards the back of a brownish black colour, and filled with more of the blood above mentioned. The throat, the glands, and the small intestines, were all inflamed. The fingered leathery-coloured spots penetrated the skin only. In 48 hours the body was so much corrupted, that they could scarcely get it into a coffin.

From the dangers to which persons and buildings are distance at exposed from lightning, it becomes an object of importance to ascertain the distance at which they may be considered as secure from its influence. The following dangerous observations of Mr. G. Morgan on this subject are related with ingenuity and good sense.

"The greatest danger of a thunder-storm lies between Morgan's two nearest extremities of the correspondent parts of the charged atmosphere, or in that interval of un-electrified air which is always found to separate the positive from the negative portion of the loaded cloud: but on either side of this interval, the further you get into the positive or the negative, the more does the power of injuring diminish.

"The idea which I now wish to impress, will be illustrated by the following circumstances of fact.

"Take a Leyden phial, five inches in diameter, and thirteen or fourteen inches in height. On the inside, let the coating rise till its upper edge be two inches and a half from the rim of the vessel. On the outside, let the coating rise no higher than one inch from the bottom. When the phial is thus coated, let it be charged, and a spark will pass from the tin-foil on the outside to that on the inside; but its form will resemble that of a tree, whose trunk will increase in magnitude and brilliancy, and consequently in power, as it approaches the edge, owing to ramifications which it collects from all parts of the glass. Within two inches of the edge it becomes one body or stream, and along that interval its greatest force acts.

"When two clouds, or the two correspondent parts of a cloud, have their equilibrium restored by a discharge, the appearances are exactly similar to those of the preceding experiment. Each extremity of the flash is formed by a multitude of little streams, which gather into one body, whose power is undivided in that interval only which separates the positive from the negative.

"In this country these appearances are frequently seen; but they are most commonly hidden by intervening clouds. While I was passing over Mount Jura, one night during a thunder-storm, the flashes succeeded each other so rapidly, that about thirty struck within each minute, but owing to the height of my situation at that time, not one of them appeared otherwise than partially or generally, according to the description I have just given. Sometimes a lower cloud would hide one of the two charged parts, and in this case the lightning assumed the form of a tree, whose trunk and branches only appeared. Sometimes the trunk was hidden, and then the ramifications on each side were alone visible. Frequently intervening clouds would hide all but the trunk, and the lightning then appeared as it commonly does to a spectator in a low situation.

"It must be obvious from the preceding statement of circumstances, that the greatest deviation of lightning must take place in that interval through which the whole body of the fluid passes, and that as you penetrate further and further into the cloud, the stream that is formed becomes less and less, like a river which diminishes..." nithes by entwining itself as you approach its fountain. Hence to us placed on the ground, no danger can ever occur, till the clouds are so low, that the striking distance through air, or the aerial interval between the charged parts, resists the passage more powerfully than the body of earth, and any additional portion of atmosphere which may lie in the direction of the earth from the striking interval.

"If the charged cloud lies in contact with the ground, its passage to the earth will be that of several streams, and the danger will be great, in proportion to the magnitude of these separate stream which passes through any given part of the earth; and several distinct situations may be thus unequally endangered at the same time. Hence it happens, that the same stroke will frequently injure several distinct buildings, which are very near to each other, and that different degrees of injury are always observed in the different tracts.

"The striking distance, or the length of the interval of greatest danger, will vary with the height of the charge, and not with the dimensions of the charged body. This is clear from a multitude of facts already illustrated and applied. We may hence safely conclude, that the longer any charged cloud is in the vicinity of the wet ground, the more will the length, and consequently the danger, of its striking distance be diminished, provided the points and prominences, which are active on the ground, discharge the fluid more abundantly than it is accumulated by the producing cause.

"From what I have already said, it is clear that all the parts of the circuit, through which a thunder cloud may discharge its contents, are not equally dangerous, and that the maximum of danger is confined within much narrower limits than those of the interval, within which it may be felt in one inferior degree or another. You must however perceive, that as the cloud enlarges, the number of additions increases, by which the great body of the flash is formed, and that the length of the most dangerous interval will always increase with, and bear a certain proportion to, the diameter of the cloud. In our attempts to estimate this diameter, we may follow two methods, which have been recommended; but I cannot say that either of these methods has any great pretensions to accuracy.

"1st, If you measure the space on which the thunder-shower falls, it is said that you measure what is commensurable with the dimensions of the thunder-cloud. In a mountainous country this measurement is very possible; for the body of the shower may be seen at a small distance, well described upon the elevated grounds whose parts it separates from the eye. Its diameter, therefore, may be correctly estimated from the distance of those well known objects by which it is bounded. Those thunder-showers, which I have observed, have varied in their diameter, from five hundred yards to two miles. It is, however, to be observed, that the partial vacuum, produced by the collapse attending the removal of the electric fluid, may extend its influence to a greater distance, and cause the fall of rain, by rarefying the atmosphere, far beyond the bounds of the charged cloud.

"2dly, The velocity of a cloud may be known by measuring its height, and the time which any fixed appearance in it takes to describe a certain angle. This may be done in a very small portion of time, and when it is done, you are next to watch the moment at which it begins to affect your elevated conductor, and with equal accuracy you are to mark the evanescence of its signs. The knowledge of these circumstances, united with that of the cloud's velocity, will correctly determine its dimensions.

"From a diary in my possession, made by Mr. Brook, it does not appear that the same electricity ever lasted more than fifteen minutes. When the symptoms of approaching thunder were decisive, the opposite electricity generally lasted as long, and the interval of time between the two electrics seldom exceeded one-tenth of the whole.

"If we allow, that the cloud in this case moved at the rate of eight or ten miles in an hour, its diameter must have been four miles. However, in many instances, all the above-mentioned changes of electricity took place in two minutes. This happened several times successively, and each series of changes terminated by a flash of lightning. In all instances of this kind, to make the diameter of the cloud half a mile, we must suppose that it moves at the rate of thirty miles an hour; and in such a case, one-tenth of the whole, or the interval of greatest danger, would not exceed a hundred and eighty yards. But on the supposition that the size of the cloud were such as to strike over the distance of two miles, many are the circumstances which, on its descent towards the ground will encroach upon its offensive powers, change its direction, or decrease and perhaps altogether annihilate its violence.

"1. Innumerable points and prominences rise from the whole surface of the earth over which it hangs. These act as so many channels, through which its contents will find a rapid evacuation. In the power of carrying off the fluid gradually, I have been able to discover but little difference between partial and metallic conductors. It should be added, that the torrent through an elevated wire is such, when the cloud approaches it, as would discharge a battery, whose surface equalled four or five acres, in twenty or thirty seconds. When, therefore, millions of other conductors are acting with equal effect at the same moment, that must be an immense cloud indeed, whose striking distance in such circumstances is not much lessened, or whose striking powers are not altogether exhausted.

"2. Metals alone conduct the fluid better than charged surfaces. If a plate of glass, coated on one side with tin-foil, be charged and placed in a circuit, so that the contents of a jar may pass over the other side uncoated, the luminous striking distance will be quadruple what it is in air. Such a combination of changes as that which I have now described must frequently occur in the upper regions of the atmosphere; for the charged clouds must lie in strata above each other; and in the varieties of their motions, produced by their mutual attractions, and by the innumerable causes which affect their different currents, they must be perpetually serving as discharging-rods to one another. We consequently find that nine hundred and ninety-nine flashes out of a thousand, strike from cloud to cloud through the intervening air."

Severe shocks have been sometimes experienced from... a flash of lightning, when the person or building struck has been at a very considerable distance from the cloud in which the discharge appeared to take place. A person at Vienna received a terrible shock from a thunder-rod, on which his hand reeled during an explosion that happened at the distance of three miles from the place where the conductor was erected; and it is supposed that a shock might be felt, or even a person killed, at a distance "prodigiously greater." It is certain that during a thunder-storm, the insulated conductor is affected at every explosion, however great, so as to emit sparks.

It is supposed by most electricians that no direct stroke is adequate to the production of these effects, and they have therefore had recourse to what Lord Stanhope calls the returning stroke. The following is an abridgment of this theory.

Let PC, fig. 122, represent a conductor charged positively; and AB a conductor in its natural state, placed so that one of its extremities A may just enter the atmosphere of PC. In this case, Lord Stanhope says, that the superabundance of PC will cause some of the natural share of AB to pass from A to B, where it is stopped and accumulated. By this change A is left in a different or negative state, and B by the addition it has received becomes positive. But when the superabundance at P is taken off, the positive fluid at B rushes back to its natural place at A, and this restoration is called the returning stroke.

Again, let us suppose PC to be negative; and A placed as before just within its atmosphere. Now part of the fluid in AB will rush from B to A, and there being stopped will produce an accumulation; but when PC is discharged, this accumulation will disappear, and the returning stroke will be from A to B.

To apply this to the present case. Let us suppose two clouds horizontally distant, A and B (in the annexed diagram), the one A electrified positively and the other B negatively, to be incumbent over the surface of the earth at a and b; they will here tend to produce the opposite states, or the part of the surface a will be negative and b positive. If now a discharge take place between the clouds A and B, the fluid will rush back from b to a; and if conductors are fixed at these places, the fluid will rush down the conductors at b, and up that at a. The same effects, though in a less degree, will be produced, if we suppose the negative cloud B placed above the positive cloud A.

By this theory, Lord Stanhope undertook to explain how the man and two horses were killed in the thunder-storm described by Mr Brydone, and his Lordship presented a very able paper on this subject to the Royal Society.

This theory of Lord Stanhope has been well received, and it is no small testimony in its favour that it has obtained the support of so able a philosopher as Professor Robison. Mr G. Morgan, however, strenuously objects to this theory, on the very serious grounds that its principle is erroneous, its effects overrated, and its application unnecessary. Our limits will not permit us to detail all Mr Morgan's objections, but we must confess they do not convince us of the fallacy of the theory, although they certainly tend to invalidate the effects attributed to the returning stroke.

"Let us allow," says Mr Morgan, "that the force required by the theory is rendered active in the manner which I have just described, what reason have we for believing that it would be active to the degree supposed? Lord Stanhope has estimated, that what is effects separated from our natural share without injuring us, and the return may be absent for hours without being felt, is so great in quantity as to destroy us by its motion in returning. But what are the grounds of this estimate? As yet it has been justified by no appeal, either to fact or experiment; and the person who could say, that the greatest possible loss from our natural share is little or nothing, would certainly stand upon equal, I think rather better, grounds, than those who would make it adequate to the fusion of metals and the destruction of life. I would add, that when the power of the returning stroke is magnified as it is in this theory, the rationale of this bold estimate is not only neglected, but it is neglected where it might have been made without much trouble.

"If the returning stroke of a thunder-cloud will destroy large edifices, surely artificial electricity could produce a similar stroke which would destroy a bird or a mouse, or act on some scale analogous to that which it is said to resemble. If, I say, the returning stroke in nature will melt the irons of a waggon wheel, surely, with the grand machines which we are now able to construct, such a returning stroke might be caused as would melt a capillary thread of metal. But nothing of this kind has ever been done or attempted by those who support the theory, and I am bold enough to prophecy, from the details of my own experience, that nothing of the kind ever will be done."

Sect. III. Of the means of preventing Accidents from Lightning.

It has been well observed, that knowledge is invaluable chiefly in proportion as it is useful; a maxim of conduct which no man ever exemplified better than Dr Franklin against lightning. No sooner was the real nature of lightning ascertained by experiment, than it was naturally suggested to Franklin that this grand discovery might be rendered beneficial to mankind, by affording means for preserving buildings from the formerly inevitable devastations of that powerful instrument of nature. Here too, the genius of Franklin led the way; and as he certainly deserves the greatest share of the merit due to the discovery of the identity of lightning and electricity, we are also chiefly indebted to him for the means of applying this knowledge to advantage. He was led to propose the use of pointed metallic conductors attached to the building, as a security against the effects of lightning; and this proposal, like most of Dr Franklin's ingenious contrivances in electricity, was the result at once of acute reasoning and accurate observation.

Dr Franklin considered the earth as performing the office of a conductor, in restoring to the atmosphere the ions for electrical equilibrium, that had been disturbed by the conflagrations which tend to produce atmospherical electricity. In its course, he observes, that the lightning will commonly strike the best conductors; and accordingly, as a metallic rod is a much more perfect conductor than the stones, bricks, &c., of which buildings are chiefly composed, the lightning will strike the rod in preference.

Chap. I.

Atmospheric Electricity.

Requisites to be observed.

Should be of the best conductors.

Should be of sufficient diameter, and perfectly continuous.

Thunder-houfe.

ence to the materials of the building. He therefore advised, that a metallic rod should be fixed to some part of the building, penetrating for some distance into the moist earth, and, as lightning does not in every case strike the highest parts of a building, that the rod should extend for some feet above these, in order, as it were, to solicit the lightning. As lightning has been found to destroy metallic rods of a considerable diameter, he advises, that these conductors should be at least half an inch thick, that they may the better resist the destructive power of the lightning.

From a comparison of numerous experiments and observations, the following rules have been laid down for the construction of conductors.

1. That the rods be made of such substances as are in their nature the best conductors of electricity.

It is found that all metals do not conduct equally well, and that lead and copper are the best fitted to serve as conductors against lightning; but as lead is exceedingly destructible by electricity, and therefore would require to be of a very considerable diameter, copper is to be preferred, as well on account of its greater conducting power, as from its being less liable to contract rust than iron, which is commonly employed.

2. That the rods be of a sufficient diameter.

3. That they be perfectly uninterrupted, or, if formed of several pieces, that their junctions be as nearly in contact as possible.

The effect of interruptions in conductors, as well as the effects of lightning in general on buildings, may be illustrated by the following experiments.

Exper. 1.—Fig. 123. shews an instrument representing the side of a house, either furnished with a metallic conductor, or not; by which both the bad effects of lightning striking upon a house not properly secured, and the usefulness of metallic conductors, may be clearly represented. A is a board about three-quarters of an inch thick, and shaped like the gable end of a house. This board is fixed perpendicularly upon the bottom board B, upon which the perpendicular glass pillar CD is also fixed, in a hole about eight inches distant from the basis of the board A. A square hole, LMK, about a quarter of an inch deep, and nearly one inch wide, is made in the board A, and is filled with a square piece of wood, nearly of the same dimensions. We say nearly of the same dimensions, because it must go so easily into the hole, that it may drop off by the least shaking of the instrument. A wire, LK, is fastened diagonally to this square piece of wood. Another wire, IH, of the same thickness, having a brass ball, H, screwed on its pointed extremity, is fastened upon the board A; so also is the wire MN, which is looped in a ring at O.

From the upper extremity of the glass pillar CD, a crooked wire proceeds, having a spring socket F, through which a double-knobbed wire slips perpendicularly, the lower knob G of which falls just above the knob H. The glass pillar DC must not be made very fast into the bottom board; but it must be fixed so as to be pretty easily moved round its own axis, by which means the brass ball G may be brought nearer or farther from the ball H, without touching the part of EFG. Now when the square piece of wood LMIK (which may represent the shutter of a window or the like) is fixed into the hole so, that the wire LK stands in the dotted representation IM, then the metallic communication from H to O is complete, and the instrument represents a house furnished with a proper metallic conductor; but if the square piece of wood LMIK is fixed so, that the wire LK stands in the direction LK, as represented in the figure, then the metallic conductor HO, from the top of the house to its bottom, is interrupted at IM, in which case the house is not properly secured.

Fix the piece of wood LMIK, so that its wire may be as represented in the figure, in which case the metallic conductor HO is discontinued. Let the ball G be fixed at about half an inch perpendicular distance from the ball H, then, by turning the glass pillar DC, remove the former ball from the latter: by a wire or chain connect the wire EF with the wire Q of the jar P, and let another wire or chain, fastened to the hook O, touch the outside coating of the jar. Connect the wire Q with the prime conductor, and charge the jar; then, by turning the glass pillar DC, let the ball G come gradually near the ball H, and when they are arrived sufficiently near one another, you will observe that the jar explodes, and the piece of wood, LMIK, is pushed out of the hole to a considerable distance from the thunder-house. Now the ball G, in this experiment, represents an electrified cloud; which, when it is arrived sufficiently near the top of the house A, the electricity strikes it, and, as this house is not secured with a proper conductor, the explosion breaks part of it, i.e., knocks off the piece of wood IM.

Repeat the experiment with only this variation, viz., that this piece of wood IM is situated so, that the wire LK may stand in the situation IM; in which case the conductor HO is not discontinued; and you will observe, that the explosion will have no effect upon the piece of wood LMIK; this remaining in the hole unmoved; which shews the usefulness of the metallic conductor.

Further: Unscrew the brass ball H from the wire HI, so that this may remain pointed, and, with this difference only in the apparatus, repeat both the above experiments; and you will find that the piece of wood IM is in neither case moved from its place, nor any explosion will be heard; which demonstrates the preference of conductors with pointed terminations to those with blunted ones.

Exper. 2.—This apparatus is sometimes made in the powder-shape of a house, as represented fig. 124, where, for the house's sake of distinctness, the side and part of the roof next the eye are not represented. The gable end AC represents that of the thunder-house, and may be used in the same manner with that above described, or more readily by the following method. Let one ball of the discharging rod touch the ball of the charged jar, and the other the knob A of the conductor AC of the thunder-house; the jar will then of course explode, and the fluid will act upon the conductor just mentioned. The conducting wire at the windows HH must be placed in a line. The sides and gable AC of the house are connected with the bottom by hinges; and the building is kept together by a ridge on the roof. To use this model, fill the small tube a with gunpowder, and ram the wire c a little way into the tube; then connect the tube e with the bottom of a large jar or battery. When the jar is charged, form a communication from the hook at C, on the outside, to the top of the jar, by discharging. discharging the rod; the discharge will fire the powder, and the explosion of the latter will throw off the roof, with the sides, back and front, so that they will all fall down together. The figures f and g in the side of the house represent a small ramrod for the tube a, and a pricker for the touch-hole at C.

Mr Jones of Holborn makes the front of the common thunder-houses, as well as the powder-house above described, with two pieces of wood or windows h h, which, by being placed in proper situations, the one to conduct and the other to reflect the fluid, will illustrate by one discharge the usefulness of good conductors for securing buildings or magazines from the explosion of thunder, as well as the danger of using imperfect ones.

Exper. 3.—Fig. 125 represents a wooden pyramid, made in several pieces, with a wire through each, so that their ends may touch, as at s s s. Let one corner of the pedestal d be loose, and have the safety wire pass almost but not quite through it. Let the wire passing through the rest of the pedestal join by a chain the outside coating of a Leyden phial. If the cloud x be supported by a wire from the prime conductor, and hang half an inch from the knob q of the pyramid; when the phial is discharged, a spark will take place between x and g; the spark will pass along the wires s s s, till it comes to the break at d; there an explosion will take place, that will drive out the corner-stone d, and overthrow the fabric.

Abundant observation has proved the danger of having discontinuous conductors either attached to a building, or forming part of the materials. About the middle of the last century, the steeple of St Bride's church in London was struck by lightning, and greatly injured. In the construction of this steeple a great deal of iron work had been employed; the stones having been fastened together in many places by iron cramps, the ends of which were covered with small stones. The lightning seems first to have struck the vane of the spire, from which it was safely conducted down the shaft by which the vane was supported; from the extremity of this shaft, it leaped to two cross iron bars which were at the base of the obelisk, shattering the obelisk in its way. Hence it passed to one of the above mentioned cramps, and thus from cramp to cramp throwing out or demolishing the stones as it passed along.

The principles of electricity afford us an easy explanation of the manner in which the interruption of conductors acts. We know that at the extremity of all long rods there is a considerable accumulation of electricity, and this has here a tendency to fly off with great force, especially if there is another conductor at hand. This other conductor also assists the accumulation in the former by acquiring at its adjacent extremity the opposite electricity. Supposing a positive cloud to be over the upper conductor, this conductor will be electrified positively at its lower extremity, and this accumulation being increased by the negative electricity of the upper and of the lower conductor, will tend to fly off with great violence into the air, or if any obstruction oppose its passage, this will be removed by the bursting or displacing the resisting body.

4. It is necessary that the connection between the conductor and the common stock, or the earth, be as complete as possible.

It has been said, that the lower extremity of the conductor should be inserted some feet below the surface of the ground; it is also proper that it should be turned in a direction away from the foundation; and as moisture is one of the best conductors, it would be advisable, where this can conveniently be done, to connect the extremity of the metallic rod with some neighbouring piece of water.

5. That the rod be carried from the top of the building to the common stock in the shortest convenient direction, as straight as possible.

6. That the upper extremity be finely tapered, and terminative in a sharp smooth point.

There is no question in electricity, that has been argued with more keenness, than whether thunder-rods pointed should terminate in a sharp point, or in a round ball. Dr Franklin, we have seen, decidedly gave the preference to a pointed conductor, and he has been followed by most of the electricians of Europe; Dr Wilson standing almost alone in support of the round ball. This controversy was renewed with great warmth on the occasion of a house at Purfleet belonging to the board of ordnance, having been struck by lightning, although guarded by a pointed conductor. A set of very ingenious experiments were made, both by Mr Nairne and Dr Wilson, to estimate the comparative merits of pointed and obtuse conductors; but by these the question was not decided; Mr Nairne's experiments always concluding in favour of pointed conductors, while those of Dr Wilson as constantly favoured the obtuse termination. Most electricians, however, still prefer the pointed conductor.

Let B (fig. 126) represent the position of a charged cloud; A, the part that is oppositely charged, or that is connected with it; FG a pointed wire. In this case, the electric fluid must pass either through the series of partial conductors, a, b, c, &c., or through the body of earth, AF.

Now when, on the one hand, we consider the dryness of that foil which is generally selected for the foundation of buildings, the probability there is that nothing but the foil, thus dry, may separate A from the wire FG, and the certainty that if water should connect A and FG, its resistance is very considerable; when, on the other hand, we take into consideration, the nails, bolts, iron bars, strips of lead, bell wires, and metallic utensils that are scattered through all buildings, we shall, I think, perceive the much greater probability there is of the lightning's passing through a, b, c, d, &c., to the cloud, than of its passage through the ground.

2. Let us erect another wire, HI, and fill the danger is almost as great; for now the possible circuits of the lightning are four, and of those, that leading through the house appears to be the easiest; if HI convey it harmlessly, then it must pass through the body of the air FG, or over the roof of the house. We well know from past experiments, that the insulating power of the air makes the resistance in the direction IG very considerable; and even on the supposition that j were wet, the resistance over the roof of the house is not much considerable. If the house were covered or coped with lead, the probability of a stroke would then be diminished, but not taken away; for, suppose the easiest circuit should lie in the direction KM, then, rather than pass through the body of earth HK, or FK, it might might find an easier passage through the house than either of the conductors. This would not be the case, if a strip of lead, or metallic substance of any kind, extended from K to H, and from K to F. "I hence thought," says Mr Morgan, "at one time, that a house would be perfectly safe, if a strip of lead were carried around the top, and all the bottom of the building, and then connected by two or three metallic strips extending from the one to the other.

"Let us suppose that a house were erected over a stratum of moisture, or any other conducting substance, which dipped considerably, at a little distance from the house, and then suddenly rose just below it; in that case, if the stratum became the circuit of a charge, the stroke would rise immediately in the centre or body of the house, and in all directions would force its way with devastation, towards the conductors on the outside."

To prevent all possible danger, Mr G. Morgan proposes, that, while the house is building, the foundation of each partition wall be laid on a strip of lead, or that such a strip be fastened to the sides of these partition walls. The strips should be two inches wide, and at least a quarter of an inch thick, and they should be closely connected with each other. A perpendicular strip on each side of the house, should rise from this bed of conductors, to the surface of the ground; whence a strip should be continued round all the house, and carefully connected with water-pipes, &c. The strips on the sides of the house, should then be continued to the roof, which should be guarded in the same manner as the foundation. The top should be surrounded by a strip, whose connection should spread over every edge and prominence, and should continue to the summit of each separate chimney. It is particularly necessary to guard the chimneys; for Mr Morgan was witness to a case, in which a house that had been guarded, in most respects, according to the foregoing directions, except that the chimneys were unprotected, was struck with lightning which entered by one of the chimneys; here it spent its fury; but the chimney falling on the roof, did considerable damage.

The principal objection to this method, is the expense attending it; but this may be, in a great measure avoided, by making proper use of the leaden pipes, gutters, and copings, which belong to most houses.

Ships, from the height and construction of their masts, and from their being such inflated conducting objects as must necessarily attract the lightning from a cloud that is very near, are peculiarly exposed to danger. It is, therefore, still more necessary to guard vessels by proper conductors. Chains are very commonly employed for this purpose, from their being more conveniently disposed among the rigging; but it is found, that from the want of continuity in the links, chains are very imperfect conductors, and have not unfrequently been broken by a severe shock. Strips of lead, are therefore, to be preferred, both as they are cheaper, and less liable to be injured by the weather and salt water, than iron chains. One strip should surround the deck, and another the bottom or side of the keel, and these should be connected with other strips, embracing the ship in various parts. If the ship be copper-bottomed, it will only be necessary to connect the copper with the deck; but in every case, a strip should

Vol. VII, Part II. prevent our being injured by the splinters of wood, if the tree should be strucken. It is particularly necessary to avoid rivers and brooks, as these are excellent conductors.

Perhaps the best protection in the open air, is a carriage made so large, as that a person may fit in it at a distance from the tides, especially if it be surrounded at the top and bottom with metallic fillets connected with each other by a strip of the same substance.

If overtaken in a storm, it is safer to be completely wet than dry.

**CHAP. II. Experiments and Observations on the Spontaneous Electricity of the Atmosphere.**

The first person who observed the spontaneous electricity of the atmosphere, was M. Momnier, who found that even when there was no appearance of lightning, some degree of electricity might generally be observed in the atmosphere. His experiments were made at St Germaine en Laye, and published in a memoir read at the Royal Academy of Sciences at Paris in 1752.

But more accurate experiments were made upon the electricity of the air by the abbé Mazeas, at Chateau de Maintenon, during the months, June, July, and October, of 1753, and communicated to the Royal Society in a letter to Dr Hales.

The abbé's apparatus consisted of an iron rod, 370 feet long, raised 90 feet above the horizon. It came down from a very high room in the castle, where it was fastened to a silk cord fixed feet long; and was carried from thence to the steeple of the town, where it was likewise fastened to another silk cord of eight feet long, and sheltered from rain. From the extremity of this rod, a large key was suspended to receive the electric fluid.

When he began his experiments, viz. on the 17th of June, the electricity of the air was sensibly felt every day, from sunrise till seven or eight in the evening, except in moist weather, when he could perceive no signs of electricity. In dry weather, the rod attracted minute bodies at no greater distance than three or four lines. He repeated the experiment carefully every day, and constantly observed, that in weather void of storms, the electricity of a piece of sealing wax of two inches long, was above twice as strong as that of the air. This observation inclined him to conclude, that in weather of equal dryness, the electricity of the air was always equal.

It did not appear to him that hurricanes and tempests increased the electricity of the air, when they were not accompanied with thunder; for that during three days of a very violent continual wind in July, he was obliged to put some dust within four or five lines of the conductor, before any sensible attraction could be perceived.

No sensible alteration in the electricity of the air was observed under different directions of the winds, except when these were moist.

He could observe no electricity in the air during the driest nights of summer, but it returned in the morning with the sun, disappearing again soon after sunrise.

The strongest common electricity of the atmosphere during that summer, was observed in July, on a very dry, clear, warm day.

On the 27th of June about noon, he perceived some stormy clouds rising above the horizon, and observed that the electricity of the atmosphere occasioned by them, was increased as the clouds reached the zenith. He at this time drew considerable sparks from his apparatus, though there was neither thunder nor lightning.

The electricity observed during the appearance of these stormy clouds, was not diminished by a very heavy rain, till the clouds began to dissipate.

Mr Kinnerley observed, that when the air was in its driest state, there was always a quantity of electricity in it, and which might be easily drawn from it. This, he says, may be proved by a person in the negative state of electricity extending his arm into the air in the dark while holding a pointed needle in his hand; this, however, can only be observed when the air is very dry.

Whether the electricity in the air, in clear dry weather, be of the same density at the height of two or three hundred yards, as on the surface of the earth, Mr Kinnerley thought might be easily ascertained by Dr Franklin's old experiment with the kite. The twine, he says, should have throughout a very small wire in it, and the ends of the wire, where the several lengths are united, ought to be tied down with a waxed thread, to prevent their acting in the manner of points.

Mr Canton made several ingenious experiments on atmospheric electricity, by means of his pith-ball electrometer, described in No. 66. According to this philosopher, desiccated atmospheric air, when heated, becomes negatively electric, and when cooled, the electricity is of the positive kind, even when the air is not permitted to expand or contract; and the expansion or contraction of atmospheric air occasions changes in its electrical state.

But no electrician, in the earlier stage of the science, Beccaria, conducted his observations in this way with greater accuracy, or purified them farther than Sig. Beccaria. He observed, that, during very high winds, his apparatus gave no signs of being electrified. Indeed he found, that in three different states of the atmosphere, he could find no electricity in the air: viz. in windy and clear weather; in weather when the sky was covered with distinct and black clouds, that had a slow motion; in moist weather, not actually raining. In a clear sky, when the weather was calm, he always perceived signs of a moderate electricity, but interrupted. In rainy weather without lightning, his apparatus was always electrified a short time before the rain fell, and during the time of the rain, but it ceased to be affected a little before the rain was over.

The higher his rods reached or his kites flew, the stronger signs they gave of electricity. Also longer strings and cords, extended and insulated in the open air, acquired electricity sooner than those which were shorter. A cord 1500 Paris feet long, stretched across the river Po, was strongly electrified during a shower without thunder, as a metallic rod, employed to bring lightning into his house, had been in any thunderstorm.

Having Having two rods for bringing the lightning into his house, 140 feet afar, he observed, that if he took a spark from the higher of these, the spark from the other, which was 30 feet lower, was at that instant lessened; but its power again revived, though he kept his hand upon the former.

He imagined that the electricity communicated to the air might sometimes furnish small sparks to his apparatus; since the air parts with the electricity it has received very slowly, and therefore the equilibrium of the electric fluid in the air, will not be restored so soon as in the earth and clouds.

Among the effects of a moderate electricity in the atmosphere, Signor Beccaria considers rain, hail, and snow.

Clouds that bring rain, he thought, were produced in the same manner as thunder-clouds, only by a more moderate electricity.

He notes several circumstances attending rain without lightning, which make it very probable, that it is produced by the same cause as when it is accompanied with lightning. Light has been seen among the clouds by night in rainy weather; and even by day, rainy clouds are sometimes seen to have a brightness evidently independent of the sun. The uniformity with which the clouds are spread, and with which the rain falls, he thought were evidences of a uniform cause, like that of electricity. The intensity of electricity in his apparatus, generally corresponded very nearly to the quantity of rain that fell in the same time.

Sometimes all the phenomena of thunder, lightning, hail, rain, snow and wind, have been observed at one time; which shews the connection they all have with some common cause.

Signor Beccaria supposes, therefore, that previous to rain, a small quantity of electric fluid escapes out of the earth, in some place where there was a redundancy of it; and in its ascent to the higher regions of the air, collects and conducts into its path a great quantity of vapours. The same cause that collects will condense them more and more, till in the places of the nearest intervals they come almost into contact, so as to form small drops, which uniting with others as they fall, come down in rain. The rain will be heavier in proportion as the electricity is more vigorous, and the cloud approaches more nearly to a thunder cloud.

He imitated the appearance of clouds that bring rain, by inflating himself between the rubber and conductor of his electrical machine, and with one hand dropping colophonium into a spoon fastened to the conductor, and holding a burning coal, while his other hand communicated with the rubber. In these circumstances, the smoke spread along his arm, and by degrees all over his body, till it came to the other hand that communicated with the rubber. The lower surface of this smoke was everywhere parallel to his clothes, and the upper surface was swelled and arched like clouds replete with thunder and rain. In this manner, he supposed, the clouds that bring rain diffuse themselves from over those parts of the earth which abound with the electric fluid, to those parts that are exhausted of it; and by letting fall their rain, restore the equilibrium between them.

This ingenious philosopher supposes hail to be formed in the higher regions of the air, where the cold is intense, and where the electric fluid is very copious.

In these circumstances a great number of particles of water are brought near together, where they are frozen, and in their descent collect other particles, so that the density of the substance of the hailstone grows less and less from the centre; this being formed first in the higher region, and the surface being collected on the lower. Agreeable to this it is observed, that in mountains, hailstones, as well as drops of rain, are very small; there being but small space through which to fall and thereby increase their bulk. Drops of rain and hail also agree in this circumstance, that the more intense is the electricity that forms them, the larger they are.

Clouds of snow differ in nothing from clouds of rain, but in the circumstance of cold which freezes them. Both the regular diffusion of snow, and the regularity in the structure of its particles, shew the clouds of snow to be actuated by some uniform cause like electricity. All these conjectures about the cause of hail and snow were confirmed by observing, that his apparatus never failed to be electrified by snow, as well as by rain.

A more intense electricity unites the particles of hail more closely, than the more moderate electricity does those of snow. In like manner, we see thunder clouds more dense than those that merely bring rain, and the drops of rain are larger in proportion, though they often fall not from so great a height.

Mr Romayne observed, that the air in Ireland was generally electrified in a fog, and even in a mist, and that both day and night, but principally in winter; Romayne seldom in summer, except from positive clouds or cool fogs. The electricity of the air in a frost or fog is always positive. He says, that he has often observed, during what seemed the puffing of one cloud, successive changes from negative to positive, and from positive to negative. It may be remarked that most fogs have a smell like an excited glass tube.

Mr Henly has shewn, that fogs are more strongly electrified in or immediately after a frost than at other times; and that the electricity of fogs is often the strongest soon after their appearance. Whenever there appears a thick fog, and at the same time the air is sharp and frothy, that fog is strongly electrified positively.

Though rain is not an immediate cause, yet Mr Henly is inclined to consider it as a remote consequence of atmospherical electricity; and he generally found, that in two or three days after he had discovered the air to be strongly electrified, there was either rain or snow.

If, in clear weather, a low cloud, which moves slowly and is considerably distant from any other, passes over the apparatus, the positive electricity generally grows very weak, but does not become negative; and when the cloud is gone, it returns to its former state. When many whitish clouds keep over the wire, sometimes uniting with and then separating from each other, thus forming a body of considerable extent, the positive electricity commonly increases. In all the above circumstances the positive electricity never changes to negative.

The clouds which lessen the electricity of the exploring wire, are those which move; though those that are low, seem also to have the same effect.

Mr Cavallo has considerably improved our knowledge with respect to atmospherical electricity, and by his apparatus, has greatly facilitated the means of making. making experiments. His first experiments were made by means of a kite; and after bestowing much pains in constructing kites of various dimensions, &c., he found that a common school-boy's kite, about four feet high, and two wide, answered as well as any other. The string of his kite was formed by twisting together two threads of common twine, and one of copper thread, such as is used for trimmings. When a kite constructed in this manner was raised, he always found the string give signs of electricity, except once, when the weather was warm, and the wind very weak, that the kite could scarcely be raised, and could be kept up only for a few minutes. Afterwards, however, when the wind increased so that he could easily raise the kite, he obtained, as usual, pretty strong signs of electricity.

As making experiments on atmospheric electricity is often attended with more or less danger, it is necessary to observe the following directions given by Mr Cavallo.

"In raising the kite when the weather is very cloudy and rainy, in which time there is fear of meeting with a great quantity of electricity, I generally use to hang upon the string AB, fig. 127, the hook of a chain C, the other extremity of which falls upon the ground. Sometimes I use another caution besides, which is, to stand upon an insulating stool; in which situation I think, that if any great quantity of electricity, suddenly discharged by the clouds, strikes the kite, it cannot much affect my person. As to insulated reels, and such-like instruments, that some gentlemen have used to raise the kite, without danger of receiving any shock; fit for the purpose as they may appear to be in theory, they are yet very inconvenient to be managed. Except the kite be raised in time of a thunder-storm, there is no great danger for the operator to receive any shock. Although I have raised my electrical kite hundreds of times without any caution whatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunder-storm, if the kite has not been raised before, I would not advise a person to raise it while the stormy clouds are just overhead; the danger at such time being very great, even with the precautions above mentioned: at that time, without raising the kite, the electricity of the clouds may be observed by a cork-ball electrometer held in the hand in an open place; or, if it rains, by my electrometer for the rain; which will be described hereafter.

"When the kite has been raised, I generally introduce the string through a window in a room of the house, and fasten it to a strong silk lace, the extremity of which is generally tied to a heavy chair in the room. In fig. 128, A.B represents part of the string of the kite which comes within the room; C represents the silk lace; D.E, a small prime conductor, which, by means of a small wire, is connected with the string of the kite; and F represents the quadrant electrometer, fixed upon a stand of glass covered with sealing-wax, which I used to put near the prime conductor, rather than to fix it in a hole upon the conductor, because the string A.B sometimes shakes so as to pull the prime conductor down; in which case the quadrant electrometer remains safe upon the table; otherwise it would be broken, as I have often experienced before I thought of this method. G represents a glass tube, about eighteen inches long, with a knobbed wire cemented to its extremity; with which instrument I use to observe the quality of the electricity, when the electricity of the kite is so strong that I think it not safe to come very near the string. The method is as follows:—I hold the instrument by that extremity of the glass tube which is the farthest from the wire, and touch the string of the kite with the knob of its wire, which, being insulated, acquires a small quantity of electricity from it; which is sufficient to ascertain its quality when the knob of the instrument is brought near an electrified electrometer.

"Sometimes, when I raise the kite in the night-time, out of the house, and where I have not the convenience of observing the quality of its electricity by the attraction and repulsion, or even by the appearance of the electric light, I make use of a coated phial, which I can charge at the string, and, when charged, put it into my pocket; wherein it will keep charged even for several hours. By making use of this instrument, I am obliged to keep the kite up no longer than is necessary, to charge the phial, in order to observe the quality of the electricity in the atmosphere; for after the kite has been drawn in and brought home, I can then examine the electricity of the inside of the phial, which is the same as that of the kite.

"When the electricity of the kite is very strong, I fix a chain, communicating with the ground, at about six inches distance from the string; which may carry off its electricity, in case that this should increase too much as to put the bystanders in danger.

"Besides the above-described apparatus, I have occasionally used some other instruments, which I have often varied, according as some particular experiments required; but, as they are of no great consequence, I shall omit to describe them. It is only necessary, to give an idea of the standard of my quadrant electrometer; which may, very probably, show the same intensity of electricity under a number of degrees different from the other instruments of the same kind. When the kite is flying, and the apparatus is disposed as in fig. 128, I bring, under the extremity E of the prime conductor, a little bran, held upon a tin plate, and observe, that when the index of the electrometer is at ten degrees, the prime conductor begins to attract the bran at the distance of about three-fifths of an inch; when the index is at twenty degrees, the prime conductor attracts the bran at the distance of about one inch and a quarter; when the index is at thirty degrees, the bran begins to be attracted at the distance of two inches and one-fifth. These distances vary, as the weather changes its degree of dryness; but in frosty weather I observe them constantly as above."

Mr Cavallo has given copious extracts from a journal which he kept of his experiments with the kite; great danger from these we shall give his account of one experiment, which is peculiarly interesting from the danger to which the experimenters appear to have been exposed.

"October the 18th. After having rained a great deal in the morning, and night before, the weather became a little clear in the afternoon, the clouds appearing separated, and pretty well defined. The wind was west, and rather strong, and the atmosphere in a temperate degree of heat. In these circumstances, at three P.M. I raised my electrical kite with three hundred..." After that the end of the string had been inflated, and a leather-ball, covered with tin-foil, had been hung to it, I tried the power and quality of the electricity, which appeared to be positive, and pretty strong. In a short time a small cloud passing over, the electricity increased a little; but the cloud being gone, it decreased again to its former degree. The string of the kite was now fastened by the silk lace to a post in the yard of the house wherein I lived, which was situated near Illington, and I was repeatedly charging two coated phials, and giving shocks with them—while I was doing so, the electricity, which was still positive, began to decrease, and in two or three minutes time it became so weak, that it could be hardly perceived with a very sensible cork-ball electrometer. Observing at the same time that a large and black cloud was approaching the zenith (which, no doubt, caused the decrease of the electricity) indicating imminent rain, I introduced the end of the string through a window, in a first-floor room, wherein I fastened it by the silk lace to an old chair. The quadrant electrometer was set upon the same window, and was, by means of a wire, connected with the string of the kite. Being now three quarters of an hour after three o'clock, the electricity was absolutely unperceivable; however, in about three minutes time, it became again perceivable, but now upon trial was found to be negative; it is therefore plain, that its flopping was nothing more than a change from positive to negative, which was evidently occasioned by the approach of the cloud, part of which by this time had reached the zenith of the kite, and the rain also had begun to fall in large drops.—The cloud came farther on; the rain increased, and the electricity keeping pace with it, the electrometer soon arrived to $15^\circ$. Seeing now, that the electricity was pretty strong, I began again to charge the two coated phials, and to give shocks with them; but the phials had not been charged above three or four times, before I perceived that the index of the electrometer was arrived at $35^\circ$, and was keeping still increasing. The shocks now being very smart, I desisted from charging the phials any longer; and, considering the rapid advance of the electricity, thought to take off the inflation of the string, in case that if it should increase farther, it might be filently conducted to the earth, without causing any bad accident, by being accumulated in the inflated string. To effect this, as I had no proper apparatus near me, I thought to remove the silk lace, and fasten the string itself to the chair; accordingly I disengaged the wire that connected the electrometer with the string; laid hold of the string; untied it from the silk lace, and fastened it to the chair; but while I effected this, which took up less than half a minute of time, I received about a dozen, or fifteen, very strong shocks, which I felt all along my arms, in my breast, and legs; shaking me in such a manner, that I had hardly power enough to effect my purpose, and to warn the people in the room to keep their distance. As soon as I took my hands off the string, the electricity (in consequence of the chair being a bad conductor) began to snap between the string and the shutter of the window, which was the nearest body to it. The snappings, which were audible at a good distance out of the room, seemed first isochronous with the shocks which I had received, but in about a minute's time, oftener; so that the people of the house compared their sound to the rattling noise of a jack going when the fly is off. The cloud now was just over the kite; it was black, and well defined, of almost a circular form, its diameter appearing to be about $40^\circ$; the rain was copious, but not remarkably heavy. As the cloud was going off, the electrical snapping began to weaken, and in a short time became inaudible. I went then near the string, and finding the electricity weak, but still negative, I inflated it again, thinking to keep the kite up some time longer; but observing that another larger and denser cloud was approaching apace towards the zenith, as I had then no proper apparatus at hand, to prevent every possible bad accident, I resolved to pull the kite in; accordingly a gentleman, who was by me, began pulling it in, while I was winding up the string. The cloud was now very nearly over the kite, and the gentleman, who was pulling in the string, told me, that he had received one or two slight shocks in his arms, and that if he were to feel one more, he would certainly let the string go; upon which I laid hold of the string, and pulled the kite in as fast as I could, without any farther observation; being then ten minutes after four o'clock.

"N.B. There was neither thunder nor lightning perceived that day, nor indeed for some days before or afterwards."

From his experiments with the kite, Mr Cavallo deduces the following conclusions:

1. The air appears to be electrified at all times; its electricity is constantly positive, and much stronger in frosty, than in warm weather; but it is by no means less in the night than in the daytime ($\kappa$).

2. The presence of the clouds generally lessens the electricity of the kite; sometimes it has no effect upon it; and it is very seldom that it increases it a little.

3. When it rains, the electricity of the kite is generally negative, and very seldom positive.

4. The aurora borealis seems not to affect the electricity of the kite.

5. The electrical spark taken from the string of the kite, or from any inflated conductor connected with it, especially when it does not rain, is very seldom pungent. When the index of the electrometer is not higher than $20^\circ$, the person that takes the spark will feel the effect of it in his legs; it appearing more like the discharge of an electric jar, than the spark taken from the prime conductor of an electrical machine.

6. The

---

($\kappa$) In all his experiments, it happened only once that the string of the kite gave no signs of electricity; it was one afternoon, when the weather was warm, and the wind so weak, that the kite was raised with difficulty, and could hardly be kept up for a few minutes; in the evening, however, the wind, which in the daytime had been north-west, shifted to the north-east, blowing a little stronger: he then raised the kite again, being half past ten o'clock, and obtained, usual, a pretty strong positive electricity. 6. The electricity of the kite is in general stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it: the electricity, for instance, brought down by a string of a hundred yards, may raise the index of the electrometer to 25°; when, with double that length of string, the index of the electrometer will not go higher than 25°.

7. When the weather is damp, and the electricity is pretty strong, the index of the electrometer, after taking a spark from the string, or presenting the knob of a coated phial to it, rises surprisingly quick to its usual place; but in dry and warm weather, it rises exceedingly slow.

Mr Bennet observed with his electrometer, that in very clear weather, when no clouds were visible, on applying the instrument to the insulated string of kites without metal, their positive electricity caused the slips of gold-leaf to strike the sides of the glass; but when a kite was raised in cloudy weather, with a wire in the string, and when it gave sparks about a quarter of an inch long, the electricity was sensible by the electrometer, at the distance of about ten yards from the string; but when placed at the distance of five feet, the gold-leaf continued to strike the sides of the electrometer for more than an hour together, with a velocity increasing and decreasing with the density or distance of the unequal clouds that passed over.

Sometimes the electricity of an approaching cloud has been sensible without a kite, though in a very unfavourable situation for it, being in a town surrounded with hills, and where buildings encompassed the wall on which the electrometer was placed. A thunder cloud passing, caused the gold-leaf to strike the sides of the glass very quick at each flash of lightning.

Mr Bennet relates the following instance of the danger sometimes incurred in making experiments with the kite. Having on the 5th of July 1788, raised a kite with two hundred yards of string, when it had been flying for about an hour, a dark cloud appeared at a great distance, and changed the electricity from positive to negative. The electric power increased till the cloud became nearly vertical, when some large drops of rain fell; and Mr Bennet attempting to secure the string from wet, received such a strong shock in his arm, as deprived it for a few seconds of sensation. The explosion was heard at the distance of forty yards, like the loud crack of a whip.

The following curious phenomenon was observed by Mr Loammi Baldwin, while raising an electrical kite in July 1771, during the approach of a severe thunderstorm. He observed himself to be surrounded by a rare medium of fire, which, as the cloud rose nearer the zenith, and the kite rose higher, continued to extend itself with some gentle faint flashes. Mr Baldwin felt no other effect than a general weakness in his joints and limbs, and a kind of little feeling; all which, he observes, might possibly be the effect of surprise, though it was sufficient to discourage him from pursuing any farther attempt at that time. He therefore drew in his kite, and retired to a shop till the storm was over, and then went to his house, where he found his friends much more surprised than he had been himself; and who, after expressing their astonishment, informed him, that he appeared to them (during the time he was raising the kite, to be in the midst of a large bright flame of fire, attended with flasings; and, that they expected every moment to see him fall a sacrifice to the flame. The atmosphere was observed by some of his neighbours, who lived near the place where he stood.

Fig. 129 represents a very simple instrument, contrived by Mr Cavallo for making experiments on atmospheric electricity, and which, on several accounts, seems to be the most convenient for that purpose.

AB is a common jointed fishing-rod, without the last or smallest joint. From the extremity of this rod proceeds a slender glass tube C, covered with sealing-wax, atmospheric and having a cork D, at its end, from which a pith-ball cal electrometer is suspended. HG is a piece of twine fastened to the other extremity of the rod, and supported at G by a small string FG. At the end I of the twine a pin is fastened, which, when pushed into the cork D, renders the electrometer E uninsulated.

When he would observe the electricity of the atmosphere with this instrument, he thrust the pin I into the cork D, and holding the rod by its lower end A, projects it out from a window of the upper part of the house into the air, raising the end of the rod with the electrometer, so as to make an angle of about 50° or 60° with the horizon. In this situation he keeps the instrument for a few seconds, and then pulling the twine at H, disengages the pin from the cork D; which operation causes the string to drop in the dotted situation KL, and leaves the electrometer insulated and electrified, with an electricity contrary to that of the atmosphere.—This done, he draws the instrument into the room, and examines the quality of the electricity, without obstruction either from wind or darkness.

With this instrument he made observations on the electricity of the atmosphere, several times in a day for several months, and from them he deduces the following general observations, which seem to coincide with those made with the electrical kites.

1. That there is in the atmosphere, at all times, a quantity of electricity; for, whenever he used the above-described instrument, it always acquired some electricity.

2. That the electricity of the atmosphere, or fogs, is always of the same kind, namely positive; for the electrometer is always negative, except when it is evidently influenced by heavy clouds near the zenith.

3. That in general, the strongest electricity is observable in thick fogs, and also in frosty weather; and the weakest, when it is cloudy, warm, and very near raining; but it does not seem to be less by night than in the daytime.

4. That in a more elevated place, the electricity is stronger than in a lower one; for, having tried the atmospheric electrometer, both in the stone and iron gallery on the cupola of St Paul's cathedral, Mr Cavallo found that the balls diverged much more in the latter than in the former less elevated place; hence it appears, that, if this rule takes place at any distance from the earth, the electricity in the upper regions of the atmosphere must be exceedingly strong.

Mr Cavallo has also contrived an instrument, which he calls his electrometer for the rain; this is merely an insulator for insulated instrument to catch the rain, and, by means of a pith-ball electrometer, to show the degree and quality of its electricity.

At fig. 130, is represented an instrument of this kind, which Mr Cavallo frequently used, and after several several observations, found to answer very well. ABCI is a strong glass tube about two feet and a half long, having a tin funnel, DE, cemented to its extremity, which funnel defends part of the tube from the rain. The outside surface of the tube from A to B is covered with sealing-wax; so also is the part of it which is covered by the funnel. FD is a piece of cane, round which several braids wires are twisted in different directions, so as to catch the rain easily, and at the same time to make no resistance to the wind. This piece of cane is fixed into the tube, and a slender wire proceeding from it goes through the bore of the tube, and communicates with the strong wire AG, which is thrust into a piece of cork fastened to the end A of the tube. The end G of the wire AG is formed into a ring, from which is suspended a more or less sensible pith-ball electrometer, as occasion requires.

This instrument is fastened to the side of the window-frame, where it is supported by strong brass hooks at CB, which part of the tube is covered with a silk lace, in order to adapt it better to the hooks. The part FC is out of the window, with the end F a little elevated above the horizon. The remaining part of the instrument comes through a hole in one of the lights of the sash, within the room, and no more of it touches the side of the window than the part CB.

When it rains, especially in passing showers, this instrument, standing in the situation above described, is frequently electrified; and, by the diverging of the electrometer, the quantity and quality of the electricity of the rain may be observed, without any danger of a mistake. With this instrument, he observed, that the rain is generally, though not always electrified negatively, and sometimes to strongly, that he has been able to charge a small coated phial at the wire AG.

This instrument should be fixed in such a manner, that it may be easily taken off from the window, and replaced again, as occasion requires; for it will be necessary to clean it very often, particularly when a shower of rain is approaching.

Mr Cavallo has also shewn how the electricity of the atmosphere may be observed by means of his multiplier, described in No. 255.

In order to examine the electricity of the atmosphere, he at first used to fix a long pointed wire into the socket of the plate A, and then exposed it to the open air. But he has lately used a much better method of accomplishing that object. He exposes, out of the window, an inflated stick of about five feet in length, and covered with tin-foil; and while he holds this apparatus by the extremity of its inflating handle, he touches with the other hand, for about two or three seconds, the lower part of the stick. By this means, the stick being free from points, acquires an electricity contrary to that of the surrounding air. Mr Cavallo then brings it within the room, and communicates that electricity to the plate A of the multiplier, &c. But the electricity so acquired by the inflated stick, is generally sufficient to affect an electrometer without the use of the multiplier. To examine the electricity of the rain, snow, hail, &c., the same apparatus must be exposed out of a window, but the stick must not be touched, for in this case, it acquires the same sort of electricity as that of the rain, snow, &c., and not the contrary sort, as when exposed to the air.

Mr Read, in his "Summary View of the Spontaneous Electricity of the Earth and Atmosphere," observes, that the electricity of the atmosphere, in moderate weather, was always found to be positive; in storms and disturbed states of the air frequently negative; and suddenly and repeatedly changing from the one state to the other. Warm small rain, was found to be very slightly electric; large drops, strongly; hail showers, the most intensely of all. In an easterly wind of long continuance, the electricity was so faint, as to require the nicest of all known tests for discovering its existence. The vapour of water, as soon as it had attained the height of five or six inches of inflation in the air, was found to be permanently and positively electrified; and the surface from which it evaporated, negatively. According to Mr Read, vapour has a greater capacity for electricity, or absorbs and requires more of fluid, than water in its denser state; and, therefore, rarefaction must diminish, and condensation increase, the sensible electric charge of the vapour. Hence, in serene weather, the atmosphere is subject to a regular flux and reflux, or increase and diminution of electricity twice in every twenty-four hours, depending on the action of the sun, and the consequent evaporation and state of the vapours. This diligent observer further remarks, that a limited portion of the earth's surface is often sensibly electrified; over it, there is always a proportionate state of the contrary electricity in the atmosphere; and when an electrified cloud is carried forward by wind, an equal and opposite electric charge keeps pace with it on the earth, till the two charges, becoming more augmented or approaching nearer to one another, or meeting with some conducting eminence, rush together, and produce an explosion.

We shall conclude our account of experiments on atmospheric electricity, with those made by M. Sauflaire in his excursions among the Alps. The instrument employed by M. Sauflaire is a modification of Cavallo's atmospheric electrometer, and shall be described under the article Electrometer.

The following are M. Sauflaire's observations on the electricity of the atmosphere.

Aerial electricity varies according to the situation; it is generally stronger in elevated and inflated situations; not to be observed under trees, in streets, in houses, or electricity, any inclosed places; though it is sometimes to be found pretty strong on quays and bridges. It is also not so much the absolute height of the places, as their situation: thus a projecting angle of a high hill will often exhibit a stronger electricity than the plain at the top of the hill, as there are fewer points in the former to deprive the air of its electricity.

The intensity of the atmospheric electricity is varied by a great many circumstances, some of which may be accounted for, others cannot. When the weather is serene, it is impossible to assign any rule for their variation, as no regular correspondence can then be perceived with the different hours of the day, nor with the various modifications of the air. The reason is evident; when contrary and variable winds reign at different heights, when clouds are rolling over clouds, these winds and clouds, which we cannot perceive by any exterior sign, influence however the strata of air in which we make our experiments, produce these changes of which we only see the result, without being ing able to assign either the cause, or its relation. Thus, in stormy weather, we see the electricity strong, then null, and in a moment after arise to its former force; one instant positive, the next negative; without being able to assign any reason for these changes. M. Sauflure says, that he has seen these changes succeed with such rapidity, that he had not time to note them down.

When rain falls without a storm, these changes are not so sudden; they are however very irregular, particularly with respect to the intensity of force; the quality thereof is more constant. Rain, or snow, almost uniformly gives positive electricity.

In cloudy weather, without rain or storms, the electricity follows generally the same laws as in serene weather.

Strong winds generally diminish its intensity; they mix together the different strata of the atmosphere, and make them pass successively towards the ground, and thus distribute the electricity uniformly between the earth and the air; M. Sauflure has observed a strong electricity with a strong north wind (la bise).

The state of the air, in which the electricity is strongest, is foggy weather: this is always accompanied with electricity, except when the fog is going to resolve into rain.

The most interesting observations, and those which throw the greatest light upon the various modifications of electricity in our atmosphere, are those that are made in serene weather. In winter, (during which most of M. Sauflure's observations were made) and in serene weather, the electricity was generally weakest in an evening, when the dew had fallen, until the moment of the sun's rising; its intensity afterwards augmented by degrees, sometimes sooner, and sometimes later; but generally before noon, it attained a certain maximum, from whence it again declined, till the fall of the dew, when it would be sometimes stronger than it had been during the whole day; after which, it would again gradually diminish during the whole night; but is never quite destroyed, if the weather is perfectly serene.

Atmospherical electricity seems, therefore, like the Periodical sea, to be subject to a flux and reflux, which causes it to flux and reflux twice in 24 hours. The moments of its greatest force are some hours after the rising and setting of the sun; those when it is weakest, precede the city of the rising and setting thereof. This will be further explained in the following pages.

M. Sauflure has given an instance of this periodic flux in electricity, on the 22nd of February, 1785, (one of the coldest days ever remembered at Geneva); the hygrometer and thermometer were suspended in the open air, on a terrace exposed to the south-west; the electrometer, from its situation, indicated an electricity equal to what it would have shown if it had been placed on an open plain. The height of the barometer is reduced to what it would have been if the mercury had been constantly at the temperature of 19 degrees of Reaumur's thermometer. The place of observation was elevated 60 feet above the level of the lake. The observations of the day preceding and following this great cold, are inserted in the following table; because it is pleasing to have the observations which precede and follow anyingular phenomena. There was a weak south-west wind during the whole three days; and it is rather remarkable, that most of the great colds, which have been observed at Geneva, were preceded by, or at least accompanied with, a little south-west breeze.

### TABLE

| Barometer, feet in height | Thermometer | Hygrom. | Electrom. | |---------------------------|-------------|---------|-----------| | h. m. | | | | | Feb. 21st | | | | | 9 15 M | 26 6 7 | 8 3 | 89 3 | 2 0 Pale sun, cloudy. | | 11 10 M | 26 6 5 | 4 3 | 83 0 | 1 6 Bright sun. | | 2 10 E | 26 6 1 | 0 2 | 69 6 | 1 1 The same. | | 5 E | 26 6 1 | 2 3 | 77 2 | 1 1 Setting sun. | | 6 E | 26 6 0 | 5 2 | 85 | 1 0 Cloudy in the S.W. | | 7 E | 26 6 2 | 6 8 | 89 | 1 8 Perfectly clear. | | 8 E | 26 6 3 | 10 0 | 95 | 2 0 Idem. | | 9 E | 26 6 3 | 10 6 | 97 5 | 1 8 Idem. | | 10 E | 26 6 1 | 9 9 | 95 | 1 2 Little cloud at horizon S. | | 11 E | 26 6 0 | 12 3 | 99 1 | 1 5 Idem more to S.W. | | 12 E | 26 5 15 | 12 5 | H r frost | 1 2 Idem. | | 22d, 1 M | 26 6 0 | 14 3 | Idem | 0 9 Idem. | | 2 M | 26 6 8 | 14 5 | Id. | 1 2 Clouds increase and approach. | | 6 15 M | 26 5 7 | 15 0 | Id. | 0 8 Clear. | | 7 30 M | 26 5 4 | 14 7 | Id. | 1 2 Light fog. | | 8 10 M | 26 5 2 | 14 2 | Id. | 1 1 Idem. | | 9 10 M | 26 4 15 | 10 7 | Id. | 1 6 Idem. | | 10 10 M | 26 4 13 | 8 2 | Id. | 2 2 Thicker fog. | | 11 10 M | 26 4 3 | 4 8 | Id. | 1 8 Idem. | | 1 10 E | 26 4 0 | 4 9 | Id. | 1 7 Idem. | | 2 20 E | 26 3 14 | 6 6 | 82 | 1 4 Weak fog, pale sun. | ### Table

| h. m. | Barometer, feet in height | Thermometer | Hygrom. | Electrom. | |-------|--------------------------|-------------|---------|-----------| | Feb. 22d, 3 30 | E 26 3 13 | — 0 9 | 81 9 | 1 1 | Cloudy pale sun. | | 5 | E 26 3 13 | — 4 3 | 89 | 1 2 | Less cloudy. | | 6 | E 26 3 14 | — 4 4 | 91 2 | 2 2 | More fo. | | 7 | E 26 3 14 | — 6 1 | 94 | 1 7 | Idem. | | 8 | E 26 3 13 | — 5 9 | Id. | 3 7 | Cloudy foggy in S. W. | | 23d o 45 M | — 4 1 | Id. | 1 0 | | Cloudy with more fog. | | 8 5 M | E 26 5 0 | — 1 0 | 81 3 | 1 2 | Idem. | | 10 7 M | E 26 5 5 | — 0 0 | 76 | 0 8 | Idem. | | 3 45 | E 26 6 8 | + 0 5 | 76 | Id. | Cloudy pale sun. | | 5 | E 26 6 14 | — 0 3 | 75 3 | 1 0 | Cloudy. | | 6 | E 26 7 3 | — 0 7 | 74 | 0 8 | Idem. | | 7 | E 26 7 9 | — 1 7 | 79 7 | 2 2 | Very clear. | | 8 | E 26 7 14 | — 3 7 | 87 3 | 1 7 | Cloudy. | | 12 | E 26 9 1 | — 3 0 | 92 | 0 5 | More fo. |

M for Morning, E for Evening.

---

From the first 18 observations of this table, when the sky was quite serene, we see that the electricity was pretty strong at nine in the morning; that from thence it gradually diminished till towards six in the evening, which was its first minimum; after which it increased again till eight, its second maximum; from whence it again gradually declined till six the next morning, which was the time of its second minimum; after which, it again increased till ten in the morning, which was the first maximum of the following day: as this was cloudy, the electric periods were not so regular.

The electricity of serene weather is much weaker in summer than in winter, which renders it more difficult to observe these gradations in summer than in winter; besides a variety of accidental causes, which at the same time render them more uncertain. In general, in summer, if the ground has been dry for some days, and the air is dry also, the electricity generally increases, from the rising of the sun till three or four in the afternoon, when it is strongest: it then diminishes till the dew begins to fall, which again reanimates it; though after this it declines, and is almost extinguished during the night.

But the serene days that succeed rainy weather in summer, generally exhibit the same diurnal periods or states of electricity, as are to be observed in winter.

The air is invariably positive in serene weather, both in winter and summer, day and night, in the sun or in the dew. It would seem, therefore, that the electricity of the air is essentially positive, and that whenever it appears to be negative, in certain rains or in storms, it probably arises from some clouds, which have been exposed to the influence of the electric fluid contained in the upper part of the atmosphere, or to more elevated clouds, that have discharged a part of their fluid upon the earth, or upon other clouds.

In order to find out the cause of these phenomena, Mr. Saussure instituted a set of experiments on evaporation, avoiding the use of Volta's condenser.

To produce a strong evaporation, he threw a mass of red hot iron into a small quantity of water, which was contained in a coffee-pot, with a large mouth, and suspended by silk strings; by this he obtained a strong positive electricity, though, according to M. Volta's system, it ought to have been negative. The experiment was repeated several times, varying some of the circumstances, but the result was always the same.

As it was not easy to think so able a philosopher as M. Volta was deceived, it was necessary to try the experiment in a manner more analogous to that of M. Volta. A small chafing-dish was therefore inflated by silk cords, and the coffee-pot, with a small quantity of water, placed on it; one electrometer was connected with the coffee-pot, and another with the chafing-dish; the fire was raised by a pair of bellows: when the water had boiled strongly for a few minutes, both electrometers exhibited signs of electricity, which, on examination, was found to be negative; proving the truth of M. Volta's experiment. The evaporation produced by the effervescence of iron in the sulphuric acid, and by that of chalk in the same acid, gave also negative electricity.

It was now necessary to inquire, why the vapour, excited by the heated iron, produced positive electricity; while that from boiling water, in any other way, produced a negative electricity.

M. Saussure suspected, that the intensity of heat to which the water is exposed, by the contact of a body in a state of incandescence, was the cause of the electricity produced by its evaporation; and that a combination was then formed, by which a new quantity of the electric fluid was produced. This conjecture may at first sight seem improbable; but the quantity of electricity produced by this experiment will amply show that it is true; and this quantity is more surprising, because, if it is true, according to the system of M. Volta, that the vapours absorb, while they are forming, a quantity of the electric fluid, there must, therefore, be enough developed in this experiment, for the formation of the great quantity of vapours produced by the heated iron, and afterwards a sufficient quantity to electrify strongly the apparatus, and all these vapours. This experiment shows clearly the cause of that prodigious quantity of electricity, which is unfolded in the eruption of volcanoes; as it is probable, that the water in these, from many circumstances, acquires a much greater degree of heat than is given to it in our experiments.

To verify this conjecture, that it was in some measure the combustion of the water, or the iron, that produced the positive electricity, it was proper to try whether, by a regular moderation of the heat of the iron, positive electricity would always be obtained. This was essayed in the following manner: A large iron crucible, five inches high, four in diameter, and six lines thick, was heated red hot; then insulated; after which, small quantities of water were thrown into it, each projection of the water cooling more and more the crucible; thus defending by degrees till there was only sufficient heat to boil the water; carefully observing, and then destroying the electricity produced at each projection. The electricity was always positive or null; at the first projections it was very strong; it gradually diminished to the twelfth, when it was scarce sensible, though always with a tendency to be positive.

On repeating this experiment, and varying it in different ways, a remarkable circumstance was observed: When a small quantity of water was thrown into the crucible, the moment it was taken from the fire, while it was of a pale red, approaching what is called the white heat, no electricity was obtained.

This fact seemed to have some connection with another mentioned by Mufchenbroek, that water evaporates more slowly on a metal, or any other incandescent body, than on the same body, heated only a small degree above boiling water. To examine this relation, and to find whether there was any between the periods of evaporation, and the production of electricity, M. Sauffire made a great number of experiments, which are most accurately described in his excellent work; but as the detail would be much too long to be introduced in this article, we must content ourselves with presenting the reader with the heads thereof, and a description of the apparatus.

The apparatus consisted of a pot of clay, well baked or annealed, fifteen lines thick, and four inches in diameter; this was insulated by a dry glass goblet; upon this pot was placed the crucible, or any other heated substance, on which the water was to be thrown, in order to be reduced into vapours; the crucible was contiguous to a wire connected with an electrometer; a measure containing 54 grains weight of distilled water, was thrown upon the heated crucible; the time employed in the evaporation thereof was observed by a second watch; the electricity produced by this evaporation was noted. When this measure of water was reduced into vapour, the electricity of the apparatus was destroyed, and a fresh measure of water was thrown into the crucible; proceeding in the same manner till the crucible was almost cold.

The first experiment was with an iron crucible, from which it was found, that Mufchenbroek was not right, in saying that the evaporation was slower when the iron was hottest; for at the instant it was taken from the fire it required 19 seconds to evaporate the water, and took more time till the third projection, when it took 35 seconds, though from that period it employed less time, or in other words, the evaporation accelerated in proportion as the iron cooled.

With respect to the electricity, it was at first 0, then positive, afterwards negative, then 0, and afterwards positive to the end of the experiment. The vapour was not visible till the 7th projection.

In the second experiment with the same crucible, though every endeavour was made use of to render them as similar as possible, the electricity was constantly positive.

The third experiment was with a copper crucible; here also the electricity was positive, and the longest time employed in evaporation was not the instant of the greatest heat. It was very curious to see the water endeavouring to gather itself into a globule, like mercury on glass; to be sometimes immovable, and then to turn on itself horizontally, with great rapidity; sometimes throwing from some of its points a little jet, accompanied with a hissing noise.

The fourth experiment was with the same crucible; the electricity was at first negative, then constantly positive.

The fifth was with a crucible of pure silver; a considerable time was employed here in evaporating the same quantity of water; even in the instant of the greatest heat it took five minutes, five seconds; the electricity was weak, three times no electricity was perceived, five times negative electricity was discovered.

In a sixth experiment with the same crucible, a positive electricity was obtained, at the second projection; after which none of any kind was perceived.

The seventh with the same, gave at first strong negative electricity, the second and third projection gave a weak positive electricity.

The eighth was made with a porcelain cup; here the evaporation was slower at the second, than the first projection; but from this it took longer time till it was cold, contrary to what happened with the metals; the electricity was always negative.

The ninth and tenth experiments, with the same cup, produced similar effects.

The eleventh experiment was with spirits of wine in a silver crucible; here there was no electricity produced at the two first projections, and what was afterwards obtained was negative.

Twelfth experiment with ether; here the electricity was also negative. These two inflammable fluids, in evaporating, followed the same laws as water, being diffused at first most rapidly in the greatest heat, afterwards taking a longer and longer time before they were evaporated, to a certain period, then employing less time, or evaporating quicker, till the crucible was nearly cold.

Now as china and silver always produced negative electricity, while iron and copper have generally given positive electricity, we may conclude, that electricity is positive with those bodies that are capable of decomposing water, or of being decomposed themselves by their contact with the water; and negative with those which are not at all decomposed or altered.

If in the foregoing experiment, those substances which were susceptible of oxidation had constantly given a positive electricity, and those which do not oxidise had always given the negative; everything would have been explained by these principles, and they would thence have acquired a greater degree of probability. But the phenomena have not always followed this law. We have seen iron and copper sometimes give a negative electricity, and silver the positive. The first case is not difficult to account for; it is well known with what facility iron and copper are oxidized in a brick fire; they become covered with a scaly crust, which is not susceptible of any further alteration with the same heat. If the bottom of the crucible acquires this crusty coating, the drop of water placed thereon will be no longer in contact with an oxidizable substance; there will be no further decomposition, no generation of the electric fluid: the vapors, however, which are still formed, will absorb a part of the fluid naturally contained in the apparatus, and this will therefore be electrified negatively. If some of the scales should be so far detached, that the water may gain some points of contact, the quantity thus generated may compensate for what is absorbed by the vapors, and thus the electricity will be null. If more are detached, it will super-abound and be positive. From the same reasons, a large mass of water, by attacking the iron in a greater number of points, always gives positive electricity; and hence, also, a strong positive electricity is obtained, by throwing a piece of red-hot iron into a mass of water.

It is not easy to explain why silver gives sometimes a positive electricity, but by supposing it to have been mixed with some substances capable of oxidation; and this the more, as the white porcelain always gave negative electricity. This supposition was verified by some subsequent experiments, in which the same silver, when purified, always gave a negative electricity.

M. Sauflure owns himself incapable of explaining why heated charcoal always gives negative electricity; unless it can be attributed to the promptitude with which it rare a substance loses its heat, by the contact of water.

One fact astonished him, namely, that by combustion properly so called, although it is an evaporation, nay, the highest degree of evaporation, he never obtained any signs of electricity; though he tried to obtain it in a variety of ways. Probably, the current produced by the flame, disperses and dissipates the electricity as soon as it is formed. The case, however, must not be looked upon as general, because M. Volta obtained signs of electricity from bodies in combustion, by means of his condenser.

Another singular fact was, his not being able to obtain electricity without ebullition, though he endeavored to compensate by the quantity of surface for the quantity of vapors that were elevated by boiling water; and indeed, the same quantity of water, if extended over too large a surface, will not give any electricity.

We shall resume this subject immediately, but must first conclude our observations on the phenomena of atmospheric electricity.

The following axioms with respect to atmospheric electricity, deduced by M. Cotte after a long course of observations, merit attention.

1. Electricity manifests itself oftener without storms than with them. 2. It is produced more frequently by dry than by rainy clouds. 3. It is more frequently positive than negative, especially when occasioned by stationary clouds. 4. The atmosphere exhibits signs of electricity at all times by night or day.

In our endeavors to explain the production of natural electricity, we have nothing more to do, than to discover the various circumstances of the atmosphere, in which moisture is absorbed or precipitated.

It is necessary to recollect the proof furnished by numerous experiments, that when any portion of the atmosphere is in a state to take up an additional quantity of moisture, it is in a state at the same time to take up more electric fluid; and vice versa, when it is parting its portion with its water, it is at the same time parting with its electric fluid. But in these cases, neither the superabundance nor the deficiency can produce a charge, unless there be some other part of the air contemporaneously in an opposite state, or in a disposition either to receive or give. It is, however, scarcely possible that this should not always happen; for our atmosphere is, throughout its vast dimensions, each moment agitated by millions of co-instantaneous changes, and for our purpose, it is of no consequence where the required change takes place. Were it New Holland, or at the Antipodes, a connection would be instantly formed between the remote but opposite situations, by the conducting power of the earth.

It is a necessary conclusion from what we have just said, that if the absorption of moisture by air, or the storms copious evaporation of it from the earth, be attended with a new accumulation of the fluid; then where this cause operates most powerfully, there its correspondent effect will be most sensible. We consequently find, that the most tremendous electrical phenomena belong to the countries within the tropics, or to that portion of our atmosphere which is loaded with moisture by the most powerful influence of the sun's rays. In like manner, within the limits of our own and other similar climates, electrical phenomena are greatest, both in force and frequency, during the hottest months of the year, or during the season in which our atmosphere is most copiously and rapidly charged, by absorbing the humidity of the ground.

In the neighborhood of Etna and Vesuvius, during the period of their volcanic fury, surfaces, covering the dimensions of several square leagues, are sometimes volca- noes, scorched with red-hot lava, and every atom of their moisture is rapidly dissipated. At the same time the surrounding air is heated to a vast extent, and in this state swallows up an immense quantity of aqueous vapor; but contemporaneously with the operation of these powers, according to the reports of all natural historians, an immense quantity of the electric fluid is accumulated and discharged.

Again, a dry wind passing over a moist soil is another modification of the cause we are applying; it produces a copious and rapid solution of the aqueous particles, and its consequent alteration of attractive force. Let us for instance, suppose a wind, which had passed over the deserts of Arabia, or that had been torrefied in its passage over a large extent of burning sands, to come in contact with a similar extent of marshy soil, or of any surface well drenched with water, a most abundant evaporation would necessarily take place, and with it an immense accumulation of the electric fluid. But subsequently, in case any power operated, which would take away the aqueous particles thus dissolved, and of course alter the degree of attractive force by which the collected electric fluid is fufilled, we should find that the most dreadful thunder-storms would take place. This is really the case; for there is scarcely a region in the vast circle surrounding the immeasurable lands of Africa, which is not remarkable for storms and tempests.

On the side of Abyssinia, when the warm winds that have passed over the neighbouring deserts are condensed on its mountains, those deluges are collected, which form the inundations of the Nile.

On the coast of Guinea, the harmattan, which is a current of air so dry, as to wither and pulverize, by a complete absorption of all its juices, every substance that occurs in its passage, is no sooner mixed with that body of air which is cooled by the ocean, than it forms most terrific hurricanes of wind and lightning that are described by navigators. Along the Syrian regions, we learn from sacred authority, that the storms gather with such rapidity, that a cloud, which this instant might be covered with the hand, is within the interval of a few minutes, charged with water adequate to the inundation of a whole country.

The thunder that attended these impetuous storms, provoked the sublimest expressions of their poets. Indeed, whenever their minds attempt the description of celestial greatness, or the sudden and overwhelming approach of divine power in its triumph, or in its fury, they have recourse for imagery to those thunder-clouds, which they justly represented as extinguishing the light of the sun, and as involving the world in a few instants in the darkness of midnight.

Having specified the two most general causes of evaporation on the surface of this earth, let us now attend to the possible changes of the atmosphere, when by the operation of either, or both, it is charged with the electric fluid. All these changes are but different degrees of the same effect, viz. the condensation of moisture, and this condensation is in every case produced by an alteration of temperature, which may proceed,

1. From a mixture, or even the contact, of a colder with a warmer air. When the smallest clouds are formed by such a mixture, an electrical charge takes place, so that one part of the cloud has more, and the other less, than its natural share. Fogs, dews, and the slightest change of clear for hazy weather, commonly arise from a warmer atmosphere coming in contact with one of a lower temperature; but even these trifling degrees of condensation are always followed by signs of electricity.

In this country, from its insular situation, which exposes it to the perpetual influence of varying winds, the air changes its appearances often many times in one day. But there is no degree of thick cloudiness or perfect clearness, of scattered clouds succeeding embodied masses of clouds, of small rain increasing to heavy, or vice versa, that is not attended with changes in the expressions of the elevated conductor, which never fails to vary with all the atmospheric condensations and rarefactions that take place.

It is, however, obvious that the effect must be in proportion to the quantity and rapidity of the condensation. When, therefore, any body of air has been for a long time suspended over a surface of ground previously drenched with showers, and at the same time exposed to the violence of the sun's rays, a change in the direction of the wind, or such a change in the weight of the air as mixes the upper with the lower regions of the air, is almost always attended with a thunder-storm.

In tropical climates, for months together, scarcely a day passes, in which the calm atmosphere is not loaded by successive additions of moisture, till at last, it becomes the reservoir of vast rivers and lakes, and of all the moisture that is spread over whole continents. But when this drought has reached its crisis, the sun crosses the line, the wind takes a new direction, a colder air mixes with that which is thus charged with vapours, and the condensation becomes so copious, as to inundate all the subjacent country; but the deluge is not more destructive than its attendant storm; for, according to the reports of spectators, our imaginations, confined to the proceedings of nature in this frozen region, have no images from which any such comparison can be made, as will communicate the least idea of the thunder attending a tropical hurricane.

The cause which we are now applying to the explanation of these natural appearances, will furnish us with an easy solution of a difficulty which has oppressed several theories of electricity, namely, that rapid generation and increase of the electric fluid which takes place in some thunder-storms. Even in this country, the succession of flashes is sometimes so quick, that one hundred and twenty have been known to follow each other in a minute. In Asia, this celerity of accumulation and discharge was so great, that Homer uses it as part of a simile, by which he paints the quick repetition of Agamemnon's sighs and pantings in an hour of distress.

It may be asked, if each distinct cloud is loaded with a distinct charge, and if each flash is a separate discharge of such a cloud, what is there, in our knowledge of natural powers, that will account for an innumerable repetition of these accumulations and discharges within a very short space of time, more especially when each of them is connected in our minds with the necessity of a distinct part of that time for its process? In other words, do we know of any cause that is adequate to the filling and emptying of the same portion of air every instant, for hours together?

On a hot summer's day it not unfrequently happens, that a fine blue sky will, within five seconds, be changed into one mass of clouds. If the cause which produces so great an effect, were supposed to be doubled in its power of condensation, the degree of electricity shown by the elevated conductor would be rather more than doubled, and its signs would be much stronger than in a common storm; we may hence conclude, that the whole mass which might be thus formed in five seconds, would be loaded so as to have every part of it at the discharging height; but the mass might consist of hundreds of distinct clouds all in the same state, and consequently adequate to the production of several hundred flashes within a minute.

The collapse of aqueous particles, which would necessarily follow such a rapid succession of discharges as have been now proved to be possible, would produce a partial vacuum of great extent, and on all sides the heavier air would rush into it, and the upper and colder regions would press downwards, and by their condensing temperatures, would renovate all the accumulations and discharges which have been already described; a second collapse would follow a second series of thunder-strokes, thunder-strokes, and a partial vacuum, additional to the former; a fresh portion of warm air would again rush in from all quarters, and a fresh mixture of cold air from the upper regions. It is scarcely necessary to show that this repetition of condensations may continue for hours, or till the air, which rushes in laterally, becomes of such a temperature, that its mixture with the colder air will not produce the condensations adequate to the collection of that quantity of electric fluid which is necessary for a discharge.

From this explanation, it is obvious that a central point must exist, at which the violence of every storm begins, and from which it is spread in all directions. A hurricane in the West Indies, though ruinous to many, is generally the distinguishing calamity of one island, at which alone the wind is described as blowing from every point of the compass; while in every other island, it is represented as bearing down decisively from one quarter.

2. The precipitation of aqueous particles when suspended by heat in air, is frequently the consequence of the loaded atmosphere's coming in contact with portions of the earth that are colder than itself. Such, particularly, are the summits of mountains, whose effect is great in proportion to the degree of their cold and the extent of their surface. It is, however, certain that condensations, when thus produced, are invariably attended by thunder-storms.

The uproar, and the splendour of the innumerable lightnings, which dart through all the entangled circuits of an abyss of thunder-clouds, are the immutable attributes of grandeur which belong to the Cordilleras; for they dam up, as it were, an immense flow of air, which is almost saturated with moisture by passing over several thousand leagues of land, exposed to the fury of a tropical sun.

In summer, the north-westerly winds that pass over France, are always condensed by the Alps; and in the night, during such a state of the atmosphere, to all those who live along the Saone and the upper part of the Rhone, these mountains are always brightened by electrical flashes and coruscations.

All ridges or chains of very high grounds, especially those which terminate extensive plains lying in the direction of their most common winds, are perpetually beclouded; and with a good conductor, fixed on their summit, we should find that the signs of electricity were as constant as the condensations by which they are enveloped. But in proportion to the coldness, so is the subsequent change of temperature on the eminences diminished, and the electrical effect dependant on that change. It hence happens, that there are countries in the northern parts of Europe, the gloom of whose mists is never dispelled by a thunder-storm, excepting in the hottest season of summer.

3. When the sun, by directing its rays with force and abundance upon the earth for any length of time, has produced a considerable evaporation, the mere interruption of its influence will be attended with a discharge of the electric fluid; for the great source of change in our atmosphere is the ready influence of its upper regions, which are cold, on its lower regions when warmed; and any cause which mixes these together, must bring on a condensation of aqueous vapour. This mixture, however, takes place on the mere approach of night, as is evident from the change of temperature expressed by the thermometer, and the usual fall of the dew; we consequently find, that as night comes on, the signs of electricity always increase. When the weather is tolerably settled, or such that no other cause is active than that proceeding from the change of day for night, or night for day; then the signs of electricity gradually decrease from twelve o'clock at night till six in the morning; from this hour till nine, they gradually increase, when they become exceedingly weak, and continue so till four in the afternoon; the increase at this time recommences, and is very decisive in its appearance till about two hours after sunset, when it becomes stationary, and remains in this state, or decreasing, so farcely to be sensible, till the morning.*

The cause, whose operation we have now investigated in the production of its most feeble effects, may be easily applied to many other causes, in which similar, but greater powers are displayed by nature. Let us suppose, that on a wide surface of ground, previously warmed by the sun, copious showers of rain had fallen, followed by a return of the sun's influence; in this case, the evaporation is necessarily very rapid, and the signs of electricity expressed by an elevated conductor are very strong.

When a copious production of electric fluid has attended a copious evaporation continued for several successive hours, a thunder-storm, or some striking electrical appearance will come on with the approach of night; for unless the barometer should suddenly rise, the condensation attending the evening's cold must be very considerable, and its usual consequences proportionally great. Such a day as we have just described, is usually followed by a violent thunder-storm. Indeed, there is scarcely an instance in which a moist ground, operated on for some hours by a clear sun, provided the wind continues to blow from the south-west or west, is not attended the following night by the appearance of falling stars, flashes of lightning, or the Aurora Borealis.

We have alluded to the connection of winds with the phenomena of atmospheric electricity. The influence of winds must depend on various circumstances; in some cases, they will tend to diminish electrical appearances, and in others, they may altogether destroy them. The current of air which proceeds from a mixture of two winds of different temperatures, is the effect of a condensation of vapour, that may be succeeded by the most violent storms. But if there should be two neighbouring regions, in one of which, the rays of the sun should co-operate with the moisture of the ground, in producing electricity, while in the other there should prevail a condensation favourable to the discharge of the electric fluid; a current of air would be produced that would act like a communicating rod between two opposite electrified surfaces, would exchange the situations of the charged bodies, and would consequently cause the new situation to counteract the effects produced in the last. This effect would be more sensible in proportion as the exchange has been more rapid, and accordingly we find, that during high winds, the electricity of the atmosphere is very small.*

* Vide Read on Spontaneous Electricity.

Morgan's Lectures, vol. ii. Most of the luminous appearances in the atmosphere have of late been attributed to electricity. Of these we shall at present only consider the Aurora Borealis, or Northern Lights, referring the account of other meteors for the article Meteorology.

The aurora borealis is usually of a reddish colour, inclining to yellow, sending out frequent coruscations of pale light, which seem to rise from the horizon in a pyramidal undulating form, shooting with great velocity towards the zenith. This light sometimes appears remarkably red, as it happened December 5, 1737.

The aurora borealis appears frequently in the form of an arch; chiefly in spring and autumn, after a dry year. This arch is partly bright, partly dark, but generally transparent; and no change is found to be produced by it on the rays of light which pass through it. It sometimes produces a rainbow.

This kind of meteor, which becomes more uncommon as we approach towards the equator, is almost constant during the long winter of the polar regions, and appears there with the greatest lustre.

In the Shetland isles, the merry dancers, as the northern lights are there called, are the constant attendants of clear evenings, and afford great relief amid the gloom of the long winter nights. They commonly appear at twilight, near the horizon, of a dim colour, approaching to yellow; sometimes continuing in that state for several hours, without any perceptible motion; and afterwards breaking out into streams of stronger light, spreading into columns, and changing slowly into numberless different shapes, and varying in colour from all the tints of yellow, to the most obscure russet brown. They often cover the whole hemisphere, then exhibiting the most brilliant appearance. Their motions at this time are exceedingly quick, and they astonish the spectator with the rapid change of their form. They break out in places where none were seen before, skim briskly along the heavens, are suddenly extinguished, and are succeeded by a uniform dusky tract. This again is brilliantly illuminated in the same manner, and as suddenly becomes a dark space. In some nights, they assume the appearance of large columns, on one side of the deepest yellow, and on the other gradually changing, till it becomes indistinguishable from the sky. They have generally a strong tremulous motion from one end to the other, continuing till the whole vanishes. As for us, who see only the extremities of these phenomena, we can have but a faint idea of their splendour and motions. They differ in colour according to the state of the atmosphere, and sometimes assuming the colour of blood, they make a dreadful appearance. The rustic sages who observe them become prophetic, and terrify the spectators with alarm of war, pestilence, and famine; nor indeed were these superstitious preludes peculiar to the northern islands: appearances of a similar nature are of an ancient date; and they were distinguished by the appellations of phaenata, trabea, and bolides, according to their forms and colours. In old times they were either more rare or less frequently noticed; but when they occurred, they were supposed to portend great events, and the timid imagination formed of them aerial conflicts.

In the northern latitudes of Sweden and Lapland, the aurora borealis is not only an object of pleasing curiosity from the singular beauty of its appearance, but is extremely useful in affording to travellers, by its almost constant effulgence, a very brilliant light. In Hudson's bay, it is said to possess a variegated splendour, equalling that of the full moon. "In the north-eastern parts of Siberia," says Gmelin, "these northern lights are observed to begin with single bright pillars rising in the north, and almost at the same time in the north-east, which gradually increasing, comprehend a large space of the heavens, rush about from place to place with incredible velocity, and finally almost cover the whole sky up to the zenith, and produce an appearance as if a vast tent was spread in the heavens, glittering with gold, rubies, and sapphire. A more beautiful spectacle cannot be painted; but whoever should see such a northern light for the first time, could not behold it without terror. For however fine the illumination may be, it is attended, as I have learned, with such a hissing, cracking, and rushing noise through the air, as if the largest fireworks were playing off. To describe what they then hear, they make use of the expression spolochi chodiat, i.e., 'the raging hoist is paling.' The hunters who pursue the white and blue foxes in the confines of the icy sea, are often overtaken with these northern lights. Their dogs are then so frightened, that they will not move, but lie obstinately on the ground till the noise has passed. Commonly clear and calm weather follows this kind of northern lights. I have heard this account, not from one person only, but confirmed by the testimony of many who have spent part of several years in these very northern regions, and inhabited different countries from the Yenisei to the Lena; so that no doubt of its truth can remain. This seems, indeed, to be the birthplace of the aurora borealis."

This account of the noises attending the aurora borealis, allowing for some degree of exaggeration, has been corroborated by other testimonies.

Similar appearances have likewise been observed towards the south pole, and are therefore called aurora australis australis. The best account of these is given by Mr Forster, who in his voyage round the world with Captain Cook, says that he observed them in high southern latitudes, though attended with phenomena somewhat different from those observed here. "On February 17, 1773, in south lat. 58°, a beautiful phenomenon (he says), was observed during the preceding night, which appeared again this and several following nights. It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spreading on the whole southern part of the sky. These columns were sometimes bent sideways at their upper extremities, and though in most respects similar to the northern lights (aurora borealis) of our hemisphere, yet differed from them in being always of a whitish colour, whereas ours assume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point." The periods of the appearances of the aurora borealis are very inconstant. In some years they occur very frequently, and in others they are more rare; and it has been observed that they are more common about the time of the equinoxes than at other seasons.

Dr Halley has given us a sort of chronological history of the appearances which may be ranked under the aurora borealis; but for his account of the individual cases we must refer to his paper in the Philosophical Transactions Abridged, vol. iv.

The particular part of the atmosphere in which these appearances take place, or the height above the earth to which they extend, is by no means certain; various philosophers have attempted to ascertain the height of various aurora borealis by trigonometrical calculation; some have estimated them at a few hundreds, others at some thousands of miles above the earth; but the results of their measurements are so contradictory, that they cannot be relied on.

"Several of the most celebrated inquirers into nature have given their authority to some of the most extravagant theories, in attempting to assign its proper cause to the aurora borealis. Their imaginations have kindled bonfires in the poles of the earth, and they have represented the northern lights as the effects of flames, to which those lights have scarcely any similarity, and from which they are distinguished by numberless diversities.

"The salt-pits of the north were at one time regarded as emitting a luminous effluvium from their entrails, copious enough to pervade the whole of our northern atmosphere. The discoveries of electricians have confirmed all these reveries to a shade; whence they would never return to excite the wonder of modern philosophers, if the authors of them had not brought forth other productions, whose merits have made even their mistakes immortal."

The evidence which we have for considering the aurora borealis as an effect of electricity, chiefly consists of the following arguments.

1. If lightning be an effect of electricity, the same cause must, at a certain height in the atmosphere, produce such an appearance as is exhibited by the aurora borealis. The passage of the electric matter through air rarefied to a certain degree, is attended with all the undulating coruscations of this meteor. Indeed there is scarcely a single circumstance attending the passage of a spark or a charge through an exhausted tube, that does not bear a resemblance to something observed in the northern lights. The same peculiar motion, the same variety of colour, the same rapid alternations of flashes, occur both in the experiment, and in the natural phenomenon; the streams of light in both are vivid and pointed; and if, in the experiment, the exhaustion has been properly managed, some parts of the light will be marked with that reddish tinge, which in the aurora borealis has so often struck the vulgar mind with terror and consternation. The experiments to which we particularly allude are those of the conducting glass tube, the luminous conductor and the aurora borealis described in No. 188—190.

2. The striking distance of a charge of electric fluid passing through the air, increases according to the rarefaction of that medium. If, therefore, two clouds in opposite states of electricity have no other circuit, it is probable that they will be discharged through the higher regions of the atmosphere, more especially if they are at such an elevation, as renders their communication with the earth impracticable.

3. The same causes which tend to produce such an accumulation of electricity in the atmosphere as will bring on a thunder-storm, have been found, in certain seasons, and in the more northern climates, to be attended with an aurora borealis.

It must be confessed, however, that Mr Brook and Mr Bennet, in their observations on the electricity of the atmosphere during an aurora borealis, could observe no particular signs of increased electricity, more than would have occurred in a serene sky without any such appearance.

4. A magnetic needle commonly appears a little disturbed during a strong aurora borealis.

We have already hinted at the connection between magnetism and electricity, and we shall fully illustrate this in the article MAGNETISM. Till this connection is fully explained, the force of this last argument can scarcely be seen.

A considerable difficulty attends even the most received theories of the aurora borealis, viz. the light appearing always to strike from the poles towards the equator, rather than in the contrary direction. Perhaps this may be explained in the following manner. We shall assume the three following axioms.

1. That all electrics when considerably heated, become conductors of electricity.

2. That, conversely, non-electrics when subjected to violent degrees of cold, ought to become electrics.

3. That cold must also increase the electric powers of such substances as are already electric.

The air, all round the globe, at a certain height above its surface, is found to be exceedingly cold, and, as far as experiments have yet determined, exceedingly electrical. The inferior parts of the atmosphere between the tropics, are violently heated during the daytime, by the reflection of the sun's rays from the earth. Such air will, therefore, be a kind of conductor, and much more readily part with its electricity to the clouds and vapours floating in it, than the colder air towards the north and south poles. Hence the prodigious appearances of electricity in these regions, shewing itself in thunder and other tempests of the most terrible kind. Immense quantities of the electric fluid are thus communicated to the earth; and the inferior warm atmosphere having once exhausted itself, must necessarily be recruited from the upper and colder region. This becomes very probable from what the French mathematicians observed when on the top of one of the Andes. They were often involved in clouds, which, sinking down into the warmer air, appeared there to be highly electrified, and discharged themselves in violent tempests of thunder and lightning; while in the mean time, on the top of the mountain, they enjoyed a calm and serene sky. In the temperate and frigid zones, the inferior parts of the atmosphere, never being so strongly heated, do not part with their electricity so easily as in the torrid zone, and consequently do not require such recruits from the upper regions; but notwithstanding the difference of heat observed in different parts of the earth near the surface, it is very probable that at considerable heights the degrees of cold are nearly equal all round. the globe. Were there a like equality in the heat of the under part, there could never be any considerable loss of equilibrium in the electricity of the atmosphere; but as the hot air of the torrid zone is perpetually bringing down vast quantities of electric matter from the cold air that lies above it; and as the inferior parts of the atmosphere lying towards the north and south poles do not conduct in any great degree; it thence follows, that the upper parts of the atmosphere lying over the torrid zone will continually require a supply from the northern and southern regions. This easily shews the necessity of an electric current in the upper parts of the atmosphere from each pole towards the equator; and thus we are also furnished with a reason why the aurora borealis appears more frequently in winter than in summer; namely, because at that time the electric power of the inferior atmosphere is greater on account of the cold than in summer; and consequently the abundant electricity of the upper regions must go almost wholly off to the equatorial parts, it being impossible for it to get down to the earth; hence also the aurora borealis appears very frequent and bright in the frigid zones, the degree of cold in the upper and under regions of the atmosphere being much more nearly equal in these parts than in any other. In some parts of Siberia particularly, this meteor appears constantly from October to Christmas, and its corruptions are said to be very terrifying. Travellers agree that here the aurora borealis appears in its greatest perfection; and it is to be remarked, that Siberia is one of the coldest countries in the world. In confirmation of this, it may also be observed, that from the experiments hitherto made with the kite, the air appears considerably more electrical in winter than in summer, though the clouds are known to be often most violently electrified in the summer time; a proof, that the electricity naturally belonging to the air, is in summer much more powerfully drawn off by the clouds than in winter, owing to the excess of heat.

A considerable difficulty, however, still remains from the upright position which the streams of the aurora borealis are generally supposed to have; whereas, according to our hypothesis, they ought rather to run directly from north to south. Dr Halley answered this difficulty by supposing his magnetic effluvia, (to which he attributed this phenomenon), to pass from pole to pole in arches of great circles, rising to a great height above the earth, and consequently darting from the places whence they arose like the radii of a circle; in which case, being let off in a direction nearly perpendicular to the surface of the earth, they must necessarily appear erect to those who see them from any part of the surface, as is demonstrated by mathematicians. It is also reasonable to think that they will take this direction rather than any other, on account of their meeting with less resistance in the very high regions of the air than in such as are lower.

But the greatest difficulty still remains; for we have supposed the equilibrium of the atmosphere to be broken in the daytime, and restored only at night; whereas, considering the immense velocity with which the electric fluid moves, the equilibrium ought to be restored in all parts almost instantaneously; yet the aurora borealis never appears except in the night, although its brightness is such as must sometimes make it visible to us did it really exist in the daytime.

In answer to this it must be observed, that though the passage of electricity through a good conductor is almost instantaneous, yet through a bad conductor it takes some time in passing. As our atmosphere, therefore, unless very violently heated, is but a bad conductor of electricity; though the equilibrium in it is broken, it can by no means be instantaneously restored. Add to this, that as it is the action of the sun which breaks the equilibrium, so the same action, extending over half the globe, prevents almost any attempt to restore it till night, when flashes arise from various parts of the atmosphere, gradually extending themselves with a variety of undulations towards the equator.

PART VI.

OF THE EFFECTS OF ELECTRICITY ON VEGETABLE LIFE.

IT has been much disputed whether electricity produces any effects on vegetables; and the experiments that have been made with the view of ascertaining this point are most contradictory.

The first electrician who seems to have attended to this subject of inquiry was Mr Maimbray of Edinburgh, who, in the year 1746, electrified two myrtles during the whole month of October (i.e. we suppose, for some hours every day). The consequence was, that in the following summer, these electrified myrtles put forth buds and blossoms sooner than their neighbours who had been left to nature*.

Mr Maimbray was soon followed by the Abbé Nollet, who made some comparative experiments on the germination of seeds under similar circumstances, except that one pot was electrified three or four hours every day for fifteen days. The result of these experiments was similar to that of Mr Maimbray's†.

Similar experiments were made by M. Achard of Berlin, and several other philosophers, but still with the same result; till Dr Ingenhousz instituted a very complete set of experiments on the electrification of plants, which were communicated to the world through the medium of Rozier's Journal, at first by M. Swankhardt, and afterwards by Dr Ingenhousz himself, in consequence of some severe animadversions which the communication of M. Swankhardt had called from M. Duvernier. By these experiments the faith of philosophers with respect to the effect of electricity on vegetation was staggered, as they were attended with results very opposite to those of Maimbray, Nollet and Achard.

Experiments Experiments have also been made by Dr Carmonay and the Abbé D'Ornoy, rather more favourable to the first opinion; but the manner in which the electricity was applied appears very equivocal, as it is found that even shocks do not pass through the body of the plant, but merely over its surface.

But the most complete set of experiments on this subject has been made by the Abbé Bertholon, and these we shall give more in detail.

"In the first place (says the Abbé), there is continually and everywhere diffused in the atmosphere (particularly in the upper regions) a considerable quantity of the electric fluid.

"This principle being granted: in order to remedy the deficiency of electric fluid which is supposed hurtful to vegetation, we must erect in the spot which we want to fecundate the following new apparatus, which has had all possible success, and which I shall call by the name of the electro-vegetometer. This machine is as simple in its construction as efficacious in its manner of acting; and I doubt not but it will be adopted by all those who are sufficiently instructed in the great principles of nature.

"This apparatus is composed of a mast AB, fig. 131, or a long pole thrust just so far into the earth as to stand firm and be able to resist the winds. That part of the mast which is to be in the earth must be well dried at the fire; and you must take care to lay on it a good coat of pitch and tar after taking it from the fire, that the resinous particles may enter more deeply into the pores of the wood, which will then be dilated, at the same time that its humidity will be expelled by the heat. Care must likewise be taken to throw around that part fixed in the earth a certain quantity of coal dust, or rather a thick layer of good cement, and to build besides a base of masonry-work of a thickness and depth proportionable to the elevation of the instrument, so as to keep it durable and solid. As to the portion it above the ground, it will be sufficient to put upon it some coats of oil paint, except one chooses rather to lay on a coat of bitumen the whole length of the piece.

"At the top of the mast there is to be put an iron console or support C; whose pointed extremity you are to fix in the upper end of the mast, while the other extremity is to terminate in a ring, in order to receive the hollow glass tube which is seen at D, and in which there is to be glued an iron rod rising with the point E. This rod, thus pointed at its upper extremity, is completely insulated, by reason of its keeping a strong hold of a thick glass tube, which is filled with a quantity of bituminous matter, mixed with charcoal, brick-dust, and glass-powder; all together forming a sufficiently good and strong cement for the object in view.

To prevent rain wetting the glass tube, care must be taken to folder to the rod E a funnel of white-iron; which consequently is entirely insulated. From the lower extremity of the rod E hangs a chain G, which enters into a second glass tube H, supported by the prop I. The lower end of the above-mentioned chain rests upon a circular piece of iron wire, which forms a part of the horizontal conductor KLMN. In L is a breaker with a turning joint or hinge, in order to move to the right or left the iron rod LMN; there is likewise another in Q, to give still greater effect to the circular movement. O and P are two supports terminating in a fork, where there is fixed a filken cord tightly stretched, in order to infuse the horizontal conductor: in N are several very sharp iron points.

"In fig. 132, you see an apparatus in the main like the former, but with some difference in the construction. At the upper extremity of the mast ab there is bored another hole into which enters a wooden cylinder c, which form of this has been carefully dried before a great fire, in order to extract its humidity, dilate its pores, and saturate it with tar, pitch, or turpentine, applied at repeated intervals. The more heat the wood and bituminous matter receives, the more the substance penetrates, and the infusion will be the more complete. It is moreover proper to besmear the circumference of the little cylinder with a pretty thick coat of bitumen. This preparation being made, we next insert the cylinder c into the hole b of the mast; and it is easy to join together these two wooden pieces in the most perfect manner.

"At the upper extremity of the cylinder c we strongly attach an iron rod gf, which, instead of one, is terminated by several sharp points, all of gilded iron. In e you see a branch of iron resembling the arm of an iron crown, from whence hangs an iron chain hi, at the end of which there is hooked a piece of iron resembling a man's square, and ending in a fork. The piece of iron i is a ring with a handle entering a little into the glass tube m filled with mastic, in the same manner as does the iron rod n. The conductor p is to be considered as an additional piece to act in that marked p. There are likewise put iron splices in q: the support s resembles those of O and P in the former figure. In this new machine you can lengthen or shorten the horizontal conductor as you please; and as the iron ring l turns freely in a circular gorge made in the mast, the conductor is enabled to describe the entire area of a circle.

"The construction of this electro-vegetometer once well underflooded, it will be easy for us to conceive its effects, these infra. The electricity which prevails in the aerial regions will soon be drawn down by the elevated points of the upper extremity.

"The electric matter brought down by the point E, or by those marked fff, will be necessarily transmitted both by the rod and chain; because the insulation produced at the upper extremity of the mast completely prevents its communication with the timber. The electric fluid passes from the chain to the horizontal conductor KM or no; it then escapes by the points at P and q.

"The manner of using this instrument is not more difficult than the knowledge either of its construction or effects. Suppose, for example, we are to place it in the midst of a kitchen garden. By making the horizontal conductor turn round successively, you will be able to carry the electricity over the whole surface of the proposed ground. The electric fluid thus drawn down, will extend itself over all the plants you want to cultivate; and this at a time when there is little or no electricity in the lower regions near the surface of the earth.

On the other hand, when it happens that the electric fluid shall be in too great abundance in the atmosphere, in order to take off the effect of the apparatus in K, fig. 131, and in m, fig. 132, you have only to hang to it an iron chain reaching to the ground, or else... Effects of Electricity on Vegetation.

Electricity effects, viz., that of destroying the insulation, and of an infinitely transmitting the electric fluid in the same proportion as it is drawn by the points; so that there shall never be an overcharge of this fluid in the instrument, and its effect shall be either something or nothing, according as you add or remove the second chain or the additional rod.

"There will be nothing to fear from the spontaneous discharge of this apparatus, because it is terminated below by proper points in P and q of both machines; however, it will be easy to furnish one, by means of which we may approach the apparatus with perfect security; it is only necessary to hold the hand before it. This has the form of a great C, and is of a height equal to the distance that takes place between the horizontal conductor and the surface of the earth. This discharger near the middle must be furnished with a glass handle; and at that extremity which is directed towards the conductor, there must hang an iron chain made to trail on the ground. This instrument is an excellent safeguard. Sec fig. 133.

"By means of the electro-vegetometer just now described, one may be able to accumulate at pleasure this wonderful fluid, however diffused in the regions above, and conduct it to the surface of the earth, in those seasons when it is either scantily supplied, or its quantity is insufficient for vegetation; or although it may be in some degree sufficient, yet can never produce the effects of a multiplied and highly increased vegetation. So that by these means we shall have an excellent vegetable manure or nourishment brought down as it were from heaven, and that too at an easy expense; for after the construction of this instrument, it will cost nothing to maintain it: It will be moreover the most efficacious you can employ, no other substance being so active, penetrating, or conducive to the germination, growth, multiplication, or reproduction of vegetables. This heavenly manure is that which nature employs over the whole habitable earth; not excepting even those regions which are deemed barren, but which, however, are often fecundated by those agents which nature knows so well to employ to the most useful purposes. Perhaps there was nothing wanting to bring to a completion the useful discoveries that have been made in electricity, but to show this so advantageous an art of employing electricity as a manure; consequently, that all the effects which we have already mentioned depend upon electricity alone; and lastly, that all these effects, viz., acceleration in the germination, the growth, and production of leaves, flowers, fruit, and their multiplication, &c., will be produced, even at a time when secondary causes are against it; and all this is brought about by the electric fluid, which we have the art of accumulating over certain portions of the earth, where we want to raise those plants that are most calculated for our use. By multiplying these instruments, which are provided at little expense (since iron rods of the thickness of one's finger, and even less, are sufficient for the purpose), we multiply their beneficial effects, and extend their use ad infinitum.

"This apparatus having been raised with care in the midst of a garden, the happiest effects were perceived, viz., different plants, herbs, and fruits, in greater force than usual, more multiplied, and of better quality. At the same time it was observable, that during the night, the points P and q, as well as the upper extremities, were often garnished with beautiful luminous sparks. These facts are analogous to an observation which I have often made, viz., that plants grow best and are most vigorous near thunder-rods, where their situation favours their development. They likewise serve to explain why vegetation is so vigorous in lofty forests, and where the trees raise their heads thunder far from the surface of the earth, so that they seek rods, as it were, the electric fluid at a far greater height than plants less elevated; while the sharp extremities of their leaves, boughs, and branches, serve as so many points granted them by the munificent hand of nature, to draw down from the atmosphere that electric fluid, which is so powerful an agent in forwarding vegetation, and in promoting the different functions of plants.

"It is not only by means of the electricity in the atmosphere, collected by the above apparatus, that augment one can supply the electric fluid, which is so necessary to vegetation; but the electricity named air of vegetation answers the same purpose. However although artificial electricity may be, or however impossible it may appear to realize it, yet nothing will be found more easy upon trial. Let us suppose that one wants to augment the vegetation of trees in a garden, orchard, &c., without having recourse to the apparatus destined to pump down as it were the electricity from the atmosphere, it is sufficient to have a large inflating stool. This may be made in two ways; either by pouring a sufficient quantity of pitch and melted wax upon the above stool, whole borders being more raised than its middle, will form a kind of frame; or more simply, the stool (which is likewise called the inflator) shall only be composed of a plate longer than broad, supported by four glass pillars, like those used for electrical machines. One must take care to place above the inflator a wooden tray full of water, and to cause mouth upon the stool a man carrying a small pump in the form of a syringe. If you establish a communication between the man and an electrical machine put in motion (which is easily done by means of a chain that connects with the conductor of the machine), then the man thus insulated (as well as everything upon the stool) will be able, by pushing forward the sucker, to water the trees, by pouring upon them an electrical shower; and thus diffusing over all the vegetables under its influence a principle of fertility that exerts itself in an extraordinary manner upon the whole vegetable economy: and this method has moreover this advantage, that at all times and in all places it may be practised and applied to all plants whatever.

"Every one knows that the electricity is communicated to the water thus employed; and it would be easy to obtain the most ample conviction (if any one doubted it), by receiving upon his face or hand this electrical shower; he immediately feels small punctures or strokes, which are the effects of the sparks that issue from each drop of water. This is perceived most sensibly if there is presented a metal dish to this electrical dew; for at the very instant of contact, brilliant flashes are produced.

"That the electricity received by the man from the chain..." Effects of chain may be communicated to the tray, we must put Electricity a small cake of white iron, upon the end of which he may place his foot. The tray filled with water is a kind of magazine or reservoir to serve as a continual supply to the pump. After watering one tree, you transport the flood to a second, a third, and so on successively; which is done in a short time, and requires very little trouble.

Instead of the chain, it is better to employ a cord or twist of pinchbeck or any other metal; by means of which there can be no loss of the electric matter, as there is in the case of the chain by the ring points. Moreover, this metal cord or thread being capable of being untwisted and lengthened, there will be no occasion for transporting too often the electrical machine. It is almost needless to add, that this string or metallic cord, which is always insulated, may rest upon the same kind of supports with those which have been exhibited in OP and figs. 131. and 132. This method is simple, efficacious, and not expensive, and cannot be too much employed.

If one wants to water either a parterre or common garden, beds and platforms of flowers, or any other plots in which are sown grain or plants of different ages and kinds, no method is more easy and expeditious than the following: Upon a small carriage with two wheels there is placed a framed insulator in form of a cake of pitch and rosin, as we have mentioned before in No. 538. The carriage is drawn the whole length of the garden by a man or horse fixed to it. In proportion as you draw the carriage, the metallic cord winds itself upon a bobbin, which turns as usual. This last is insulated, either because the little apparatus that sustains the bobbin is planted in a mat of rosin (when you choose the axle to be of iron), or else because this moveable axis is a tube of solid glass. There must also be a support, which serves to prevent the gold thread or the metallic cord from trailing on the ground, and thus dissipating the electricity; and, moreover, it serves as an insulator. To accomplish this last purpose, it is necessary that the ring into which it passes be of glass. One may likewise employ the insulators and supports marked OP and figs. 131. and 132. If a gardener, mounted upon an insulator, holds in one hand a pump full of water, and with the other takes hold of a metallic cord, in order to transmit the electricity which comes from the conductor; in this case, the water being electrified, you will have an electrical shower; which falling on the whole surface of the plants which you want to electrify, will render the vegetation more vigorous and more abundant. A second gardener is to give additional pumps full of water to him who is upon the insulator, when he shall have emptied those he holds; and thus in a little time you will be able to electrify the whole garden. This method takes hardly longer time than the ordinary one; and although it should be a little longer, the great advantages resulting from it will abundantly recompense the small additional trouble.

By repeating this operation several days successively, either upon seed sown or plants in a state of growth, you will very soon reap the greatest advantages from it. This operation, equally easy with the preceding described upon the subject of watering trees, has been put in practice with the greatest success. Several other effects of methods answering the same purpose, might be devised; but they are all of them pretty similar to that just described.

I cannot finish this article without mentioning another method relative to the present object, although it be much less efficacious than the preceding ones. It consists in communicating to water kept in basins, reservoirs, &c. (for the purpose of watering), the electric fluid, by means of a good electrical machine. To this end, one must plaster over with a bituminous cement all the interior surface of the basin destined to receive the water that serves for irrigation; the nature of this cement answering the purpose of insulation, will prevent the electric fluid that communicates with the water from being dissipated; and the water thus charged with electricity will be the more fitted for vegetation.

If the deficiency of the electric fluid, or rather a small quantity of it, is apt to be hurtful to vegetables, injured by a too great abundance of this matter will likewise sometimes produce pernicious effects. The experiments made by Messrs Nairne, Banks, and other learned men of the Royal Society of London, prove sufficiently this truth. An electric battery, very strong, was discharged upon a branch of balsam still holding its trunk. Some minutes after, there was observed a remarkable alteration in the branch, of which the least woody parts immediately withered, dropped towards the ground, died next day, and in a short time entirely dried up; at the same time that another branch of the same plant that had not been put under the electric chain, was not in the smallest degree affected.

This experiment repeated upon other plants showed the same effects; and it was remarked that the attraction, occasioned by a strong discharge of the electricity, produced an alteration different according to the different nature of the plants. Those which are least woody, more herbaceous, more aqueous, experience in proportion, impressions that are stronger and much more speedy in their operation.

A branch of each of the following plants, composing an electrical chain, it was observed by these able philosophers, that the balsam was affected by the discharge of the battery in a few moments after, and perished next day. The leaves of a marvell of Peru did not drop till the day following that; and the same phenomenon happened to a geranium. Several days elapsed before there was observed any fatal effect on the cardinal flower. The branch of a laurel did not show any symptoms till after the lapse of about 15 days, after which it died; but it was a full month before they perceived any sensible change on the myrtle; at the same time they constantly observed that the bodies of those plants and branches which had formed no part of the chain, continued to be fresh, vigorous, and covered with leaves in good condition.

It hardly ever happens that the supersaturation of the electric fluid existing in a small portion of the atmosphere where a plant is situated, can be so great as that which took place by the explosion of the strong battery of Mr Nairne, directed particularly upon one branch; or if this should happen, it can only be upon a few individual plants in a very small number. INDEX.

A

ACHARD's experiments on ice, No 5 division of the scale of an electrometer, 68 Action, electric, law of, 366—368 shown by experiment, 368 Æpinus's experiments on the tournament, 21 on sulphur, 27 theory of electricity, 294—347 defect in, supplied, 320 defended, 321 Air, effect of, in electricity, 63 decomposed by electricity, 165 rarefied by an explosion, 168 when hot, a conductor, 227 how excited, 229 by friction, 230 evaporation, 231 of a room, to electrify, 234 plate of, to charge, 235 essential to electrical phenomena, 237 Amalgam, electrical, preparation of, 13 ib. Amber, electricity of, known to Thales, page 645, note (A.) gives name to electricity, 1 Ammonia decomposed by electricity, 166 Apparatus, electrical, Part II. Atmosphere, electric, visible, 191 electricity of, constant, 495 always positive in dry weather, ib. strongest in foggy weather, 503 table of observations on the, p. 792. periodical flux and reflux observed in the, 504 causes affecting the electricity of the, 511—520 Atmospheres, electric, hypothesis of, refuted, 408 Attraction and repulsion, electrical, illustrated, Part III. Ch. i. through glass explained, 292 Aurora australis, 522 borealis, phenomena of, 521 attributed to polar fires, 523 evidence for the electrical origin of, 524 theory of borealis, experiment of, 190 Axioms on atmospheric electricity, 510

B

Baldwin, curious phenomenon observed by, 497 Battery, electrical, how constructed, 110 Battery, electrical, compendious, No 110 rules for, 112 origin of, 113 Van Marum's, 114 charge of, how estimated, 204 Beccaria's substitute for glass, 105 vindicating electricity, 422 observations on atmospheric electricity, 486 idea of the production of hail, 488 Bells, electrified, 61, 80, 126 Bennet's electrometer, 69 advantages of, 71 defects of, 74 how remedied, 75, 216 applied to kites, 496 doubler, 247 mode of remedying the defects of the doubler, 252 Bertholet's remarks on the fusion of metals by electricity, 328 Bertholon's observations on electrifying vegetables, 531 electro-vegetometer, 532 Boyle introduces the prime conductor, page 660, note (1). Brooke's method of making batteries, 111 of coating jars, 108 of repairing jars, 115 experiments on the Leyden phial, 140 on the force of batteries, 202 mode of constructing gages, 239 Brugnatelli supposes the electric fluid to be an acid, 365

C

Can and chain, electrified, 413 Canals, communication of electricity by, explained, 374—392 uniformity of fluid in very slender, 374 Canton first employs an amalgam, 13 Canton's experiments on glass, 16 electrometer, 66 Capillary syphon, 77 Carbonic acid decomposed by electricity, page 694, note (1). Carmoy's experiments on electrifying vegetables, 530 Cavallo's experiments on glass tubes, 16 on exciting powders, 31 improvement of Bennet's electrometer, 75 pocket electrometer, 76 directions for fusing metals, 171 experiments on colours, 193

Cavallo's experiments on the electrophorus, No 208 on the non-conducting power of a vacuum, 240 improvement of the condenser, 246 collector of electricity, 254 multiplier of electricity, 255 use of in observations on atmospherical electricity, 500 construction of kites, 492 means of avoiding danger from the kite, 493 conclusions from his experiments with the kite, 495 atmospherical electrometer, 498 electrometer for rain, 499 Cavendish's experiments on air, 165 theory of electricity, 348 of the action of points, 414 of charged glasses, 416 method of measuring the charge of a jar, 420 Cement for electrical apparatus, 37 for broken jars, 115 Chocolate, electricity of, 28 Cigna's experiments on silk, 18 with charged plates, 151 Coating for cylinders and globes, 40 for jars and plates, 107 Collector of electricity, 254 Colours, experiments on, 193 Compensated electricity, 411 Condenser of electricity, 245 improvement of, 246 Conductors, table of, 5 distinguished from electrics by Gray, page 646, note (B). all imperfect, 4 prime, of a machine, introduced by Boyle, page 660, note (1). capacity of, 243 conjugate, 244 against lightning, invented by Dr Franklin, 464 should be of the best conducting materials, 467 of sufficient diameters, 468 perfectly continuous, 469 interrupted, effect of, 473 should be as strict as possible, 475 Configurations, curious, produced by electricity, 275 Copper forms the best conductors against lightning, 467 Cotte's Index.

Cotte's axioms on atmospherical electricity, No 510

Cotton, electrified, 4

Coulomb ascertains the law of electric action, 367

finds that conductors are electrical only at the surface, 359

Coulomb's experiments on the density of the electric fluid, 397

Cuneus's experiment, 98

Cuthbertson's proofs of the oxidation of metals by electricity, page 701, note (z), 203

experiments on the effect of breathing into jars, 205

mode of estimating the power of machines, 224

method of distinguishing the two electricities, 266

Cylinder, advantages of, 39

directions for choosing, ib.

state of the inside of, during excitation, 212

D

Dalibard's experiment proving the identity of lightning and electricity, 442

Dancing balls, 61

Danger from lightning, means of avoiding, 469, 481

from atmospherical electricity, instances of, 494, 496, 497

approaching, signs of, 480

Darwin's moveable doubler, 249

Discharger, Henly's universal, 117

Discharging rod, construction of, 116

Morgan's, 119

Doubler of electricity, 247

manipulation of, 248

moveable by Darwin, 249

revolving by Nicholson, 250

defects of, 251

how remedied, 252, 253

Du Faye's theory of electricity, 280

E

Eggs, illuminated, 176

Electric fluid, nature of, 281, 355

materiality of, proved, 356

differs from caloric, 358

from light, 360

probably a compound, 361

generally confined to the surface of conductors, 359

supposed to be an acid, 365

law of action of, 366

disposition of, in two parallel plates, 371

in a sphere, 372

Electric power, cause of electric phenomena so called, 2

Electric power, attracts and repels, No 60

makes liquids flow through capillary syphons, 77

communication of, to electrics, 93

passage of, shown, 136—139

mechanical effects of, Part III. Ch. viii.

velocity of, 270

See Electric Fluid.

Electrical apparatus described, Part II.

machine, general construction of, Part II. Ch. i.

different parts of various, 46—51

directions for using, Part II. Ch. iii.

power of, how estimated, 221

with silk, 226

horse-race, 84

orry, 85

jack, 155

air thermometer, 168

Electricity, general idea of, 1

origin of the name, 1

properly the science, applied to the cause of phenomena, ib.

produced in various ways, 10

by friction, 11

heating and cooling, 21, 24

melting, 25—28

breaking, 29

evaporation, 32

of wood shavings, 30

powders, 31

small degrees of, how ascertained, 258—261

theories of, Part IV. Ch. i.

discovered to come from the earth, 282

direction of, doubts concerning, 284

phenomena of, depend on redundancy of fluid or matter, 305

induced, 344

compensated, 411

vindicating, 422

positive and negative, 7

how distinguished, 128

observations on attract each other, 325

produced in the same conductor, 215

vitreous and resinous, 8

Electric, what, 1

table of, 5

Electric, all imperfect, used as insulators, coated, effects of, resinous, when melted become conductors, nature of, only fo superficially, Electrometer, Nollet's, Canton's, stand of, quadrant, scale of, how best divided, Bennet's, sensibility of, rendered more flexible by a candle, defects of, improvement of, Cavallo's pocket, Lane's, by Adams similar to Brook's, Cuthbertson's, Robison's comparable mode of using, Cavallo's atmospheric, for the rain, Electrophorus described, phenomena of, experiments on, by Cavallo, by Morgan, theory of, primitive state of, common state of, neutral state of, charged state of, charging state of, activity of, how renewed, Electroscopes. See Electrometers. Electro-vegetometer, described, Evaporation produces electricity, increased by electricity, experiments on, by Saufure, a cause of atmospherical electricity, Excitation, modes of producing, of powders, remarks on, by Mr Nicholson, mode of increasing, how performed without the silk flap, No 217 velocity necessary to produce the greatest degree of, Experiments on glass by Hawkesbee, by Cavallo, on silk by Symmer, by Cigna, Experiments Experiments on paper, on the tourmaline, 24 on sulphur by Willeke, 25 by Æpinus, 26 on powders by Cavallo, 31 on attraction and repulsion, Part III. Ch. I., 80—84 on points, 80—84 on the Leyden phial, 96, 122, et seq. on the electric spark, 130 on inflammation, 158 on the glaas, 164—166 on decomposing water, 169 on fusing metals, 171—173 on electric light, 175—190 on colours, 193 on the electrophorus, 208, 209 on evaporation, 231—233 on the non-conducting power of a vacuum, 238—240 on the capacity of conductors, 243 on the velocity of the electric fluid, 271 proving the identity of lightning and electricity, 441, 442 on atmospheric electricity, Part V. Ch. II. on evaporation by Saufure, 509

Experiments, entertaining. Dancing figures, 61 Capillary syphon, 77 Electrical well, 79 horse-race, 81 orrey, 82 Electrified cotton, 84 Drawing sparks, 86 Dancing balls, 94 Leyden phial, 96 Electrified spider, 126 Lateral explosion, 127 Leyden vacuum, 138 Double jar, 139 Magic picture, 154 Electrical jack, 155 Self-moving wheel, 156 To fire rosin, 158 spirits, 159 hydrogen gas, 160 gun-powder, 162 Inflammable air lamp, 161 Electrical air thermometer, 168 To fuse wires, 171 To burn wire by electricity, 173 To illuminate water, 175 eggs, 176 Canton's phosphorus, 178 Spiral tube, 180 Luminous word, 181 conductor, 188 Conducting glass tube, 189

Aurora borealis, No 190 Visible electric atmosphere, 191 To perforate a card, &c., 196 To electrify the air of a room, 234 To charge a plate of air, 235 To imitate the planetary motions, 264 To spin sealing-wax into threads, 272 To form curious configurations, 275 Electrified ear and chain, 313 Thunder-house, 470 Powder-house, 471

Flame, a conductor, 265 Fogs have a smell like an excited glass tube, 489 Franklin discovers the seat of the charge in the Leyden phial, 124 Franklin's experiments on metals, 171 theory of electricity, 287—292 difficulty to explain the attraction between negative bodies, 288 theory of the Leyden phial, 291 conjecture on the identity of lightning and electricity, 438 comparative view of their effects, 439 proposal for proving their identity, 440 experiment proving their identity, 441 invention of conductors against lightning, 464 directions for their construction, 465 Friction excites electrics, observations on its mode of action, 364

Gases, decomposed by electricity, 135 Gilbert, Dr., discovers glass to be an electric, 12 Glass, when first shown to be an electric, excited, phenomena of, Part I. Chap. i. durability of its electricity, 16 best electric for machines, substitute for, 105, 225 red hot, a conductor, 268 powdered, a conductor, 269 impermeable to the electric fluid, 409 Graalab first employs a battery, 113 Grey distinguishes electrics from conductors, p. 446, note (b). discovers silk to be an electric, supposes a perpetual electricity in sulphur, 27

Hail, supposed to be produced by electricity, 487 Hail, Beccaria's idea of the formation of, No 488 Holdane's, Colonel, mode of eliminating the charge of a battery, 204 Hawkebee's experiments on glaas, renders sealing-wax transparent, ib. Henly's experiments on chocolate, quadrant electrometer, universal discharger, observations on charged plates, atmospheric electricity, 490 House, best means of protecting from lightning, 477 Ice, when free from air, a non-conductor, 5 Induced electricity, illustrated, may become permanent, 347 Ingenhouz's electrical machine, substitutes for glaas in machines, denies the effect of electricity on vegetation, 529 Insulation, what, 6 K Kienmayer's amalgam, p. 659, note (k). Kinnerley's electrical thermometer, observations on atmospheric electricity, 484 Kite, Franklin's experiment with, Cavallo's construction of, directions for making experiments with, Bennet's electrometer applied, 496 Kleff's, Van, discovery of the electric shock, 97 Klineck's method of estimating small degrees of electricity, 242

L Lamp, inflammable air, 161 Lane's electrometer, 199 Lateral explosion, 127 Leyden phial, construction of, charging and discharging, 96 discovery of, progressive improvement, 100 best form of, insulated, cannot be charged, charge of, where seated, gradually discharged, lateral explosion of, Brooke's experiments on, Milner's experiments on, self-charging, states of its surfaces, 134—140 Leyden Index.

Leyden phial, charged without friction, No 148 when damped within, receives a higher charge, 205 Franklin's theory of, 291 phenomena of illustrated, 415 theory of, 416 charge of, how ascertained, 420

Lichtenberg's method of estimating small degrees of electricity, 242 curious experiment with the electrophorus, 274

Light differs from electricity, electric, varieties of, 91 first seen by Otto Guericke, p. 672, note (o). star and pencil of, 129 appearance of, on paper, 133 experiments on, by Mr Morgan, 182—187 may become visible in all bodies, 182 more visible in imperfect conductors, 183 more visible in rare than in dense media, 184 more brilliant in small bodies, 185 like solar light in refrangibility, 186 affected like solar light by various media, 187 invisible in a perfect vacuum, 237

Lightning and electricity, similar effects of, 439 identity of, proved by Dr Franklin, 441 flash of, form of, 446 colours of, 447 effects of, on a building, 453 fusion of metals by, not a cold fusion, 454 sets fire to inflammable bodies, 455 tears refitting bodies, 456 kills animals, 457

Luminous word, conductor, 181 Lyncurium, the tourmalin so called by Theophrastus, No 20 Lyons's experiments to prove the permeability of glass, 409

M Machine. See Electrical. Magic picture, 154 Magnesia given to needles by electricity, 276 Mainbray first electrified plants, 526 Marum's Van, electrical machines, 48, 49 batteries, 114

Marum's Van, experiments with the gases, No 166 experiments with metals, 172 experiments on magnetic needles, 278 Mazeas's, abbe, experiments on atmospheric electricity, 483 Mercurial phosphorus, 14 Metals fused and oxidated by electricity, 171 fusibility of, comparative, by heat and by electricity, No 172, and p. 700, note (y). oxides of, reduced, 174 fusion of, by lightning not a cold fusion, 454 Milner's observations on the Leyden phial, 120 Monnier's experiments on atmospheric electricity, 482 Morgan's rules for constructing batteries, 112 for discharging rods, 118 discharging rod, 119 experiments on electric light, 182—187 experiments on the inflated electrophorus, 209 experiments on the non-conducting power of a vacuum, 238 observations on the effect of friction, 364 observations on the distance at which thunder may strike, 460 proposal for preventing danger to buildings from lightning, 477 Multiplier of electricity, 255 Multiplying wheel, its uses, 41

N Nairne's electrical machine, method of securing jars, 121 Neutral points, several in an imperfect conductor, 346 Neutrality of bodies that are redundant or deficient, 313 Nicholson's observations on the electric spark, 130 instrument for distinguishing the two electricities, 210 remarks on excitation, 220 revolving doubler, 250 spinning condenser, 257 improvement of Bennet's electrometer, 262 remarks on the glass case of ditto, 263 Nitric acid formed by electricity, 165 Nitrous gas decomposed by electricity, 166

Nollet's, abbe, electrometer, theory of electricity, conjectures on the identity of lightning and electricity, experiments on electrifying plants, 527 Non-conductors. Vid. Electrics.

O Opaque bodies made transparent, 13, 179 Otto Guericke, first constructs an electrical machine, p. 656, note (f).

P Paper, electricity of, 19 a good rubber, 43 use of as a coating, 108 Pearson's directions for decomposing water by electricity, 170 Peart's idea of electric atmospheres, 407 Phosphorus, Canton's, illuminated, 178 Planetary motions imitated, 82, 264 Plate-machine superior to one with a cylinder, 223 Plates of glass, their inconveniences, 38 Points, action of, 80—84 explained, 414 Polarity of magnetic needles reversed by an electric shock, 276 Powder-house, experiment, 471 Priestley's electrical machine, 46 experiments on tourmalins, 24 gales, 165

R Rain supposed to be owing to electricity, 487 Read denies hot air to be a conductor, 228 Read's observations on atmospheric electricity, 501 Reid's portable electrical machine, 51 Repulsion between negative bodies unaccounted for by Franklin, 288 attempts to account for, 389 explained, 408 Returning stroke, theory of, 402 effects of, over-rated, 403 Ribbons, experiments on, by Cigna, 18 Richman's experiment with coated plates, 153 death by lightning, 458 Robison's electrometer, improvement of Bennet's doubler, 253 ascertains the law of electric action experimentally, 367 Ronayne's observations on atmospheric electricity, 489 Roundland's silk electrical machine, 226 Rubber, first employed by Winckler p. 657, note (c).

S construction of, 42 improvement of, 43 silk flap of, the principal cause of excitation, 211 Ruffell's hypothesis, 362

Spontaneous electricity, No 25

Stanhope's, Lord, theory of the returning stroke, 462