Electricity, from the Greek word ἐλέκτρον, electron, amber, is the name given to a modern science which treats of the phenomena and effects produced by the friction of amber, and other bodies which possess analogous properties.

The science of Electricity, in its most general acceptation, may be divided into four different branches, viz.

I. Ordinary Electricity, or that which is developed by friction.

II. Galvanism, or that which is produced by chemical action.

III. Magneto-Electricity, or that which is developed by magnets; and

IV. Thermo-Electricity, or that which is developed by heat.

Under these four articles we shall be able, not only to give a perspicuous and condensed view of those splendid discoveries which have illustrated the present age, but to render our account of them much more complete than if we had treated the subject under the titles of Electromagnetism and Galvanism, which occur so early in our alphabetical arrangement.

In giving a succinct and popular view of the science of Electricity in the ordinary acceptation of the word, the subject naturally divides itself into two parts: 1st, On the phenomena and laws of electricity; and, 2ndly, on the instruments and apparatus used in electrical experiments. Before we proceed, however, to these topics, we shall give a brief history of the origin and progress of the science.

HISTORY OF ELECTRICITY.

The name of the philosopher who first observed that amber when rubbed possesses the property of attracting and repelling light bodies has not been handed down to our times. Thales of Miletus is said to have described this remarkable property, and both Theophrastus (B.C. 321) and Pliny (A.D. 70) mention the power of amber to attract straws and dry leaves. The same authors speak of the lapis luminumis, which is supposed to be a mineral called tourmaline, as possessing the same property.

The electricity of the torpedo was also known to the ancients. Pliny informs us, that when touched by a spear it paralyses the muscles and arrests the feet, however swift; and Aristotle adds that it possesses the power of benumbing men, as well as the fishes which serve for its prey. The influence of electricity on the human body, and the electricity of the human body itself, were also known in ancient times. Anthero, a freedman of Tiberius, was cured of the gout by the shocks of the torpedo; and Wolliner, the king of the Goths, was able to emit sparks from his own body.

Eustathius, who records this fact, also states that a certain philosopher, while dressing and undressing, emitted occasionally sudden crackling sparks, while at other times flames blazed from him without burning his clothes.

Such are the scanty gleanings of electrical knowledge which we derive from the ancient philosophy; and though several writers of the middle ages have made occasional references to these facts, and even attempted to speculate upon them, yet they added nothing to the science, and left an open field for the researches of modern philosophers.

Our countryman Dr Gilbert of Colchester may therefore be considered as the founder of the science of electricity, as he appears to have been the first philosopher Gilbert, who carefully repeated the observations of the ancients, A.D. 1600, and applied to them the principles of philosophical investigation. In order to determine if other bodies possessed the same property as amber, he balanced a light metallic needle on a pivot, and observed whether or not it was affected by causing the excited or rubbed body to approach to it. In this way he discovered that the following bodies possess the property of attracting light substances: Amber, gugates or jet, diamond, sapphire, carbuncle, rock crystal, opal, amethyst, vincentina or Bristol stone, beryl, crystal, paste for false gems, glass of antimony, flags, beryllites, sulphur, gum-mastic, sealing-wax of lac, hard resin, arsenic, sal gem, mica, and alum.

These various bodies attracted, with different degrees of force, not only straws and light films, but likewise metals, stones, earths, wood, leaves, thick smoke, and all solid and fluid bodies. Among the substances which are not excited by friction, Gilbert enumerated emerald, agate, carnelian, pearls, jasper, chalcedony, alabaster, porphyry, coral, marble, Lydian stone, flints, lucemattes, mugris (emery or corundum), bones, ivory, hard woods, such as cedar, ebony, juniper, and cypress, metals and natural magnets.

Having thus determined the bodies which were capable, as well as those which were incapable, of electrical excitation, Dr Gilbert was desirous of ascertaining the circumstances which were most favourable to the production of electricity. When the wind blew from the north and east, and when the air was dry, the body was excited in about ten minutes after the friction commenced; but when the wind was in the south, and the air moist, the attractive power of the body was greatly diminished, and in some cases it could not be excited at all.

The celebrated Mr Boyle added many new facts to the Boyle science of electricity, and he has given a full account of them in his Experiments on the Origin of Electricity. By means of a suspended needle, he discovered that amber retained its attractive virtue after the friction which excited it had ceased; and though smoothness of surface had been regarded as advantageous for excitation, yet he found a diamond which in its rough state exceeded all the polished ones and all the electrics which he had tried, having been able to move a needle three minutes after he had ceased to rub it. He found also that heat and tension (or the cleaning or wiping of any body) increased its susceptibility of excitation; and that if the attracted body were fixed, and the attracting body moveable, their mutual approach would still take place. To Dr Gilbert's list of electrics Mr Boyle added the resinous cake which remained after evaporating one fourth part of good oil of turpentine; the dry mass which remains after distilling a mixture of petroleum and strong spirit of nitre, glass of antimony, glass of lead, caput mortuum of amber, white sapphire, white amethyst, diaphanous ore of lead, carnelian, and a green stone supposed to be a sapphire.

To these discoveries of Mr Boyle, his illustrious contemporary Otto Guericke added the highly important invention of electric light. Having cast a globe of sulphur in a glass sphere, the glass was broken, and the sulphur ball mounted upon a revolving axis, and excited by the friction of... By this means he discovered that light and sound accompanied strong electrical excitation, and he compares the light to that which is exhibited by breaking lump sugar in the dark. With this powerful apparatus Guericke verified on a greater scale the results obtained by his predecessors, and obtained several new ones of very considerable importance. He found that a light body, when once attracted by an excited electric, was repelled by it, and was incapable of a second attraction until it had been touched by some other body; and that light bodies suspended within the sphere of influence of an excited electric, possessed the same properties as if they had been excited.

To our illustrious countryman Sir Isaac Newton the science of electricity owes some important observations. He seems to have been the first person who constructed an electrical machine of glass. "A globe of glass," says he, "about eight or ten inches in diameter, being put into a frame where it may be swiftly turned round its axis, will in turning shine when it rubs against the palm of one's hand applied to it; and if at the same time a piece of white paper or a white cloth, or the end of one's finger, be held at the distance of about a quarter of an inch or half an inch from that part of the glass when it is most in motion, the electric vapour which is excited by the friction of the glass against the hand will, by dashing against the white paper, cloth, or finger, be put into such an agitation as to emit light, and make the white paper, cloth, or finger, appear lurid like a glow-worm, and in rushing out of the glass will sometimes push against the finger so as to be felt. And the same things have been found by rubbing a long and large cylinder of glass and amber with a paper held in one's hand, and continuing the friction till the glass grew warm." We owe also to Sir Isaac a beautiful experiment on the excitation of electricity on the side of a disc of glass opposite to the side which was rubbed. Having fixed a round disc of glass at the distance of one third of an inch from one end of a brass hoop or ring, and one eighth of an inch from another, he placed small pieces of thin paper within the brass ring and upon a table, so that the lower surface of the glass was one eighth of an inch distant from the table. He then rubbed the upper surface of the glass, and he observed the pieces of paper "leap from one part of the glass to the other, and twirl about in the air." Upon sliding his finger upon the upper side of the glass, he also observed that the pieces of paper, as they hung under the glass, inclined this way or that according as he moved his finger.

The Royal Society had ordered this experiment to be tried at their meeting of the 16th December 1675; and, in order to ensure its success, had obtained the above account of it from Sir Isaac. The experiment however failed, and the secretary requested the loss of Sir Isaac's apparatus, and inquired whether or not he had secured the papers from being moved by the air which might have somewhere stolen in. In Sir Isaac's reply, dated 21st December, he recommended them to rub the glass "with stuff whose threads may rake its surface, and if that will not do, to rub it with the finger ends to and fro, and knock them as often upon the glass." By means of these directions, the society succeeded with the experiment on the 13th January 1676, when they used "a scrubbing brush of short hog's bristles, and the heft of a knife made with whalebone."

Mr Francis Hawksbee, one of the most active and ingenious experimental philosophers of his age, added many new facts to the science. In 1705 he communicated to the Royal Society several curious experiments on what he calls "the mercurial phosphorus." He showed that light could be produced by passing common air through mercury placed in a well-exhausted receiver. The air rushing through the mercury, blew it up against the sides of the glass that held it, "appearing all around like a body of fire, consisting of abundance of glowing globules." The phenomenon continued till the receiver was half full of air. When the mercury was made to descend in vacuo from the top to the bottom of a receiver about twenty-one inches high, it fell in minute particles, "like a shower of fire, in a very surprising manner." In repeating this experiment with about three pounds of mercury, and making it break into a shower by dashing it against the crown of another glass vessel, flashes resembling lightning, of a very pale colour, and very distinguishable from the rest of the produced light, were dashed from the crown of the glass, sometimes horizontally, and at other times upwards and downwards. Mr Hawksbee likewise showed that considerable light may be produced from mercury, by giving it motion before the receiver was quite exhausted; and that even in the open air "abundance of particles of light are discoverable by shaking quicksilver in a glass."

In a subsequent series of experiments on the attraction of bodies in vacuo, he showed that light was generated by the swift attrition of amber on woollen; that a purple light was produced by the attrition of glass on woollen; and that a considerable light was developed by the attrition of glass on glass in vacuo, and in common air, or under water. During the attrition of glass on woollen, Hawksbee "observed the light to break from the agitated glass in as strange a form as lightning," particularly when he used some list of cloth that had been drenched in spirit of wine. In all these experiments Hawksbee was not aware that the light which he observed was that of electricity.

Like Sir Isaac Newton, Hawksbee used a glass globe capable of revolving in a wooden frame, and by its assistance he made a great number of experiments, which are not sufficiently important to be given in detail. The following experiment, however, is too interesting to be omitted. Having coated more than one half of the inside of a glass globe with sealing wax, which in some places was an eighth of an inch thick, and therefore absolutely opaque, he exhausted it and put it in motion. When his hand was applied to excite it, the form of his hand was distinctly seen in the concave surface of the wax, as if it had become transparent. The same result was obtained when pitch or common brimstone was substituted in place of sealing-wax.

We have already seen that Hawksbee observed the resemblance between the electric spark and lightning. Dr Wall went a step farther, and compared the crackling and the flash of excited amber to thunder and lightning. The crackling he found to be fully as loud as that of charcoal on fire when the finger was held at a little distance from the amber after it had been drawn gently and slightly through a piece of woollen cloth.

One of the most ardent experimentalists of the present Mr Ste- tine was Mr Stephen Gray, a fellow of the Royal Society, who Gray, in his first paper, published in 1720, showed that electricity could be excited by the friction of feathers, hair, silk, linen, woollen, paper, leather, wood, parchment, and gold-beaters' skin. Several of these bodies exhibited light in the dark, especially after they had been warmed; but all of them attracted light bodies, and sometimes at the distance of eight or ten inches.

The communication of electricity to bodies not capable of excitation was the next discovery of Mr Gray. An ivory ball, and various other substances of a metallic, animal, and vegetable nature, were made to attract light bodies by connecting them with strings, wires, or pieces of wood, with one extremity of an excited glass tube; and by suspending pack-threads of different lengths with silken threads, he was able to transmit the electrical influence in any direction to distances of 50, 147, 293, and finally 765 and 886 feet.

In order to determine if the electric attraction is proportioned to the quantity of matter in bodies, Mr Gray and Mr White made two cubes of oak about six inches square, the one solid and the other hollow. When suspended by hair lines, and similarly electrified by an excited glass tube, both the cubes attracted and repelled leaf brass at the same time and to the same height. Hence Mr Gray concluded that it was the surface of the cubes only which attracted.

The conducting powers of fluids and of the human body were next ascertained by Mr Gray. Having blown a soap bubble with an electrified tobacco pipe, he found that the lower part of the bubble attracted small pieces of Dutch metal; and when a boy eight or nine years old, weighing 47 lbs., was suspended upon hair lines, he found that every part of his body exercised a strong electrical action upon light bodies, and hence he concluded "that animals receive a greater quantity of electrical effluvia." When an excited tube was held above water or quicksilver placed in little ivory dishes, the fluid was attracted upwards into little conical mounds, accompanied with a snapping noise and a discharge of light from their summit. In sunshine small particles of water rose from the top of the fluid cone, and sometimes a fine stream of water like a fountain, from which there arose a fine steam or vapour. Hot water was attracted much more powerfully, and at a much greater distance, and the steam was more distinctly visible. Mercury did not rise so high as the water; but the snapping noise was louder, and continued much longer, than when water was employed.

Mr Gray now set himself to discover "whether there might not be a way found to make the property of electrical attraction more permanent in bodies." Having procured iron ladles of several sizes, he melted the substances given in the following table. They were then set by in the ladle to cool and harden, and afterwards being replaced on the fire so as to allow what was next the bottom and sides of the ladle to melt, the ladle was inverted, and the substance taken out. These bodies at first would not attract light substances till their temperature was nearly that of a hen's egg; but when cold they attracted ten times farther than at first. In order to preserve these bodies in a state of attraction, he wrapped them up in flannel or white paper or black worsted stockings, and then put them into a large fir box till they were used. The following is Mr Gray's list of the electrics thus formed:

| Names | Weight Avord. | Time when made | |------------------------------|--------------|----------------| | Fine black rosin | 2 0 | Jan. 31 | | Stone pitch, and black rosin | 2 2 | Jan. 31 | | Fine rosin and bees' wax | 2 1 | Feb. 1 | | Stone pitch | 1 7 | Feb. 1 | | Stone sulphur | 3 6 | Feb. 4 | | Shell lack | 10 0 | Feb. 10 | | Fine black rosin | 10 4 | Feb. 10 | | Bees' wax and rosin | 9 0 | Feb. 12 | | Rosin 4, gum lac 1 part | 10 0 | Feb. 12 |

Mr Gray continued for thirty days to observe every one of these bodies, and at the end of that time he found that all of them attracted as vigorously as at the first or second day, and some of them continued to preserve their attraction for more than four months.

While Mr Gray was pursuing his career of discovery in England, M. Dufay, of the Academy of Sciences, and superintendent of the Royal Botanic Gardens, was actively employed in the same researches. He found that all bodies, whether solid or fluid, could be electrified by an excited tube, by setting them on a glass stand slightly warmed, or only dried; and that those bodies which are in themselves least electrical, received the greatest degree of electricity from the approach of the glass tube. He found that electricity was transmitted more easily along pack-thread when it was wetted, and that it might be supported upon glass tubes in place of silk lines; and in this way he conveyed it along a string 1256 feet long.

M. Dufay repeated Mr Gray's experiments on the human body, by suspending a child on silken strings; but having suspended himself in a similar manner, he discovered that an electrical spark, accompanied with a crackling noise, took place when any other person touched him, and he has described the prickling sensation like the burning from a spark of fire, which is at the same time felt either through the clothes or on the skin. He found that the same effects took place in other living animals; but that if the carcass of an animal was suspended, there were no slippings or sparks, but merely a still uniform light observed in the dark.

The great discovery of M. Dufay, however, was that of Vitreous two different kinds of electricity, to which he gave the and resinous name of vitreous and resinous, and the importance of these electricities which he did not fail to recognise. He has given the name of vitreous electricity to that which is produced by exciting glass, rock crystal, precious stones, hair of animals, wool, and many other bodies; and the name of resinous to that which is produced by exciting resinous bodies, such as amber, copal, gum-lac, silk, paper, thread, and a number of other substances. The characteristic of those two electricities was, that a body with vitreous electricity attracted all bodies with resinous electricity, and repelled all bodies with vitreous electricity; while a body with resinous electricity attracted all bodies with vitreous electricity, and repelled all bodies with resinous electricity.

Two electrified silk threads, for example, repel each other, and also two electrified woollen threads; but an electrified silk thread will attract an electrified woollen thread. Hence it is easy to determine whether any body possesses vitreous or resinous electricity. If it attracts an electrified silk thread, its electricity will be vitreous; if it repels it, it will be resinous. This important discovery seems to have been made about the same time by Mr White, by a series of independent observations. Mr Gray repeated and varied the experiments of M. Dufay, and made many new ones, which our limited space will not permit us to detail. Like Hawksbee and Dr Wall, he recognised the similarity between the phenomena of electricity and those of thunder and lightning; and he expresses a hope "that there may be found out a way to collect a greater quantity of electric fire, and consequently to increase the force of that power, which, by several of these experiments, si licet magnis componere parva, seems to be of the same nature with thunder and lightning."

The discoveries which we have now recounted began to rouse the activity of the German and Dutch philosophers. To the electrical machine used by Newton and Hawksbee, Professor Boze of Wittemberg added the prime conductor, which at first consisted of an iron or tin tube supported by a man standing upon cakes of rosin; but it was afterwards suspended by silk strings. Professor Winkler of Leipzig substituted the cushion in place of the hand for exciting the revolving globe; and Professor Gordon of Erfurt, a Scotch Benedictine monk, first used a glass cylinder, eight inches long and four broad, which he caused to revolve by means of a bow and string. By these means electrical sparks of great size and intensity were produced, and by their aid various combustible substances, both fluid and solid, were inflamed. In 1744 M. Ludolph of Berlin succeeded in firing, by the electrical spark, the ethereal spirit of Probenius. Winkler did the same by a spark from his finger; and he succeeded in inflaming French brandy and other weaker spirits after they had been heated. Mr Gordon kindled spirits by a jet of electrified water. Dr Miles inflamed phosphorus by the electric spark; and oil, pitch, and sealing-wax, when strongly heated, were set on fire by similar means.

These striking effects were all produced by the electricity obtained immediately from an excited electric phial, but a great step was now made in the science by the discovery of a method of accumulating and preserving the electric fluid in large quantities. The author of this great invention is not distinctly known; but there is reason to believe that a monk of the name of Kleist, a person of the name of Cuneus, and Professor Muschenbroeck of Leyden, had each the merit of an independent inventor. The invention by which this accumulation was effected was called the Leyden Jar or Phial, because it was principally in that city where it was either invented or tried. Having observed that excited electrics soon lost their electricity in the open air, and that their loss was accelerated when the atmosphere was charged with moisture or other conducting materials, Muschenbroeck conceived that the electricity of bodies might be retained by surrounding them with bodies which did not conduct it. In putting this idea to the test of experiment, they electrified some water in a glass bottle, and a communication having been made between the water and the prime conductor, while the bottle was held by an assistant, who was trying to disengage the communicating wire, he received a sudden shock in his arms and breast, and thus established the efficacy of the Leyden jar.

Sir William Watson made some important experiments at this period of our history. He succeeded in firing gunpowder by the electric spark; and by mixing the gunpowder with a little camphor he discharged a musket by the same power. He also fired inflammable air by the electric spark; and he kindled both spirits of wine and inflammable air by means of a drop of cold water, and even with ice. In the German experiments the fluid or solid to be inflamed was set on fire by an electrified body; but Sir William Watson placed the fluid in the hands of an electrified person, and set it on fire by causing a person not electrified to touch it with his finger.

Sir William Watson first observed the flash of light which attends the discharge of the Leyden phial, and it is to him that we owe the present improved form of the Leyden phial, in which it is coated both without and within with tinfoil. Dr Bevis indeed had suggested the outside coating, and, at Mr Smeaton's recommendation, he coated a pane of glass on both sides, and within an inch of the edge, with tinfoil; but still the idea of coating the jar doubly belongs to Sir William Watson.

A party of the Royal Society, with the president at their head, and Sir William Watson as their chief operator, entered upon a series of magnificent experiments, the Royal Society of London for the purpose of determining the velocity of the electric fluid, and the distance to which it could be conveyed. The French savans had conveyed the influence of the Leyden jar through a circuit of 12,000 feet; and in one case the basin at the Tuileries, containing about an acre of water, formed part of the circuit; but the English philosophers made a more complete series of experiments, of which the following were the results:

1. That in all their operations, when the wires have been properly conducted, the electrical commotions from the charged phial have been very considerable only when the observers at the extremities of the wire have touched some substance readily conducting electricity with some part of their bodies.

2. That the electrical commotion is always felt most sensibly in those parts of the bodies of the observers which are between the conducting wires and the nearest and the most non-electric substance; or, in other words, so much of their bodies as comes within the electrical circuit.

3. That on these considerations we infer that the electrical power is conducted between these observers by any non-electric substances which happen to be situated between them, and contribute to form the electrical circuit.

4. That the electrical commotion has been perceptible to two or more observers at considerable distances from each other, even as far as two miles.

5. That when the observers have been shocked at the end of two miles of wire, we infer that the electrical circuit is four miles, viz. two miles of wire, and the space of two miles of the non-electric matter between the observers, whether it be water, earth, or both.

6. That the electrical commotion is equally strong, whether it is conducted by water or dry ground.

7. That if the wires between the electrifying machine and the observers are conducted on dry sticks, or other substances non-electric in a slight degree only, the effects of the electrical power are much greater than when the wires in their progress touch the ground, or moist vegetables, or other substances in a great degree non-electric.

8. That by comparing the respective velocities of electricity and sound, that of electricity, in any of the distances yet experienced, is nearly instantaneous.

In the following year these experiments were resumed with the view of ascertaining the absolute velocity of electricity at a certain distance, and it was found, "that through the whole length of a wire 12,276 feet, the velocity of electricity was instantaneous."

One of the most important discoveries of the present period was that of Sir W. Watson, who proved "that the glass tubes and globes had not the electrical power in themselves, but only served as the first movers or determiners of that power." In rubbing a glass tube while standing upon a cake of wax, he was surprised to observe that no spark could be obtained from his body by any other person touching any part of him. But if a person not electrified held his hand near the tube while it was rubbing, the snapping was very sensible. The great discovery of plus and minus electricity which was afterwards made by Franklin, was distinctly announced by Sir W. Watson. He lays it down as a law, that in electrical operations there is an efflux of electric fluid to the globe and the conductor, and also an efflux of the same matter from them. In the case of two insulated persons, the one in contact with the rubber and the other with the conductor, he observed that either of them would communicate a much stronger spark to the other than to any bystander. The electricity of the one, he says, became more rare than it is naturally, and that of the other more dense, so that the density of the electricity in the two insulated persons differed more than that between either of them and a bystander.

Our limits will not permit us to give a detailed account of the various ingenious experiments which were about this time made by Le Monnier, Nollet, Winckler, Ellicott, Jallabert, Boze, Menon, Smeaton, and Miller. In 1746 Le Monnier confirmed the result previously obtained by Mr Gray, that electricity is communicated to homogeneous bodies in proportion to their surfaces only. M. Boze discovered that capillary tubes which discharged water by drops afforded a continued stream when electrified. The Abbé Nollet ascertained that electricity increases the natural evaporation of fluids, and that the evaporation is hastened by placing them in non-electric vessels. M. Jallabert confirmed the result previously obtained by Watson, that electricity passes through the substance of a conducting wire, and not along its surface. Smeaton found that the red-hot part of an iron bar could be as strongly electrified as the cold parts on each side of it. Dr Miles kindled common lamp spirits by a stick of black sealing-wax excited by dry flannel. Mr Ellicott conceived that the particles of the electric fluid repel each other, while they attract those of all other bodies. Mr Mowbray discovered that the vegetation of two myrtles was hastened by electrifying them; a result which Nollet confirmed in the case of vegetating seeds. The Abbé Menon found that cats, pigeons, sparrows, and chaffinches, lost weight by being electrified for five or six hours, and that the same result was true of the human body; and hence it was concluded that electricity augments the insensible perspiration of animals.

Passing over the scientific fables of John Pivati of Venice, we arrive at that auspicious period when Dr Franklin raised electricity to the dignity of a science, and connected it with that tremendous agency which had so often terrified the moral and convulsed the physical world. The thunderbolt had frequently descended from heaven upon its victims; but mortal genius had now learned to bring it down in chains, to disarm its fury, and to convert it into an useful and even a friendly element.

One of the first labours of the American philosopher was to present, in a more distinct form, the theory of plus and minus electricity, which Sir W. Watson had been the first to suggest. He showed that electricity is not created by friction, but merely collected from its state of diffusion through other matter by which it is attracted. He asserted that the glass globe, when rubbed, attracted the electrical fire, and took it from the rubber, the same globe being disposed, when the friction ceases, to give out its electricity to any body which has less. In the case of the charged Leyden jar, the inner coating of tinfoil had received more than its ordinary quantity of electricity, and was therefore electrified positively or plus, while the outer coating of tinfoil having had its ordinary quantity of electricity diminished, was electrified negatively or minus. Hence the cause of the shock and spark when the jar is discharged, or when the superabundant plus electricity of the inside is transferred by a conducting body to the defective or minus electricity of the outside. This theory of the Leyden phial Franklin established in the clearest manner, by showing that the outside and the inside containing possessed opposite electricities, and that, in charging it, exactly as much electricity is added on one side as is subtracted from the other. The copious discharge of electricity by points was observed by Franklin in his earliest experiments, and also the power of points to conduct it copiously from an electrified body. Hence he was furnished with a simple method of collecting electricity from other bodies; and he was thus enabled to perform those remarkable experiments which we shall now proceed to explain.

Hawksbee, Wall, and Nollet had successively suggested the similarity between lightning and the electric spark, and between the artificial snap and the natural thunder. Previous to the year 1750 Dr Franklin drew up a statement, in which he showed that all the general phenomena and effects which were produced by electricity had their counterpart in lightning. Like the electric spark, lightning moves in a crooked and irregular direction. Lightning strikes the highest and most pointed bodies, and electricity does the same. They both inflame combustibles, fuse metals, destroy animal life, produce blindness in animals, render common sewing needles magnetic, and reverse the polarity of needles that have been magnetised. Notwithstanding these points of resemblance, direct experiment was still necessary to establish his views. He waited anxiously for the erection of a spire at Philadelphia, by means of which he might bring down the electricity of a thunder-storm; but his patience being exhausted, he conceived the idea of sending up a kite among the clouds themselves. With this view he made a small cross of two light strips of cedar, the arms being sufficiently long to reach to the four corners of a large thin silk handkerchief when extended. The corners of the handkerchief were tied to the extremities of the cross, and when the body of the kite was thus formed, a tail, loop, and string were added to it. The body was made of silk to enable it to bear the violence and wet of a thunder-storm. A very sharp pointed wire was fixed at the top of the upright stick of the cross, so as to rise a foot or more above the wood. A silk ribband was tied to the end of the twine next the hand, and a key suspended at the junction of the twine and silk. In company with his son, Franklin raised the kite like a common one, in the first thunder-storm, which happened in the month of June 1752. To keep the silk ribband dry, he stood within a door, taking care that the twine did not touch the frame of the door; and when the thunder-clouds came over the kite he watched the state of the string. A cloud passed without any electrical indications, and he began to despair of success. He saw, however, the loose filaments of the twine standing out every way, and he found them to be attracted by the approach of his finger. The suspended key gave a spark on the application of his knuckle, and when the string had become wet with the rain, the electricity became abundant; a Leyden jar was charged at the key, and by the electric fire thus obtained spirits were inflamed, and all the other electrical experiments performed which had been formerly made by excited electrics. In subsequent trials with another apparatus, he found that the clouds were sometimes positively and sometimes negatively electrified, and thus demonstrated the perfect identity of lightning and electricity.

Having thus succeeded in drawing the electric fire from the clouds, Franklin immediately conceived the idea of protecting buildings from lightning, by erecting on their highest parts pointed iron wires or conductors communicating with the ground. The electricity of a hovering or a passing cloud would thus be carried off slowly and silently; and if the cloud was highly charged, the electric fire would strike in preference the elevated conductors.

The attention of European philosophers was now directed to the great discovery of Franklin, and various individuals fearlessly sought to repeat his experiments. Among these Professor Richman of St Petersburg was one of the most enterprising. He had undertaken a series of experiments on the electricity of the atmosphere, and for this purpose he had erected an electrical gnomon, which consisted mainly of a Leyden jar, communicating with an iron rod, which rose four or five feet above the roof of his house, and an electrometer formed of a linen thread with half a grain of lead, the angular ascent of which on the face of a divided quadrant indicated the force of the accumulated electricity. On the 9th August 1752 Professor Richman obtained from the end of the rod electrical flashes which could be heard at several feet of distance; and if any person touched the apparatus, a sharp stroke was felt in the hand and arm. On the 31st May 1753 the electric fire exploded from the apparatus with such a force that it was heard at the distance of three rooms from the apparatus. On the 6th August 1753 the professor had prepared and adjusted his apparatus, and having heard the sound of distant thunder, he left a meeting of the Academy of Sciences, and took with him his engineer, Mr Sokolow, to draw any interesting phenomena that might occur. On their arrival at the professor's house, the plummet of the electrometer was elevated four degrees from the perpendicular; and while the philosopher was describing to his friend the dangerous consequences that might take place if the thread should rise to 45°, a tremendous burst of thunder terrified the imperial city. Richman leant his head over the gnomon to observe the indications of the electrometer, and in this stooping position, with his head a foot from the iron rod, a huge globe of bluish-white fire, about the size of Mr Sokolow's fist, shot from the iron rod to the professor's head, with a report like that of a pistol. The blow was fatal; he fell back upon a chest and instantly expired. Sokolow was stupified and benumbed by a sort of steam or vapour, and the red hot fragments of a metallic wire struck his clothes and covered them with burnt marks. As soon as he recovered from his surprise, Sokolow ran out of the house, acquainting every person with the accident which had taken place. In the mean time Madam Richman, alarmed by the thunder-stroke, hastened to the chamber, and found her husband apparently lifeless, in the attitude of sitting upon the chest, and leaning against the wall. Medical aid was instantly obtained, but though a vein was opened, from which no blood would flow, and though every attempt was made to restore life by violent friction and other means, they were all fruitless. A small quantity of blood dropped from the mouth when the body was turned, and on the forehead there was a red spot, from the pores of which a few drops of blood oozed out. Several red and blue spots, like leather shrunk by burning, were found on the left side, the back, and other parts of the body. The shoe upon the Professor's left foot was burst open, and a blue mark appeared on the foot beneath the aperture. There was no corresponding hole in the stocking, and the coat was uninjured. When the body was opened, twenty-four hours after death, there was no appearance of injury either in the brain or the cranium; a little extravasated blood appeared in the cavities below the lungs, and in the lungs towards the back, which were of a dark brown colour. The heart, glands, and smaller intestines, were all inflamed; but the entrails were uninjured. About seventy rubles of silver which were in the left coat pocket were not altered by the electric fluid.

Immediately after the accident the house was filled with a sulphurous vapour. A clock which stood in the corner of the adjoining room was stopped; the ashes from the hearth were scattered about the room; the door-case of the room was rent asunder, and a piece of the door itself actually torn off. The Leyden jar was also broken, and the metallic filings which it contained thrown about the room.

One of the most active and ingenious labourers in the field of electrical science was our countryman Mr John Canton. Canton. Before his time it had been assumed as indisputable that the same kind of electricity was invariably produced by the friction of the same electric; that glass, for example, yielded always vitreous, and amber always resinous electricity. Having roughened a glass tube by grinding its surface with emery and sheet lead, he found that it possessed vitreous or positive electricity when excited with oiled silk, but resinous electricity when excited with new flannel. He found, in short, that vitreous or resinous electricity may be developed at will in the same tube, by altering the surfaces of the tube and the exciting rubber, and according as the one or the other is most affected by their mutual friction. This he illustrated by the very beautiful experiment of removing the polish from one half of the tube. In this case the different electricities were excited with the same rubber at a single stroke, and, what is very curious, the rubber was found to move much easier over the rough than over the polished half.

Mr Canton likewise discovered that glass, amber, sealing-wax, and calcareous spar, were all electrified positively when taken out of mercury; and hence he was led to the important practical discovery, that an amalgam of mercury and tin was most efficacious in exciting glass when applied to the surface of the rubber. Mr Canton found also that any body placed within the electric atmosphere of another body acquired the electricity opposite to that of the body in whose atmosphere it was placed; and that the whole air of a room could be electrified either positively or negatively, and made to retain it for a considerable time.

Signor Beccaria, a celebrated Italian, kept up the spirit of electrical discovery in Italy; and in his work on natural and artificial electricity, he has given us the results of many important original investigations. He showed that water is a very imperfect conductor of electricity, that its conducting power is proportional to its quantity, and that a small quantity of water opposes a powerful resistance to the electric fluid. He succeeded in making the electric spark visible in water, by discharging shocks through wires that nearly met in tubes filled with water. In this experiment the tubes, though sometimes eight or ten lines thick, were burst in pieces. Beccaria likewise demonstrated that air adjacent to an electrified body gradually acquired the same electricity; that the electricity of the body is diminished by that of the air; and that the air parts with its electricity very slowly. He considered that there was a mutual repulsion between the particles of the electric fluid and those of air, and that in the passage of the former through the latter a temporary vacuum was formed.

The science of electricity owes several practical as well as theoretical observations to our countryman Mr Robert Symmer. In pulling off his stockings in the evening, Mr Symmer had often remarked that they not only gave a cracking noise, but even emitted sparks in the dark. The electricity was most powerful when a silk and a worsted stocking had been worn on the same leg, and it was best exhibited by putting the hand between the leg and the stockings, and pulling them off together. The one stocking being then drawn out of the other, they appeared more or less inflated, and exhibited the attractions and repulsions of electrified bodies. Mr Symmer's first trials were accidentally made with black silk stockings, but he was surprised to find that white ones produced no electricity. Two white silk stockings, or two black ones, when put on the same leg and taken off, gave no electrical indications. When a black and a white stocking were put on the same leg, and at the end of ten minutes taken off, they were so much inflated when pulled asunder, that each of them showed the entire shape of the leg, and at the distance of a foot and a half they rushed to meet each other. With worsted stockings, also, nothing but the combination of black and white produced electricity. As it was troublesome to electrify the stockings by putting them on and taking them off the leg, Mr Symmer excited the stockings by drawing them on the hand, which, however, produced a weaker degree of electricity. The electricity was in this case more permanent, and the effects were more powerful, when the stockings were new or newly washed. When an excited white and black stocking are presented to each other, they attract one another, inclining to each other at the distance of three feet, catching hold of each other within two feet, and at a less distance rushing together with surprising violence, becoming as flat as so many folds of silk when they are joined. "But what appears most extraordinary is, that when they are separated, and removed at a sufficient distance from each other, their electricity does not appear to have been in the least impaired by the shock they had in meeting. They are again inflated, again attract and repel, and are as ready to rush together as before. When this experiment is performed with two black stockings in one hand, and two white in the other, it exhibits a very curious spectacle; the repulsion of those of the same colour, and the attraction of those of different colours, throws them into an agitation that is not unentertaining, and makes them catch each at that of its opposite colour, at a greater distance than one would expect. When allowed to come together, they all unite in one mass. When separated, they resume their former appearance, and admit of the repetition of the experiment as often as you please, till their electricity, gradually wasting, stands in need of being recruited."

In the course of these experiments Mr Symmer accidentally threw a stocking out of his hands, and some time afterwards he found it sticking to the paper hangings of the room. They stuck also to the painted panelling, and often continued for a whole hour suspended upon the hangings.

Mr Symmer's attention was next directed to the force of cohesion between stockings of black and white silk, and he found that from ten ounces to nine pounds weight was necessary to separate the stockings, according to their weight, or according as the rough or smooth surfaces were in contact.

Mr Symmer likewise found that a Leyden jar could be charged by the stockings either positively or negatively, according as the wire from the neck of the jar was presented to the black or white stockings. When the electricity of the white stocking was thrown into the jar, and on that the electricity of the black one, or vice versa, the jar will not be electrified at all. With the electricity of two stockings he charged the jar to such a degree that the shock from it reached both his elbows; and by means of the electricity of four silk stockings he kindled spirits of wine in a tea-spoon which he held in his hand, and the shock was at the same time felt from the elbows to the breast. Independent of these curious experiments, Mr Symmer had the merit of having first maintained the theory of two distinct fluids, not independent of each other, as Dufay supposed them to be, but co-existent, and, by counteracting each other, producing all the phenomena of electricity. He conceived that when a body is said to be positively electrified, it is not simply that it is possessed of a larger share of electric matter than in a natural state; nor, when it is said to be negatively electrified, of a less; but that, in the former case, it is possessed of a larger portion of one of these active powers, and in the latter, of a larger portion of the other; while a body, in its natural state, remains unelectrified, from an equal balance of these two powers within it.

Contemporary with Symmer were Delaval, Wilson, Cigna, Kinnersley, and Wilcke. M. Delaval found that the sides of vessels that were perfect conductors, were non-conductors, and that animal and vegetable bodies lost their conducting power when reduced to ashes. Mr Wilson discovered that when two electrics are rubbed together, the harder of the two is generally electrified positively, and the other negatively, but always with opposite electricities. Cigna made many curious experiments by using silk ribbands in place of the silk stockings of Symmer. Kinnersley, the friend of Franklin, made some important experiments on the elongation and fusion of iron wires, when a strong charge was passed through them in a state of tension; and Wilcke brought to light many new phenomena respecting the spontaneous electricity produced by the melting of electric substances.

The pyro-electricity of minerals, or the faculty possessed by some minerals of becoming electric by heat, and of exhibiting negative and positive poles, now began to attract the notice of philosophers. There is reason to believe that the lycaenaria of the ancients, which, according to Theophrastus, attracted light bodies, was the tourmaline, a Ceylon mineral, in which the Dutch had early recognised the same attractive property, whence it got the name of Ascheattrikker, or attracter of ashes. In 1717 M. Lemery exhibited to the Academy of Sciences a stone from Ceylon which attracted light bodies; and Linnaeus, in mentioning the experiments of Lemery, gives the stone the name of Lapis Electricus. The Duke de Noya had heard at Naples that Count Pichetti had seen at Constantinople a stone called tourmaline, which attracted and repelled light bodies; and in 1758 he purchased some of them in Holland, and, assisted by MM. Daubenton and Adanson, he made a series of experiments with them, which were published separately.

This curious subject, however, had engaged the attention of M. Epinus, a celebrated German philosopher, who published an account of them in 1756. Hitherto nothing had been said respecting the necessity of heat to excite the tourmaline; but it was shown by Epinus that a temperature between 94° and 212° of Fahrenheit was requisite for the development of its attractive powers. Mr Benjamin Wilson, Priestley, and Canton, continued the investigation; but it was reserved for the Abbé Haüy to throw much light on this curious branch of the science. He found that the electricity of the tourmaline decreased rapidly from the summits or poles towards the middle of the crystal, where it was imperceptible; and he discovered that if a tourmaline is broken into any number of fragments, when excited, each fragment has two opposite poles. Haüy discovered the same property in the Siberian and Brazilian topaz, borate of magnesia, mesotype, prehnite, sphene, and calamine. He also found that the polarity which minerals receive from heat has a relation to the secondary forms of their crystals, the tourmaline, for example, having its resinous pole at the summit of the crystal which has three faces, and its vitreous pole at the summit which has six faces. In the other pyro-electrical crystals above mentioned, Haüy has detected the same deviation from the rules of symmetry. in their secondary crystals which occurs in tourmaline. Mr Brard discovered that pyro-electricity was a property of the axinite; and more recently Sir David Brewster has detected it, as we shall afterwards see, in a variety of other minerals.

In repeating and extending the experiments of Haüy, Sir David Brewster discovered that various artificial salts were pyro-electrical; and he mentions the tartrate of potash and soda, and the tartaric acid, as exhibiting this property in a very strong degree. He likewise made many experiments with the tourmaline when cut into thin slices, and reduced to the finest powder, in which state each atom preserved its pyro-electricity; and he has shown that scolcite and melocite, even when deprived of their water of crystallization, and reduced to powder, preserve their property of becoming electrical by heat. When this white powder is heated and stirred about by any substance whatever, it collects in masses like new fallen snow, and adheres to the body with which it is stirred.

In addition to his experiments on the tourmaline, Epinus made several on the electricity of melted sulphur; and, in conjunction with Wilcke, he investigated the subject of electric atmospheres, and discovered a beautiful method of charging a plate of air by suspending large wooden boards coated with tin, and having their surfaces near each other, and parallel. Epinus, however, has been principally distinguished by his ingenious theory of electricity, which he has explained and illustrated in a separate work which appeared at St Petersburg in 1759. This theory is founded on the following principles. 1. The particles of the electric fluid repel each other with a force decreasing as the distance increases. 2. The particles of the electric fluid attract the particles of all bodies, and are attracted by them, with a force obeying the same law. 3. The electric fluid exists in the pores of bodies; and while it moves without any obstruction in non-electrics, such as metals, water, &c., it moves with extreme difficulty in electrics, such as glass, resin, &c. 4. Electrical phenomena are produced, either by the transference of the fluid from a body containing more to another containing less of it, or from its attraction and repulsion when no transference takes place.

The electricity of fishes, like that of minerals, now began to excite very general attention. The ancients, as we have seen, were acquainted with the bewildering power of the torpedo, but it was not till 1676 that modern naturalists attended to this remarkable property. The Arabians had long before given this fish the name of raad or lightning; but Redi was the first who communicated the fact that the shock was conveyed to the fisherman by means of the line and rod which connected him with the fish. Lorenzini published engravings of its electrical organs; Reaumur described the electrical properties of the fish; Krompfer compared the effects which it produced to lightning; but Bancroft was the first person who distinctly suspected that the effects of the torpedo were electrical. In 1773 Mr Walsh and Dr Ingenhouz proved, by many curious experiments, that the shock of the torpedo was an electrical one; and Dr Hunter examined and described the anatomical structure of its electrical organs. Humboldt, Gay Lussac, and M. Geoffroy, pursued the subject with success; and Mr Cavendish constructed an artificial torpedo, by which he was able to produce artificially the actions of the living animal. The subject has been more recently investigated by Dr Todd, Sir Humphry Davy, and Dr John Davy.

The power of giving electric shocks has been discovered also in the Gymnotus Electricus, the Silurus Electricus, the Trichurus Indicus, and the Tetradon Electricus. The most interesting and the best known of these singular fishes is the Gymnotus or Surinam eel. Its electrical organs have been minutely described by Hunter and Geoffrey; Dr Williamson, Dr Gordon, and Mr Walsh have published interesting details of its electrical powers; and Humboldt has more recently given the most romantic account of the combats which are carried on in South America between the gymnott and the wild horses in the vicinity of Calabozo.

Among the modern cultivators of electricity, our countryman, the late Mr Cavendish, is entitled to a distinguished place. Before he had any knowledge of the theory of Epinus, he had composed and communicated to the Royal Society a theory of electrical phenomena nearly the same as that of the German philosopher. As Mr Cavendish, however, had carried the theory much farther, and considered it under a more accurate point of view, he did not hesitate to give his paper to the world.

Mr Cavendish made some accurate experiments on the relative conducting power of different substances. He found that the electric fluid experiences as much resistance in passing through a column of water one inch long, as it does in passing through an iron wire of the same diameter 400,000,000 inches long; and hence he concludes that rain or distilled water conducts 400,000,000 times more than iron wire. He found that the water, or a solution of one part of salt in one of water, conducts a hundred times better than fresh water; and that a saturated solution of sea-salt conducts seven hundred and twenty times better than fresh water. Mr Cavendish likewise determined by nice experiments that the quantity of electricity in coated glass of a certain area increased with the thinness of the glass; and that in different coated plates the quantity was as the area of the coated surface directly, and as the thickness of the glass inversely.

Although electricity had been employed as a chemical agent in the oxidation and fusion of metals, yet it is to electricity that we owe the first of those brilliant enquiries which have done so much for the advancement of modern chemistry. By means of the electric spark he succeeded in decomposing atmospheric air. By using different proportions of oxygen and hydrogen, and examining the product which they formed after explosion with the electric spark, he obtained a proportion when the product was pure water. He was equally successful in the more difficult experiment of exploding oxygen and nitrogen; but when he combined seven measures of oxygen with three measures of nitrogen, he obtained from their explosion nitric acid. As several foreigners had failed in repeating this interesting experiment, Mr Cavendish, aided by Mr Gilpin, exhibited it publicly before the leading members of the Royal Society on the 6th of December 1787.

The decomposition of water by the electric spark was first effected by MM. Paets, Troostwyk, and Deiman; and improved methods of doing it were discovered and used by Dr Pearson, Mr Cuthbertson, and Dr Wollaston.

As a chemical agent, however, electricity was now destined to transfer its supremacy to another science. The greatest discovery made by Galvani in 1790, that the contact of metals produced muscular contraction in frogs, and the invention of the Voltaic pile, in 1800, by M. Volta of Como, have led to the establishment of a new science, called Galvanism or Voltaic Electricity, which, though now proved to be identical with common electricity, requires to be treated in a separate article. The chemical effects of the Voltaic pile far transcended those of ordinary electricity, and enabled Sir Humphry Davy to decompose the earth's Electricity.

History, and the alkalies, and thus to create a new epoch in the history of chemistry.

Coulomb, contemporary with Mr Cavendish was M. Coulomb, born 1736, one of the most eminent experimental philosophers of the last century. Anxious to determine the law of electrical action, he invented for this purpose an instrument called a torsion balance, which has since his time been universally used in all delicate researches, and which is particularly applicable to the measurement of electrical and magnetical actions. Æpinus and Cavendish had considered the action of the electrical fluid as diminishing with the distance; but M. Coulomb proved, by a series of elaborate experiments, that it varied like gravity in the inverse ratio of the square of the distance. Our countryman Dr Robison had previously determined, without, however, having published his experiments, that in the mutual repulsion of two similarly electrified spheres, the law was slightly in excess of the inverse duplicate ratio of the distance, while in the attraction of oppositely electrified spheres the deviation from that ratio was in defect; and hence he justly concluded that the law of electrical action was similar to that of gravity.

Adopting the hypothesis of two fluids, Coulomb investigated experimentally and theoretically the distribution of electricity on the surface of bodies. He determined the law of its distribution between two conducting bodies in contact; he measured the density of the electricity in different points of two globes in contact; he ascertained the distribution of electricity among several globes (whether equal or unequal) placed in contact in a straight line; he measured the distribution of electricity on the surface of a cylinder, and its distribution between a globe and cylinder of different lengths but of the same diameter. His experiments on the dissipation of electricity possess also a high value. He found that the momentary dissipation was proportional to the degree of electricity at the time; and that when the electricity was moderate, its dissipation was not altered in bodies of different kinds or shapes. The temperature and pressure of the atmosphere did not produce any sensible change; but the dissipation was nearly proportional to the cube of the quantity of moisture in the air. In examining the dissipation which takes place along imperfectly insulating substances, he found that a thread of gum-lac was the most perfect of all insulators; that it insulated ten times better than a dry silk thread; and that a silk thread covered with fine sealing-wax insulated as powerfully as gum-lac when it had four times its length. He found also that the dissipation of electricity along insulators was chiefly owing to adhering moisture, but in some measure also to a slight conducting power.

Towards the end of the last century a series of experiments was made by MM. Laplace, Lavoisier, and Volta, from which it appeared that electricity is developed when solid or fluid bodies pass into the gaseous state. The bodies which were to be evaporated or dissolved were placed upon an insulating stand, and made to communicate by a chain or wire with a Cavalli's electrometer, or with Volta's condenser, when it was suspected that the electricity increased gradually. When sulphuric acid diluted with three parts of water was poured upon iron filings, inflammable air was disengaged with a brisk effervescence; and at the end of a few minutes the condenser was so highly charged as to yield a strong spark of negative electricity. Similar results were obtained when charcoal was burnt on a clashing dish, or when fixed air or nitrous gas was generated from powdered chalk by means of the sulphuric and nitrous acids.

M. Volta, who happened to be at Paris when these experiments were made, and who took an active part in them, had subsequently observed that the electricity produced by evaporation was always negative. He found that burning charcoal gives out negative electricity; and in other kinds of combustion he obtained distinct electrical indications.

In this state of the subject M. Saussure undertook a series of elaborate experiments on the electricity of evaporation and combustion. In his first trials he found that the electricity was sometimes positive and sometimes negative when water was evaporated from a heated crucible of iron; but he afterwards found it to be always positive both in an iron and a copper crucible. In a silver and in a porcelain crucible the electricity was negative. The evaporation of alcohol and of ether in a silver crucible also gave negative electricity. M. Saussure made many fruitless trials to obtain electricity from combustion, and he likewise failed in his attempts to procure it from evaporation without ebullition.

Many valuable additions were about this time made to electrical apparatus, as well as to the science itself, by Van Marum, Cavalle, Nicholson, Cuthbertson, Brooke, Bennet, Read, Morgan, and Henley; but our limits will not permit us to do anything more than thus notice their labours.

The application of analysis to electrical phenomena may be dated from the commencement of the present century. Coulomb had considered only the distribution of electricity on the surface of spheres; but Laplace undertook to investigate its distribution on the surface of ellipsoids of revolution, and he showed that the thickness of the coating of fluid at the pole was to its thickness at the equator as the equatorial is to the polar diameter, or, what is the same thing, that the repulsive force of the fluid, or its tension at the pole, is to that at the equator as the polar is to the equatorial axis.

M. Biot has extended this investigation to all spheroids differing little from a sphere, whatever may be the irregularity of their figure. He likewise determined analytically that the losses of electricity form a geometrical progression when the two surfaces of a jar or plate of coated glass are discharged by successive contacts; and he found that the same law regulates the discharge when a series of jars or plates are placed in communication with each other.

It is to M. Poisson, however, that we are mainly indebted for having brought the phenomena of electricity under the dominion of analysis, and placed it on the same level as the more exact sciences. By assuming the hypothesis of two fluids, he has deduced theorems for determining the distribution of the electric fluid on the surface of two conducting spheres when they are either placed in contact or at any given distance; and the truth of these theorems has been established by experiments performed by Coulomb long before the theorems themselves had been investigated.

The cultivation of the new science of Voltaic electricity had now withdrawn the attention of experimental philosophers from that of ordinary electricity. The splendour of its phenomena, as well as its association with chemical discovery, contributed to give it popularity and importance; but the discoveries of Galvani and Volta were destined, in their turn, to pass into the shade, and the intellectual enterprise of the natural philosophers of Europe was directed to new branches of electrical and magnetical science. Guided by theoretical anticipations, Professor H. C. Oersted of Copenhagen, in 1820, laid the foundations of the science of Electro-magnetism. He found that the electrical current of a galvanic trough, when made to pass through a platina wire, acted upon a compass needle placed below the wire; and upon repeating the experiment, he discovered the fundamental law, that the magnetical effect of the Voltaic current had a circular motion round the current, or round the conductor, or the wire through which the current passed. M. Ampere of Paris soon afterwards made the important discovery, that two wires conducting electrical currents, when suspended so as to be capable of motion, attracted each other when the currents moved in the same direction, and repelled each other when they moved in opposite directions; or, to express the fact more simply, two points of electrical currents repel each other by their similar sides, and attract each other by their opposite sides; so that, as Professor Oersted remarks, an electric current contains a recoiling action, exhibiting every appearance of polarity.

In 1820 M. Arago, Sir H. Davy, and Dr Seebeck of Berlin, without being acquainted with each other's labours, discovered the power of the electric current to impart magnetism to iron and steel needles; but the most singular discovery on this branch of the subject was made by M. Savary, who found that small steel needles placed at different but very short distances from a wire conducting an electrical current, are magnetised in different directions. Needles in contact with the wire are magnetised in the usual or positive direction; while needles at the distance of 1-1 millimeter, or 1/36th of an inch, are magnetised in an opposite direction, which he calls negative. At the distance of two inches from the wire there was a neutral line in which the needles were not magnetised at all. When the distance of the unmagnetised needle was increased from three to eight millimeters it again became positively magnetic, the maximum effect taking place at the distance of 5½ millimeters. Between the distance of 8½ and 21½ millimeters the magnetism was a second time negative, the effect increasing from 8½ to 14½, and again reaching the vertical or zero point at 21½. Beyond the distance of twenty-three millimeters the magnetism was again positive. With different conducting wires M. Savary found, that within certain limits the maximum intensity is produced at a greater distance from the wire, and the number of alternations of positive and negative direction is also greater in proportion, as the wire is shorter in proportion to the length of the helix. When needles are placed parallel to the axis of a helix of narrow windings, they all receive the same kind of magnetism; but when the electrical charge is increased from one jar to a battery of twenty-two superficial feet, six alternations, viz. three positive and three negative, are obtained. When Voltaic electricity is substituted for ordinary electricity, the alternations are destroyed by a continued current, but appear when the current is established only for an instant.

These curious experiments were followed by those of Professor Erman of Berlin, who found that when an electrical discharge passes through the centre of a circular disc of steel, and in a line perpendicular to its surface, no apparent magnetism is developed; but when a slit is made in the plate, or a sector cut out of it, the side of the disc opposite to the slit, or the sectoral opening, exhibits the opposite magnetism. MM. Gay Lussac and Welther obtained the same result with a steel ring.

The discovery of thermo-electricity by Dr Seebeck in 1822 gave a new impulse to this branch of science. In studying the influence of heat in Galvanic arrangements, he was led to believe that magnetism might be developed in two metals forming a circuit when the equilibrium of heat in them was disturbed. He accordingly joined a semicircular piece of bismuth with a similar piece of copper, so as to form a circle by their union; and when one of the junctions was heated an electrical current was produced, which could show its existence only by the magnetic needle, and which exhibited all the magnetical properties of an electrical current.

In the same year in which Dr Seebeck made this remarkable discovery, the rotation of a magnetical needle day's duration around an electrical current, and of a body transmitting coveries, an electrical current round a magnet, were exhibited in a series of beautiful and highly ingenious experiments by Dr Faraday, whose subsequent discoveries place him at the head of the cultivators of this most interesting science.

These experiments were followed by those of Arago, Experiment Barlow, Seebeck, Herschel, and Babbage, in which a revolving plate of copper gives a rotatory motion to a magnetic needle conveniently suspended; but notwithstanding the ingenuity and talent with which this subject was treated by these eminent individuals, it is to Dr Faraday that we owe a complete analysis and explanation of this curious phenomenon.

This explanation was founded on the great discovery of the evolution of electricity from magnetism, by which Dr Faraday laid the foundation of the new science of magneto-electricity. By means of a series of simple and beautiful experiments with the celebrated magnets of Dr Go win Knight, and with the powerful magnet of Professor Daniel, Dr Faraday obtained the most unequivocal and striking electrical effects, though the intensity of the electricity was very feeble, and its quantity small. He obtained a distinct though feeble spark; he succeeded in convulsing the limbs of a frog by means of a magnet; and he perceived also the sensation on the tongue and the flash before the eyes, but he could not effect chemical decomposition by magnetism. Besides obtaining these important results, Dr Faraday has clearly established the laws according to which a magnet develops magnetic currents. He applies these laws to the explanation of the reciprocal action of revolving magnets and metals, and he adduces unquestionable proofs of the production of electricity by terrestrial magnetism.

These important results have been more recently extended by Dr Faraday and others. M. Pixii observed attractions and repulsions in the electricity evolved by magneto-electric induction; and by an ingenious and powerful apparatus he obtained a great degree of divergence in the gold leaves of an electrometer. At the meeting of the British Association at Oxford in June 1832, Dr Faraday, by means of Mr Snow Harris's electrometer, subsequently described, succeeded in heating a wire by magneto-electric induction. By means of the magneto-electric apparatus of M. Pixii already referred to, he and M. Hachette decomposed water, and obtained the oxygen and hydrogen in separate tubes.

In the progress of his electrical researches, Dr Faraday found it necessary for their further prosecution to establish either the identity or the distinction of the electricities excited by different means; and in a paper of great value, he has established beyond a doubt the identity of common electricity, Voltaic electricity, magneto-electricity, thermo-electricity, and animal electricity. The phenomena exhibited in these five kinds of electricity do not differ in kind, but merely in degree; and in this respect they vary in proportion to the variable circumstances of quantity and intensity, which can at pleasure be made to change in almost any one of the kinds of electricity, as much as it does between one kind and another. Dr Faraday has given the following interesting table of the experimental effects common to the electricities derived from different sources:

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1 The cross indicates that the effect at the top of this table is produced by the electricity mentioned in the column at the side. Dr Faraday was anxious to determine the relation by measure of ordinary and voltaic electricity; and after various excellent experiments, he obtained as an approximation, and judging from magnetical force only, "that two wires, one of platinum and one of zinc, each one-eighteenth of an inch in diameter, placed five-sixteenths of an inch apart, and immersed to the depth of five-eighths of an inch in acid consisting of one drop of oil of vitriol, and four ounces distilled water at a temperature about 60°, and connected at the other extremities by a copper wire eighteen feet long and one-eighteenth of an inch thick (being the wire of the galvanometer coils), yield as much electricity in eight hours of my watch, or in 7½ this of a minute (or 3-2 seconds), as the electrical battery (of fifteen jars) charged by thirty turns of the large machine in excellent order. The same result was found to be true in the case of chemical force."

In the course of his investigations relative to electrochemical composition, Dr Faraday was led to observe the effects due to a very general law of electric conduction which had not formerly been recognised. He found that solid bodies assume the power of conducting electricity during liquefaction, and lose this conducting power during congelation. The voltaic electricity produced by a battery of fifteen troughs, or a hundred and fifty pairs of four-inch plates powerfully charged, was incapable of passing through a thin film of ice three-sixteenths of an inch thick; but when the ice was melted, the electricity passed in such quantities as to deflect the magnetic needle 70°. This insulation, however, exhibited by ice is not effective with electricity of exalted intensity. In making this experiment with other solid bodies, Dr Faraday chose those which, being solid, at common temperatures, were fusible, and of such a composition as, for other reasons connected with electrochemical action, led to the conclusion that they would be able to replace water. When the electric current passed through the solid body employed, there was no chemical decomposition; but when the body was liquefied or fused, the decomposition took place. The bodies which Dr Faraday found to be subject to this law will be found in our section on Electrical Conduction. The degree of conducting power conferred upon bodies by liquidity is generally very great. In water it is the feeblest of all; and in the various oxides, chlorides, salts, &c., it is given in a much higher degree, some a hundred times greater, than in the case of pure water.

In studying the phenomena of induction, Mr Faraday has been led to a beautiful theory of inductive action, which throws a new light upon every department of electricity. While Cavendish, Poisson, and other distinguished cultivators of the science have considered induction as a force excited at a distance, and in straight lines, he has been led to regard it as an action of contiguous particles, or particles near each other, consisting of a species of polarity in the particles of the dielectric or insulating medium through or across which the electric forces are acting. When an electrified body induces electricity in a conductor, it does it by Faraday's polarizing all the intermediate particles of the dielectric, theory of that is by exciting the opposite electricities in the particles statical near it, and by these particles producing the same effect, electricity, from particle to particle, till it is transferred to the conductor. The particles which are thus polarized are not supposed to be material atoms of the dielectric, but merely points or centres of force pervading all space and penetrating all material bodies. When such contiguous particles communicate their forces slowly to one another, insulation or coercion, as it has been called, is produced; and when rapidly, conduction is the result. This polarization of dielectrics has been placed beyond a doubt by the experiments of Faraday and Matteucci in the case of solids and fluids, if not for air and the gases. By means of this fine theory, which has been confirmed by recent experiments, M. Mosotti has succeeded in explaining the law of electrical forces discovered by Coulomb.

In an able memoir published at Turin in 1845, M. Plana Researches has given the results of his researches on the distribution of M. of the electric fluid on the surface of conductors. With Plana, taking into consideration the cause of the retention of electricity on the surface of conductors, he has treated the problem of its distribution in three cases,—in the case where the spheres are in contact, in the case where they are separated by any interval, and in the case where the separation is very small compared with the distance of their centres. He has also given a more rigorous demonstration of certain principles, on the relations which exist between the thickness of the electric wire and the forces which emanate from it. From the simple fact, that free electricity distributes itself on the surface of conductors, he demonstrates that the repulsive force in the case of simple physical points is inversely as the square of the distance.

The distribution of electricity on spherical conductors has been successfully pursued by English mathematicians. One of Caven-celebrated countryman Mr Cavendish had, so early as 1773, made great progress in the inquiry. He demonstrated the remarkable proposition, that unless the electrical force was in the inverse proportion of the square of the distance, the electricity would be distributed through the interior of a charged conductor; and consequently that this law must be true, as the electricity is confined to an infinitely thin film on its surface. We owe also to Cavendish an approximation to the true theory of the Leyden phial, and a determination of the effects produced by connecting conductors with fixed wires.

It is by our countryman Mr Green, a self-taught mathematician, that the greatest advances have been made in the mathematical theory of electricity.1 His researches, as Green, Professor William Thomson observed, "have led to the elementary proposition which must constitute the legitimate foundation of every perfect mathematical structure that is to be made from the materials furnished in the experimental laws of Coulomb. Not only do they afford a natural and complete explanation of the beautiful quantitative experiments

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1 Essay on the application of Mathematical Analysis to the Theories of Electricity and Magnetism, Nottingham, 1828. which have been so interesting at all times to practical electricians, but they suggest to the mathematician the simplest and most powerful methods of dealing with problems which, if attacked by the mere force of analysis, must have remained for ever unsolved." One of the simplest applications of these theorems was to perfect the theory of the Leyden phial, a result which (if we except the peculiar action of the insulating solid medium since discovered by Mr Faraday) we owe to his genius. He has also shown how an infinite number of forms of conductors may be invented, so that the distribution of electricity in equilibrium on each may be expressible in finite algebraical terms,—an immense stride in the science, when we consider that the distribution of electricity on a single spherical conductor, an uninfluenced ellipsoidal conductor, and two spheres mutually influencing one another, were the only cases solved by Poisson; and, indeed, the only cases conceived to be solvable by good mathematical writers.

The work of Mr Green, which contained these fine researches, though published in 1828, had escaped the notice not only of foreign, but even of British mathematicians; and it is a singular fact in the history of science, that all his general theorems were rediscovered by Professor William Thomson of Glasgow, MM. Charles and Sturm of Paris, and M. Gauss of Göttingen. Professor Thomson, however, pushed his researches much farther than his fellow-labourers. He pointed out in 1845 the consistency of Mr Faraday's experiments with the new theory; and, guided by an analogy between the uniform motion of heat and the distribution of electricity on conductors, or the attractive and repulsive forces excited by electrified bodies, he has shown how the peculiar electric polarization discovered by Mr Faraday in dielectrics, or solid insulators subjected to electric force, is to be taken into account in the theory of the Leyden phial, so as to supply the deficiency in Green's investigations.

From the elementary propositions of Mr Green, Mr Thomson was led to the beautiful principle of what he calls electrical images, which is a new and admirable method of treating a great variety of problems in reference to the distribution of electricity on spherical conductors. "The effect," says Mr Thomson, "of a body electrified in any given manner upon an uninsulated sphere, is shown to be completely represented by what may be called the image of the electrified body in the sphere; and a simple geometrical construction is given by which this image may be described. When an electrified body is placed in the neighbourhood of two uninsulated spheres, an inductive effect is produced which may be represented by an infinite series of successive images in each sphere. An algebraic expression of this result leads to solutions, by means of converging series, of the various problems which occur with reference to the distribution of the induced electricity, and the attractions exerted by the two spheres. Or, when a single conductor, bounded by segments of two spherical surfaces cutting at an angle which is a submultiple of two right angles, is electrified by the influence of a charged body, the effect may be represented by a finite number of images disposed in a symmetrical manner in the circumference of a circle passing through the exciting body, and cutting the two spherical surfaces at right angles. The principle of electrical images in these two cases may be illustrated by a reference to the successive images of a candle placed between two parallel plane mirrors, and to the symmetrically arranged images which are seen in the kaleidoscope."

One of the most remarkable discoveries in electricity which has been made in our own day is that of the hydro-electric machine, which we owe to Mr Armstrong of Newcastle. On the 29th September 1840 William Paterson, who attended a steam-engine at Cramlington Colliery, 8 miles from Newcastle, happening to take hold with one hand of the lever of the safety valve of the boiler, while his other strong hand was in the steam which was issuing from a fissure, received an electric shock. The same effect was produced if he touched any part of the boiler, or any iron work connected with it. The engine-man also found that when one hand was immersed in the jet of steam he gave a shock to every person he touched with the other, whether the person was in contact with the boiler or merely stood on the brickwork which supports it, though the shock was greater when the person touched the boiler. Having heard of these remarkable facts, Mr Armstrong went to Cramlington Colliery and observed the phenomena, and having constructed a steam apparatus, he found that the place where the electricity was produced was that at which the steam was subjected to friction, and that when the boiler was of wrought iron the electricity was always positive, excepting when he made the steam pass to the discharging aperture through a considerable surface of polished brass. In this case the electricity became very feeble; but when the inside of the brass tube was moistened with dilute nitric acid, the steam from the iron boiler became for the first time negatively electrified. By an insulated brass discharging-rod, consisting of a metallic plate at one end and a brass ball at the other, he obtained from 60 to 70 powerful sparks in a minute, the plate being plunged in the issuing steam, and the ball brought into contact with the boiler. Mr Armstrong was thus led to construct a hydro-electric machine, which gives sparks 22 inches long, and so large, dense, and rapid that they frequently resembled a continuous flame. This machine, with a boiler 63 feet long by 3½ feet wide, was constructed for the Polytechnic Institution, where it has long excited the admiration of the public.

Mr Armstrong at the very commencement of his inquiry Mr Faraday communicated his discovery to Mr Faraday, who, with his day's usual ability, investigated the theory of the machine. He found that the electricity was produced by the friction of the particles of water upon the discharging tube, and that electricity was not the electricity caused by evaporation. When machines of glass, metal, or wood were used, the electricity of the steam was always negative, and that of the boiler positive, but when a quill tube or a tube of ivory was employed, "the boiler received scarcely any charge, and the stream of steam was also in a neutral state," showing that the electricity was not produced by evaporation. Mr Faraday also found that the electricity of the issuing current was negative, and that of the boiler positive, when oil or oil of turpentine was carried forward by the current of steam. When acetic acid was used in the steam globe, the electricity was neutralized.

Among the able and active cultivators of electrical science, Sir Snow Harris deserves a prominent place. He was the first who introduced accurate quantitative measures into the investigation of the laws of statical electricity—the unit measure by which quantity is minutely estimated—and the hydro-electrometer and scale-beam balance by which its intensity and the laws of attractive forces at all distances are demonstrated. Of not less value is the thermo-electrometer, by which the heating effects of given quantities of electricity are measured and rendered comparable with the varying conditions of quantity and intensity. Besides these instruments, we owe to the same philosopher the discovery of a new reactive force, by which repulsion and other small physical forces are investigated and determined by means of his bifilar balance, founded upon the reactive force of two vertically suspended parallel threads when twined upon each other.

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1 See Cambridge and Dublin Mathematical Journal, Nov. 1845; and Phil. Mag., July 1854, pp. 42-62. 2 Reports of the British Association, 1847, Trans. Sect., pp. 6, 7.

other at a given angle, and acted upon by a suspended weight. With the aid of these ingenious instruments he has carried on a variety of successful inquiries into the laws of electrical forces, and the laws and operations of electrical accumulation. In studying the laws of electrical discharge in the form of lightning, Sir Snow Harris was led to the invention of a permanent system of conductors for ships, so perfect that during the last twenty-five years in which they have been introduced into the British navy no instance has occurred of the least damage by lightning. Although the nation has thus gained many thousand pounds per annum, and hundreds of valuable lives have been saved, Sir Snow Harris has received only the paltry reward of L5000. Our limits will not permit us to enumerate the various individuals by whom the science of statical electricity has been advanced, either by their researches or their inventions. Among these we may mention the distinguished names of Peltier, Pouillet, Becquerel, Matteucci, Fusinieri, Grove, Wheatstone, Delarive, Melloni, and Dubois Remond.

PART I.

PHENOMENA AND LAWS OF ELECTRICITY.

Elementary Phenomena and Definitions.

1. If a smooth glass tube, or the glass of a watch, or a piece of sealing-wax, or amber, be rubbed upon the sleeve of a cloth coat, or, what is still better, if it be rubbed with a piece of dry flannel or woollen cloth, it will be found to have acquired from this friction a new physical property. This property will be exhibited by holding the body which has been newly rubbed above small shreds of paper, gold leaf, or any thin light substances placed upon the table. These bodies will be instantly attracted to it, some of them adhering to its surface, others falling back to the table, and others being thrown off from the body as if they were repelled from it.

The property which has thus been communicated by friction is called electricity, the body which acquires the property is called the electric, the attraction which it exercises over light bodies is called electric attraction; and when the attractive power is produced by friction, the body rubbed, or the electric, is said to be excited by friction, or electrified or electrized, and the body by which it is excited is called the rubber.

2. In order to study these phenomena with more precision, let a small ball B, the size of a pea, made of cork, or the dry pith of elder, or, what is better still, of the finely porous pith of the sala tree from India, be suspended from a stand ACD by a dry silk thread AB, or a fibre of raw silk. Having rubbed a large glass tube with a piece of dry silk, present it to the ball B, and the ball will be instantly attracted to the tube, and will adhere to it. After they have continued in contact for a second or two, withdraw the glass tube, taking care not to touch the ball with the finger. If the excited glass tube is now a second time brought near the ball, the ball will recede from it, or will be repelled by the tube. If, after touching the ball with the finger, so as to deprive it of its electricity, the above experiment is accurately repeated with a stick of sealing-wax in place of glass, the very same phenomena will be exhibited; the ball will, in the first instance, be attracted, and on the second application of the sealing-wax it will be repelled. Hence we draw two conclusions, first, that both glass and sealing-wax attract the ball B before they have communicated to it any of their own electricity; and,

second, that both these electrics repel the ball after each of them has communicated to it some of their own electricity.

3. Let us now examine what takes place when the excited sealing-wax is presented to the ball after the ball has received electricity from the excited glass, and vice versa. For this purpose excite the glass tube, present it to the ball B, and after it has been a few seconds in contact, withdraw it. The ball has now received electricity from the glass tube. Let the sealing-wax be now excited and presented to the ball, the ball, in place of being repelled, will be attracted by the wax. Reverse this experiment, by first presenting the excited wax to the ball, and then the excited glass, and it will be in like manner found that the glass repels the ball. Hence it follows that

Excited glass repels a ball electrified by excited glass. Excited wax repels a ball electrified by excited wax. Excited glass attracts a ball electrified by excited wax. Excited wax attracts a ball electrified by excited glass.

From which we conclude that there are two opposite electricities, namely, that produced by excited glass, to which the name of vitreous or positive electricity has been given, and that produced by excited wax, to which the name of resinous or negative electricity has been given.

4. If, when the pith ball B is electrified, either with excited glass or wax, we touch it with a rod of glass, its property of being subsequently attracted or repelled by the excited glass or wax will suffer no change; but if we touch it with a rod of metal it will lose the electricity which it had received, and will be attracted both by the excited glass or wax, as it was when they were first applied to it. Hence the rod of glass and the rod of metal possess different properties, the former being incapable, and the latter capable, of carrying off the electricity of the pith ball. The metal is therefore said to be a conductor, and the glass a non-conductor, of electricity.

In the few elementary experiments which we have now described, the electricity has been produced by friction; but the pith ball could have been electrified by a great variety of other methods, which will be explained in a subsequent part of this article. In all these cases the effects are precisely the same, whatever be the source from which the electricity is obtained; but as friction is the simplest means of generating electricity, and as machines and apparatus have been invented, by means of which it can be thus produced in great abundance, and accumulated in large quantities, we shall proceed to describe the phenomena and laws of electricity as produced by friction.

CHAP. I.—ON THE PHENOMENA OF ELECTRICITY PRODUCED BY FRICITION.

Sect. I.—Description of the Electrical Machine for generating Electricity.

Although the friction produced by the strength of the human arm is sufficient to produce abundance of electricity for ordinary experiments, yet the aid of mechanism has been found essential for carrying on electrical investigations, and producing powerful electrical effects. The various forms which have been given to the electrical machine will be described in the second part of this article, under the head of Electrical Apparatus, so that we shall chiefly confine our attention at present to a description of the plate-glass machine.

This machine, in its common form, is represented in Plate CCXXII. fig. 1, where AB is a circular disc of plate-glass. glass from eighteen inches to two or more feet in diameter, and from two to three eighths of an inch thick. This disc is fixed perpendicular to a horizontal axis, supported by two uprights E, F, of a mahogany frame, and is capable of being turned round with any ordinary degree of velocity, by means of the handle or winch W. The rubbers by which this disc of glass is rubbed or excited are placed at the upper and lower end of the disc, as seen in the section, fig. 2. The two upper rubbers above A, viz. G, H, are suspended from the top of the frame, and are fixed by screws to two flat pieces of wood m, m, which can be pressed together or slackened by turning the screw s so that the rubbers G, H may be made to press with the requisite degree of force against the disc AB which revolves between them. The lower rubbers M, N below B, are supported upon the stand, and are similarly put together. The rubbers are generally flat cushions of silk or soft leather stuffed with hair.

The prime conductor CD is a semicircle of hollow brass, supported on the upright E by means of the solid glass cylinder R. The two extremities of this conductor, one of which is seen below A, and the other above B, carry each a row of brass points, and the transverse piece of brass tube in which the points are inserted terminates in a varnished wooden ball.

From the upper rubbers an oil silk flap, embracing both surfaces of the plate, extends to a little above the row of points on the conductor; and from the lower rubbers a similar flap extends to a little below the other row of points. One of these flaps is seen in the figure, but the other is partially hid by the upright F.

As it has been found that electricity is developed more copiously when the rubbers are covered with an amalgam of one part of tin and two of mercury, various compositions have been tried by philosophers. The following amalgam, recommended by Singer, is equal in efficacy to any that has yet been proposed. Melt two ounces of zinc and one of tin, and pour into the crucible six ounces of mercury. Shake the whole together till it is cold, in an iron or thick wooden box; and when it has been reduced to a fine powder in a mortar, mix it with as much lard as will form it into a paste. The amalgam thus formed must then be thinly spread on the surface of each cushion; and when the disc of glass has been well cleaned from dust, and from black specks or lines, by means of a little spirits of wine, the mixture is ready for use.

When a very powerful excitation is required, it is usual to cover a piece of smooth leather, four or five inches broad, with the amalgam, and apply it with the hand to the revolving disc; and it has been found very useful to apply previously a rag with a little tallow, so as just to give a slight dimness to the glass.

Although the plate-glass machine is generally regarded as the best, yet, from its greater cheapness and facility of construction, the cylinder machine is most commonly used. We shall therefore describe at present one of those machines as improved by Mr. F. Ronalds. This machine is represented in fig. 3, where A is a cylinder of blue glass about a quarter of an inch thick, supported by the two mahogany uprights B, B, fixed to the box or case C, which forms the base of the machine. DD is a copper pipe which supports the semicylinder E, which is also hollow, and into which the pipe D opens. This semicylinder carries on its flat side the cushion or rubber, the surface of which is made concave to suit the convexity of the cylinder AA. A small spirit-lamp F, the burner of which consists only of a single cotton thread, is placed, as shown in the figure, immediately beneath the mouth of the copper pipe DD. The prime conductor G, which stands parallel to the cylinder, is a cylindrical tube of thin copper, rounded at both ends, and carrying at its middle a row of metallic points, which nearly touch the surface of the glass cylinder. The conductor G is supported by a hollow glass support H, opening into the hollow conductor G. Its lower end at H is fixed to the wooden case C by means of three screws, one of which is seen at a passing through a circular piece of hard boxwood, the inside of which, as well as that of the perforation in the case C, is lined with leather. The lower end of the glass tube H terminates, like that of D, within the case C, and a spirit-lamp is in like manner placed beneath it.

By these ingenious contrivances the rubber and the conductor are kept warm and dry, and in damp weather, or in a close room, where the air is rendered moist by the breath of the audience, the machine will be found always effective and in working order. Mr. Ronalds is of opinion that the excitation of the cylinder is promoted by the excitation of the amalgam by means of the heat. If similar means are not taken to heat the interior of the glass cylinder, the development of electricity may be promoted by holding a hot piece of cloth or flannel beneath the cylinder while it is in operation.

In using an electrical machine a singular but not disagreeable odour is invariably felt, which increases with the number of sparks taken from the conductor or from electrical jars and batteries. The same smell is felt when lightning strikes terrestrial objects, and particularly objects in the interior of an apartment. The origin of this remarkable odour has been discovered by M. Schoenbein of Basle, the discoverer of gun cotton, and he has given the name of ozone to the substance which produces it. Ozone is highly odoriferous, and is nothing more than oxygen gas possessing a particular chemical activity which it receives from electricity. It has very remarkable properties as a chemical agent which are not possessed by pure oxygen, into which it is converted at a temperature of 300° Centigrade.

Sect. II.—On the Phenomena of Electrical Attraction and Repulsion.

If the electrical machine, when thus prepared, is put in electrical motion, the two rows of points will collect the electricity attraction which is generated by friction, and the brass conductor CD, fig. 1, will be filled with the electricity thus produced. By means of this electricity the following experiments may be readily performed.

Exp. 1. If we suspend a pith ball by a slender wire, and bring the ball near the conductor, it will be instantly attracted by, and adhere to, the conductor, as long as there is any electricity left. In this case the electricity imparted to the ball by the conductor is carried off by the conducting wire to the hand, and through the body to the earth.

If the pith ball is suspended by a dry silk thread, and held near the conductor, it will at first be attracted to it as formerly; but after it has received as much as it can take, it will then be repelled by the conductor, from the repulsive action of the two similar electricities, and it will not again approach the conductor, till either its own electricity or that of the conductor has been carried off by the contact of some conducting body. In either of these cases the pith ball will be again attracted by the conductor.

Exp. 2. Suspend a little brass ball by a silk thread, and bring the ball near the conductor, so as to receive electricity from it, and be repelled, as in experiment 1. Then with the other hand bring another brass ball near to the first, but on the side of it opposite to the conductor. The first ball will now be attracted to the conductor in conse-

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1 Moscato gold and the deuto-sulphur of tin is a good amalgam. 2 It would be easy to improve this construction by introducing the heated air of other two spirit-lamps into each end of the cylinders. sequence of having given out its electricity to the second ball; but having received a new charge of electricity, it will be repelled from the conductor and attracted to the hand or fixed ball. In this way it will oscillate like a pendulum between the conductor and the fixed ball. If in place of the fixed ball we substitute a bell, the ball will oscillate as before, and cause the bell to ring by its successive strokes.

Exp. 3. The beautiful experiment of the electrical bells is exhibited in fig. 4, where AB is a solid glass rod surrounded by a brass ball A, and supported upon a wooden stand. Two arms of brass crossing at right angles are also supported by the glass rod, and, by means of wires or chains hanging from their extremities, are suspended four bells b, b, b, b. From the middle part of each of these cross arms is suspended a brass ball by silk threads, so that each ball when put in motion and made to oscillate in a plane passing through its own cross arm, may strike alternately the middle bell b' and the one adjacent to it. If the brass ball is now placed close beneath the brass knob of the prime conductor, or made to communicate with it, the electricity of the conductor will be transmitted through the brass arms to the balls, and the balls giving out their electricity to the bells, will strike them alternately, and cause them to ring, the electricity passing off through the central bell b' into the earth.

The experiment may be made more simply by suspending three bells, one from the middle, and one from each extremity of a brass rod, which is hung by its middle part from an electrified conductor. Two brass balls are hung by a silk thread between the central bell and the outer ones. The outer bells are supported by a wire or chain, and the central one by a silk thread. This central bell, however, must communicate with the ground by a chain. When the machine is put in motion, the electricity passes to the outer bells, and the insulated balls, being attracted and repelled, strike the outer bells and the inner one, by which last the electricity passes into the earth.

A still simpler form of the experiment consists in placing two small bells on separate glass stands, at a quarter of an inch distance, one of the bells communicating with the prime conductor, and the other with the ground. A brass ball is then suspended between them by a silk thread, and when the machine is wrought, the electricity will pass to the earth through the bells and the ball, the latter oscillating between them, and ringing them, as long as the current of electricity is kept up.

Exp. 4. Take a dozen of threads about a foot long, and having tied them together at both ends, hang them, by a loop attached to the upper knot, to the prime conductor. When the machine is wrought, the threads will separate from each other, swell out at the middle, and assume a form approaching to that of a sphere. If the threads are merely joined at each end, so that their extremities point to two poles, which may easily be done, they would swell out, and form, as it were, the meridians of a hollow globe. This pretty experiment we owe to Mr. Wheeler.

Exp. 5. Having fastened a piece of sealing-wax to a wire, and inserted the wire in the hole at the end of the prime conductor, soften the sealing-wax by the flame of a candle, and work the machine.—fine fibres of wax like those of wool will be thrown off, and may be received on paper. By gently heating the paper, the result of the experiment may be fixed. These fibres are thrown off by the repulsion of the electrified particles of wax, which becomes a conductor when melted. The same experiment might no doubt be made with melted sugar, rosin, and other substances.

Exp. 6. The experiment of the dancing figures is one of the finest illustrations of electrical attraction and repulsion. Take two circular discs of wood or pasteboard, E, F, like those shown in fig. 5. Cover them with tinfoil, and having suspended the uppermost from the prime conductor (or from the end D of a metallic rod CD insulated by the glass stand AB, and whose other extremity C communicates by a chain with the prime conductor), place the other upon a stand G, so that, by means of the screw nut n, it can be raised or depressed. Place, upon the lower disc small painted figures cut out of paper, and as soon as the machine is wrought the figures will spring upon their feet, and execute the most extraordinary movements, sometimes dancing on their heads, sometimes hanging by the upper plate, and sometimes flying into each other's arms. If these figures are cut out of the pith of the sola tree, and if the arms and legs are made separate, and attached by threads to the body, the effect surpasses all description. The circular discs will answer equally well if made of metal.

Exp. 7. Suspend from the prime conductor a small metallic cup nearly full of water, and having placed in it the shorter end of a syphon made with a capillary glass tube, of such a bore that the water will with difficulty drop from it. When the water is electrified by working the machine, it will be discharged in a continuous stream from the larger arm of the syphon; and if the electricity is powerful, the current of water will divide itself into several branches. In like manner, if a condensed air fountain is electrified, the jet will subdivide itself into minute parts, and suffer great expansion; but the moment the machine stops it resumes its original form. In like manner, if a sponge filled with water discharges the fluid only by drops, it will, when electrified, let fall an abundant shower, which in the dark will be luminous.

Exp. 8. In a metallic cup place a piece of lighted camphor, and when the cup communicates with the electrified conductor, the camphor will throw off numerous ramifications, shooting forth its branches like a vegetable in growth.

An immense number of similar experiments may be made by placing pith balls under inverted tumblers, and thin balls of glass within metallic rings; and when the tumblers and the rings are electrified, the most varied movements are produced; and the effect is greatly heightened by the accompanying luminosity, which displays itself in the dark.

The theory of the phenomena which we have now described will be given in a subsequent section.

SECT. III.—On the Phenomena of Positive and Negative Electricity.

We have already seen that there are two opposition Positive electricities, which have received the name of positive or negative, and negative or resinous electricity, the former being generated by excited glass, and the latter by excited wax.

In order to examine the properties of these two kinds of electricity, take four stands like that shown in page 577, consisting of a vertical rod of glass fixed in a wooden base. From the top of each stand suspend a single pith ball by a slender wire, and place the four stands, which we shall call P, P', N, N' at some distance from each other on a table. Electrify the pith balls on P, P' by excited glass, so as to make them positively electrical; and the pith balls on N, N' with excited wax, so as to make them negatively electrical. The following phenomena will then be observed. If the balls P, P' or N, N' are brought near each other, they will repel one another, but if P or P' is brought near to N or N', the balls will attract each other. Hence it follows,

That two SIMILARLY electrified bodies P, P' or N, N' repel each other, while two DISSIMILARLY electrified bodies of positive P, N or P', N' attract each other.

If, in place of electrifying the balls with the glass and the sealing-wax, we had electrified them with the rubber with which they had been excited, we should have found that the rubber which excited the glass gave out resinous electricity, and the rubber which excited the wax vitreous electricity. Hence we learn,

That in electrical excitation positive and negative electricity are simultaneously produced.

In all electrical machines, therefore, where the plate or cylinder is made of glass, the conductor which takes the electricity from the glass will be charged with positive electricity; and as the rubber is negatively electrified, we may obtain negative electricity from it in the same abundance, by placing a conductor behind the rubber, and insulating them both by a glass stand. In the cylinder machine this is easily done, as shown in fig. 6, which represents a machine driven by a wheel and pulley, where E is the negative conductor placed at the back of the rubber R, and S the glass stand by which they are both supported and insulated.

In the plate-glass machine it is more difficult to unite a conductor to the rubbers. In Van Marum's beautiful electrical machine, shown in fig. 1-5 of Plate CCXXIII., and which will be more minutely described afterwards, the positive and negative electricity can be obtained only in succession; but Dr Hare, of the university of Pennsylvania, has removed this difficulty by the very ingenious contrivance shown in fig. 7, of making the plate revolve horizontally, and thus allowing the positive and negative conductors B, F to stand like arches in two vertical planes at right angles to each other.

The circumstances of surface and structure under which bodies yield the two opposite electricities by friction are still very imperfectly understood. Mr Canton found that the same body gave out opposite electricities when rubbed with different substances. Polished glass, for example, was always positively electrified when excited with flannel or silk; but always negatively electrified when excited by the back of a cat. But, what was still more strange, he found that rough glass acquired negative electricity when excited by flannel, and positive electricity when excited by dry oiled silk. Rough quartz has been found to exhibit the same difference.

A still more extraordinary and instructive anomaly was observed by Haüy, in exciting a mineral called kyanite. Some of the crystals he found to acquire positive electricity by friction, while others acquired negative electricity. Saussure had announced that they were negatively electrified by friction; and when Haüy obtained an opposite result in his first experiment, he was led to examine the subject more carefully, and to make his trials both with the natural faces and with those produced by cleavage. "I have," he remarks, "in my collection a crystal whose opposite faces have presented me with these opposite effects (electricities), and I can assign no other cause for this singular result than a certain alteration in the contexture of one of the surfaces." Hence Haüy has given the name of diasthene, or two powers, to this mineral. The remarkable property which Haüy discovered in the individual crystal above referred to may have arisen from some composite structure which he did not recognise.

As the property of giving positive or negative electricity by friction has been used as a mineralogical character, we shall lay before our readers a general view of the experiments which have been made on the subject, which we have collected from a great variety of sources.

| Names of the Excited Substances | Nature of the Electricity produced | |---------------------------------|-----------------------------------| | Smooth Glass | Positive | | | Negative | | Rough Glass | Positive | | | Negative | | Quartz, smooth | Positive | | Quartz, rough | Negative | | Topaz, smooth | Positive | | Topaz, rough | Negative | | Back of a living cat | Positive | | Hare skin | Positive | | | Negative | | White Silk | Positive | | | Negative | | Black silk | Positive | | | Negative | | Woollen cloth | Positive | | | Negative | | Sealing-wax | Positive | | | Negative | | Baked wood | Positive | | | Negative | | Sulphur | Positive | | | Negative | | Resinous bodies | Positive | | | Negative |

Substances used for Excitation:

- Every substance yet tried but the back of a cat and mercury. - Back of a cat, and sometimes caoutchouc. (Nich. Journ. xxviii. p. 11.) - Dry oiled silk, metals, wax, and resinous matters. - Woollen cloth, human hand, back of a cat, wood, paper, quills. - Flannel, &c. - Human hand, wessel's skin, paper, hair. - Sealing-wax. - Harp's, weasel's, and ferret's skin, white silk, human hand, brass, silver, iron, lodestone. - Zinc, silver, bismuth, copper, lead, olgiest iron. - Platina, gold, tin, antimony, grey copper, grey cobalt, tellurium, &c. - Rough glass, white wax, sulphur, and all metals except iron, steel, plumbago, lead, and bismuth. - Harp's, wessel's, and ferret's skin, human hand, leather, smooth glass, flannel, wood, paper, iron, steel, plumbago, lead, and bismuth. - Silk, paper, rough glass, wax, lead, sulphur, and the metals. - Flannel, human hand, smooth glass, quills. - All metals but lead. - Lead and all other substances. - All resinous substances. - All bodies but resinous ones.

The most accurate and numerous observations on the development of electricity by friction were made by the Abbé Haüy in reference to the discovery of new characters of minerals. He rubbed the minerals on a woollen cloth, and when it was necessary to insulate them, he fixed them by wax to the end of a stick of gum-lac or Spanish wax. In this way he divided the mineral kingdom into four classes of bodies in reference to the electrical character of the minerals.

Class I.—Containing minerals which possess the insulating property, and acquire vitreous electricity by friction.

- Borosite. - Topaz. - Axinite. - Tourmaline. - Mesotype. - Prehnite. - Oxide of zinc. - Sphene. - Carbonate of lime. - Carbonate of magnesia. - Arragonite. - Apatite. - Fluete of lime. - Gypsum. - Anhydrite. - Sulphate of barytes.

1 That is, that they do not require to be insulated or placed upon a substance which does not conduct or carry off electricity, in order to exhibit their electricity. **ELECTRICITY**

**Phenomena and Laws**

| Class II.—Containing minerals which possess the insulating property (excepting anthracite), and acquire resinous electricity by friction. | |---| | Sulphate of strontian. | | Carbonate of barytes. | | Carbonate of strontian. | | Silica of magnesia. | | Siliceous borax of lime. | | Nitrate of potash. | | Sulphate of potash. | | Muriate of soda. | | Glanberite. | | Hyalin quartz. | | Zircon. | | Corundum. | | Cynophane. | | Spinels. | | Emerald. | | Euclase. | | Cordierite. | | Garnet. | | Essoelite. | | Iodocrase. |

| Class III.—Containing conducting substances which acquire, when they are insulated and rubbed, the one order vitreous electricity, and the other resinous electricity. | |---| | Order 1. Substances which acquire vitreous electricity. | | Pure silver. | | Native silver. | | Silver coin. | | Pure lead. | | Pure copper. | | Native copper. | | Copper coin. | | Pure zinc. | | Brass. | | Native bismuth. | | Native amalgam. | | Order 2. Substances which acquire resinous electricity. | | Pure platinum. | | Native platinum. | | Palladium. | | Pure gold. | | Native gold. | | Gold coin. | | Pure nickel. | | Native iron. | | Hammered iron. | | Palladium. | | Amalgam of tin and mercury. | | Native arsenic. | | Pure antimony. | | Native antimony. | | Tellurium of Naygag. | | Antimonial silver. | | Arsenical nickel. | | Arsenical iron. | | Oxidated iron. | | Metallic oxide of manganese. | | Sulphuret of silver. | | Sulphuret of lead. | | Copper pyrites. | | Grey copper. | | Sulphuret of copper. | | Graphite. | | Common sulphuret of iron. | | White sulphuret of iron. | | Magnetic sulphuret of iron. | | Sulphuret of tin. | | Sulphuret of bismuth. | | Sulphuret of manganese. | | Sulphuret of antimony. | | Sulphuret of molybdenum. | | Chromate of iron. | | Oxide of iron. | | Jenite. | | Black oxide of cobalt. | | Oxidated uranium. | | Wolfram. | | Tantalite. | | Yttrio-tantalite. | | Black oxide of cerium. |

| Class IV.—Containing Substances which acquire resinous electricity by friction. The insulating property is limited to the very transparent varieties. | |---| | Ruby silver. | | Sulphuret of mercury. | | Red oxide ore. | | Oligist iron ore. | | Sulphuret of arsenic. | | Titanite. | | Anatase. | | Muriate of mercury. | | Chromate of lead. | | Phosphate of lead. | | Molybdate of lead. | | Green carbonate of copper. | | Blue carbonate of copper. | | Arseniate of copper. | | Dioxypase. | | Fluoride of copper. | | Hydrate of copper. | | Sulphate of copper. | | Phosphate of iron. | | Arseniate of iron. | | Sulphate of iron. | | Sulphuret of zinc. | | Red cobalt. | | Green oxide of uranium. | | White oxide of antimony. | | Red oxide of cerium. |

As the causes which determine the production of positive or negative electricity by friction are wholly unknown, and require to be carefully investigated, we must warn the philosopher against the implicit adoption of all the preceding determinations. Different results have been obtained in many cases by different observers, and even by the same observer while using the same materials; and we could have greatly enlarged the first of the preceding tables had we inserted the opposite results of different observers. There are two points, however, which require to be attended to in such inquiries: 1st, There is a tendency to the production of negative electricity in the substance which has the least extent of surface; and, 2ndly, there is a tendency to the production of an opposite electricity when the surface of the body is even minutely scratched.

Besides the chemical nature of the bodies, the kind of Faraday's electricity produced by friction depends on various physical circumstances in the two bodies. Polish, for example, tends to produce vitreous, and heat resinous electricity. Mr Faraday obtained interesting results by rubbing together the substances in the following table in pairs.

1. Calfskin and Bearskin. 2. Flannel. 3. Ivory. 4. Quill. 5. Rock crystal. 6. Flint Glass. 7. Cotton. 8. Linen, Canvas. 9. White Silk. 10. The hand. 11. Wood. 12. Lac. 13. Metals—Iron, copper, brass, tin, silver, platinum. 14. Sulphur.

If any one of these substances is rubbed against the substance above it, it becomes negatively electrified, and if against the substance below it, the electricity is positive. Mr Faraday, however, found many exceptions to this rule. One part of a cat's skin, for example, was very negative to another part, and even to rock crystal, and different pieces of flannel also differed much from each other; a change in the mode of rubbing, too, made a great difference. A feather struck lightly against a dry canvas becomes strongly negative, while the same feather drawn with a little pressure between the folds of the same canvas will be strongly positive. When a piece of flannel is halved, and the two pieces drawn across each other, the electricity of the two will be of a different kind irregularly; or the same piece will have both electricities in different parts; or, sometimes, both pieces will be negative, in which case Mr Faraday thinks that the air must have been rendered positive and then dissipated.

When we come to describe the hydro-electric machine, we shall find that water is positive when carried through tubes of wood and metal by a stream of steam, the tubes becoming positive; and Mr Faraday thinks it will probably be found to stand above all other substances, even cats' hair and oxalate of lime.

Mr Wilson had found that a stream of air directed against a tourmaline, or glass, or resin, electrified these substances positively; but Mr Faraday has shown that no electrical effect is produced unless the air is damp, or holds dry powders in suspension. In these cases, the electricity is produced by the friction of the particles of water in the one case, and the particles of powder in the other.

In order to account for the production of electricity by friction, Dr Wollaston ascribed it to an oxidation; but this cannot be the cause of it, as electricity is produced by friction in vacuo, as was shown long ago by Stephen Gray; and what is a still more decisive objection to this theory, Gay Lussac found that electricity could be developed by friction in dry carbonic acid.

**Sect. IV.—On Electrical Conduction.**

It is obvious, from all the phenomena described in the preceding sections, that electricity is communicated from conduction. one body to another. The excited glass or wax communicate, as we have seen, their electricity to a pith ball; and the electricity of the machine is conveyed first to the prime conductor, and from that to the bells or other apparatus which have been already described. If we touch an electrified pith ball, or any other electrified body, with a rod of metal of any kind, the electricity of the pith ball will be instantly carried off; but if we touch it with glass or wax it will not be carried off. Hence metals are said to be conductors, and glass and wax non-conductors, of electricity.

Bodies vary greatly in the degree in which they conduct electricity; and many of them owe their conducting power to the water which they contain. The conducting power of any substance depends on the state of the atmosphere at the time with regard to humidity, and on the intensity of the electricity employed. The following tables of conductors and non-conductors have been collected from different authors. The bodies are placed in the order of their conducting or non-conducting power; but it is probable that this order would be greatly changed if the bodies were all submitted to a new and uniform examination.

List of Conductors.

| All metals. | River water. | |------------|-------------| | Silver. | Ice above —13° Fahr. | | Copper. | Snow. | | Lead. | Living vegetables. | | Gold. | Living animals. | | Brass. | Flame. | | Zinc. | Smoke. | | Tin. | Steam. | | Platinum. | Soluble salts. | | Palladium. | Rarefied air. | | Iron heated. | Vapour of alcohol. | | Iron cold. | Vapour of ether. | | Charcoal well burned. | Moist earths and rocks. | | Plumbago. | Anthracite. |

All the substances and minerals in the third class of Häuy's list, as given in Sect. II.

List of Non-Conductors or Electric.

| Gutta percha. | Dry paper. | |---------------|-----------| | Shell-lac. | Parchment. | | Amber. | Leather. | | Resins. | Air, and all dry gases. | | Salphur. | Baked wood. | | Wax. | Dry vegetable bodies. | | Jet. | Porcelain. | | Glass. | Dry marble, and Siliceous and argillaceous stones in Class I of Häuy's list. | | Vitreifications. | Camphor. | | Mica. | Coesitehene. | | Diamond. | Lycopodium. | | Transparent gems, and All the minerals in Class I of Häuy's list. | Dry chalk. | | Raw silk. | Lime. | | Bleached silk. | Phosphorus. | | Dried silk. | Ice at 0° Fahr. | | Wool. | Ashes of animal bodies. | | Hair. | Ashes of vegetable bodies. |

Glass of soda is the best conductor, and glass of potash the worst. M. Mathiessen found a species of glass which was absolutely incapable of receiving any electric charge, and he found it to be a glass of soda almost entirely free of potash. The conducting power of glass increases with its temperature. M. Buff has found from very accurate experiments that the glass of tubes used for chemical experiments, which is a good insulator, had its insulating power increased by heat nearly as in the following table:

| Temp. Cent. | Insulating power. | |-------------|-------------------| | 200 | 2882 | | 250 | 168 | | 300 | 17 |

When made in large quantities this substance preserves the form of the sheet of cotton wool from which it is made. When it is well dried a cloud of sparks will be produced by drawing it through the hand.

Oil, the heaviest being the best conductors. Gun cotton. Phenomena and Laws.

The most perfect non-conductors of electricity are also called insulators, from their power of insulating an electrified body, or preventing any of its electricity from escaping along its support. It is to Coulomb that we owe the useful discovery that shell-lac is the most perfect of all insulators; and hence it is of great value in electrical inquiries. Coulomb found that the electricity of a pith ball five or six lines in diameter could be completely insulated by a cylinder of sealing-wax or gum-lac about half a line in diameter, and eighteen or twenty lines long; that a very fine silk thread, penetrated and covered with melted wax so as to form a cylinder one-fourth of a line in diameter, had the same insulating power when its length was five or six inches; and that an equal degree of insulation could not be obtained by a fine thread of glass five or six inches long, or by a hair or a fibre of silk, unless the electricity insulated was very weak, or the air very dry. Coulomb found also that the density of electricity insulated by a fibre of gum-lac was ten times as great as that which could be insulated by a silk fibre of the same length and diameter; and he established the following general law, that the densities of electricity insulated by different lengths of fine cylindrical fibres, such as those of gum-lac, hair, silk, &c., vary as the square root of the lengths of the fibre.

In examining whether or not positive and negative electricity were conveyed with equal facility by conducting tion of bodies, M. Erman found that there were some bodies which allowed completely obstructed the passage of one kind of electricity, while they afforded a ready passage to the other. As this result, however, was obtained by weak galvanic electricity, the question is still open to examination in reference to ordinary electricity.

Although some bodies are said to be perfect non-conductors, yet this is not strictly true. A strong electrical charge can be made to pass through a thin film of the worst conductor. Dr Ritchie found that electricity permeated thin balls of blown glass; and though in one case he found that a small invisible aperture had been made in the glass, yet in other experiments he could not by any known method detect the smallest perforation.

It has been long known that imperfect conductors have influence their conducting power increased by heat; gases, charcoal, heat in glass, ice, and resins when melted, are proofs of this. Dr promoting Ritchie, on the authority of some accurate experiments, is of opinion, that if the body be naturally a pretty good conductor, the ratio of its conducting power will not be so much increased by heat as in the case of a less perfect conductor. Mariamini found this to be true with fluid conductors, and Dr Ritchie thinks that it is universally true.

An interesting paper on this subject by Sir Snow Harris, will be found in the Edinburgh Transactions for 1832; in which the author describes a new electrometer, See plate and measures the degree of heat excited in metallic bodies CCXXVII by voltaic electricity. The results of his inquiries are—figs. 1 & 2.

"That for certain given small forces the differences in the conducting powers of the several metals vanish, each metal being equally efficient. 2d. That differences in conducting power become more apparent within a certain limit, as the force of the battery increases;" the exact proportions in which the differences increase with the increased power the author had not accurately ascertained. In reasoning on the several experimental results arrived at, our author traces the source of these phenomena to certain unknown relations subsisting between the causes of heat and electricity, and is led to adopt the principle advocated by Sir H. Davy, that the excitation of heat in the metal diminishes its conducting power. He found by several striking experiments that the electrometer became most sensibly affected by changes of temperature in the wire transmitting the charge, and that whether by the common means of heat or cold directly applied to it, or otherwise by means of an electrical current; so that it does not appear to be of any consequence how the heat is derived by which the conducting power is diminished. Hence it follows that the heat excited in a metallic body during the time of its conduction would tend to impede the transmission of the electrical current. In certain cases, therefore, of electricity of low tension, the heat excited in different metals may be so small as not to interfere with the transmission of the current, and they may all appear to conduct equally well in that particular case. With respect to currents differing in intensity, he found that when the force was very considerable, the best conductor became the most heated, from the circumstance of its permitting a larger quantity of electricity to pass through it. When the force of the battery became less, then the inferior conductor became the most heated; because it is now capable of transmitting the whole current.

There is however a contingency attendant on experiments of this kind, which, as observed by Sir Snow Harris in his inquiries into the Elementary Laws of Electricity, demands very special attention. The author has shown, in the course of these investigations, that rarefied air does not restrain the electrical discharge to the same extent as dense air. He found that a given accumulation on coated glass would strike through twice the distance in air of half the density; and, further, that an electrical accumulation was transmitted with much greater facility over a wire when the atmospheric pressure was removed from its surface; so much so, that an extremely fine steel wire became beautifully luminous in a closely exhausted receiver, in transmitting without fusion a powerful accumulation which in the atmosphere would dissipate the metal in red-hot balls. When an electrical discharge therefore is caused to pass over a heated surface in the atmosphere, as for example, that of iron heated to redness, we have to take into account the necessarily rarefied state of the air in contact with the metal, and which may permit a free passage along its surface, thereby more than compensating for any diminished power of conduction in the metal itself. Unless therefore such experiments be conducted in a space devoid of air, or nearly so, they can not be well received in opposition to a large class of well established facts, and must be always more or less inconclusive.

A question arises here, bearing on the relations between heat and electricity, of no small interest to these branches of physical science. Is the source of heat material, and if a material element, is heat a conductor or insulator of ordinary electrical action? The following are some singularly interesting experiments relative to this question, by Sir Snow Harris, communicated to the Royal Society in 1834. Having examined the laws of electrical discharge of high tension in air varying in density, and having fully shown that the resistance of the atmosphere to the passage of free electricity is not greater through one discharging distance than through another, and in no case greater than the existing atmospheric pressure; he proceeds to examine the influence of an atmosphere of variable density and temperature in arresting the progress of electrical discharge; and he finds, 1. That the quantity requisite to force a given interval, varies in a simple ratio of the density, so that when the density was only one-half as great, the discharge occurred with one-half the quantity accumulated. 2. That the distance through which a given accumulation could discharge was in a simple inverse ratio of the density. Thus, as already observed, in air of half the density the discharge from a given accumulation occurred at twice the distance. Having determined this, the author endeavours to find whether the influence of heat was such as to impair or augment the insulating power of the air, which he does by forcibly retaining a given volume of air within a glass receiver, and causing given electrical accumulations to discharge through it, under very considerable variations of its temperature, viz., from 50 to 300 degrees of Fahrenheit.

The manipulation and apparatus by which all these experiments were effected may be thus briefly explained:—used by Sir Snow Harris.

Two smooth balls of brass of about an inch and a half in diameter were directly opposed to each other within a large glass receiver connected with an effective air-pump. These balls could be set at given measured distances between their nearest points, by means of a sliding brass rod passing through an air-tight collar and glass plate on the top of the receiver acted on by a micrometer screw and index. The air-pump was fitted with a long mercurial gage and a delicate Fahrenheit thermometer placed within the receiver. Given quantities of electricity, measured by the unit jar (Plate CCXXIX, fig. 17), were accumulated on coated glass, the thermo-electrometer (Plate CCXXXVII, figs. 1 and 2) being used for measuring the effect of the discharge occasionally placed CCXXXVII. in the circuit, and discharges caused to pass between the figs. 1 & 2. brass balls within the receiver under a variety of different circumstances. The temperature of the air contained within the receiver was raised at pleasure, by means of an external metallic envelope and a lamp, so contrived as to be removed at pleasure without disturbing the fixed pieces, which, under the ordinary pressure of the atmosphere without, resisted the expansive power of the air contained within the receiver, when the volume was required to be the same under different temperatures, and at the density of the external air.

As a preliminary experiment, a given quantity of electricity was accumulated and discharged between the brass balls within the receiver in air of constant temperature, but varying in density. By diminishing the density, the distance of discharge within the receiver could be extremely increased. The results of thirty successive experiments give an invariable effect on the wire of the thermo-electrometer, at whatever distance the discharge passed between the brass balls; showing that the resistance to the passing shock must have been in each case alike. In order to effect discharges between the balls in the receiver within the limit at which the accumulated electricity could force a passage, the battery was discharged externally to the receiver by a drop ball falling with force on a small plate of varnished glass resting on an opposed ball, in connection with the positive coating; thus impeding the discharge of the battery up to a given instant. The experiments being now arranged solely with reference to heat, an accumulated quantity was effected adequate to discharge over a certain distance between the balls in the receiver, in a volume of air of a noted temperature. This being determined, and the volume fixed by closing the stop-stock connected with the receiver, the temperature within was varied from 50° to 300° of Fahrenheit, but without in any way affecting the result. The discharge of the battery invariably occurred when the same quantity had accumulated, or very nearly. The experiments were sufficiently approximate to show that the heat had in no degree affected the restraining or insulating power of the air.

---

1 Phil. Trans. for 1834, p. 230. 2 On some Elementary Laws of Electricity, First Series. The heated air was now permitted to expand, by opening the stop-cock under the receiver, and allowing an escape through the long mercurial gage from beneath the surface of the mercury below. When full expansion had taken place, the cock was again closed. This done, the quantity of accumulated charge in the battery now requisite to pass between the balls in the receiver was again determined, which, although necessarily reduced, was found still constant through all succeeding depressions of the temperature, from 280° down to the temperature of the external air. When this was reached, the cock was again opened so as to allow the mercury to rise in the gage, by which the final density could be pretty well ascertained. The comparative quantitative accumulations were then found to be as the diminished density, or very nearly so.

The author describes several other forms of these experiments on the power of heated air to restrain electrical discharges, but the results were found the same. The insulating power of the air was found quite independent of the temperature, and to have reference only to the condition of density.

The author thinks that we may fairly conclude from these experiments—

1. That heated air is not, as frequently stated, a conductor of electricity; and that heat in no way facilitates electrical discharge through air, except by affecting its density.

2. Supposing the cause of heat to be a material element, it must necessarily be an electric or non-conductor; because the incorporation of a conducting with a non-conducting substance is always found to impair its insulating property—as for example in the case of air charged with free vapour; whereas in the intimate union of two non-conductors the insulating power is in no way impaired. Since, then, heat does not impair the insulating property of a given volume of air, heat, if a substance, should therefore be an electric, and have insulating properties.

The converse of this reasoning furnishes additional evidence of this. It is a well-known fact, as we shall hereafter find, that the excitation of heat in good conductors, such as the metals, greatly impairs their conductivity; a result invariably attendant on the mixture of a conducting with an insulating substance, and even evinced on amalgamating a good conducting metal with a metal low in the scale of conducting power.

It appears from some recent experiments made by Professor Delarive of Geneva, that the degree of conductivity of bodies for electricity depends on the quantity of electricity which traverses them. Hence it follows, that, of two conducting bodies, that which is the most perfect for an electric current of a given intensity may be the worst conductor for either a stronger or a weaker current. The conducting powers of bodies, therefore, ought to be re-examined in reference to electric currents of different intensities; and when such experiments are made with accuracy, we may expect that they will lead to great improvements in our electrical apparatus.

Much light has been recently thrown on the conducting power of bodies by the researches of Dr Faraday, of which we have already given a general account in our history of electricity. He found that a great number of solid bodies which were incapable of conducting electricity of low tension, acquired by liquefaction or fusion the power of conducting it in a very high degree. The following is a list of the bodies which possessed this property:

- Water. - Oxides: Potassa, protoxide of lead, glass of antimony, protoxide of antimony, oxide of bismuth. - Chlorides of potassium, sodium, barium, strontium, magnesium, manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver. - Iodides of potassium, zinc, and lead; protiodide of tin, protoperiodide of mercury, fluoride of potassium, cyanide of potassium, sulpho-cyanide of potassium. - Salts: Chlorate of potass; nitrates of potassa, soda, baryta, strontia, lead, copper, and silver; sulphates of soda and lead; proto-sulphate of mercury; phosphates of potassa, soda, lead, copper, phosphoric glass, or acid phosphate of lime; carbonates of potassa and soda, mingled and separate; borax, borate of lead, perborate of tin; chromate of potassa, bichromate of potassa; chromate of lead; acetate of potassa. - Sulphurates: Sulphuret of antimony, sulphuret of potassium made by reducing sulphate of potassa by hydrogen, ordinary sulphate of potassa. - Silicated potassa; chameleon mineral.

In those substances which soften before they liquefy, Dr Faraday found it highly interesting to watch the increase of conducting power as they approached to perfect fluidity. When borate of lead, for example, is heated by the lamp upon glass, it becomes as soft as treacle, without gaining the power of conduction; and it was only when brought to a fair red heat by the blowpipe that it conducted. When it was quite liquid, it conducted with extreme facility.

The following bodies were found by Dr Faraday to acquire no conducting power when they assumed the liquid state:

| Substance | Conducting Power | |--------------------|------------------| | Sulphur | Adipocere | | Phosphorus | Stearine of coco-nut oil | | Loddle of sulphur | Spermaceti | | Protiodide of tin | Camphor | | Orpiment | Naphthaline | | Bealgar | Resin | | Glacial acetic acid| Gum sandarach | | Mixed margaric and oleic acids | Shell-lac | | Artificial camphor | Perchloride of tin | | Caffeine | Chloride of arsenic | | Sugar | Hydrated do |

Boracic acid and green bottle glass raised to the highest heat by an oxyhydrogen flame acquired no conducting power. Flint glass conducted a little when so heated.

When a solid becomes fluid it loses almost wholly its power of conducting heat, and gains in a high degree that of conducting electricity, and vice versa; and hence Dr Faraday concludes that there is a natural dependence between the two classes of facts.

Dr Faraday concludes his very interesting researches on this subject with the following summary of conditions of conduction in bodies, which, though they apply chiefly to voltaic electricity, are yet true within certain limits for ordinary electricity.

1. All bodies conduct electricity in the same manner, from metals to lac and gases, but in different degrees. 2. Conducting power is in some bodies powerfully increased by heat, and in others diminished, yet without our perceiving any accompanying essential electrical difference, either in the bodies or in the changes occasioned by the electricity conducted. 3. A number of bodies insulating electricity of low intensity when solid, conduct it very freely when fluid, and are then decomposed by it. 4. There are many fluid bodies which do not sensibly conduct electricity of this low intensity; there are some which conduct it and are not decomposed, nor is fluidity essential to decomposition. 5. There is but one body yet discovered (periodide of mercury) which, insulating a voltaic current when solid, and conducting it when fluid, is not decomposed in the latter case. 6. There is no strict electrical distinction of conductors which can as yet be drawn between bodies supposed to be elementary and those known to be compounds.

The experiments of Dr Faraday with ice, in which it appeared that electricity of exalted intensity passed through it, while it completely stopped voltaic electricity, confirms the observations of M. Delaville on the relation between the conducting power and the quantity of electricity which traverses the conductor; and the phenomena seem to indicate that the electric fluid or matter may consist, like solar light, of different parts possessing different powers of conductivity and other properties, which may facilitate or obstruct their passage through solid, fluid, or gaseous bodies. An electric current, composed of different currents, may have some of its component currents entirely stopped by some bodies, while other currents are transmitted with the greatest facility, in the same way as certain rays both of light and heat are entirely absorbed by coloured bodies, while other rays are copiously transmitted. Non-conductors, like black bodies, stop every electrical current. Perfect conductors, like colourless transparent bodies, may transmit every electrical current, or absorb a small portion of all of them in an equal degree; while there may be imperfect conductors, which, like coloured bodies, stop some currents and transmit others. If this should prove correct, two bodies which, when used separately, conduct electricity, would be insulators when joined so as to transmit the electricity in succession, in the same manner as two transparent coloured bodies which separately transmit light copiously are opaque when combined, the light which each transmits being absorbed by the other.

We have already seen that electricity was conveyed through a distance of four miles. On the ground that these experiments were made imperfectly, and that an electric charge will prefer a short passage through air to a passage of twenty or thirty feet through thin wire, Mr Singers has expressed his conviction that the results of the experiments referred to are incorrect. We are unable, we confess, to appreciate the reasons on which this opinion is founded; but, even if they have any force, the original fact has been more than confirmed by Mr F. Ronalds, who erected at Hammersmith an electrical telegraph on which the inflections of the wire composed one continuous length of more than eight miles. "When a Canton's pith-ball electrometer was connected with each extremity of this wire, and it was charged by a Leyden jar, both electrometers appeared to diverge suddenly at the same moment; and when the wire was discharged by being touched with the hand, both electrometers appeared to collapse as suddenly. When any person took a shock through the whole length of wire, and the shock was compelled to pass also through two insulated inflammable air pistols, one connected with each extremity of the wire, the shock and the explosion seemed to occur simultaneously. But when the shock was compelled to pass through the gas pistols, and any one closed his eyes, it was impossible to distinguish more than one explosion, although both pistols were discharged. When people did not look at the pistols, and when I sometimes charged only one highly, and sometimes both lowly, they could never guess, except by mere chance, whether one or both were fired. Thus, then, three of the senses, namely, sight, feeling, and hearing, seemed to receive absolute conviction of the instantaneous transmission of electrical signs through my pistols, my eight miles of wire, and my own proper person."

These results, interesting as they were some years ago, through telegraphic wires and submarine cable, sink into insignificance compared with the extraordinary facts with which the operations of the electric telegraph have made us acquainted. The transmission of electricity through hundreds of miles of wire passing through the atmosphere, and through 300 miles of submarine wires stretching from Dover to Ostend, are facts familiar to everybody. Experiments have not yet been made to ascertain the distance to which a given electric force will pass through wires of different lengths and diameters, when insulated and uninsulated; but from the opportunities which we now possess from the extension of the telegraphic system, we may expect that very interesting results will ere long be obtained. The completion of the electric current, too, through immense distances in the ground and in water—as first noticed through distances of a few miles by Sir William Watson—is a remarkable fact which requires farther investigation in reference to Mr Lindsay's proposal of a transmarine telegraph.

Various attempts have been made to measure or rather velocity of to estimate the velocity of electricity in passing along a conducting wire. Sir William Watson considered it to be less than any measurable portion of time in passing through a distance of 12,276 feet or 2½ miles, or as he expressed it, that it passed instantaneously through such a length of wire. The first attempt to measure the velocity of electricity by any accurate method, was made by Mr Wheatstone, who, by an ingenious apparatus, concluded that it moved along a copper wire at the rate of 288,000 miles in a second. According to the experiments of MM. Fizeau and Gouillet, it moves through a similar wire at the rate of 112,080 miles per second, and through an iron wire at the rate of only 62,000 miles. Professor Mitchell of Cincinnati found its velocity to be only 28,500 miles in a second; and Professor Walker of the United States only 16,000 miles through an iron wire. Along the copper wire between Greenwich and Edinburgh, its velocity was only about 6500 miles per second; and along the copper wire (a great part of which was plunged in water), between Greenwich and Brussels, only 2300 miles in a second. Dr Faraday ascribes the enormous differences in these measures, to a certain extent, to the influence of the conducting bodies in contact with the wire; and he is of opinion that the velocity may vary more than the hundredth of its velocity according as the electricity passes through a wire immersed in water, or through one suspended at a great distance from the ground, or one carried along a solid conducting wall. These experiments on the electric telegraph wires confirm, in a remarkable degree, the sagacious anticipation of Dr Faraday, who announced, at the time of the publication of Wheatstone's experiments, that the velocity of electricity in the same metallic wire would vary much "with the tension or intensity of the first urging force, which tension is charge and induction. So if the two ends of the wire in Professor Wheatstone's experiment were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction, after making the contact for discharge, might be in part removed from the internal portion of the wire at the first instant, and disposed for the moment on its surface, jointly with the air and the surrounding conductors, then I venture to anticipate that the middle part would be more retarded than before; and if these two plates were the inner and the outer coating of a large jar or a Leyden battery, then the retardation of that spark would be still greater."

These interesting anticipations have been proved by a series of fine experiments made by Dr Faraday with the means of telegraphic lines of wire between London and Manchester. Faraday's wire, which is 1400 miles long, is buried in the ground, and consists of four wires, each 350 miles long. At the Manchester station the extremities of the first and second wire were united, and also the extremities of the third and fourth. At the London station a galvanometer was attached to the end of the first wire, the ends of the second and third wire were united by a second galvanometer, and at the end of the fourth wire was attached a third galvanometer commu-

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1 Description of an Electrical Telegraph, &c., p. 4. Lond. 1823. 2 Experimental Researches, &c., vol. i., No. 1, 1833. 3 See North British Review, No. xlv., p. 547. nicating with the ground. The first galvanometer was then put in connection with one of the poles of a pile, the other pole of which communicated with the ground. The needle of the first galvanometer immediately deviated, but that of the second did not move till after a sensible interval, and that of the third a little later still. About two seconds elapsed before the electric current was propagated from the first to the third galvanometer. Upon cutting off the communication between the first galvanometer and the pile, the needle of the first galvanometer immediately returned to zero, that of the second was not displaced till after a short time, and that of the third still later. By establishing or cutting off the communication between the first galvanometer and the pile, at intervals not very distant, we may in some degree project along the wire successive electric waves, so that the three galvanometers are traversed at the same instant with three different waves. Finally, if after cutting off the communication between the pile and the first galvanometer, we connect this galvanometer with the ground, the electricity with which the wire is charged will discharge itself simultaneously by its two extremities, so that the first and the third galvanometer are traversed by electric currents in opposite directions. With a telegraph wire suspended in the air, the three galvanometers deviate from and return to zero almost exactly at the same instant.

Similar results were obtained by Dr Faraday by the beautiful experiment of using three of Bain's telegraphic apparatuses, in which messages are written chemically by lines or strokes of different lengths. When the circuit was interrupted, as before, at intervals not very distant, the apparatus nearest the pile traced a discontinuous line, composed of full and distinctly separated strokes, thus, — — — — — — produced during the intervals when there was a communication with the pile. The second and third apparatus, on the contrary, gave a line composed of full and distinct strokes united by very delicate ones, thus, — — — — — — ; which showed that the electricity took a certain time to escape from the wire. When the interruptions of the current are very near one another, the fine lines become equal to the full ones, and a continuous line — — — — — — is produced.

**Sect. V.—On the Electric Spark.**

Since the discovery of electric light by Otto Guericke and Dr Wall, the subject has attracted the particular attention of philosophers. In exciting a glass tube, or in working an electrical machine in the dark, sparks and streams of light are distinctly visible; but the phenomenon is best seen when the knuckle or a brass ball is brought near to an electrified conductor. A bright light, called the *electric spark*, passes from the conductor to the knuckle or ball, and exhibits a great variety of phenomena, varying with the nature and intensity of the electricity, and with the form, magnitude, distance, and nature of the bodies between which it passes.

The electric spark is produced by the action of the positive electricity in the conductor, upon the neutral electricity in the knuckle or ball that receives the spark, the former decomposing the latter. When the attraction between the two electricities is sufficiently powerful to overcome the resistance of the air, the two fluids on the knuckle recombine with a noise, accompanied by a brilliant spark like the forked lightning in a thunderstorm. This mode of discharging the electricity of conductors is called the *disruptive discharge*, and the distance of the bodies between which it is made is called the *striking distance*.

**Exp. 1.** Having screwed into the prime conductor a brass ball about two inches in diameter, and projecting about three inches, electrify the conductor positively, and hold another ball near the first. Long ramified zigzag sparks will pass between the two balls, as shown in fig. 6, where pos. is the positively electrified ball, and not. the one held in the hand in a natural state of electricity. If the ball on the conductor is very small, the spark will become a faint divided brush of light. If the ball on the conductor is electrified negatively, the spark will be as shown in fig. 7, clear, straight, and more luminous. If one of the balls is positively, and the other negatively electrified, the forms shown in fig. 6 and 7 will be combined, as in fig. 8. When, in this last experiment, the distance of the balls is not too great, the positive zigzag spark will strike the negative straight spark about one-third of the length of the latter from its point, the other two-thirds becoming very luminous. Sometimes the positive spark strikes the negative ball at a distance from the negative spark.

**Exp. 2.** If two conductors P; M, fig. 9, three-fourths of an inch in diameter, and having spherical ends, are placed parallel to each other, at the distance of two inches, so as to have their ends pointing in different directions six or eight inches asunder; then, if P is positively electrified, its spark will strike the other conductor M in its natural state, as in fig. 9. If M is electrified negatively, and P connected with the earth, the conductor M will send the negative spark to P, as in fig. 10; and if the conductors have opposite electricities, the positive spark will appear at one end, and the negative at the other, as shown in fig. 11.

**Exp. 3.** Upon the brass stem bc, fig. 12, having a fine Fig. 12. point at c, place a brass ball A, about three inches radius, so that the point c can be protruded to any distance beyond the ball, or be drawn within it, as shown in the figure. In this last state the point produces no effect and the zigzag spark appears between the balls.

In proportion, however, as the point is protruded, its transmitting power is increased, and it may be made to have the same effect as any ball, from the smallest size to one three inches radius. When the point projects to a particular distance, it acts as if no ball were present.

**Exp. 4.** Hold an insulated sheet of paper at a small distance from a positively electrified conductor, and a beautiful star with distinct radiations will be thrown upon the paper. If the conductor is negatively electrical, a cone of rays, with its base on the paper and its apex on the conductor, will replace the star.

**Exp. 5.** If the point of a needle is presented to a positively electrified conductor in the dark, the point will be illuminated with a star; but if the conductor is negative, the needle will exhibit a pencil or brush of light.

The following experiment illustrates the effect of distance on the spark.

**Exp. 6.** Fix a sharp-pointed wire to the end of the prime Influence conductor, and having electrified it positively, hold an un-distance insulated ball of metal very near the metallic point; a on the succession of small and brilliantly white sparks will pass spark between them. The white colour will tend to red as the distance of the ball and the point is increased, and at a certain distance the sharp explosions will cease, and a feebly violet light will diverge from the extremity of the point, covering with its base the nearest half of the sphere.

The influence of the form of the body upon the spark Influence which it gives is considerable. Professor Hildebrand found that an obtuse cone with an angle of 5° gave a much more luminous spark than one with an angle of 36°, and he found that the parabolic rounding of the summit, or slight inequalities of surface, are particularly advantageous in the production of a strong light. The influence of points on the spark has been already described.

The nature of the body by which the spark is taken Influence exercises also an influence upon its magnitude and its colour. Professor Hildebrand made some interesting experiments on this subject. The pieces of metal had a spark. conical form, and of the same shape and size. When they were fixed in the same manner at the end of an insulated conductor, the sparks which they yielded differed much in extent. The following table exhibits the results of these experiments, the metals at the head giving the greatest sparks.

| Regulus of antimony | Salpharet of copper | Lead | |---------------------|--------------------|------| | Gold | Tin | Steel | | Silver | Zinc | Tempered steel | | Brass | Iron | |

When the spark is white by taking it with a metallic body, it will under the same circumstances be violet if taken with the finger. If the spark is taken with ice or water, or a green plant, its light will be red; and if it is taken with an imperfect conductor, such as wood, the light will be emitted in faint red streams.

The medium through which the spark is transmitted exercises also a remarkable influence on its colour and form. A spark capable of passing through only half an inch in common air, will pervade six inches of the Torricellian vacuum. The apparatus used by Sir H. Davy for examining the influence of a vacuum, &c., is shown in fig. 13, where ABC is a bent glass tube, A the wire for communicating electricity, D the surface of quicksilver or fused tin for producing a vacuum, B the tube to be exhausted by the stop-cock C, after being filled by means of the same stop-cock when necessary with hydrogen, and EF the moveable tube connected with the air-pump. Sir H. Davy found, that in all cases when the mercurial vacuum was perfect, it was permeable to electricity, and rendered luminous either by the common spark or the charge of a Leyden jar. The intensity of these phenomena varied with the temperature. When the tube ABC was very hot, the electric light appeared on the vapour of the mercurial vacuum of a bright green colour, and of great density. As the temperature diminished it lost its vividness. At 20° below zero of Fahrenheit it was perceptible only in considerable darkness. When the minutest quantity of rare gas was introduced into the mercurial vacuum, the colour of the electric light changed from green to sea green, and by increasing the quantity, to blue and purple. At a low temperature the vacuum became a much better conductor. A vacuum above fused tin exhibited nearly the same phenomena. At temperatures below zero the light was yellow, and of the palest phosphorescent kind, just visible in great darkness, and not increased by heat. When the vacuum was formed by pure olive oil, and by chloride of antimony, the electric light through the vapour of the chloride was more brilliant than that through the vapour of the oil; and in the last it was more brilliant than in the vapour of mercury at common temperatures. The light was of a pure white with the chloride, and of a red inclining to purple in the oil.

Upon rarefying the air five hundred times in a glass vessel a foot long and eight inches in diameter, Mr Smeaton made the vessel revolve rapidly on a lathe, at the same time exciting it with the palm of his hand. A large quantity of lambent flame appeared under his hand, variegated with all the colours of the rainbow. Though the light was steady, every part of it was continually changing colours.

In carbonic acid gas the light of the spark is white and brilliant, and in hydrogen gas it is red and faint. When the sparks are made to pass through balls of wood or ivory they are of a crimson colour. They are yellow when taken over powdered charcoal, green over the surface of silvered leather, and purple from imperfect conductors.

The following experiments on the spark and electrical light are both instructive and entertaining.

Exp. 1. Cover a metallic wire with silk, and form it into a close flat spiral of not more than twenty-four revolutions, with the different coils in contact. When a considerable electric charge (of about two square feet of coated surface) is passed through it, a vivid light resembling that of an artificial fire-work will be seen, even in daylight, originating in the centre of the spirals. M. Nobili considers this light as electro-magnetic light, on account of its relation to the magnetic state of the spiral, and as similar to the aurora borealis.

Exp. 2. Take a bound book covered with rich gilding, and, holding it in one hand, bring it near the prime conductor. The spark will immediately shoot along the gilding in sparks or streams of green light, and will exhibit the pattern in the dark, and enable the observers to read the gilt title of the book.

Exp. 3. In the preceding experiment the letters themselves were covered with a metallic film; but if we construct an apparatus like that in fig. 14, on which the word fig. 14. light is left blank in a continuous line of narrow tinfoil pasted upon glass, and forming several parallel lines, and apply the ball B to an electrified prime conductor, the word light will be seen in the dark in luminous letters formed by the electric spark passing from one piece of tinfoil to the opposite one. Figures of all kinds may in a similar manner be delineated electrically.

Exp. 4. Another beautiful experiment, called the luminous spiral tubes, is shown in fig. 15, where a number of spirals, round pieces of tinfoil are pasted spirally upon four glass tubes a, b, c, d, fixed on a board round a central rod of glass AB, supporting horizontally a wire mn with brass balls, and capable of turning round the pivot A. Electricity by sparks the wire at A, and pushing the wire mn gently round, the ball at the top of each tube will receive electricity from a or b, and a brilliant line of light will appear to surround each ball in succession, in consequence of the spark appearing between each of the small circles of tinfoil.

Exp. 5. The luminous jar shown in fig. 16 is a still more beautiful experiment. In one which is now before us, fifty-jar, five squares of tinfoil an inch square, and each perforated fig. 16. with a hole four-tenths of an inch in diameter, are pasted in five rows on the outside and inside of a glass jar AB, fig. 16, about five inches in diameter and eleven inches high. The diagonals of the square pieces are placed horizontal and vertical, and their points or angles are separated by about one-twelfth of an inch. The rows of the tinfoil squares are similarly placed on the inside of the jar, with this difference only, that their horizontal points nearly touch one another at the centres of the circular holes of the outer squares. The brass ball A communicates with the inside squares by a wire, and when it is charged by the prime conductor, a hundred and ten sparks will be seen at once in a horizontal, and a hundred and ten in a vertical direction, when the jar is discharged.

Exp. 6. Take a glass cylinder three inches wide and three feet long, so fitted up that a brass plate may be let down from the top of the cylinder, so as to stand at any distance from another brass plate fixed at the bottom of the cylinder. When the cylinder is exhausted of air in the usual manner, and the upper plate communicates with the prime conductor, and the lower one with the ground, a brilliant sheet of light will pass from the upper to the lower plate. If the distance of the plates is ten inches, and if the charge of a Leyden jar is made to pass from the one to the other, a continuous body of the most brilliant fire will pass between them.

The course taken by the electricity in the disruptive discharge is always in the line of least resistance, or that in which there is the smallest amount of insulating power; and if a number of bodies of different degrees of conducting power lie in its path, it will find its way through some and avoid others, as if it had the power of choosing the easiest and shortest line. The annexed drawing, taken from an experiment made by Sir W. Snow Harris, is an excellent example of this phenomenon. lent illustration of this interesting property. In figure A, \(a, b, c, d, \ldots\) are detached fragments of leaf gold laid down on a sheet of paper. A charge of about ten square feet of coated glass is then passed through this interrupted circuit from P to N, and the effect produced is shown in fig. B, where the shaded parts are the portions of the leaf gold burned up, and marking the path of the electricity, or the line of least resistance, \(P, b, d, e, f, g, h, i, N\). The pieces \(e\) and \(k\) are left untouched, and also the white portions of the other pieces, while the portion \(h\) and \(N\) are wholly destroyed. This experiment finely illustrates the course taken by the lightning which strikes buildings and passes through the apartments of a house.

From ordinary electrical machines, sparks ten or twelve inches long have been obtained in rapid succession, by affixing a ball about two inches in diameter to the conductor, and projecting three or four inches from it, and then presenting to it a large ball connected with the earth or with the opposite conductor. When the discharge is made from a single jar, the striking distance is generally about an inch; but if we arrange a number of equal and similar jars, as shown in the annexed figure, where the course of the discharge is \(P, p, a, b, c, n, N\), we obtain a long striking distance between the outer coating of the last jar and the knob of the first. Professor Dove of Berlin has shown that when such a succession of positive and negative surfaces is used the length of the spark increases with the square of the number of jars. If the jars are charged separately and equally, as suggested by Mr Baggs, and placed quickly as above directed, very near one another, but so as not to be in contact, a spark of great length and brilliancy will be produced.

When the electricity is discharged by pointed conductors, Dr Faraday has given it the name of the *convective* or *carrying discharge*, which consists in the motion of charged particles either of insulating or conducting matter from place to place.

Another species of discharge, called the *conductive discharge* by Dr Faraday, takes place when the electricity is carried off by the intervening dielectric communicating their forces either rapidly as in the metals, or slowly as in spermaceti, air, glass, shell-lac, &c.

A very pretty experiment has been made to show the short duration of the light of a spark. The experiment was, we believe, first made by Mr Wheatstone by painting three unequal sectors of a circular disc with the three prismatic colours, yellow, red, and blue. When it is made to revolve rapidly round its centre, the colours are mixed and form a grayish white tint. But if we darken the room and illuminate the revolving disc with an electric spark, the three colours will be distinctly seen. During the brief duration of the flash there was not time for the colours to make a combined impression on the retina and obliterate their individual action. This very interesting effect is not owing to the velocity of the electric light, but to its short duration, and may be produced by the flash of a copper cap or even of gunpowder. If we inclose a bright light, or even a candle, in a box having an aperture in one of its sides prolonged so that it can pass instantaneously before the light, and allow it to fall on the revolving disc, the same effect will be obtained.

This experiment is much more interesting when we use the revolving discs of the phenakistoscope, or when we place upon any one of them a few large and distinct letters. If a light of short duration is thrown upon the revolving discs when every figure or word is obliterated, the figures and words will be distinctly seen.

In like manner a current of fluid or of solid particles discharged with such a velocity as to appear a continuous stream, would if illuminated in the dark be exhibited in separate and distinct portions. The same effect is produced in daylight or any other light by viewing through revolving apertures a continuous stream of this kind, or the shoot flame of a coal fire, all of which derive their continuity from the duration of the impression made on the retina by the successive action of their different portions.

**Sect. VI.—On the Nature and Origin of Electrical Light.**

Dr Wollaston seems to have been the first person who made an accurate examination of electric light, and the following is all that he has published as the result of his experiments. "When the object viewed is a blue line of electric light, I have found the spectrum to be separated into several images; but the phenomena are somewhat different from the preceding (viz. the spectrum of the blue portion of the flame of a candle). It is, however, needless to describe minutely appearances which vary according to the brilliancy of the light, and which I cannot undertake to explain." M. Biot, in speaking of electric light, remarks, "that if we look through a prism at the sparks which pass between two conductors oppositely electrified, we shall find all the colours which compose common light."

M. Fraunhofer examined the electric spark in a more philosophical manner. In order to obtain a continuous line of electrical light, he brought to within half an inch of each other two conductors, and united them by a very fine glass thread. One of the two was connected with an electrical machine, and the other communicated with the ground. In this manner the light appeared to pass continuously along the fibre of glass, which consequently formed a fine and brilliant line of light. When this luminous line was expanded by refraction, Fraunhofer saw that, in relation to the lines of its spectrum, electric light was very different both from the light of the sun and from that of a lamp. In this spectrum he met with several lines partly very clear, and one of which in the green space seemed very brilliant compared with other parts of the spectrum. He saw in the orange another line not quite so bright, which appeared to be of the same colour as that in lamp-light spectra; but in measuring its angle of refraction, he found that its light was much more strongly refracted,

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1 Faraday's Electrical Researches, vol. i. p. 418. 2 Phil. Trans., 1802. 3 Traité de Physique, tom. ii., p. 459. and nearly as much as the yellow rays of lamp light. In the red rays towards the extremity of the spectrum, he saw a line of very little brightness, and yet its light had the same degree of refrangibility as the clear line of lamp light. In the rest of the spectrum he saw other four lines sufficiently bright. In a subsequent paper read at Munich in 1823, Fraunhofer observes, that, by means of the large electrical machine in the cabinet of the Academy of Munich, he obtained a spectrum of electric light, in which he recognised a great number of light lines, and that he had determined the relative places of the lightest lines, and the ratios of their intensities. What these positions and ratios were we have no means of knowing, as we believe that this distinguished philosopher has not given them to the public.

The nature of electric light has been more recently examined by Sir David Brewster. He had long ago shown that the light of electricity was refracted singly and doubly, and polarized exactly like all other light; but his recent observations were made, like those of Fraunhofer, on the dark and luminous lines which appear in the spectrum formed from it by a prism. Fraunhofer examined the electric light produced in the manner he has described. In this species of electric light Sir David Brewster observed the lines described by Fraunhofer, and also its remarkable difference from that of the sun and a lamp in relation to the fixed lines of the spectrum. This difference he found to arise from the fact that certain colorific rays which exist in solar light do not exist in electrical light, though, in some parts of the spectrum, other rays of equal refrangibility are visible. The extreme red space, for example, is wanting, and, generally speaking, much of the red and yellow light. Hence the light of the electric spectrum is green at a line or point of a given refrangibility, where it is yellow in the solar spectrum. These facts confirm in a remarkable manner the discovery that the spectrum consists of three superimposed spectra of blue, yellow, and red light, of equal lengths, and receive from that discovery a complete explanation.

Sir David Brewster examined electric light of various colours, and produced under different circumstances, and he found it to differ in its composition in a very remarkable manner, each variety of electric light varying in the number, intensity, and position of its bright and defective rays. One species of electric light is as different from another as the coloured lights produced in the flame of alcohol in which different saline substances have been dissolved. It would require almost the lifetime of an individual to examine and make drawings of this class of phenomena while the light passes from a violet, through blue, green, yellow, and red, up to white light. The bright lines which occur in the green space have a most singular appearance. They shine, in reference to the rest, with the metallic brilliancy of silver; and each successive spark, obtained under nearly the same circumstances, will often present to us these lines under different intensities and characters.

It has been the general opinion of philosophers that the electric spark was the electric fluid itself; or, as Biot expresses it, "a modification of electricity itself, which had the faculty of becoming light at a certain degree of accumulation." This eminent writer, however, considered this opinion as erroneous, and has devoted a whole chapter to prove that electric light has the same origin as the light disengaged from air by mechanical pressure, "and that it is purely the effect of the compression produced on the air by the explosion of electricity." In order to establish this theory, M. Biot has stated, on the authority of several experiments, "that the intensity of electric light depends always on the ratio which exists between the quantity of electricity transmitted and the resistance of the medium; and he has shown, by an experiment with Kinnersley's air thermometer, "that at each spark the air of the cylinder, driven by the repulsive force, presses on the surface of mercury, which rises suddenly in the small tube, and falls back again immediately after the explosion." "This indication," he adds, "proves the separation produced between the particles of the mass of air where the electricity passes; and from what we know of its extreme velocity, it is certain that the particles exposed immediately to its shock ought in the first moment to sustain individually all the effect of the compression. They ought, then, from this cause alone to disengage light as when they are subjected to any other mechanical pressure. Thus one part at least of the electric light is necessarily due to this cause; and this being the case, there is no experiment which can lead us to conjecture that it is not all due to this cause."

These arguments, whatever may be their weight, carry no conviction to our mind. When we possess two series of accurate experiments, by which it is proved that light produced by mechanical pressure in air and gases, of different bulks, and of different degrees of temperature, rarefaction, and condensation, has the same colour, the same composition, and the same general character, as the light produced by electricity in passing through air and gases under the very same circumstances, we shall regard this theory as deserving of consideration.

M. Biot, anticipating the objection that electrical light is produced in the best vacuum, replies, that a vacuum such as we can produce is filled with vapours and gas, and that the barometric one is filled with mercurial vapour. Still, however, the light produced is not produced by air, and it should be shown that mechanical pressure is capable of eliciting light in such a vacuum; at all events, the light ought to bear some proportion to the degree of rarefaction, whereas Sir H. Davy obtained a bright light in the best vacuum with mercury, and the same light in the best vacuum with melted tin.

Anticipating another objection from the fact that the electric spark, when intense, passes through water, M. Biot gives a double reply, 1st, that the water itself is probably compressible, and therefore, we presume, gives out light during its compression; and, 2dly, that water always contains in combination a certain quantity of air, which may also contribute to the result. It would be desirable, therefore, to ascertain if water, and water with much air, give out light by mechanical compression.

In explaining the fact that electrical light is violet when the electricity is feeble, and of a brilliant whiteness when it is produced by a violent discharge, M. Biot remarks that "this variation of tints discloses still further the origin of the light; for we observe it in substances which burn according as the combustion is slow or rapid; that is to say, according as the oxygen of the air which this combustion absorbs is more or less rapidly condensed. The light which sulphur discharges when it begins to inflame is as violet as that of feeble electricity, but that of sulphur in vigorous combustion is white." The views upon which this argument is founded are themselves hypothetical. It is by no means made out that the colour passes from violet to white as the intensity of combustion increases; and, in the very case of sulphur referred to by M. Biot, Sir John Herschel has proved, that when it is inflamed in a white hot crucible, it gives out neither blue, green, nor red rays, but solely homogeneous yellow light, of a very definite refrangibility, and which contains few of the elements of white light.

A more philosophical view of the probable origin of electric light has been hinted at by Sir Humphry Davy, in Davy's theory. his paper of 1822, already quoted. "The circumstance," says he, "that the intensity of the electrical light in the mercurial vacuum diminishes as it is cooled to a certain point, when the vapour must be of infinitely small density, and is then stationary, seems strongly opposed to the idea that it (electrical light) is owing to any permanent vapour emitted constantly by the mercury. The results with tin must be regarded as more equivocal; because, as this substance cannot be boiled in vacuo, it may be always suspected to have emitted a small quantity of the rare air or gas to which it has been exposed; yet, supposing this circumstance, such gas must be at least as highly expanded as the vapour from cooled mercury, and can hardly be supposed capable of affording the dense light which the passage of the charged Leyden phial through the vacuum produces.

"When the intense heat produced by electricity is considered, and the strong attractive powers of differently electrified surfaces, and the rapidity of the changes of state, it does not seem at all improbable that the superficial particles of bodies, which, when detached by the repulsive power of heat, form vapour, may be likewise detached by electrical powers, and that they may produce luminous appearances in a vacuum, free from all other matter, by the annihilation of their opposite electrical state.

"In common cases of electrical action, the quantity of the heat generated by the annihilation of the different states depends upon the nature of the matter on which it acts; and in cases where electrical sparks are taken in fluids, vapour or gas is always generated; and in elastic fluids, the intensity of the light is always greater the denser the medium."

About the same time that Sir H. Davy was occupied with these researches, Dr Fusinieri was engaged in those beautiful experiments on electric light which have added so greatly to our knowledge of its nature and origin. The results of these experiments, which seem to have been commenced in 1821, were published in successive years. In 1825 there appeared in the Journal of Pavia a most interesting communication, of which the following is a brief abstract, relative to the transport of ponderable matter in the electrical discharges of ordinary machines; and, in 1831, another of equal importance on the transport of ponderable substances by lightning.

Dr Fusinieri has proved that the electric spark which issues from a brass conductor, and traverses air, contains brass in the state of fusion, and incandescent molecules of zinc.

When the spark issues from a globe of silver, it contains in its passage through air silver in fusion, and incandescent molecules of the same metal.

If the spark which issues from silver traverses a plate of copper, the silver which it contains passes also through the copper in perforating it, and in traversing even a space of several centimeters, if the passage is oblique from the one surface to the other.

In this passage a portion of the transported silver is detained in the aperture which is made in the copper, and another portion follows the current, and penetrates the ball which receives the electric spark.

When the electric spark issues from a ball of gold, and passes into air, it contains gold in a state of fusion, and also incandescent molecules of gold.

If the spark from gold traverses a plate of silver, the gold contained in the spark traverses the plate in piercing it; and in traversing a space of several centimeters in the silver, if the direction of the passage is oblique, a part of the transported gold remains in the silver, and spreads itself over the two surfaces of the plate, and another part accompanies the electric current, and penetrates the ball which receives the spark. The gold spread over the polished surface of the silver appears in the form of a thin circular stratum upon the surface where it enters, and upon the surface where it leaves the plate. The very same result takes place if the spark passes from brass to silver.

These strata or metallic spots are so exceedingly thin, that after a certain time they are volatilized and disappear.

Dr Fusinieri also found that in each passage of the spark there was an opposite and reciprocal transport of the two metals. In a spark from silver to copper, part of the copper is transported to the silver, as well as the silver to the copper; and the same reciprocal transfer takes place in a spark from gold to silver.

Accompanying this reciprocal transport, there are two strong and opposite percussions produced by the transported metal, one at the point where it is detached, the other at the point where it enters the other metal. These two percussions show themselves, by two opposite cavities which contain the same metal, in such a state as to indicate fusion. Here the transported metal exerts two pressures in opposite directions.

In passing from one metal to another, the electric current leaves the first metal in the second, and takes with it a small quantity of the second.

The electric spark which issues from a metal into air contains a group of molecules, the most central of which are in a state of simple fusion, and the exterior ones are in a state of greater or less combustion, from their contact with oxygen, according as the metal is more or less oxidizable; and the matter thus contained in the spark is endowed with a force of spontaneous expansion.

From these highly interesting facts Dr Fusinieri draws the following important conclusions respecting the nature and origin of electric light. 1. The electric spark is not formed by a pure fluid, or by any imperceptible fluid. 2. The heat and light of the spark proceed from the ignition and combustion of the particles of ponderable matter. 3. The presence of air produces on the spark two distinct effects, the one to hinder its free expansion in space, the other, by supplying oxygen, to promote the combustion of the exterior molecules of the group, while the central molecules are luminous from ignition and fusion alone. 4. In gases without oxygen, the material molecules which compose the spark ought to be simply in a state of incandescence and fusion, without any combustion of the exterior particles of the group, in the same manner as this takes place for the central parts of the spark in common air. 5. In gases deprived of oxygen, as well as in a vacuum, the molecules which compose the spark ought to be incandescent; that is, in a state which fits them to emit light and heat; a phenomenon of the same kind as those inflammations which chemical experiments prove to take place even without the aid of oxygen, in so great a number of other combinations, or even without there being any new combinations, by the sole effect of division of parts.

In a later memoir on the transport of ponderable substances by lightning, published in the Ann. delle Scienze del Regno Lomb. Veneto for 1831, Dr Fusinieri has shown, by a series of laborious and beautiful observations, that lightning leaves in houses and on trees traces of ferruginous and sulphurous substances which it contains; and he infers that iron exists in the clouds, having been attracted from the earth, and principally from mountains, where the mines

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1 Phil. Trans., 1822, pp. 72, 73. 2 Dr Fusinieri likewise found that the electric spark obtained between the two poles of the voltaic pile, terminated either by metals or charcoal, contains also particles of these substances extremely divided, and in a state of combustion. 3 The iron may be carried off by the lightning which issues from the ground in cases where the clouds are negatively electrified. are more abundant, and where storms generally begin to form. Hence, as Dr Fusinieri supposes, we may connect this fact with meteoric stones, and with the magnetic currents which surround the globe.

That the electric spark is a flame, and consists, like all other flames, of incandescent molecules in a state of minute subdivision, will, we think, be now admitted by every philosopher; and it cannot fail to be observed how singularly this result harmonizes with the varied composition of electric light of different kinds and colours, as ascertained by Sir David Brewster by means of prismatic analysis:—and from the comparison which he is making between the composition of electric light consisting of different ponderable substances, and that of flames in which the same ponderable substances exist in a state of incandescence, there is reason to expect that these two widely separated classes of phenomena may be strictly identified.

Sect. VII.—On the Law of Electrical Attraction and Repulsion, and the Attraction of Spheres and Planes.

It is obvious, from the simplest experiments, that the force of electrical attraction and repulsion diminishes with the distance. In the theories of Epimus and Cavendish nothing more than this simple fact has been assumed. Newton had supposed that the forces of electricity and magnetism decreased with the cube, or some higher power, of the distance. Lord Stanhope inferred, from reasonings not very conclusive, that the law was the same as that of gravity; and Dr Robison, so early as 1769, ascertained, from more than a hundred experiments, that the repulsive force diminished according to a power of the distance whose exponent was 2.06, or very nearly as the square of the distance.

The accurate determination, however, of the law of electrical attraction and repulsion was left to Coulomb. The apparatus which he employed for this purpose, and which is known by the name of the torsion balance, is represented in Plate CCXXXIV, fig. 1, where ABCD is a glass cylinder, which is covered with a plate of glass AB thirteen inches in diameter. This plate is perforated with two holes e and a, the former being intended to receive a tube of glass eG two feet high, carrying on its upper end a torsion micrometer, consisting of a graduated circle MN, an index M, and a pair of pincers, opened and shut by a ring, for holding a slender silver wire GH, whose lower end H is also grasped by a similar pair of pincers made of copper, and about a line in diameter. Through a hole in these copper pincers there passes a horizontal needle cd. This needle consists of a silk thread or a straw covered with sealing-wax, but the end of it, at d, about eighteen lines long, is a cylinder of gum-lac. It is terminated at c by a ball of the path of elder about two or three lines in diameter, and at d by a vertical plane of paper covered with turpentine. A circular band of paper EF, divided into 360°, is pasted round the cylinder on a level with the needle, and at the hole a there is introduced a small cylinder ab, the lower end of which, made of gum-lac, carries another ball b of the path of elder. The instrument is adjusted when a line passing through the centre of the silver wire GH at P passes also through the centres of the balls b and c, and points to the zero of the graduated circle EF.

Having fixed a brass pin BC, fig 2, with a large head B, into a handle of sealing-wax AC, and having electrified the ball BA, Coulomb communicated its electricity to the balls b, c. They accordingly repelled each other, and the needle cd turned round through a certain arch. By turning, however, the micrometer button in the direction NP, he twisted the wire GH, and caused it to return to its first position, and point to the zero of the scale. When this is done, the force of torsion has been made to balance the repulsive force of the two balls b, c; so that, by comparing the forces of torsion which balanced the repulsive forces at different distances of the balls, he obtained measures of the repulsive force at these distances. When the distances were 36°, 18°, and 18°, he found the angle of torsion, or the force of torsion, which is proportional to the angle, to be 36°, 144°, and 57°; that is, at half the distance the force is four times greater, and at a fourth of the distance the force is nearly eight times as great. Hence he concluded that the repulsive force of two small globes charged either with positive or negative electricity is inversely as the squares of the distances of the centres of the globes.

In applying the same method to determine the law of the attractive force which takes place between two oppositely electrified bodies, M. Coulomb met with a difficulty, arising from the attractive force increasing in a greater ratio than the force of torsion. From this cause it was difficult to prevent the balls from coming into contact, and a delay was created, during which part of the electricity had escaped. By providing against this difficulty, he obtained results which led to the conclusion that the attractive force of two small globes, one electrified positively and the other negatively, was in the inverse ratio of the squares of the distances of their centres.

In order, however, to confirm this result by an entirely different method, he employed the apparatus shown in fig. 3, where BC is a vertical stand of wood, carrying a horizontal arm of wood AB, divided into inches, upon which there slides another piece of wood ED, to which is suspended, by a fibre of silk fc, a horizontal needle of gum-lac cd, fifteen lines long, carrying at one end, and perpendicular to the needle, a circle of gilt paper d, seven lines in diameter, and at the other end a ball c of gum-lac. A globe of copper a foot in diameter, or a globe of paper covered with tinfoil, resting on four insulating cylinders of glass, coated with sealing-wax, is then placed upon a stand, so that it can be raised or depressed, and fixed in any position, its horizontal diameter passing through de.

This apparatus is adjusted by placing the globe so that, when the moveable piece E is at zero of the scale on BA, the centre of the circle d may just touch the globe. When this is done, place the piece E at three inches on the scale, so that the distance of d from the globe will be three inches, and then the distance of d from the centre of the globe will be nine inches. Let the globe be now electrified by the spark of a Leyden jar; then, if a conductor is made to touch the plate d, the globe will communicate to it the opposite electricity upon removing the conductor, and the globe and the plate will attract one another. Cause the needle cd to oscillate through an arch of about 20° or 30° from the line where the force of torsion is nothing, and observe the time in which a given number of oscillations, suppose fifteen, is performed. Repeat the very same experiment when the piece E is placed at twelve and eighteen inches on the scale; that is, when the distances of the centres of the attracting bodies are eighteen and twenty-four inches. In doing this, Coulomb obtained the following results:

| Distances of centres | Number of oscillations | Time in which they were performed | |----------------------|------------------------|----------------------------------| | 9 | 15 | 20 | | 18 | 15 | 40 | | 24 | 15 | 60 |

As the oscillations in the preceding experiments are produced by the attraction of the globe and plate d, in the same manner as the oscillations of a pendulum are produced by the force of gravity; then, since the time in which a given number of oscillations is performed is inversely as the square root of the attractive force, and if we assume that the attractive force is inversely as the squares of the distances, or 9, 18, 24, or 3, 6, 8, then it will follow that the time of oscillation is proportional to these distances. These times will consequently be 20°, 41°, and 54°, if the attractive force is inversely as the square of the distance; but by experiment the times were 20°, 40°, and 60°. The difference is, therefore, almost nothing at 18 inches of distance, but it is nearly 1/3rd at 24 inches. Coulomb has applied a correction to the number 54°, in consequence of the loss of electricity by the two bodies during the four minutes which the experiment occupied. He found by experiment that the action was diminished 1/3rd of the whole per minute, and consequently 1/3rd of the whole in four minutes. Hence, \( \sqrt{10} : \sqrt{9} = 60 : 57 \), a result which now differs only 1/3rd from 60°, the time determined by experiment. Hence it follows, that by both methods of observation, the law of action is the same for attractive as it is for repulsive forces.

We are not aware that these experiments of Coulomb have been repeated and confirmed by other philosophers. Experiments with the torsion balance and contact plane are very difficult and precarious, and it is almost impossible to estimate with accuracy the loss of electricity in the two charged conductors during the performance of the experiment.

Under these circumstances, Sir Snow Harris of Plymouth has resumed the subject, and, by new methods of observation and instruments of great accuracy, he has confirmed the law given by Coulomb, both in the case of simply electrified conductors, and in bodies upon which given quantities of electricity have been accumulated. A more particular account of these instruments, and of the method of applying them, will be given in a subsequent part of this article.

The law of the attractive force is easily obtained when the opposed surfaces are parallel planes or rings; but in the case of spherical conductors and bodies of other forms, the conditions become more complicated. Considering the distribution of the electricity on the spheres to be uniform, and the whole force exerted to be as the number of attracting points directly, and as the squares of the distances between the respective points inversely, Sir Snow Harris has shown that the forces between two spheres will be inversely as the distances between their nearest points multiplied into the distances between their centres.

In order to submit this result to the test of experiment, he used two spheres whose radius was an inch, and obtained by means of his electrical balance the following results:

| Distance of Centres of Spheres | Distance of nearest Points by experiment | Calculated Distance of the Points in each Sphere within which the Force may be supposed to be collected | Force in Grains | |-------------------------------|------------------------------------------|-------------------------------------------------|---------------| | 2.2 | 0.2 | 0.664 | 12.0 | | 2.5 | 0.5 | 1.117 | 4.25 | | 2.8 | 0.8 | 1.496 | 2.25 | | 3.0 | 1.0 | 1.732 | 1.75 |

These results confirm the law deduced from theory, and Sir Snow Harris has established its truth more completely by extending it to several new cases, the most important of which, with the deductions, are as follows:

1. Two spheres at the distances in column 1, exert the same force as two circular planes of equal areas at the distances in column 3.

2. The attractive force of two opposed conductors is not influenced by the form or disposition of the unopposed portions. The attractive force, for example, is the same, whether the opposed bodies are merely circular planes, or planes backed by hemispheres or cones. Two hemispheres also attract each other with the same force as the spheres of which they are hemispheres.

3. The force between two opposed bodies is directly as the number of attracting points, the distance being the same. Thus two circular planes of unequal diameter do not attract each other with a greater force than that of two similar areas, each equal to the lesser. In like manner, the attractive force between a ring and a circular area of the same diameter is equal to that exerted between two similar rings, each equal to the former.

4. The attractive force between a spherical segment and an opposed plane of the same curvature, is equal to that of two similar segments on each other.

Sect. VIII.—On the Dissipation of Electricity by the Contact of Air, and by Imperfect Insulation.

If we place an electrified body upon a mass of gum-lac, Dissipation which is the worst of all conductors, or the best insulator of electricity, we shall find that, in a certain time, the whole electricity of its electricity has disappeared. In like manner, if we by air suspend the same body under the same circumstances by a long fibre, or very small cylinder of gum-lac, we shall also find that in this last case the electricity will wholly disappear in a certain time; but the time in this last case will be much longer than in the first case. If we perform the same experiments in rarefied, moist, or hot air, we shall find that the electricity disappears faster than in condensed, dry, and cold air.

In all these cases the electricity is said to be dissipated; and it becomes an interesting as well as a most useful inquiry to determine the separate influence of these different causes in carrying off the electricity of electrified bodies.

The only observations which we possess on this subject we owe to the ingenuity and industry of M. Coulomb.

By means of the torsion balance he determined, in four days—two in May, one in June, and the last in July—the ratio of the electric force lost per minute to the total mean electrical force of the body, the electrical density varying in the five or six experiments which were made in each day. The following were the results:

| Date | Ratio of the Force Lost | |----------|-------------------------| | May 28 | 1/40 | | May 29 | 1/38 | | June 22 | 1/42 | | July 2 | 1/42 |

Hence, in reference to the state of the atmosphere on the days of observation, we have

| Date | Mean Ratio of Force Lost per Minute | Barometer Inches | Thermometer of Reaumur | Hygrometer of Banneux | |----------|-------------------------------------|------------------|------------------------|-----------------------| | May 28 | 1/40 | 28.3 | 15.5 | 75 | | May 29 | 1/38 | 28.4 | 15.4 | 69 | | June 22 | 1/42 | 27.11 | 15.3 | 87 | | July 2 | 1/42 | 28.2 | 15.4 | 80 | By examining the results for each day in the first of the preceding tables, it will appear that the ratio of the electric force to the whole force is a constant quantity during the same day, or when the air has the same degree of moisture. Hence it follows,

1st, That the loss of electricity is proportional, in the same state of the air, to the electrical density; from which it follows, as Coulomb has shown,

2d, That the ratio of the force lost in a minute to the total force, is double of the ratio of the loss of intensity of each body to the total density.

From a great number of experiments made with balls of different magnitudes, and when the quantity of electricity, as well as the electrical density of each ball, were very different, he found,

3d, That the ratio of the dissipation of the electric force during a minute, to the total force, is uniformly a constant quantity.

By using a globe a foot in diameter, cylinders of all lengths and magnitudes, circles of paper and of metal, &c., he found,

4th, That when the air was dry, and the degree of electricity not great, the ratio of the decrease of the electrical density to the density itself is always a constant quantity, whatever be the form or the magnitude of the electrified body.

By using pith balls, and balls of copper and sealing-wax, he found,

5th, That the law of dissipation is not influenced by the nature of the body.

It appears, from the second of the preceding tables, that the dissipation increases with the degree of moisture, as indicated by Saussure's hygrometer; and, by comparing the observations, he concluded,

6th, That the diminution of the repulsive force, or, what is the same, of the electric density, is proportional to the cube of the weight of the quantity of water dissolved in a given quantity of air. He found also,

7th, That the dissipation of electricity increases with the temperature.

In the course of these valuable researches Coulomb ascertained that there was no dissipation along the fibre which supported the electrified bodies which he employed; and he found also that there were other causes of dissipation, which produced effects of a considerable amount, and which yet remain to be discovered.

But though electricity is thus retained on the surface of bodies by the bad conducting power of air, it is not, as might have been expected, entirely dissipated in a vacuum. M. Becquerel has proved that in a vacuum so perfect that where the atmospheric pressure is only the 25th of an inch, a body preserves its electricity for ten days; and he has more recently shown in his Treatise on Electro-Chemistry by means of a gold-leaf electroscope, that if an electrified body is placed in a perfect vacuum, at a distance from objects that could act upon it electrically by influence, such a body will preserve a certain quantity of electricity for an indefinite time.

Having thus determined the laws of dissipation by the contact of air, Coulomb proceeded to inquire into the causes of dissipation along imperfectly insulating bodies. The experiments which he performed for this purpose were made on the same days with those made on the dissipation by air, so that he was able to determine by calculation the portion which was lost by aerial contact, and the portion lost by imperfect insulation.

When a highly electrified ball was suspended by a silk fibre, the dissipation of its electricity was much more rapid than it should have been by the contact of the air, and therefore a part of it was owing to the imperfect insulating power of the silk thread. But when the intensity of the electricity was diminished to a certain degree, the silk fibre was as good an insulator as the gum-lac. A cylinder of gum-lac eighteen lines long did not cease to insulate perfectly till the degree of electricity was nearly triple of that which is insulated by the silk fibre.

Coulomb likewise found, that when a silk thread, or hair, or any fine cylindrical electric, began to insulate perfectly, the electrical density of the body which was insulated was proportional to the square root of the length of the support; that is, if a silk fibre one foot long insulates perfectly when the electrical density is D, it will require a fibre four feet long to insulate perfectly when the electrical density is 2 D, or double.

M. Coulomb's experiments seem to have been made only with one kind of electricity. M. Biot, however, found that the dissipation was nearly the same, whether the insulated body was electrified negatively or positively.

Sect. IX.—On the Distribution of Electricity.

When any body is electrified by presenting it to the distributing conductor, the electricity, though it enter at one tion of part of the body, is obviously distributed over the whole electricity of it, as every part of the body gives distinct indications of its new state. It becomes an interesting inquiry, therefore, to ascertain by what powers the electricity is thus distributed over the body; to determine whether it is distributed throughout the substance of the body, or only on its surface; and to discover the laws of its local distribution, whether it exists on single bodies, on two or more equal or unequal bodies placed in contact, and on bodies of different forms.

These various topics have been treated by Coulomb with that ingenuity and sagacity which distinguish all his labours; and his torsion balance is the principal apparatus which was found necessary.

In order to determine whether electricity was distributed over conductors by a repulsive force between the particles of the electric fluid, or by some affinity or electric attraction for one body in preference to another, he found, by using a pith ball and a ball of copper, that the pith ball received exactly one-half of the electricity of the ball of copper, and that the ball of copper had no more affinity or electric attraction for the electric matter than the pith ball. This experiment was varied by using a disc or circle of iron ten lines in diameter, and a paper disc of the same size. In this case also he found that the electricity was equally distributed between the two discs; and he obtained the same result by using various other substances, and performing the experiments with a large torsion balance, with globes of five or six inches diameter.

In all experiments of this kind, the two balls must be allowed to remain a short time in contact, as several seconds elapse before an imperfectly insulating ball is capable of acquiring from the other half of its electricity. When the experiment is made with circular discs, the surface of the one must be placed symmetrically on the surface of the other.

In order to determine whether the electricity pervaded superficial the whole substance of the conductor, or was distributed on distribu its surface, Coulomb provided an electrometer, consisting of a small circle of tinsel, suspended by a fibre of gum-lac, electricity, which, when suspended in a cylinder of glass, is so extremely sensible that a force equal to the sixty thousandth part of a grain was sufficient to repel the ball of the needle through an arch of more than ninety degrees.

The conductor whose electrical state he proposed to examine was a solid cylinder of wood four inches in diameter, and pierced with several holes four lines wide and four deep. This cylinder was then supported upon an insulating stand, and electrified by sparks from a leaden jar. He then took a small circle of gilt paper called the proof plane, one and a half line in diameter, and about the eighteenth part of a line in thickness, and he insulated it at the extremity of a cylinder of gum-lac a line in diameter.

Having electrified the tinsel of his electroscope, he brought the proof plane into contact with the surface of the electrified wooden cylinder, and upon presenting the circle to the electroscope it repelled the tinsel with great force. The proof plane was then introduced into one of the holes of the cylinder, so as to come in contact with the bottom of the hole, and rest upon it. When it was taken out, without touching the sides of the hole, and presented to the electroscope, it gave no indications of electricity. In the first case, the proof plane carried off electricity from the part of the surface which it touched; but in the second case it carried off none, so that there was no electric matter in the interior of the cylinder, even at the depth of four lines.

These curious results, thus established by accurate observation, may be proved by two very pretty experiments, which have been given by Biot. Let S, fig. 4, be a spheroid of conducting matter, suspended by a perfectly insulating fibre A of gum-lac. Form two cups B, C, made of gilt paper or tinfoil, or any other conducting material, so as to fit exactly the spheroid when united, and fix to each of them an insulating handle L, L, of gum-lac. Electrify the spheroid S, and holding a cup in each hand by the handles L, L, apply them, as in the figure, to the surface of the spheroid. Upon withdrawing the cups, it will be found that they have abstracted from the spheroid S all its electricity, and that so completely, that it will not affect the most delicate electroscope, while the cups will be found to possess the same quantity of electricity which originally existed in the spheroid.

The other experiment of M. Biot is shown in fig. 5, where A B is an insulated cylinder, moveable round a horizontal axis, and which may be turned by the winch H, composed of several rods of glass. Around the cylinder there is wrapped a metallic ribbon CD, whose extremity D terminates in a semicircle, and is attached to a silk cord F. This apparatus is made to communicate with an electroscope E, composed of two linen threads carrying two pith balls. When the metallic ribbon is electrified, the balls and the threads will diverge. Upon unrolling the metallic ribbon, by pulling the silk thread F, the pith balls at E collapse, and indicate a diminution of the electrical repulsion; and if the ribbon be sufficiently long, compared with the electric charge given to the apparatus, the separation of the balls may become quite insensible; but they will again diverge, and indicate an increase of electrical intensity, if we again roll up the ribbon upon the cylinder.

Dr Faraday has demonstrated the superficial distribution of electricity by some elegant experiments. Having made a cylinder of metallic gauze, or a trellis of iron wire with meshes not very wide, he places it on a horizontal disc of metal resting on a pillar of glass. When the interior surface of the cylinder is electrified, the electricity passes to its exterior surface notwithstanding its easy communication with the inner surface. If the cylinder is so powerfully electrified as to yield vivid sparks, a mouse or any other animal placed within it will experience no shock.

The same truth is finely demonstrated by another experiment of Dr Faraday. By means of a ring of wire AB, he forms a conical muslin bag ACB, and supports it upon a glass stand BD. When the bag is electrified, its interior surface is found by the proof plane to be entirely free of electricity; but if by means of the insulated silk thread SC, we pull it outside in, it will be found that the electricity has passed to the outside of the cone. In like manner, if by means of the thread CS we pull it back to its first position, we shall find that the electricity has again passed into the outer surface of the bag.

Having thus ascertained that electricity occupies the surfaces of conductors, the next point to determine is the law of its distribution in bodies of different forms, that is, to ascertain its intensity, or the electrical density, at different points of the surface.

The following was the method used by Coulomb for this purpose: In the balance with which he made his first experiments he suspended his needle by a fine silver wire. He then took a cylinder of gum-lac, and having bent it as Plate shown at ced, he attached to it a circle of gilt paper d, five CCXXIV. or six lines in diameter, and the eighteenth part of a line fig. 6. thick. Having electrified the body whose electrical density was to be ascertained, he electrified the disc, carried by the needle by means of an insulated pin as formerly, and then touched the circle d with any part of the body where he wished to ascertain the electrical density. This circle was then placed in the balance, and the quantity of its electricity measured. Hence, as the quantity which the circle acquires by its tangential contact with the body is either the same as that of the point which it touches, or proportional to it, it became easy to ascertain the electrical density of different points of conductors, by touching those points with the circle d, and subsequently measuring its electricity.

During the time, however, which elapses between different observations, a part of the electricity will be dissipated, so that an error is necessarily introduced in comparing the electrical density of any two points a and b. In order to correct this error, he proceeds thus: Having measured the electricity in a, he then, after an interval, suppose of three minutes, measures the electricity in b. He then re-measures the electricity in a three minutes after he measured the electricity of b, and the mean of these two measures for a will be the electrical density of a at the time when that of b was measured.

By the method now described Coulomb measured the distribution of electricity on a conducting sphere, and he found that the electrical density was the same on every part of it, trinity on the globe; and in a similar manner Coulomb found that the electrical density on the middle of a cylinder is to that of its extremity as 1:00 is to 2:30; that the density of the middle cylinder, was to that of a point two inches from the extremity, as 1:00 to 1:25; and that the density in the middle was to that of a point situated in the hemisphere which terminated the cylinder, and one inch from its extremity, as 1:00 to 1:80.

Hence it appears that the electricity is very much condensed upon the two last inches at the extremity of the cylinder, and that it varies very slightly from the middle to within two inches of the extremity.

From various experiments, conducted in a way analogous to that already described, Coulomb obtained the following results relative to the manner in which the electrical fluid densities of two globes distributes itself between two globes A, B, of different diameters, after they have been placed in contact, and se-

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1 The employment of the proof plane, shown in Plate CCXXIV., fig. 6, as used by Coulomb, has been objected to by Sir Snow Harris. M. Delaville agrees with Sir Snow that in bodies of an irregular form it may not give correct results, and proposes to use a simple electroscope with a very light ball, and having the same electricity as the body to be examined. The degree of repulsion which it experiences at different points will be the measure of their electricity.—Treatise on Electricity, vol. I., p. 81. In order to explain this table, we shall take the case of two globes 64 inches and 24 inches, which were actually used by Coulomb. The small globe of 64 inches having been electrified, it was touched with the other globe of 24 inches, and when they were separated, so that the electricity of each was uniformly diffused over their surfaces, it was found that the quantity of electricity possessed by the large globe was to that possessed by the small one as 11:1 to 1; but as the surfaces of the two globes are as 14:8 to 1, a greater ratio than the other, it follows that the two globes are not charged with electricity in a ratio as great as that of their surfaces; that is, a given area on the small globe contains a greater quantity of electricity, or has a greater electrical density, than the same area in the large globe. The electrical densities in the third column are therefore found by dividing the ratio of their surfaces by the ratio of the quantities of fluid which they contain, and the quotients will be the ratio of the densities given in the third column. Thus, in the present case, \( \frac{14}{8} = \frac{11}{1} = 13333 \), the electrical density of the small globe 64 inches in diameter, that of the large one of 24 inches being 1.

Such is the electrical state of two electrified globes when placed at a distance. It now becomes a curious point to ascertain how the electricity is distributed when one or more equal or unequal globes are in contact. When two equal globes are in contact, the thickness of the stratum of electricity, if it varies in thickness, or the electrical density if it is equally thick, is nothing at the point of contact, but increases from the point of contact equally in different azimuths to the opposite point of the globes, where it is a maximum. This law of increase varies with the ratio of the diameters of the globe.

In the case of two equal globes, the electrical densities at different distances from the point of contact were as follows:

| Distances from the Point of Contact | Ratio of Electrical Densities | |-------------------------------------|-----------------------------| | 0° | 0 | | 20° | 0 | | 60° | 1 | | 90° | 372 | | 120° | 478 | | 150° | 5-03 |

When two unequal globes are in contact, the one being twice the size of the other, the density of the small globe was almost nothing at 30°. From 60° to 90° it increased in the ratio of 10 to 17, and from 90° to 180° in the ratio of 75 to 100.

When the one globe was four times the size of the other, the density of the small one was nothing up to 30°, from 30° to 45° it rose to 1, at 90° it was 4, and at 180° it was 572. The density of the large globe was nothing to the fourth or fifth degree from contact. From this point it increased rapidly, and from 30° to 180° it was almost uniform.

If we separate the two unequal globes, a curious phenomenon takes place. At a certain distance, which is not great, the point of the little globe which was in contact with the larger globe, and which had no electricity, now shows negative electricity till they are farther separated. At a certain distance the electricity becomes again nothing, and at a greater distance the same point becomes positive.

When the large globe is eleven inches in diameter, and the small one eight, and both positively electrified, the point of the large globe which touched the small one is always positively electrified, whatever be the distance of the two. The similar point of the small globe, however, will be negatively electrified till the distance of the two is one inch, at which distance the electricity becomes nothing, and beyond it becomes positive. If the small globe is only four inches in diameter, the same phenomena take place, but at the distance of two inches and five lines.

When six equal globes, two inches in diameter, were several placed in one line in contact, and electrified, and then examined by the torsion balance, Coulomb found that the globes' electrical density of the first was to that of the second as 148 to 100, and that of the first to that of the third as 156 to 100. When twelve similar globes were similarly placed, the density of the first was to that of the second as 150 to 100, and that of the first to that of the sixth as 170 to 100. When twenty-four similar globes were similarly placed, the electric density of the first was to that of the second as 156 to 100, and to that of the twelfth as 175 to 100. At equal distances from the extremities of the row the electric densities were equal, and the density always least in the middle.

The last series of Coulomb's experiments which we shall notice at present, are the highly important ones relative to the distribution of electricity between a globe and cylinder. When the globe was eight inches in diameter, and the cylinder thirty inches long, he obtained the following results:

| Diameter of Cylinder | Mean Electric Density of the Globe to that of the Cylinder | |----------------------|----------------------------------------------------------| | 24 lines | 1 to 150 | | 12 | ... 90 | | 2 | 1 ... 900 |

Hence the electrical densities of different cylinders are in the inverse ratio of the power \( \frac{1}{2} \)ths of their diameter, which approaches very much to unity when the diameter of the globe is very much greater than that of the cylinder.

When the globes are different, and the cylinders remain different, the electric density of the cylinders will vary as globes and the diameters of the globes, if their diameters are much greater than that of the cylinder. Hence, calling \( D \) the mean electric density of the globe, \( d \) that of the cylinder, \( R \) the radius of the globe, and \( r \) that of the cylinder, we have \( d = \frac{mDR}{r^2} \) or \( d = \frac{mDR}{r^2} \), when \( R \) is much greater than \( r \). Coulomb found the constant coefficient \( m \) to be \( \frac{9}{48} \).

Sect. X.—On the Action of Points, and on Electrical Rotations.

The influence of points in silently drawing off electricity from a conductor has already been mentioned, and also points their influence in discharging electricity from any conducting body in which they are fixed. Both these effects are distinctly seen if a person insulates himself by standing on a stool with glass feet, placed near an electrified prime conductor. If he takes in his hand a rod of metal with a ball at one end and a sharp point at the other, and holds the point at a certain distance from the conductor, he will be able to electrify himself in consequence of drawing the electricity from the conductor, whereas if he holds the ball at the same distance, he will receive no electricity at all. On the contrary, if he connect himself with the prime conductor by a chain till he is charged with electricity, and then throws aside the chain, he will not be able to discharge the electricity quickly from his body by holding out the ball, whereas if he holds out the rod with the point, the electri- city will be rapidly discharged from it, and will be seen streaming out from it in the dark.

The experiments contained in the preceding section afford a beautiful and satisfactory explanation of the action of points. We have already seen that the electricity communicated to a cylinder is so distributed that the electrical density at the extremity is 2:30, while that at the middle is 1; and that when the electrical density of a globe is 1, that of a cylinder two lines in diameter and thirty inches long is 9. But we may consider points as cylinders of small diameter and great length, and, following the result now mentioned, we shall find that the electrical density at the rounded extremity of a cylinder two lines in diameter will be $9 \times 2:3 = 20:7$, while that of the globe which the cylinder touches is only one. In order to make this plain, we have represented in fig. 7 a cylinder or rod AB, in which the ordinates of the curve McN represent the electrical density at different points of the cylinder, or the thickness of the stratum of electricity at these points. The ordinate cd being 1, the ordinates AM and BN will be 2:3. But it may be shown, from the law of repulsion, that the re-action of the electric fluid upon the adjacent air varies as the square of the thicknesses of the electric strata, or as the squares of the electric densities. Hence the squares of the ordinates cd, AM, or 1, and $2:30 \times 2:30 = 5:27$, will represent the re-action at d and A; that is, the electric fluid will have five times the tendency to escape at A, from what it has at d.

When the point A is connected with a ball B, as in fig. 8, the tendency of the electric fluid to escape at A will be seen from the ordinates of the curve BM, the ordinate at A being very great. We have already seen that the ordinate AM, or the electrical density at A, is 20:7 times as great as the electrical density at B. Hence $20:7 \times 20:7 = 428:49$ will represent the tendency of the electricity to escape from A, the tendency to escape from B being only one. But this tendency to escape is resisted by the air; and as the amount of resistance varies with the density, moisture, and temperature of the air, there will obviously be some degree of electrical density which will overcome that resistance. This result experience completely confirms, for even in the common state of the air a very great quantity of electricity is not necessary to make its way from a pointed conductor.

This tendency of points to discharge their electricity against the resisting air, enables us to perform some beautiful electrical experiments, in which a motion of rotation is effected.

Exp. 1. If one, two, or any other number of wires are placed, as in fig. 9, so as to have beneath their centre of gravity, A, a hollow cup, which rests on the top of an insulated stand AB; and if the points m, o, n, p of these wires are made short, and are turned in the same tangential direction; then, if we connect them with the prime conductor by a chain C, so as to electrify them, the electricity will issue from each point; and as it will be resisted by the air against which it presses, the arms will turn round in a direction opposite to that in which the electric fluid is discharged, in the very same manner as the rotatory motion is effected in Barker's mill. In the dark a stream of light will exhibit the discharge of the electricity, and when the velocity of rotation becomes sufficiently great, the four streams will form a beautiful circle of light.

Exp. 2. The Electrical Orrery, as it is called, is founded on the same principle. A spherical ball of metal S, fig. 10, representing the sun, has its inner concave surface supported on a pivot on the top of an insulated stand CD. From the ball S extends a wire SE, the turned-up extremity of which supports upon a pivot another ball E, which represents the earth, having a wire passing through it, and carrying at one end a small ball M, representing the moon, while the other end is bent into a sharp point m. A sharp point H is also fixed to the arm EF. If these balls are electrified as in the last experiment, by a chain A connecting them with the prime conductor, the discharge of electricity from the point H will give a rotatory motion to the arm CE and the earth E, while the electrical discharge from the point m will give a rotatory motion to the moon M round the earth E. In this manner the moon revolves round the earth, while the earth and moon are together carried round the sun.

Exp. 3. By the same principle a chime of bells may be rung in a more elegant manner than that which is exhibited in fig. 4, Plate CCXXII. Five cross arms of wire are made to revolve upon the pivot A of an insulated stand AB, as shown in fig. 11, and each wire has its extremity pointed Fig. 11, and turned in the same direction. To one of these arms C, which is purposely made longer than the rest, is suspended a glass ball or clapper b, by a silk thread ab, and immediately behind it a rod CD. Eight bells are placed upon the stand, and if a chain connects the point A with the prime conductor, the discharge of the electricity from the points will move the cross arms round, and cause the clapper b to ring the bells during its revolutions.

Exp. 4. The electrical inclined plane, shown in fig. 12, acts upon the same principle. Two straight parallel wires inclined MO, NP, are stretched upon the insulating stands M, N, O, P, fixed on a base of wood. Across these wires is placed a wire ab, having another wire cd at right angles to it, terminated by two bent points lying in a plane passing through cd, and at right angles to ab. When the apparatus is electrified by a chain, the electricity is discharged at the points a, b in a vertical plane, the wires revolve, and the wire cd rolls up the inclined plane, in opposition to the force of gravity.

Sect. XI.—Explanation of the Phenomena of Electrical Attraction and Repulsion.

In order to explain the phenomena of attraction and repulsion which have been already described, we must avail ourselves of several principles which have been either previously deduced from experiment, or which may be readily proved.

1. The electric fluid has a tendency to escape from all electrified bodies, whether conductors or non-conductors, in consequence of the mutual repulsion of its particles.

2. The electric fluid is prevented from escaping from bodies so rapidly as it would otherwise do, by the pressure of the air with which they are surrounded, and which is itself a bad conductor of electricity.

3. If the pressure of the air is increased, the escape of the electricity is diminished; and if the pressure of the air is diminished, the escape of the electricity is increased.

4. In conductors the electric fluid passes with the utmost facility and rapidity among the material particles, and does not seem to be in any way acted upon by them.

5. In non-conductors the electric fluid escapes from them, and moves among their material particles with difficulty; so that there is some force by which the electric fluid adheres to or is detained by the material particles of non-conducting bodies.

With the aid of these principles, we are now able to explain the three different cases of electrical attraction and repulsion.

1. When the two bodies are non-conductors. Let A be a fixed electrified non-conducting body, and B another of the same kind capable of moving. The particles of the electric fluid in A will repel each other; but this repulsive force cannot produce any motion on the centre of gravity of the ball, as their united tendency is to produce rest. The same is true of the repulsive force of the electric fluid. Let us suppose that A and B are both electrified positively, or both negatively, then the repulsion between the electric fluid in A and that in B will cause B to recede from A, because the electric fluid in B adhering as it were to the particles of B, cannot recede from A without taking the body along with it. In like manner, if A is positive and B negative, or vice versa, the attraction of the positive electric fluid for the negative electric fluid will cause the electric fluid in the moveable body B to approach to that in A, and by its bringing the material particles along with it, will produce the phenomena of attraction.

Hence it follows that the attractions and repulsions of non-conducting bodies are produced by the attractions and repulsions of the electric fluid, which, from its adhesion to their matter, causes them to partake in its motion.

2. When the one body is a non-conductor, and the other a conductor. Let A, fig. 14 and 15, be a fixed and non-conducting body, and B a moveable and conducting body. When these two spheres are separate, the electric fluid is distributed on the surface of each in a stratum or thin shell of equal thickness; but when they are brought near each other, the fluid is distributed as in fig. 14, when A and B are oppositely electrified, and as in fig. 15, when they are similarly electrified; the space between the dark circles and the dotted outlines representing the section of the stratum of electrified fluid upon each sphere. The arrangement of the fluid in fig. 14 is produced by the attraction of the fluid in A for the fluid in B, and vice versa, producing an accumulation of it on each sphere on the sides nearest one another; and the arrangement of the fluid in fig. 15 is produced by the repulsion of the two opposite fluids, producing an abstraction of the fluid from the sides nearest one another, and an accumulation of it on the sides farthest from each other. But since the non-conducting sphere A is fixed, the adhesion of its fluid to its material particles cannot produce any motion; and since there is no adhesion between the fluid in the conductor B and its material particles, these particles, or the body which they compose, cannot move along with the fluid. The accumulated fluid, however, at the points O, O, figs. 14 and 15, tends to escape from the spheres in virtue of the mutual repulsion of its own particles; but it is restrained by the pressure of the air, which re-acts upon it. But the pressure of the air is an uniform force on every part of the sphere; and as the force with which the electric fluid resists this uniform pressure is greatest at the sides O, O, the ball B, in fig. 14, will recede in virtue of this force from A; and the ball B, in fig. 15, will from the same cause approach to A. The attraction, therefore, of the two opposite fluids in fig. 14 produces, through the agency of the atmosphere, a repulsion of the moveable sphere; and the repulsion of the similar electric fluids in fig. 15 produces, through the same agency, an attraction of the moveable to the fixed sphere.

Hence it follows that the attractions and repulsions of two bodies, one a conductor and the other a non-conductor, are merely apparent, and are produced solely through the agency of the atmosphere.

3. When the two bodies are conductors. In this case the phenomena will be nearly the same as in the last; for, by making A a conductor, we have only removed the adhesion between its fluid and the particles of which the body is composed, a force which was not brought into play in case 2, owing to A being fixed.

In the preceding observations we have taken no notice of the decomposition of the natural electricities of the two bodies, as the reader is not yet prepared for this consideration. We have supposed one of the spheres to be fixed and the other moveable, merely to simplify our illustrations; but it is obvious that the same effects would have been produced, but only with different degrees of intensity, if the two spheres had been moveable.

In order to show that apparent attractions and repulsions may be produced by the mere resistance of the air, and without any mutual action between the particles of the two bodies which are attracted and repelled, M. Biot has employed a very happy illustration, on which we have ventured to make a slight improvement. Let B, fig. 16, be a glass globe filled with water, and suspended by a string A. Make a hole in two opposite points of it C and D, from which the water can flow, and having closed them with wax, fill the globe with water. With a burning mirror MN, whose focus is at C, condense the sun's rays RR, and melt the plug of wax at C. The water will instantly rush out, and the globe B will move away from M as if it had been repelled by the mirror. Repeat the same experiment by placing the mirror at M'N', and throwing the sun's rays upon the opposite plug D by reflexion from the plain mirror MN. The plug D being melted, the water will flow out at D, and the globe B will approach to M, N, the mirror having appeared to repel the globe in the first case, and to attract it in the second, though the motion in both cases arises neither from attractive nor repulsive forces, but merely from an unbalanced pressure at D when the water flowed out at C, and an unbalanced pressure at C when the water was discharged at D.

Sect. XII.—On Electrical Induction, or the Decomposition of the Combined Electricities by Actions at a distance.

In the preceding sections we have considered the phenomena of electricity as produced by friction, and as communicated or transmitted by conductors to other bodies. But it has been found that electricity may be developed in bodies by the mere influence of an electrified body placed at a distance, and we shall now proceed to investigate the laws which regulate this interesting class of phenomena.

Let AB be a cylindrical conductor supported horizontally upon an insulating stand S, and having hemispherical CCXXV. ends at A and B. Suspend from the points A, B, C, D, fig. 1, E, F, similar pairs of pith balls attached to wires or linen threads, and, having insulated it carefully by the stand S, touch it with the finger in order to see that it contains no free electricity. Let an electrified sphere M be now brought near it, so that A, B, M are in the same straight line, and that no spark can pass from M to B. When this has been done, it will be observed that the pith balls diverge as in the figure, the divergency being a maximum at A and B, and equal at these points, becoming less at C and D, where it is also equal, and still less at E and F, where the equality of divergence still exists. Between E and F there will be found some neutral point where the pith balls exhibit no divergence, and this point will shift its position according to the distance of the electrified body M. If we now suspend an unelectrified pith ball by a silk thread, and bring it near to different parts of the cylindrical conductor, we shall find that it is attracted to it in all places except the neutral point between E and F.

This neutral point is never found in the exact middle of the cylinder between E and F. Its position varies with the distance of the body M, and with the intensity of its charge. In every case, however, it is nearer to the extremity B next the sphere M, than the distant extremity A.

From these experiments we are led to the important and curious result, that an unelectrified body may be electrified by the influence of an electrified body acting upon it at a distance. The electricity is in this case said to be induced, and the phenomenon is called electrical induction.

If we now electrify the pith ball which was suspended by a silken thread, and bring it near to the cylinder AB, we shall find that it is attracted by one half of the cylinder. from A, for example, to the neutral point between E and F, and repelled by the other half from B to the same neutral point.

From this experiment we infer that the electricity on one half of the cylinder, from one extremity to the neutral point, is positive, while the electricity in the other half is negative.

Bring the electrified pith ball near the electrified body M, and it will be found that, if it was formerly repelled from B, it will be attracted by M, and vice versa; so that we conclude that the electricity induced upon the half of the cylinder nearest the electrified body is always opposite to that of the electrified body.

If we now measure the electricity of the body M, both before and after the preceding experiments, and make allowance for the dissipation of it through the agency of the adjacent air, we shall find that no part of its electricity has been communicated to the cylinder AB; and if, while the cylinder AB is electrified by the inductive influence of M, we either remove M to a distance, or discharge its electricity by touching it with the finger, the electricity of the cylinder AB will instantly disappear. In like manner, AB will recover its electrical state the moment that M is brought near it.

Hence it follows that the positive and negative electricities developed in a conducting body by the presence of an electrified body are not communicated to it by that body, but have existed in a state of combination in the substance of the conductor, and have only been separated from their state of combination by the action of the electrified body.

As the intensity of the positive electricity, as well as its quantity, is the same in one half of the conductor as that of the negative electricity in the other half, and as there is no remaining or free electricity in the cylinder AB when the body M is withdrawn, it follows that the union or recombination of the two electricities has neutralized or saturated each other. But as the two united electricities have not been destroyed by their union, they exist in a new state, which is called the natural electricity of bodies. The electricity, therefore, which thus naturally resides in conductors, consists of equal quantities of positive and negative electricity, which neutralize each other's action, and are consequently incapable of producing any of the phenomena of free electricity, or of a portion of positive or negative electricity existing in a separate state.

With these explanations, we are now able to understand how the cylinder AB is electrified by the influence of the electrified body M. We have clearly proved, by direct experiment, that bodies similarly electrified repel each other, while bodies oppositely electrified attract each other; and we have shown in Section X., that this repulsion and attraction does not take place between the material particles of the bodies, but between their electricities, or the electric fluids which they respectively contain. Hence we may enunciate the law in the following manner:—Similar electricities repel each other, and dissimilar electricities attract each other. Now when the sphere M, which we shall suppose to be electrified positively, is brought near the cylinder AB, in which the electricity exists in its natural or combined state, it will repel all the positive electricity, and attract all the negative electricity, overcoming the tendency which each has to diffuse itself in virtue of the mutual repulsion of its own particles, and the tendency which the two opposite electricities have to recombine by their mutual attraction. Hence all the negative electricity will be attracted to and occupy the half BF of the cylinder, and all the positive electricity will be repelled, and occupy the remoter half EA. If M is negatively electrified, the opposite effects will be produced. Let the body M be now withdrawn, the repulsive and attractive forces which it exercises upon the natural electricity of AB will cease, and the two electricities, separated by its action, will recombine by their mutual attraction, as well as by the mutual repulsion of the particles of each, and the cylinder AB will be restored to its natural state of electricity.

The principle of electrical induction which we have now illustrated enables us to give a satisfactory explanation of attraction and repulsion of bodies which have been described in Section II. It was there shown that electrified bodies attracted light and unelectrified bodies that were brought near them; but it will now appear that these apparently unelectrified bodies were first electrified by induction, and, in consequence of the decomposition of their natural electricities, were attracted by the excited body. Thus, if M (Plate CCXXXV., fig. 1), is an electrified body placed in a perfect vacuum, and AB a small light body suspended near M, fig. 1, and capable of moving towards it, then AB will be so electrified by the influence of M, that the electricity of the same name as that of M will be accumulated in the half BF of the cylinder, and the other electricity in the half EA. But the electricity of M attracts that of BF more powerfully than it repels that of EA, and consequently the light body AB will be attracted to M in consequence of the previous decomposition of its native electricity. If this decomposition cannot be effected by M, or if it takes place with difficulty, the body AB will not be attracted, or will be attracted less readily.

M. Biot has illustrated this position by the following simple Biot's experiment. Suspend by fine silk threads two small balls of periment, equal dimensions, one of them being made of pure gum-lac, and the other of gum-lac either gilt on its surface or covered with a thin plate of tinfoil. When these two balls are placed beside each other, and at a small distance, bring near them an electrified tube of glass or sealing-wax, and it will be seen that the gilt ball will be more strongly and easily attracted than the other. The uncoated ball of lac will not begin to be attracted till after a certain time, when the decomposition of its natural electricity has been effected; and thus its electrical state will continue after the removal of the electrified body. The first ball, though gilt, acquires also in this manner a permanent electricity, because the gum of which it is composed is impregnated with the electricity developed at its surface, and both of them are in this respect assisted by the contact of the air, which, under the influence of the electrified body, tends especially to carry off from them the one of the two electricities, which is repelled by this body, while it has less effect upon the other, whose proper repulsive force is concealed by attraction. Hence, says M. Biot, we observe in general, that insulated bodies which have for some time been under the influence of an electrified body, end in having an excess of electricity of a kind opposite to its own, and the effects of which are seen when they are withdrawn from the influence of that body.

In examining the action of M upon AB, fig. 1, we supposed that no change took place in the electrical condition of M; but this is not the case, for the body AB, as soon as its natural electricity has been decomposed, begins to react upon M, through the agency of its separated electricities. These separated electricities not only tend by their attractive and repulsive forces to change the distribution of the free electricity which exists in M, but also to decompose its natural electricity, and thus to increase its free electricity by one of the two separated electricities. When this change has been effected upon the electrical state of M, its action upon AB will also change. It will decompose a new quantity of the natural electricity of AB, and distribute the positive and the negative electricities of which it is composed in the halves AE, BF; and these new portions will again react upon M, till a permanent equilibrium is effected.

1 Traité de Physique, tom. ii., pp. 283-4. Supposing such an equilibrium to be established between the two bodies M and AB, we shall proceed to examine the phenomena which are produced by the introduction of a third body. For this purpose let AB represent the conducting cylinder, and M the electrified body, as in fig. 2. Let an insulated conducting body O, in its natural state of electricity, be now brought near AB, so as to touch it, and let us suppose that the electricity of M is positive, and consequently that the electricity in the half BE is negative, and that in AE positive. If we now remove the body O, and examine its electrical state, we shall find that it has acquired positive electricity, and we shall observe that the divergency of the pith balls at A has diminished, while their divergency at B has increased. If we again remove the cylinder AB from the influence of M, or remove M from it, we shall find that AB is charged with negative electricity. Previous to the contact of O with A, the positive electricity in AE repels the negative electricity in M, and attracts the negative electricity in BE. Hence it contributes by both these actions to weaken the attraction of the positive electricity in M for the negative electricity in BE, and its repulsion for the positive electricity in AE. But when, by the contact of the third conductor O with the end A of AB, we withdraw a portion of the positive electricity in AE, we at the same time increase the attraction between M and BE, and the repulsion between M and AE, by diminishing the force by which that attraction and repulsion were weakened. Hence the increased action of M will decompose an additional portion of the natural electricity of AB, drawing the negative part of it to EB, and repelling the positive part of it to AE. The electricity, therefore, which is accumulated at B or in EB is greater than that accumulated at A or in EA, because the third conductor O has taken away a part of the positive electricity in AE. Hence, when we remove AB from the influence of M, so as to allow its separated electricities to re-combine, there is an excess of negative electricity, with which of course AB will be found charged. It is therefore obvious that the divergency of the balls should be greater at B than at A, as was found to be the case from the excess of negative electricity which existed at B while the cylinder was under the influence of M.

In the experiment, as above described, the third conductor O was insulated, and could therefore carry off only a portion of the positive electricity in AE, corresponding to its size; but if we use a conductor which communicates with the ground, the whole of the free electricity in AE will escape; the pith balls at A will exhibit no divergency, while those at B will diverge still more than they did formerly; and this divergency will suffer no diminution by again touching the end A with the insulated conductor. If the conductor AB is now removed out of the influence of M, the divergency of the balls at B will be still further augmented. The cause of these phenomena is very obvious. When all the positive electricity in AE has escaped into the earth, it no longer counteracts the action of M upon BE, so that this action is augmented; and the consequence of this is, that M decomposes a fresh portion of the natural electricity of AB, the positive part of which passes off, by the mutual repulsion of its own particles, into the earth, while the resinous part is collected in BF, and increases the divergency of the balls at B. Hence, when AB is removed from the influence of M, the excess of negative electricity will be greater than previously, and the divergency of the balls at B will increase conformably with observation. The very same phenomena will be observed if the body M is charged with negative electricity, and may be described in the very same words by changing only the terms positive for negative and negative for positive.

The subject of electrical induction, on which Coulomb had thrown so much light, has been recently studied by M. Mohr, a German philosopher. Having insulated a cylinder AB, Plate CCXXV., fig. 1, 65 centimeters in length, he placed a positively electrified body M at the distance of 1 centimeter from the extremity B. In this case the neutral point was found at the distance only of 1 centimeter from B, so that the negative electricity occupied only 1 centimeter in length of the surface of the cylinder, while the positive electricity occupied 64 centimeters. By increasing the distance of M from B, or by diminishing the charge of M, the space occupied by the negative electricity would have been increased, and that occupied by the positive diminished, the former, however, being always inferior to the latter.

In making experiments of this kind, great care must be taken to prevent any of the electricity of the charged sphere M passing into the insulated conductor, either by the moisture of the intervening air, or the shortness of the intervening space.

If we connect AB with the earth, after removing it from the sphere M, it will be found charged with an excess of the electricity opposite to that of M.

In all these experiments on induction, the charged sphere M, the inducing body, suffers no loss of electricity from having exercised its inductive action.

CHAP. II.—ON THE ELECTRICITY PRODUCED BY HEAT, PRESSURE, AND SEPARATION OF PARTS.

In the preceding chapter we have given a general and popular view of the phenomena of electricity, and we have explained the remarkable phenomena of electrical induction. In our experiments and observations on these subjects we have made use of the electricity which is generated by the friction of tubes of glass or sticks of sealing-wax, or which is obtained from the common electrical machine. But electricity can be obtained from various other sources, and its properties are the same, from whatever source it is obtained, provided it is used in the same quantities and of the same intensity.

As there is no part of the science more interesting to the general reader than that which relates to the different modes in which electricity can be obtained from organized and unorganized bodies, we shall enter fully into this branch of the subject, and shall treat, in successive chapters, of the electricity produced by heat and pressure, by change of form and separation of parts, by animal bodies, and by the elements of our atmosphere.

SECT. I.—On Pyro-electricity, or the Electricity produced in Minerals by Heat.

In our history of the science we have already given a general view of the progress of discovery in this interesting branch of electricity. We shall now, therefore, proceed to describe the phenomena which are developed by heat in various minerals and artificial salts.

1. On the Pyro-electricity of Tourmaline.

The tourmaline is a very common mineral, which crystallizes in long slender prisms. Its primitive form is an oblong rhomb, the axis of which coincides with the axis of the prism. It has also one negative axis of double refraction, which is coincident with the axis of the rhomb; and it possesses some remarkable properties in reference to the absorption of common and polarized light, which will be described in another article. This mineral acquires vitreous electricity by friction; and when two tourmalines are rubbed together, the one acquires vitreous and the other resinous electricity.

In order to observe the electricity which heat develops in certain minerals, we have found it convenient to use the thin internal membrane of the *Arundo Phragmites*, which was cut with a sharp instrument into the smallest pieces; or, what is still better, the thin transparent scales which cover the buds of several plants of the genus *Pinus*, and which are pushed off at the expansion of the bud in spring. These minute fragments are well dried, and the pyro-electricity of any mineral is determined by its power of lifting one or more of these light bodies after the mineral has been exposed to heat. When we wish, however, to determine the kind of electricity which is developed in any pole of a mineral, we must employ a small instrument, called an electroscope, such as that used by Häyi, which is shown in Plate CCXXV. fig. 3, where AB is a needle of silver or brass, terminated on one side by a globule B of the same metal, and on the opposite side by a small bar or narrow plate G of transparent Iceland spar, fixed to A by wax or any other means. This needle carries at its middle point D a cup of rock-crystal (garnet is preferable), by which it rests on a steel pivot at the upper end of the piece of wire D, fixed in a cylinder E of gum-lac or sealing-wax. A small weight G is made to move along the arm BD to balance the needle in a horizontal position. In order to prepare this little instrument for observation, take the lever by the end B, with the right hand, and with two of the fingers of the left hand press two of the opposite faces of the crystal G, and then place the lever upon its pivot D. Häyi calls this apparatus a vitreous or positive electroscope.

The resinous or negative electroscope, which is shown in Plate CCXXV. fig. 4, differs from the preceding only in having a simple needle of silver or copper AB, with two globules A, B at its extremities, and having a cup C of the same metal. In order to prepare this electroscope for use, a stick of sealing-wax is rubbed with a piece of woollen cloth, and then made to touch one of the globules of the needle, which is immediately repelled.

In order to determine the kind of electricity generated in any pole of a crystal by heat, we have only to apply it to either of those electrosopes. If it attracts the globule of the vitreous electroscope, or repels that of the resinous one, its electricity will be resinous; and if it repels the globule of the vitreous one, and attracts that of the resinous one, the electricity will be vitreous.

Häyi used another apparatus in his experiments with tourmaline, which he considers preferable to all others. A rectangular plate of metal kk, bent up at right angles at its two ends k, k, is balanced on a steel needle ab by a cup of agate x, which is confined by a circle of silver and two screws z, z. Towards the extremities of the lower surface of the plate kk are fixed two silver wires pr, wy, having a slightly oblique direction, and terminated by two silver globules i, y. The use of these little balls is to lower the centre of gravity of the apparatus, so that the plate kk may always remain supported during its revolution on the pivot.

Let us now suppose that we wish to determine the two kinds of electricity which exist in the poles of a tourmaline. Take one of the Spanish crystals, which is the best for the purpose, both from their thinness and their strength, and having heated it either at the fire or at the flame of a spirit-lamp, by holding it in a pair of iron pincers with a wooden handle, place it at mn, as shown in fig. 5, in the two notches made on purpose in the bent-up pieces h, k; and having held near its poles in succession a stick of excised sealing-wax, that pole, r, will be the vitreous one which is attracted by the wax, and the other, r, the resinous one which will be repelled by it.

After measuring the intensity of the electricity in different points of the tourmaline, Häyi found that the electricity was distributed nearly in the same manner as in a cylindrical conductor electrified by induction. The vitreous electricity was a maximum near one extremity of the crystal, and gradually diminished towards the middle of the crystal, where it disappeared. Here the resinous electricity appeared very faintly, and gradually increased towards the other end of the crystal, near which it was a maximum.

If tourmaline, when rendered electrical by heat, is broken in pieces, each piece will have a vitreous and a resinous pole, whether it is broken from the vitreous or the resinous end, the extremity of the fragment always possessing the same kind of electricity as that of the pole to which it was nearest when it formed part of the crystal.

It had been early noticed that the tourmaline became electrical only at a particular temperature, and that its electricity disappeared at temperatures above and below that particular degree of heat. If we heat the tourmaline beyond this temperature, and allow it gradually to cool, it will soon arrive at that temperature (between 30° and 80° of Reaumur, 99° and 212° of Fahr, according to Æpinus) at which it exhibits its electrical properties. As the temperature falls, its electricity becomes progressively feebler, and finally disappears. Häyi, however, found that other changes take place as the cooling of the mineral increases. At a certain degree of coldness its electricity re-appeared, and gradually increased till it reached its maximum, when it again disappeared gradually. But, what was very interesting, the electricity was not the same as before; the pole which was formerly vitreous was now resinous. It is extremely probable that the same changes would continue to take place both above and below the temperatures at which these two opposite states were produced. Häyi caused the foci of two burning-glasses to fall upon the poles of a tourmaline, and he observed that, after each pole had acquired its electricity, it then ceased to act, and finally exhibited an electricity of an opposite kind.

Häyi has ingeniously explained the phenomenon of each fragment of a tourmaline having two different poles, like the crystal to which it belonged, by supposing that every integrant particle of a tourmaline is itself a little tourmaline with its two poles. "Hence it follows," says he, "that in the entire tourmaline there will be a series of poles alternately vitreous and resinous; and such are the quantities of free fluid which appertain to these different poles, that in all half of the tourmaline yet unbroken, which manifests the vitreous electricity, the vitreous poles of the integrant molecules are superior in force to the resinous poles in contact with them; while the contrary obtains in the half which manifests the resinous electricity; whence it follows that the tourmaline is in the same state (speaking generally) as if each of its halves were only solicited by quantities of vitreous or resinous fluid equal to the differences between the fluids of the neighbouring poles. Now, if the stone be cut at any place whatever, as the section can only take place between two molecules, the part detached will necessarily commence with a pole of one kind, and terminate with a pole of a contrary nature."

Mr Sievright of Meggetland fitted up a tourmaline so as to bring the action of its two poles very near each other. It resembles the letter D with an opening in its round part, the straight line representing the tourmaline, and the two bent portions are pieces of silver wire rising out of two silver cups, one of which embraces each pole of the tourmaline. If a pith ball, or a ball of sola, is suspended between the two ends of the silver wires, it will vibrate in a beautiful manner, in virtue of their opposite actions. Æpinus, it appears, fitted up the tourmaline in a manner somewhat re-

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1 This inversion of the poles was discovered by Mr Canton. On the Pyro-electricity of thin Plates of Tourmaline.

The electricity exhibited by ordinary crystals of tourmaline is very feeble; and though two good tourmalines, when floated in water upon corks, will approach and recede from each other when they are excited by a suitable temperature, yet these tourmalines are not capable of lifting one another, or of adhering to an unelectrified body, by decomposing the natural electricity of the part of it with which they are brought into contact.

A method of increasing the electrical action of tourmaline, and of enabling one large piece to lift another, and even to adhere to other bodies, has been used by Sir David Brewster. He cut thin slices out of a large crystal of tourmaline so that they had parallel faces perpendicular to the axis of the original crystal, as represented in fig. 6, where VVVV is the vitreous face of the plate, corresponding to the vitreous pole of the crystal; and RRRR the resinous face; each of these faces being perpendicular to the edge VR of the prism, and consequently to the axis of the crystal. When the two faces of the plates thus formed are ground flat and well polished, one plate will readily lift another. If we place one of these plates upon a piece of flat plate glass, placed horizontally upon a table, the tourmaline will slip off the glass if the latter is slightly inclined to the horizon. But if the glass has been previously heated, the tourmaline plate will adhere to it; and by inverting the glass, the tourmaline will adhere to it even in that position, supporting its own weight by its attraction for the glass. The intensity of the electricity may be easily measured at different temperatures, by ascertaining the angle of inclination at which the weight of the tourmaline overcomes its adhesive force. The plate which exhibits this powerful action obviously consists of an infinite number of minute crystals of tourmaline, with vitreous and resinous poles; and as the point of maximum intensity is situated near the extremity of each crystal, all the vitreous poles will be situated in a plane near the vitreous surface VVVV, while all the resinous poles will be situated near the resinous surface RRRR. If a rectangular plate of the same size, like VVRR, fig. 7, is cut out of a crystal, so that its surfaces are parallel to the axis of the prism, it will adhere to the heated glass plate with much less force than in the preceding case. These plates of excited tourmaline adhere to all metallic bodies, to wax, and to all minerals that have been tried.

If there was no mistake in the experiment by Sir H. Davy, described in a former paragraph, respecting the appearance of a flash of light in a mounted tourmaline, it will doubtless be best verified by mounting several plates of equal thickness, cut out of a broad tourmaline, placing them all in the same plane, and combining their effects in two wires. A powerful little pyro-electrical battery might thus be made, from which both a shock and a flash might be obtained.

Having found that the electricity of plates of tourmaline was more powerful than crystals of it, Sir David Brewster conceived the idea of examining its pyro-electricity, when its fragments were infinitely small, or when it was reduced to the finest powder or dust. The analogy of magnetic bodies led to the notion that the pyro-electricity would disappear, while the results obtained with short prisms in the form of plates strengthened the opposite opinion, that the pyro-electricity might even be increased by this process.

I therefore "pounded, he says, a portion of a large opaque tourmaline in a steel mortar till it was reduced to the finest dust. I then placed the powder upon a plate of glass, from which it slipped off by inclining the glass, like all other hard powders, without exhibiting any symptoms of cohesion, either with the glass or with its own particles. When the glass was heated to the proper temperature, the powder stuck to the glass; and when stirred with any dry substance, it collected in masses, and adhered powerfully to the substances with which it was stirred. This viscosity, as it were, or disposition to form clotted masses, diminished with the heat, and at the ordinary temperature of the atmosphere it recovered its usual want of coherence."

M. Becquerel made some interesting experiments on the pyro-electricity of the tourmaline. He found that when the crystal was of a certain length it became electrical both by heating and cooling; and that crystals of a greater length ceased to become electrical by heating. When the length of the crystal was eight centimeters, or three inches and one ninth of an inch long, they ceased to exhibit electricity either by heating or cooling. M. Becquerel remarks, that if this law is inversely true, that is, for very small lengths, the atoms of the tourmaline ought to acquire a considerable electrical polarity by the smallest changes of temperature.

2. On the Pyro-electricity of the Borate of Magnesia.

The electricity developed in boracite by heat is considerably less than that of the tourmaline. In 1791 the Abbé magnesia, Haïzy discovered the pyro-electricity of this mineral; but he found it extremely difficult to determine the vitreous and resinous poles. He naturally expected to find two opposite poles, as in the tourmaline; but a succession of attractions and repulsions which took place very rapidly perplexed him extremely. Considering, however, that the boracite is a cubical crystal with three axes, and the tourmaline a rhombohedral one with only one axis, he conceived that the former crystal has a vitreous pole at the one end of each axis, and a resinous pole at the other end. This conjecture he verified by experiment; and the poles were found to be so placed that each alternate pole possessed the opposite electricity; the experiments, however, which are necessary to establish this result require to be made with great care, particularly in reference to the repulsive actions, which take place only within a very limited space; so that, in order to obtain the repulsion of one of the resinous poles on a body which is itself in a resinous state of electricity, we must direct this body exactly to the repulsive point, otherwise it will be attracted towards the neighbouring points, which are in their natural state, or nearly so.

It is a curious fact, in reference to the preceding results, that the boracite has been found by Sir David Brewster to possess distinct double refraction; and consequently it cannot, as he concludes, have the cube for its primitive form, or three axes of crystallization. He infers that its primitive form is a rhombohedron of ninety degrees, the form which separates the obtuse and acute rhombohedrons; and hence it is a most remarkable circumstance that its electrical poles should be arranged in the manner described by Haïzy.

3. On the Pyro-electricity of the Topaz.

The pyro-electricity of the Brazilian topaz was discovered by Mr Canton in 1760. The Abbé Haïzy detected the same property in the topaz of Siberia, and found that the poles resided in the two opposite summits of the secondary form of the crystal. Haïzy at first thought that the Saxon topazes did not possess pyro-electricity, although they often preserved excited electricity for more than half an hour when the weather was favourable. He afterwards found, however, that they became electrical by heat if previously insulated. Sir David Brewster found pyro-electricity in the greenish-blue topazes of Aberdeenshire.

A pyro-electric rotation of a very extraordinary kind was observed by Sir David Brewster in a specimen of topaz. It presented itself when he was studying the very interesting collection of crystals in the cavity AB. This cavity is filled with the dense fluid which is frequently found in topazes, in contradistinction to the highly expansible one which often accompanies it. The circle at V represents a vacancy in the fluid, which diminishes so perceptibly by the expansion of the fluid when heated, that there is reason to believe that it would disappear by an increased degree of heat, like the vacuities in the expansible fluid. The fear, however, of bursting so rare and interesting a cavity, prevented the experiment from being made. The cavity AB contains a great number of crystals of different forms, not one of which melts with heat like some of those in other cavities, and almost all of them possess double refraction. When this cavity was first placed under the microscope there were five small crystals lying between D and the vacancy V—one a flat prism, a second a hexagonal plate, a third an amorphous crystal, and a fourth and fifth two irregular halves of a hexagon. Upon the first application of heat one or two of these crystals leapt from their places, and darted to the opposite side of the cavity. In a few seconds the others quitted their places one after another, performing the most rapid and extraordinary rotations; one crystal joined another, and at last four of them thus united revolved with such rapidity, that their respective shapes were completely effaced. They afterwards separated on the withdrawal of the heat, and took the position which their gravity assigned them. On another occasion a long flat prism performed the same rotation round its middle point; and, on showing the phenomenon to different persons, the experiment was so often repeated that the small crystals have been driven between the inclined edges of the cavity so that they cannot be extricated. A fine octahedral crystal, however, truncated in its edges and angles, was afterwards conducted into the middle at D, where it performed its rotations as indicated by the concentric circle at the right hand of D. In subsequently applying a high degree of heat the cavity burst, and scattered its microscopic contents.

Hally observed that the Siberian topazes often preserve their pyro-electricity during several hours, and sometimes from twenty to twenty-four hours.

Among some topazes which Hally received from M. Langsdorf, there was one which exhibited resinous electricity at both of its poles, and indications of vitreous electricity in the middle of the crystal. This effect was probably owing to one or more strata of cavities containing fluids, which may have interrupted the distribution of the electricity in the same manner as a fissure.

4. On the Pyro-electricity of Mesotype.

Hally discovered that some crystals only of this mineral were electrical by heat; but as he was not able to obtain complete crystals he detached from its support one about five and a half lines long, and found the pyramidal summit to be resinously electrified. Mem. Instit., tom. i., p. 54—65. Phenomena and Laws.

In the first edition of his Mineralogy, however (vol. iii., p. 168), he states that the pyramidal summit exhibits vitreous electricity by heat, and the fractured end resinous electricity; but in the second edition of his mineralogy he has omitted altogether that passage, and said nothing whatever on the subject.

The mesotype of Hally's first edition included the Auvergne mesotype, the apophyllite, the scolcite, and the nadelstein; and therefore it is difficult to say to which of these minerals his observations are applicable.

Sir David Brewster found distinct pyro-electricity in the mesotype of Auvergne.

5. Pyro-electricity of the Scolcite.

The scolcite is a compound crystal, in which the faces Scolcite of composition are parallel to the axis of the prism. Sir David Brewster found it to possess pyro-electricity, the pyramidal summit having vitreous, and the fractured end resinous electricity.

6. Pyro-electricity of Mesolite.

The mesolite, which has been separated from the scolcite by both distinct chemical and optical characters, is distinguished still further by its being composed of four simple crystals, whose faces of composition are parallel to the axis of the prism, whereas the scolcite consists of two prisms separated by a thin film or vein. Sir David Brewster likewise observed the pyro-electricity of this mineral, and found that its crystallized summit possessed vitreous electricity, and its fractured end resinous electricity, when heated.

7. On the Pyro-electricity of the Powders of Scolcite and Mesolite when deprived of their Water of Crystallization.

In the experiments above recited on the powder of tourmaline, the mineral had suffered no other change by trituration than that of being reduced to minute fragments. It and Mesolite became interesting therefore to compare the pyro-electricity of such a powder with that of the powder of a pyro-electrical mineral on which an essential chemical change had been induced. With this view Sir David Brewster reduced to powder the crystals of scolcite and mesolite, and by the application of heat drove off their water of crystallization, which is doubtless an essential ingredient in every mineral species. When the powder was exposed to a proper heat on a plate of glass, it adhered to it like the powder of tourmaline; and when stirred about by any substance whatever, it collected in masses like new-fallen snow, and adhered strongly to the body which was used to displace it. "This fact," says Sir David Brewster, "is a very instructive one, and could scarcely have been anticipated. As several minerals differ only in the quantity of their water of crystallization, the powder which was thus pyro-electrical could not be considered either as scolcite or mesolite, but as another substance not recognised in mineralogy. The pyro-electrical property, therefore, developed by the powder, cannot be regarded as a property of the minerals of which the powder formed a part, but merely as a property of some of their ingredients. In which of the ingredients, or in what combination of them, the pyro-electricity resides, may be easily determined by further experiments.

8. Pyro-electricity of Axinite.

In his Manual of the Mineralogist and the Geological Traveller, M. Braard has stated that some crystals of this mineral become electric by heat. Hally has confirmed this observation, but no accurate experiments on the position and electricity of its poles have been made.

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1 See Edinburgh Transactions, 1845, vol. xvi., part i., p. 19; or Phil. Mag., Dec. 1847, vol. xxxi., p. 497. 9. Pyro-electricity of Calamine.

So early as 1785 M. Haüy discovered the pyro-electricity of this mineral, which being an oxide of zinc, is the more remarkable, as it is the only metallic body in which this property is very distinctly developed. Haüy found that every crystallized specimen which he tried was pyro-electrical, and that it acquired this property also by cooling. His first observations on the return of the electric action were made on the crystals of oxide of zinc from Limbourg, near Aix-la-Chapelle; and a portion of the acicular variety from the Brisgau. In the winter of 1819 he placed a crystal on a window where the temperature was 11 degrees Cent. below zero, and having left it there a few seconds he found that it acted very sensibly on a magnetic needle not insulated. He next placed it in a room whose temperature was four degrees above zero, and he observed that its polar action progressively diminished and disappeared. He then brought it within a yard of a fire, and had the satisfaction of observing its polarity return, the pole which was formerly vitreous being now resinous.

10. Pyro-electricity of Sphene.

Haüy has found that some crystals of this mineral possess pyro-electricity, but he has not determined the position or nature of its poles.

11. Pyro-electricity of Prehnite.

This mineral crystallizes in right rhomboidal prisms. Haüy found it to be pyro-electrical, and that its poles are in a direction corresponding with the smaller diagonal of the crystal.

12. Pyro-electricity of other Minerals.

The property of becoming electrical by heat has been found by Sir David Brewster to exist in a great number of minerals; and he has given the following list of those in which he succeeded in detecting it:

- Calcareous spar. - Beryl yellow. - Sulphate of barytes. - Sulphate of strontites. - Carbonate of lead. - Diopside. - Fluor spar red. - Fluor spar blue. - Diamond.

13. Pyro-electricity of artificial Crystals.

In examining the physical properties of artificial crystals, Sir David Brewster found that several of them, when well dried, were electrical when heated. The following is the list of those in which he detected this property:

- Tartrate of potash and soda. - Tartaric acid. - Oxalate of ammonia. - Oxymuriate of potash. - Sulphate of magnesia and soda. - Sulphate of ammonia. - Sulphate of iron.

Dr Faraday has more recently discovered a remarkable degree of pyro-electricity in oxalate of lime. Having obtained some of this salt by precipitation, and dried it, when well washed, in a Wedgewood's basin, at a temperature of about 300° Fahr. till it was so dry as not to dim a cold plate of glass held over it, Dr Faraday remarked that, when it was stirred with a platinum spatula, it became in a few moments so strongly electrical that it could not be collected together, but flew about the dish whenever it was moved from its sides into the sand-bath. This phenomenon took place whether the salt was placed in glass, porcelain or metallic basins, or stirred with glass, porcelain, or metallic rods. When the particles were well excited and shaken on the top of a gold-leaf electrometer, the leaves diverged two or three inches. The same phenomena took place when it was cooled out of the contact of air. When it was excited in a silver capsule, and left out of contact with the air, the powder continued electrical for a great length of time, proving its very bad conducting power, in which it probably surpasses all other bodies. Dr Faraday remarks, that oxalate of lime stands at the head of all other bodies yet tried, in its power of becoming positively electrical by heat.

14. On the connection between the Pyro-electricity of Minerals and their Secondary Forms.

It is well known that the opposite and corresponding sides of crystals are similar in the number, disposition, and figure of their faces. Haüy, however, found that pyro-electrical crystals deviate from this symmetry, so that there are certain supernumerary planes at one pole which are not seen at the other. This is true of tourmaline, boracite, topaz, and axinite, and may possibly be found to be a general fact among pyro-electrical crystals, though we do not expect that it will. In the crystals above mentioned, the vitreous electricity resides in that pole where the supernumerary planes are found, and the resinous electricity in the other.

This deviation from symmetry as existing in the tourmaline is shown in fig. 8, where A is the vitreous pole at the summit of a pyramid with five planes, and B the resinous pole at the summit of a pyramid with three planes.

The deviation from symmetry in the boracite is more remarkable. The resinous pole at s, fig. 9, is marked with CXXV., one plane, while the vitreous one has the same plane s with fig. 9; other three planes rrr, fig. 10.

From the preceding facts, Haüy is of opinion that, during the formation of these crystals, the two electrical fluids have influenced, in an opposite manner, the laws by which the crystallization was regulated.

Sect. II.—On the Influence of Heat upon the Electric Fluid in Metallic Bodies.

The experiments which have been made on this subject we owe chiefly to M. Becquerel, of whose labours we shall of heat, endeavour to give a brief account. It had been long ago shown by M. Desaignes that metallic bodies are capable of electric excitation by heating and cooling. By raising the temperature of one end of a plate of silver, while the other retained the temperature of the surrounding air, he succeeded in producing contractions in a frog, by making the nerve communicate with one end of the plate, and the muscle with the other end. Other philosophers had observed the influence of heat, and they believed that it increased the repulsive force of each of the two fluids. In proof of this they sealed hermetically at a lamp a tube of glass which had been previously electrified interiorly, and by raising its temperature it gave very distinct signs of electricity. M. Becquerel, however, has remarked, that the glass, becoming a better conductor when heated, allowed a portion of the fluid accumulated in the interior of the tube to pass; so that the experiment does not prove that the electrical power of the tube was increased. In order to determine if any does take place, M. Becquerel made the following experiment.

Let AB, fig. 11, be a Leyden jar, on the surface of which Fig. 11. is fixed a conductor RS. The jar is closed by a cork gg, through which there passes a rod bb, fixed at its upper end. to a small glass receiver abc, and carrying at its opposite end a mass of metal P. When the jar has been electrified interiorly, it is placed in another vessel filled with ice, so that the conducting rod RS is without it. The cork og and the metal P having been taken out, and the mass P heated and replaced as in the figure, the iron P will gradually heat the interior of the bottle without sensibly altering the temperature of its outer surface, which is surrounded with ice. If we then present the button S to an electroscope, there will be perceived an indication of free electricity, and consequently the heat has not increased the action of the electric fluid in the interior of the jar; for if it had done so, the electricity of the exterior surface would have been decomposed, and the rod RS would have communicated to the electroscope the repelled electricity.

But though heat exerts no action on the free fluid, this is far from being true with the natural fluid. When a metallic wire, which we may call mm', or a series of metallic molecules m, m', m", &c., connected together by the force of aggregation, is connected by one of its ends with a heated body, such as a piece of red-hot glass, the moment that the heat enters it this extremity becomes positively electrical, while the negative electricity is driven to the adjacent molecules; but m' receiving the heat of m, m" that of m', &c., the second molecule, which is heated at the expense of the first, takes from this last its positive electricity, and gives to it negative electricity, and so on for all the other molecules. Hence there will arise a series of decompositions and recompositions of the natural fluids while the elevation of temperature lasts.

M. Becquerel's next experiment was to place on the upper plate of Bohmenberger's gold-leaf electroscope (taking care to avoid the contact of metals) a platinum wire whose other end is coiled into a spiral. This outer end is brought to a red heat by a spirit-lamp, which is soon withdrawn, and the spiral is then touched with a band of wet paper. After having made the lower plate communicate with the common reservoir, the small band of paper is found to have carried away positive electricity, and negative electricity remains free on the surface of the metal. If we repeat the experiment in an inverse manner, that is, if we hold between the fingers the platinum wire by the end opposite to that of the spiral, and make this last communicate when it is red hot with a band of wet paper, we shall find that the band carries away positive electricity. This result, which takes place also with gold and silver, does not depend on the electricity which is disengaged during the combustion of the alcohol, since the experiment did not commence till the lamp was withdrawn. Nor can it be ascribed to the presence of water in the band of paper, nor to the alteration of the latter by the effect of heat, two causes which are capable of producing electricity, since the same result is obtained when we carry away the positive electricity of the metal by a tube of glass brought to the same temperature as the metal.

In order to make the experiment in this way, take a glass tube of a very small diameter, and whose length is little more than half an inch, and fix to one of its ends a platinum wire one fiftieth of an inch in diameter, soldering it with a lamp. A wire of the same metal, but of a very small diameter, is fixed at the other end of the glass tube, and the largest platina wire is then put in communication with one of the plates of the condenser, avoiding the contact of metals, and the free end of the other wire is held between the fingers. A red heat is then communicated to the end of the small tube to which this last wire is fixed. As its temperature is much higher than that of the other, which is larger and more distant from the focus of heat, and as the tube becomes at the same time a conductor of electricity, the natural electricity of each wire is decomposed. According to the disposition of the apparatus, we shall have the difference of the effects, which will be to the advantage of the small wire, whose end in contact with the tube possesses the highest temperature. In order to obtain this result, it is not necessary to use a heat so high as that of a red heat. By this process we avoid every foreign cause which is capable of modifying the result.

Iron and copper give similar results; but the electric effect produced by oxidation is in this case combined with that of difference of temperature. M. Becquerel has proved that the oxidation is not the sole cause of the electricity obtained with oxidable metals; and he concludes that heat exerts over the natural electric fluid of all metals a similar action, which probably varies in intensity in different metals, according to their nature. With bismuth, tin, and antimony, the effects are scarcely sensible.

The following is Becquerel's theory of the preceding phenomenon. It is an incontestable fact that all bodies contain between their molecules a neutral electric fluid; and M. Becquerel thinks that a rise of temperature establishes round two contiguous molecules an accumulation of opposite electricities, the quantity of which is proportional to this temperature, but whose recomposition is effected without there having been an apparent separation of the two electricities. It is therefore an electrical effect of motion. When the molecules are separated, each of them takes the excess of electricity relative to the portion of electricity which surrounds it.

The influence of heat on the natural electricity of metals may be shown by means of the lamp without flame, in the two following experiments given by Becquerel. Let AB, Plate fig. 12, be a copper lamp filled with alcohol, cc a tube, COXXV and dd a cork through which there passes a glass tube EF fig. 12, covered with a varnish of gum-lac. A cotton wick passes through this tube, one end of it going into the alcohol, while to the other end there is fitted a platina spiral g, which becomes incandescent throughout as soon as its temperature is sufficiently raised. By means of this construction the platina spiral communicates with the interior of the lamp only by means of the vapour of alcohol and the wick. If we now place this apparatus on the upper plate of an excellent electroscope, whose lower plate communicates with the ground, and touch the spiral with an ordinary platina wire, it is evident that we carry off the negative electricity which the spiral takes during the combustion of the alcohol, and also the negative electricity furnished by the end of the wire which has the lowest temperature. In this case the spiral will be found to have become positively electrical. If we touch the spiral with a band of wet paper, a contrary result will be obtained; the spiral will become negatively electrical, because the incandescent metal transmits positive electricity to the wet paper, which is no doubt stronger than the negative electricity acquired by the spiral during combustion.

Sect. III.—On the Electricity produced by Pressure.

The electricity produced by pressure seems to have been first observed by Epimus. The Abbé Haiiy subsequently studied it in Iceland spar, which seems to be more susceptible of this species of excitation than any other mineral. If we take into one hand a rhomb of this mineral, holding it by two of its opposite edges, and at the same time lightly touch two of its parallel faces by two fingers of the other hand, and then bring it near to the small needle of the electroscope, it will exhibit vitreous electricity. If the two opposite planes, in place of being touched, are pressed between the fingers, a still greater degree of electricity will be developed.

M. Haiiy has observed this property of becoming positively electrical by pressure in topaz, especially the variety which is colourless, cuclase, arragonite, fluor-spar, and car- bonate of lead, all of them substances which are capable of being mechanically cleaved into smooth laminae. The experiments are always most successful with pure and transparent specimens. Sulphate of lime and sulphate of barytes do not evolve electricity by pressure.

In all the minerals above named which furnish positive electricity by pressure, positive electricity is also produced by friction; and in those substances which develop resinous electricity by pressure, such as a properly shaped piece of elastic bitumen, resinous electricity is also produced by friction. Hence it has been inferred, that in pressing minerals friction is produced, and that the preceding phenomena are only those of excitation by friction.

M. Libes, however, has stated a fact which appears to be hostile to this explanation of the phenomena. He took a metallic disc insulated by a glass handle, and having pressed it on the surface of varnished silk, either when single or several times folded, the disc acquired resinous and the silk vitreous electricity, and the quantity of electricity increased with the pressure. In order to ascertain if friction was a remote cause of these effects, he set the disc lightly down upon the silk, and rubbing it backwards and forwards so as to produce the effects of friction, the disc became extremely and the silk resinously electrified, a result the very opposite to that which was produced by pressure.

This curious subject has been recently examined with much attention and success by M. Becquerel. Having constructed an apparatus for compressing two bodies with a given quantity of pressure, and also an electrical balance of Coulomb, whose platinum torsion wire is sufficiently fine to compare very small electric forces, M. Becquerel sought to determine the phenomena which took place when two bodies were placed under the action of a given pressure and then quickly separated. He found that the excess of electricity acquired by each body was proportional to the pressure as long as it was not great enough to disorganize the body; but if the two bodies are exposed to a certain pressure, and if this pressure is reduced to one half without changing the contact, the effect of the pressure lost subsists during a time which depends on the degree of conducting power, so that if we immediately withdraw the bodies from compression, each of them will carry off an excess of the opposite electricity greater than that due to the remaining pressure. In place, however, of separating the bodies when the pressure has been diminished, let the pressure taken away be restored, and let this mode of action be several times repeated, the following results will be obtained:

Let a very thin disc of cork be pressed against a plate of Iceland spar with a weight of four kilogrammes; without changing the contact, let this pressure be reduced to one half, and after a minute let the bodies be separated. The tension or intensity of the electricity of each disc is represented by 170. When the separation took place during the whole pressure of four kilogrammes, the intensity would have been 250; and during a pressure of two kilogrammes it would have been 125, or one half. Hence it appears, that in the first case the effect produced by the pressure which was lost still subsisted in part, for it would only have been 125 for two kilogrammes, in place of 170, as given by experiment.

In place of separating the bodies when the pressure has been reduced from four to two kilogrammes, let the pressure of two kilogrammes which was removed be restored, and let us repeat several times the alternate action of simple and double pressures; it will then be found that the disc of each never possesses a greater electrical intensity than 250 relative to the strongest pressure. From these results M. Becquerel draws the following conclusions: first, that the electricity developed by pressure is proportional to the pressure; and, second, that when the molecules have been compressed, the effect of the pressure lost will subsist for some time, even though the contact has not ceased to subsist. This is not the case with conducting bodies, seeing that the two electricities disengaged instantly recombine whenever the pressure ceases.

The following are some of the numerical results obtained by M. Becquerel:

| Cork pressed against | Pressures | Intensity of Electricity | |---------------------|-----------|--------------------------| | Iceland spar | | | | Polished crystals of sulphate of barytes | | | | Polished quartz | | |

When two insulated discs, one of cork and the other of caoutchouc, are pressed against each other, the cork after pressure is negatively electrical, and the caoutchouc positively electrical. When the cork is pressed against the skin of an orange, the cork is positive and the skin negative.

When cork is pressed against Iceland spar, sulphate of lime, fluor spar, sulphate of barytes, the cork is negative and the minerals positive; but when cork is pressed against kyanite, retinasphaltum, pit-coal, amber, zinc, silver, &c., the cork is positive, and the minerals or metals negative.

When insulated cork is pressed against any part of the animal body free from moisture, the cork receives an excess of negative electricity. The hair and down of animals produce nearly as much electricity by pressure as Iceland spar, but of the opposite kind. Cork pressed lightly against inspissated oil of turpentine is negatively electrified.

When two discs of the same substances, such as skin or amadou, are pressed against each other, the one becomes negative and the other positive.

The electricity thus developed by pressure is lasting. Hally found it to continue eleven days with Iceland spar. Sulphate of barytes of Royat parts with it instantly unless well insulated; but a well insulated crystal retains it half an hour. The duration of the electricity seems to be inversely as the conducting power. Becquerel supposes the internal surface of the body to be, like the Leyden jar, charged with the opposite electricity; so that dissipation is prevented by the action of the two electricities.

In these phenomena the electricity never appears till the bodies are separated.

When the temperature of any body is raised, it has the greatest tendency to acquire negative electricity by friction. In like manner, by heating Iceland spar, it may be made to give negative electricity by pressure against cork. If we cut a piece of well-dried cork into two pieces by a very sharp knife, and press the cut surfaces against each other, no electricity is developed; but if one of the pieces is heated slightly near the flame of a candle, and the pressure applied, each surface will, when separated, exhibit opposite electricities. The same is true of two pieces of Iceland spar.

Sect. IV.—On the Electricity produced by Cleavage and Separation of Parts.

It has been long known that electricity is produced during the violent disruption of a body, or by tearing it asunder, or by separating a laminated body, or by breaking a body across, or by crushing it, or even by cutting it into parts. Mr Bennet observed that when an unannealed glass tear, or Prince Rupert's drop, was put upon a book, it electrified the book negatively. Mr Wilson noticed that if a piece of wood, when dry and warm, is rent asunder, one of the separated surfaces becomes vitreously and the other resinously electrified. When a stick of sealing-wax is broken across, one of the surfaces of fracture is vitreously and the other resinously electrified.

The electricity developed by the bursting of a Prince Rupert's or unannealed glass drop was found by Sir David Brewster to be accompanied with a flash of light. "These drops," says he, "have three different cleavages, one like the lines of a melon diverging from the apex of the drop, another concentric with the surface of the drop, and another oblique to the axis. Having laid one of these drops upon a table in a dark room, and covered it with a plate of thick glass to prevent any of the fragments from reaching the eye, the drop was burst by breaking off a part of its tail, and the whole of it appeared luminous, so that at the instant of the fracture a quantity of faint light, of the same shape and size of the drop itself, was distinctly visible. The drop which gave this singular result was made of flint glass, and was the largest that he had ever seen. Every other flint glass drop produced a distinct electrical light; but in none of them except the large one could he see the luminous shape of the drop. The same light appeared when they were burst under water. The small glass drops made of bottle-glass never exhibited any light at the moment of bursting; but it was almost always visible, in small sparks, in bottle-glass drops of a larger size." The same author observed also a bright electric light when the water-proof cloth manufactured by Charles Mackintosh, Esq., was separated by tearing it into its two component pieces, which are united by a thin film of caoutchouc. He found also that the same light was produced by tearing quickly cotton and other cloths, and by separating the films of mica. The same effects are produced by breaking barley-sugar or sugar-candy.

When the plates of mica, or the laminae of sulphate of lime, are quickly separated, each of the two plates, when separated, carry off an excess of the opposite electricities, the one being vitreously and the other resinously electrified. If these two plates are again placed together in the position which they occupied previous to their separation, and a slight pressure used to make them adhere, M. Becquerel found that the same phenomena took place as at the instant of their first separation, that is, each plate took the same kind of electricity. This property continued only a few moments, perhaps till the molecules had taken their ordinary state of equilibrium, which is aided by increasing their temperature. The effects above described he found to be more distinct in proportion as the crystal was more heated previous to the cleavage.

The electrical phenomena produced by cleavage, and by tearing asunder and crushing bodies, differ in degree only from those produced by pressure, as in every case of a separation of parts there must be an approximation of the molecules in one direction. If we press, for example, a piece of caoutchouc in one direction, or draw it out in an opposite direction till it breaks, the effect of both these mechanical actions is an approximation of the molecules in the same direction. Hence the electrical phenomena are nearly the same. The light produced by the collision of hard bodies, or by the separation of the parts of bodies, is no doubt produced by the rapid recombination of the two electricities when developed at the points of pressure.

A very curious phenomenon was observed by Sir David Brewster during his numerous experiments on the cleavage of topazes, in which there were cavities containing very highly expansible fluids. His practice was to make the cleavage plane pass through a fluid cavity, and thus to open the cavity and allow its contents to be seen and examined. When this was done, the most expansible of the two fluids flowed from the cavity upon the polished and electrified face of cleavage, and continued to expand and contract itself alternately, now collecting itself into a drop, and then expanding itself into a flat disc. These motions continued till the fluid evaporated; and the effect was no doubt owing to the electricity produced by evaporation, as well as to that produced by cleavage.

The experiments of Mr Wilson on the electricity of wood shavings belong, to a certain extent, to the present shavings section. Having had occasion to work very dry wood that had lain for several hours over a very large fire, he observed the shavings adhering to the tools and to everything that they came in contact with. When the dry wood was scraped with a piece of window glass, the shavings were always vitreously electrified; but when it was chipped with a knife, the electricity of the chips was vitreous when the wood was hot and the knife not very sharp, but resinous when the wood was perfectly cold. The electricity of the knife was always opposite to that of the chips. The surface of the shaved or chipped wood was seldom electrified, but when it was, the electricity was very feeble, and of the same kind as the weakest of the other two. The wood used in these experiments was beach and cherry tree.

Sect. V.—On the Electricity of Sifted Powders.

As it has not been determined whether the electricity sifted produced by the falling of sifted powders arises from friction, pressure, or separation of parts, we have thought it best to describe them in a separate section.

In 1786 Mr Bennet observed that when powdered chalk Bennet's was blown from a pair of bellows upon the cap of his gold-leaf electrometer, vitreous electricity was produced when the cap was six inches from the pipe of the bellows, and resinous electricity when the distance of the pipe was three feet. The vitreous electricity first produced was changed to resinous by breaking the stream of air in the bellows-pipe with a bunch of wire, silk, or feathers, or by removing the pipe so as to make air issue in a wide stream.

When the plate which receives the powders at a distance of three inches was moistened or oiled, Mr Bennet found that the electricity was opposite to that produced when the plate was dry.

When powdered chalk fell from one plate to another placed upon the electrometer, resinous electricity was produced; and Mr Bennet obtained the same result when he used red ochre, yellow rosin, coal ashes, black lead, powdered quicklime, powdered sulphur, flowers of sulphur, sand, rust of iron, or iron filings.

When powdered chalk was placed on a metal plate upon the cap of the electrometer, and blown away with the mouth or bellows, it produced permanent vitreous electricity; and the same result is obtained if the chalk is merely blown over the plate, or if a chalk is drawn over a brush placed on the plate.

When chalk, or other powders, was sifted upon the cap of the electrometer, resinous electricity was produced; but when the instrument was placed in a dusty road, and the dust excited by a stick fell upon the cap, vitreous electricity was developed.

The most accurate experiments on the electricity of powders were made by Mr Singer. The following results were obtained by sifting the powders on the cap of a delicate Mr Singer's electrometer, through sieves of hair, flannel, or muslin, the sieve being cleaned after every experiment.

See Edinburgh Transactions, 1823, vol. x., p. 27. The following bodies produced negative electricity:

- The metals. - The earths. - Oxides. - Acids. - Ascar. - Sulphur. - Silica. - Alumina.

The following bodies produced positive electricity:

- Wheat flour. - Oatmeal. - Lycopodium. - Quassia. - Powdered cardamom.

Mr Singer obtained the following results by bringing an insulated copper plate repeatedly in contact with extensive surfaces of powders spread upon a dry sheet of paper; the copper plate being brought in contact with the condenser after every repetition of the touching, until a sufficient charge was communicated. Very distinct effects were produced with the alkalies by contact with a copper or silver plate, an experiment which had failed in the hands of Sir H. Davy. The pure alkali being broken into small pieces, was exposed in an open phial for a quarter of an hour to a moderate heat not sufficient to fuse the alkali. It was then reduced quickly to a powder in a dry and warm mortar, and distributed instantly over a dry sheet of card paper, which for some time continued to attract moisture from the alkali as rapidly as the alkali absorbed it from the air. The whole operation was performed as rapidly as possible. The following tables contain the substances that gave positive and negative electricity, the copper plate being always electrified oppositely to the powders.

**Electricity Positive**

- Lime. - Barytes. - Strontites. - Magnesia. - Pure soda. - Pure potash. - Compounded pearl ashes. - Carbonate of potash. - Carbonate of soda. - Tartaric acid.

**Electricity Negative**

- Benzoic acid. - Boracic acid. - Oxalic acid. - Citric acid. - Silica. - Alumina. - Carbonate of ammonia. - Sulphur. - Rosin.

From the preceding experiments, which were several times repeated with uniform results, Mr Singer infers that they are unfavourable to the idea of natural electric energy; and he considers the result with sulphur and resin, viz., that the electricity is similar to that produced by their friction, as almost establishing the opinion that the contact of dissimilar bodies is in general the primary source of electrical excitation.

**CHAP. III.—ON THE ELECTRICITY PRODUCED BY CHANGE OF FORM.**

It has been long ago observed that electricity is developed when bodies change their form, or pass from one state into another. This important fact is exhibited when melted bodies pass from the fluid into the solid state, when fluids are converted into vapour, and when bodies are decomposed by combustion. The phenomena exhibited in these cases of change of form are very interesting, and will be described in the following sections.

**Sect. I.—On the Electricity developed during the Melting and Cooling of Resinous Bodies.**

In our history of electricity we have already given a general account of the experiments by which Mr Stephen Gray discovered a method of developing electricity by the fusion and cooling of resinous bodies. In his nineteenth experiment he formed a large cone of stone sulphur of thirty ounces avoidupois, by melting the sulphur in a tall glass. The cone began to attract bodies two hours after it was taken out of the glass, and the glass itself exhibited a feeble attractive power. When the sulphur was lifted out of the glass on the following day, its attractive force was very strong, and that of the glass imperceptible. In making these experiments Mr Gray had occasion to place the cone of sulphur on its base between the two windows of his chamber, and to invert the glass over it. Whenever the glass was removed from the cone of sulphur, it exhibited electrical attraction as strongly as the cone, and they both preserved the property for several weeks. The glass, however, at last attracted at a less distance than the sulphur, that is, its attractive force diminished most quickly.

These interesting inquiries were resumed by Mr Willeke Experiment of Rostoch, who gave the name of spontaneous to the development of cooled resins. He found that the sulphur acquired a strong electricity whether the glass in which it was fused was insulated or not; but it was always stronger when the vessels were not placed on electrics, and strongest when the glass vessel had a metallic coating. The electricity of the glass was always positive, and the sulphur negative. The electricity of the sulphur did not appear till it began to cool and contract, and it was a maximum at its point of greatest contraction. At this time the electricity of the glass was a minimum, having previously reached its maximum at the time when the sulphur was shaken out of it. Melted sealing-wax becomes negatively electrical when poured into glass, and positively electrical when poured into sulphur. Sealing-wax poured into a vessel of baked wood showed negative, and the wood positive electricity. When sulphur was poured into wood it was negative, but it acquired no electricity whatever when poured into sulphur or rough glass.

Epinus pursued this subject by melting the sulphur in Of Epinus metallic dishes. The sulphur and the dish showed no electrical signs when they were cooled, but the moment they were separated the electricity of each was very strong; that of the dish being always negative, and that of the sulphur positive. The electricity invariably disappeared when the sulphur was replaced in its dish, and reappeared upon their separation.

If the electricity was abstracted either from the sulphur or from the dish when they were separated, they both exhibited, when re-united, the electricity which had not been taken away, and which always existed on the surface of the sulphur.

Mr Sanders, a maker of chocolate, having observed that the chocolate exhibited electricity during its cooling, communicated the fact to Mr Henley, who having previously repeated the experiments of Mr Gray, resumed the subject. From several experiments made by Mr Sanders under his direction, he found that by heating the chocolate over and over again, the electrical property gradually disappeared; and that it could at any time be restored by the addition of a small quantity of olive oil.

The most elaborate series of experiments on this subject were made by MM. Van Marum and Van Troostwyck. The substances which they employed were sulphur, sealing-wax, gum-lac softened with rosin, rosin, pitch, and wax. These substances were all poured when in a fluid state on the surface of mercury, and all of them, except the sulphur, were electrical after their removal from the metallic surface. These soft solids were next melted in insulated vessels of baked clay, and also in linen and gauze insulated by silk cords; but though Volta's condenser was employed, no proof could be obtained that they had lost any portion of their natural quantity of electricity.

In order to verify the suspicion that friction was the source of electricity owing to friction. of the electricity generated in the melting and cooling of soft solids, they poured them upon copper, tin, lead, glass, and porcelain, and they invariably found that they acquired the same kind of electricity as if they had been rubbed by the body on which they were poured. In confirmation of this opinion they found that the lower surface of each plate was much more strongly electrified than the upper one, and no difference of effect was perceived when the plates were even one inch and a half thick. To obtain still more complete evidence of this conclusion, they melted gum-lac and rosin, and having suspended plates of copper by silk cords, they caused the plates to come into contact with the melted gum, without producing any friction. After the gum was cooled, and the plates again raised, not a trace of electricity could be discovered.

From these results their authors infer that the electricity exhibited in this class of phenomena is not produced either by the separation of the fused substance from the electric on which it is melted, or by the fusion or subsequent cooling of the body, but that it is generated by the friction which the particles of the electric bodies undergo when they disperse themselves over the surfaces of the dishes into which they are poured. The electricity thus produced is masked or counterbalanced by the opposite electricity acquired by the dish, and therefore does not appear till the one is separated from the other.

The electricity produced during the congelation of glacial sulphuric acid and other substances has probably a similar origin; and it is likely that the electrical effects which are observed when calomel fixes itself by sublimation to the upper part of a glass vessel, may belong to the same class of facts. This branch of the subject, however, has been but very imperfectly studied, and will form a fine topic of research for some young and active philosopher.

Sect. II.—On the Electricity developed during Evaporation and the Extrication of Gases.

The development of electricity during the transition of bodies from the solid or fluid state into the state of vapours or gases, was first investigated by MM. Lavoisier and Laplace, with the assistance of M. Volta. Two kinds of apparatus were used in these experiments. In both of them the bodies to be vaporized were insulated by varnished supports of glass; and in those cases where the electricity was quickly disengaged, a common electroscope communicating with the body was used to indicate it, whereas, when the effect was likely to take place place continuously, Volta's condenser was employed.

When hydrogen gas was rapidly disengaged from iron filings by the action of sulphuric acid, the condenser of Volta afforded a strong spark, and the electricity was negative.

When carbolic acid gas was evolved from powdered chalk, no sensible spark was educed, but the electricity generated was negative.

When nitrous acid diluted with two parts of water was poured upon iron filings placed in six separate vessels, so as to generate nitrous gas, a distinct negative electricity was obtained without a spark.

During the combustion of charcoal in three insulated clashing dishes strong negative electricity was generated; and a spark could easily have been obtained, by increasing the quantity of charcoal.

Having arranged three insulated furnaces of hammered iron, and made them communicate with the electroscope, water was thrown upon them when heated. In the first experiment the electricity generated was negative, and in the other two positive; a discrepancy which they ascribed to the cooling which accompanied the evaporation, the positive electricity produced by cooling being supposed to counterbalance the negative effect occasioned by evaporation.

Mr Bennet, before whom Volta had repeated his experiments in England, published in the Phil. Trans. for 1787 the following interesting facts on the same subject. Having placed a metallic cup with a red-hot coal in it upon the cap of his gold-leaf electrometer, he threw a spoonful of water Bennet's into the cup. The cup was electrified negatively, while the ascending column of vapour exhibited positive electricity. When water is poured through an insulated culender, containing hot coals, the descending drops of water are negatively, and the ascending vapour positively electrified; and Mr Bennet regards this as a good illustration of the electricities of fogs and rain. A more simple and certain method of making these experiments consists, according to Mr Bennet, in heating the small end of a long tobacco pipe, and pouring water into the heat. The water, being allowed to run through the heated end, is suddenly expanded into steam, and, when projected upon the cap of the electrometer, exhibits signs of electricity. If the pipe, when fixed in a cleft stick, is fixed on one electrometer, while the steam is received upon the cap of another, the two opposite electricities will be simultaneously exhibited. The vapour of alcohol and ether exhibits the same phenomena as that of water, but sulphuric acid and oil generate only smoke, and exhibit no electrical indications.

M. Saussure devoted much attention to this interesting branch of electricity. He confirmed the general results obtained by Volta, Lavoisier, and Laplace, and proved that negative electricity was constantly produced by the evaporation of water. He then determined the degree and kind of electricity produced by evaporation when it was carried on in vessels of different metals, and kept at different temperatures. The apparatus which he employed consisted of a well-baked vessel of clay, four inches in diameter and fifteen lines thick, which was insulated upon a clean and dry goblet of glass. Upon this clay vessel he placed a crucible, or any other dish powerfully heated; and this crucible was made to communicate with the electrometer by means of a wire. Fifty-four grains of distilled water were then thrown upon the crucible, and, by means of a time-piece and an electrometer, he observed the duration of the evaporation, and the intensity and character of the electricity.

In his first series of experiments the crucible was of iron; the number of projections of the water varied from 1 to 21, the time of the projection from 0° to 17°, the duration of the evaporation from 2½° to 118°, and the degree of electricity from 1 to 18 tenths of a line. In ten of these experiments the electricity was positive, and in six negative. In four of the negative experiments the strongest electricity was 7, 13, 17, and 18 tenths of a line, and in four of the positive experiments the strongest was 3, 5, and 8 tenths; thus showing, as might have been thought, that the weak positive electricity was produced by some secondary cause.

But in repeating the same series of experiments with the same iron crucible, he found very different results. The projections of water varied from 1 to 23, their time from 0° to 14° 10', and the duration of the evaporation from 2½° to 120°. The electricity was now always positive, and its intensity varied from 0 to 30 tenths of a line.

When the experiment was repeated with a copper crucible 3½ inches wide at top, 2 inches wide at bottom, 3 inches high, and weighing 57 ounces, the electricity was always positive, and its intensity varied from 0 to 33 tenths of a line, the maximum effect taking place when the duration of evaporation was 165°, a mean between the shortest and longest times. In another experiment with the same copper crucible, made under the very same circumstances, the electricity was negative at the end of the first projection, but afterwards became positive, and continued so till the experiment was complete.

In the next experiment the crucible was of pure silver, 2¼ inches wide at top, 1½ at bottom, 12½ inches high, 1½

line thick, and weighing 16 ounces. At the first trial, when the evaporation was very slow, the electricity, which was always very feeble, was thrice negative, and thrice 0. In a second trial it was also negative at first, but it became positive afterwards, and then vanished. In a third trial the electricity was stronger and negative. The balls of the electrometer now diverged $3\frac{1}{2}$ lines. It then became positive, when the balls diverged $\frac{1}{2}$ths of a line; and at the third projection, when it was still positive, the separation of the balls was so great as six lines.

Saussure's next experiment was made with a cup of white porcelain, surrounded with sand in a clay crucible. The electricity was negative, and the evaporation remarkably rapid. Its intensity varied from 0 to 8 lines. The same results were obtained with different porcelain crucibles.

When alcohol and ether were substituted for water, and the silver crucible used, the electricity was negative. With the former the greatest intensity was 1 line, and with the latter 4'2 lines.

From these experiments Saussure infers, with great hesitation, that the electricity is positive with those bodies which are capable of decomposing water, or of being themselves decomposed by their contact with water; and that it is negative with those which are not decomposed. He ascribed the result with silver to its being adulterated with copper or other oxidizable metals. The negative electricity of burning charcoal he supposes to arise from the readiness with which it loses its heat in contact with water.

Saussure was unable to procure electricity either from combustion or by suddenly exploding heaps of gunpowder; and all his attempts failed to develop electricity, without ebullition, by evaporation, from large surfaces of wet linen or white iron.

M. Cavallo followed Saussure in this inquiry, though he does not seem to have been acquainted with the labours of the Swiss philosopher. He found that evaporation from iron produced negative electricity when the iron was free from rust, but positive when it was very rusty. He found also that white and clear flint glass produced positive, while bottle glass evolved negative electricity. From these various researches it is not easy to deduce anything like a general principle. The subject indeed requires to be resumed, and great attention paid to the chemical changes which take place during the progress of the experiments.

Sect. III.—On the Electricity developed in Flame and Combustion.

We have already seen in the preceding section, that MM. Lavoisier and Laplace obtained distinct indications of electricity by the combustion of charcoal, and Volta informs us that he never failed to obtain it. Saussure, on the contrary, as has been mentioned, never could develop electricity either by combustion or the explosion of gunpowder; and Sir Humphry Davy equally failed to procure it by the combustion of iron or of charcoal in air or in pure oxygen.

The electrical relations of flame have been subsequently examined by M. Erman of Berlin and Professor Brande. M. Erman concluded, from some experiments, that the insulated flames of wax, oil, alcohol, and hydrogen gas conduct only positive electricity, while the flame of phosphorus conducts only negative electricity. It was noticed by Mr Cuthbertson that when the flame of a common candle was placed halfway between two equal balls, the one positively and the other negatively electrified, the flame was attracted to the negative ball, which consequently became very warm, while the positive ball continued comparatively cold.

In pursuing this idea, Mr Brande placed the flames of various bodies between two insulated brass balls, one of which was insulated positively and the other negatively, and obtained the following results.

**Flames, &c., attracted to the Negative Ball.**

- Olefiant gas. - Sulphuretted hydrogen, slightly. - Arseniated hydrogen. - Flame of hydrogen, weakly. - Sulphuret of carbon. - Potassium in combustion, and its fumes. - Flame of gum benzoin. - Smoke of benzoin. - Charcoal emitted by camphor in combustion.

Resinous bodies in combustion exhibit the same phenomena as charcoal.

**Flames attracted to the Positive Ball.**

- Sulphurous acid vapour. - A small flame of phosphuretted hydrogen, slightly. - Flame of white arsenic, slightly. - Large flame of carbonic oxide. - Vapour of burnt sulphur. - Flame of phosphorus. - Vapour of phosphorus. - Stream of muriatic acid. - Stream of nitrous gas. - Vapour of benzole acid.

In order to explain these phenomena, Mr Brande supposes, that since some bodies are naturally negative and others positive, the positive ones will be attracted by the negative ball, and the negative ones by the positive ball.

This conjecture was not confirmed by future observation, and did not lead philosophers to any certain conclusions. The subject, however, was resumed by M. Pouillet, who arrived at a general result, which explains in a satisfactory manner the errors and contradictions of preceding observers.

The first point which occupied his attention was the combustion of charcoal; and in his earliest experiments he found with surprise that he could sometimes obtain from it M. Pouillet-positive and at other times negative electricity, while at other times he could not obtain the slightest electrical indications. In explaining these discrepancies, he supposed that one of the electricities was taken by the charcoal, and the other by the oxygen or carbonic acid; and in order to determine the truth of this supposition he made the following arrangement. Having taken a cylinder of charcoal, he placed it vertically six or eight centimeters below a plate of tin or brass which rests upon one of the discs of the condenser, charcoal. The charcoal having a communication with the ground, was lighted at its upper end without the fire reaching the lateral surface, and there arose a column of carbonic acid, which struck the plate of brass, and in a few seconds charged the condenser. The electricity which the condenser received from the carbonic acid was always positive, whereas Lavoisier, Laplace, and Volta made the electricity negative. When the charcoal was held nearly horizontally, so that the carbonic acid which was generated could rise only by ascending along the base of the charcoal, which was now vertical, no sensible effect was obtained; and when the lateral as well as the upper surface of the charcoal, placed vertically, was lighted, an uncertain result was obtained.

In order to determine the electricity of the charcoal itself, M. Pouillet placed the base of the cylinder upon the disc of the condenser; and after lighting the upper end of it, and keeping up the fire by a gentle blast of air, the condenser was charged, and showed that the electricity taken by the charcoal was negative. When the charcoal burned on all its surface, or when it touched the condenser only in a few points, no electrical effects were observed. In the

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1 When the flame was large it was equally attracted by both balls. 2 The direction of the flame could not be determined. last of these cases a small quantity of electricity only can pass by a small number of points, and in the first case the positive electricity of the ascending carbolic acid was recombined with the negative electricity. In order to produce intense and rapid electrical effects, several cylinders of charcoal of the same height should be placed on their ends, and near each other, upon a sufficiently large plate of brass; and when all the cylinders are made to burn at their upper ends, and their united columns of carbolic acid received by another brass plate communicating with the condenser, and raised a few inches or even a foot above it, a strong charge of positive electricity will in a few seconds be communicated to the brass plate. When the electricity of the charcoal is required, we have only to unite the condenser to the brass plate upon which the burning cylinders are placed, and in a few seconds the condenser will be abundantly charged with negative electricity.

When the combustion is maintained by a current of oxygen, the electricity is not only much more intense, but is much more quickly developed; and the gold leaves of the condenser separate to their maximum divergency in an instant. The first point, however, to be attended to in every form of the experiment, is to burn only the upper horizontal surface, so that the carbolic acid forms and ascends in a moment, and without touching any other body till it deposits its electricity on the brass plate. So essential is this condition, that if we burn even a deep cavity on the circumference of a vertical cylinder of charcoal, and do this even with a jet of oxygen, the electrical indications are sometimes positive and sometimes negative, just as the electricity of the gas or the charcoal predominates.

M. Pouillet next entered upon the more arduous investigation of determining whether or not electricity is produced by change of condition or chemical affinity. Volta had supposed that carbon, in becoming gaseous, absorbed the positive, and left to the remaining solid parts the negative electricity which we find in them. M. Pouillet, on the contrary, supposed that if electricity is disengaged from two elements which combine, positive electricity would be given out by the one and negative by the other; and that when these elements separate, each of them required to take up the fluid which they had lost.

By forming combinations unaccompanied by changes of condition, M. Pouillet resolved this question. He first tried that of oxygen and hydrogen. The flame of hydrogen, like charcoal, gave electricity, sometimes strong and sometimes feeble, sometimes positive and sometimes negative; and it was some time before he discovered the cause of these discrepancies. That the gases are not very good conductors of electricity, he found by the following very curious experiment. Having set a very small spirit-lamp upon a common electroscope, and about five or six feet above it a feebly charged body, such as a stick of electrified rosin or a plate of glass, he observed that the gold leaves diverged greatly, though the same charged body could produce no divergence if held even so near as an inch to the electroscope without flame. This apparatus enabled our author to discover the smallest trace of electricity. If we turn the plate of an electrifying machine, the air of the room is electrified; and the flame which ascends in that air is charged at the moment with electricity of the same name. A pile in action electrifies the air in the same manner, as the flame of the electroscope proves. A charcoal fire, or even a lighted candle, develops carbolic acid electrified positively, which is shown also by the electroscope. The atmospheric air, in short, is always electrified; and if it enters a room by any opening, it will preserve itself in an electrified state so long as to affect the results of experiments on small quantities of electricity.

These causes of error being excluded, M. Pouillet repeated his experiments on the combustion of hydrogen. The gas was emitted from a glass tube, and the flame, which was vertical, was about three inches long and four or five lines broad. The brass plate was now set aside, and the electricity conducted to the condenser by a platinum wire whose end is coiled into a spiral. The spire is vertical, and the circumvolutions are sometimes so large as to surround the flame without touching it, and sometimes so small as to be completely enveloped in the interior of the flame. When we approach this flame from the interior outline of the spire, and keep it ten millimeters distant, we obtain indications of positive electricity. As the distance of the flame diminishes, the electricity becomes more and more intense; but when the flame touches the spire, the electricity becomes weak, and its nature uncertain. The same thing is observed when the flame passes to the interior of the spire, and in the direction of its own axis. Hence there exists round the apparent flame of the hydrogen a sort of atmosphere, more than ten millimeters in thickness, charged with positive electricity. Positive electricity being thus developed in the combustion of hydrogen, Pouillet tried to discover the negative electricity which must have been set free. He placed a small spiral in the centre of the flame, and when it was enveloped on all sides, negative electricity was collected by the condenser. If we plunge the spire half-way into the bright part of the flame, no electricity is manifested. Hence it follows that the inside and outside of the flame are in opposite electrical states, the former being negative and the latter positive, and that there is an intermediate layer of the flame where the electricity disappears. On these facts M. Pouillet thus reasons. In the thickness of the exterior atmosphere of the flame, when the positive electricity appears, the combination of oxygen and hydrogen is not effected, for the hydrogen cannot arrive there. The electricity is therefore communicated, and it must come from the oxygen which predominates on the outside, and which envelopes in some measure all the jet of hydrogen. This combined oxygen must therefore disengage positive electricity, which communicates itself to the neighbouring strata of air sufficiently heated to conduct it. In like manner the hydrogen predominates in the interior of the flames, and the negative electricity must be disengaged from the hydrogen which burns, and which it communicates to the excess of uncombined hydrogen. If this view is correct, it is probable that, at a certain distance above the flame, the two opposite electricities ought no longer to appear, as they must have combined; and this is proved to be the case by the fruitless attempt to collect electricity at a distance sufficiently great above the vertical flame. At the distance, however, of a few inches, other phenomena appear. The two electrical fluids appear there in the same quantity, but they are not recombined; for if we present a soldered plate of zinc and copper, the zinc will attract the positive and the copper the negative electricity.

When the hydrogen issues from a metallic pipe in place of a glass tube, and a communication is made with the condenser and not with the ground, the metal tube, which touches the hydrogen without touching the flame, always takes the negative electricity; and, on the contrary, if the tube communicates only with the ground, it loses in this manner the negative electricity which it had before taken to the condenser, and the product of the combustion preserves an excess of positive electricity.

In pursuing this inquiry in a similar manner, M. Pouillet found that the flames of alcohol, ether, wax, the oils, fatty substances, and several vegetable bodies, present exactly the same phenomenon as the flame of hydrogen. He observed, however, that the particles of charcoal which float in all these flames, and which, according to Sir H. Davy, give them the lustre with which they burn, render them also more fitted to manifest negative electricity. From these results M. Pouillet has deduced the general conclu- The experiments of M. Pouillet were repeated by M. Becquerel in 1827, on the flames of hydrogen gas or alcohol; but he commenced them with some reserve, for, as they were made by means of platinum wires plunged in the flame, he supposed that the phenomena were not only owing to the electricity disengaged during combustion, but also to some property which the metals acquired at a certain temperature. The following is the general fact, without entering into any of the details of his experiments: A platinum wire communicates by one end, through the intermediate of a band of wet paper, with one of the plates of a condenser, the other end being plunged in one of the envelopes of a flame produced by the combustion of alcohol, contained in a vessel of copper, which the observer holds in his hand. The end of the wire may even be placed without the flame, provided it is so near it as to become red hot. The wire soon takes a considerable excess of negative electricity, which ought not to be ascribed entirely to that which the alcohol carries off during combustion. In order to prove this, let us resume the last experiment but one. As soon as the end of the platinum wire attains a red heat, let us withdraw the lamp, and touch this end of the wire with a band of wet paper, or rather with the end of a tube of hot glass; the effect is the same as when the wire touched the flame, or was at a small distance from it. It is very probable that the disengagement of the electricity is due, in this last case, in part to the difference of temperature between the two ends of the wire, and that the flame has carried off the positive electricity of the wire, or the band of wet paper, as the hot glass tube had done.

This opinion is confirmed by the circumstance that the effect is the same whether we bring the wire to a red heat in the interior or in the exterior of the flame, neither of which possesses the same kind of electricity. Notwithstanding this result, M. Becquerel still admits, that during the combustion of alcohol and hydrogen, the exterior envelope of the flame is charged with positive electricity.

M. Becquerel has endeavoured to explain the curious fact discovered by M. Erman, and already referred to. Having placed upon an electroscope a lamp without flame, whose platinum wire was kept at a red heat by the burning vapour of the alcohol, he held above the spiral the negative pole of a dry pile, and the two gold leaves instantly diverged. He next held the positive pole above the spiral, but there was now no divergence of the leaves. Hence the platinum wire afforded a passage only to the negative electricity. The contrary effect took place when the electricity passed from an incandescent wire to another which was not so; and hence M. Erman found that the incandescent wire was reciprocally a conductor and insulator of each fluid.

In order to show that this conclusion is incorrect, M. Becquerel presented successively to a red-hot platinum wire the poles of a dry pile, and it conducted equally well both kinds of electricity. Besides, as he remarks, it appears, from our knowledge of the electrical effects produced in gaseous combustion, and by increase of temperature, that part of the air which surrounds the red-hot wire of the lamp without flame ought to be in a positive state of electricity, and the wire which is in the middle of the alcoholic vapour in a negative state. Moreover, it is evident, from what has been already stated, that the part of the wire which is red hot ought easily to yield positive electricity to contiguous bodies. This being admitted, when we present to this wire the negative pole of a dry pile, there are two reasons why the negative electricity should neutralize both the positive electricity of the surrounding air, and that of the red-hot wire which tends to escape from it. The negative electricity of the wire then becoming free, manifests its action upon the electroscope. In repeating the experiment in an inverse manner, that is, by causing each of the two electricities to escape successively by the red-hot wire, as this last tends to be negative, it neutralizes the positive electricity which arises, and sets free that of the surrounding air and of the red-hot end of the wire. It is not therefore necessary to have recourse to a reciprocity of insulating and conducting action in the red-hot wire in order to explain the phenomenon, for the fact admits of an easy explanation on the properties above explained.

SECT. IV.—On the Electricity of the Solar Rays.

Our readers are no doubt aware that Dr Morichini and others succeeded in magnetizing needles by the action of the solar rays in the solar spectrum. Other philosophers have failed, even in good climates, in obtaining decided indications of magnetism, so that accurate researches are still wanting to remove this approbrium from our experimental physics. The very same observations are applicable to the development of electricity by the influence of solar light; but still it is necessary, in a work like this, that we should give some account of the experiments from which this electrical action has been inferred.

In a memoir on the influence of solar light in the production of electric and magnetic phenomena, Professor Saverio Barlocchi of Rome relates the following experiments:

Having formed the prismatic spectrum by the solar rays, he caused the red rays and the violet rays to fall upon two discs of blackened copper, each of which was attached to a copper wire. Two copper nuts sliding upon a vertical glass rod, and to which the two wires were fixed, allowed the discs to be brought near each other or separated at pleasure. A prepared frog was then suspended by the body to the upper wire, and the legs were placed upon the lower one. The red rays being made to fall on one disc, and the violet on the other, the extreme parts of the two wires were brought into contact, and distinct signs of contraction were observed in the frog.

M. Matteucci has more recently investigated the same subject. Having exposed to the sun a delicate condensing tenui, electrometer of gold leaf, he soon perceived the leaves diverge and open themselves on that side of the glass which was directly exposed to the solar action, as if they had been attracted by it. Hence he was led to suspect that glass thus exposed was electrified; and in order to ascertain this, he placed some plates of it in the sun, and having in a few minutes touched them in different places with the ball electrometer, a perceptible divergence took place. This divergence was much more apparent when he touched the plates even lightly with a flat surface, as the effects of the friction did not afford a doubtful result.

Having inferred from these results that the solar rays had the power of developing electricity in glass, M. Matteucci endeavoured to ascertain whether this was owing to the existence of electricity in the rays themselves, or to the increased temperature of the glass. He therefore heated a plate of glass repeatedly, and having tried it with the electrometer, he never could discover in it any signs of electrical action. M. Matteucci likewise observed that the glass plate exposed to the rays of the sun never became electric if placed beneath another glass plate, or if the face of the sun was obscured by a cloud.

Dr Faraday likewise made experiments on the solar spectrum, in the same manner as M. Barlocchi, with the exception that he used a very delicate galvanometer in place of a frog; but, to use his own words, "no electricity could be obtained by means of an English sun." M. Delarive has still more recently (Bibl. Univers., July 1833, p. 326) stated that, after taking every precaution to avoid the action of extraneous causes, he could not discover in the solar rays the slightest trace of electricity.

Sect. V.—On the Electricity produced by Vegetables.

Mr Read seems to be the only author who, previous to the researches of M. Pouillet, had made any distinct statement respecting the electricity of vegetable bodies. He had concluded, from several experiments, that vegetable putrefaction is always electrified negatively, while the surrounding atmosphere is electrified positively. It is to M. Pouillet, however, that we owe all our knowledge on this subject, and in the present section we shall communicate to our readers a general abstract of his researches.

That the various parts of plants act upon atmospheric air is well known. At the expense of the oxygen they sometimes form a large quantity of carbonic acid gas, which disengages itself insensibly; and sometimes they exhale pure oxygen, proceeding from some combination which goes on in the interior of the plant.

As carbonic acid gas is electrified vitreously at the moment of its formation, from charcoal in combustion, M. Pouillet conceived that a considerable quantity of electricity ought to be produced during the exhalation of this acid from growing plants. This idea was soon confirmed by experiment, and M. Pouillet was led to the important conclusion that vegetation is an abundant source of electricity, and is therefore a powerful cause in the generation of the electricity of the atmosphere.

He took twelve capsules of glass, about nine inches in diameter, and coated them externally, but only to a distance of one or two inches towards the edge, with a film of gum-lac varnish. They were then arranged in two rows at the side of each other, either on a table of very dry wood, or on a table which was itself varnished with gum-lac. When they were filled with vegetable mould, they were made to communicate with each other by metallic wires, which went from the interior of the one to the interior of the other, passing over the edges of the capsules. In this manner all the insides of the twelve capsules, and the mould which they held, formed only one conducting body. If electricity is communicated to such a system, it will be distributed over the twelve capsules, and will remain there, as it cannot pass into the ground, nor even into the exterior surfaces of the capsules, on account of the film of gum-lac round their edges. The upper plate of a condenser is now put in communication with one of the capsules by means of a brass wire, and its lower plate with the ground by the same means; and these communications are so made that they may be kept up even for several days. The grain of which we wish to study the effects is then sown in the earth in the capsules, and from this moment the laboratory must be closely shut, and neither fire nor light, nor any electrical body, admitted.

This experiment was made during the dry north and east winds of the month of March. During the first two days the surface of the mould was dried up, and the grains swelled; the germ projected about a line out of its envelope, without, however, appearing above the thin stratum of earth which covered the grain; and the condenser, after several trials, gave no signs of electricity. On the third day the germs had come out of the mould, and began to raise their points towards the window, which had no shutters. Upon now trying the condenser, M. Pouillet saw a divergence in the gold leaves, and he found the electricity to be negative in the capsules, and positive in the gases which were disengaged. Hence M. Pouillet infers that the rapid action which the rising germ exercises on the oxygen of the air disengages electricity.

The apparatus was then put into its usual state, and after the lapse of some hours the action of the germ again charged it with electricity. Upon testing the apparatus next morning, M. Pouillet found that it gave a very strong electric charge, and the electricity was of the same kind as before. During the next eight days the vegetation continued active, and at all times of observation, both during the day and night, the condenser exhibited more or less electricity, according to the time that had elapsed. After twelve hours the divergence of the gold leaves was more than an inch, and the electricity of the earth in the capsules was always negative. Damp weather followed, and it was then impossible to collect the least quantity of electricity.

M. Pouillet's next experiment was to make two vegetations of corn, two of cresses, one of gillyflower, and one of lucerne; but he was obliged to maintain in his laboratory an artificial dryness, by spreading in a very large apartment several bushels of quicklime broken into very small fragments, and he also distributed in porcelain saucers several kilogrammes of muriate of lime, and placed them near the capsules. The condenser now exhibited a more intense electricity than before, and in each operation the development of the vegetable action, and that of the accompanying electrical phenomena, were observed during ten or twelve days. So rapid was the development of electricity, that after the first three or four days of vegetation, if the condenser was put into the natural state after one observation, and it was then replaced for experiment only during one second, it was found to be charged with electricity.

"But," as M. Pouillet observes, "it is evident that, during one second, the weight of oxygen which combines and disengages during a languid vegetation, of only three or four square feet, is a weight so feeble, and a fraction of a milligramme so imperceptible, that the electricity which it disengages is not sensible to the condenser. One is apt to fear, after this, that the electricity has another source, and that it can only be developed by some foreign cause; but upon reflection we see that the earth of the capsules is so dry that it becomes an imperfect conductor, that the electricity is retained, and that it is it which charges the condenser. To be certain of this, it is sufficient to place successively in contact with the condenser, one, two, three, or a greater number of capsules, and we shall see the charge increase in proportion as the number increases; in short, it is sufficient to place them in communication with the ground for a long time, when they will no longer give a charge to the condenser, and it will be many hours after that before they communicate a sensible electricity. It is without doubt this imperfect conductibility of the dried earth which has rendered it impossible for me to observe until now any electrical charges during the periods of day or night, although I took every precaution to observe it, presuming that if the disengagement of carbuncle acid produce resinous electricity in the ground, the disengagement of oxygen ought, on the contrary, to produce vitreous electricity.

"It is perhaps the same cause which has given birth to another phenomenon, which I have not yet studied sufficiently to give an exact account of it. It happened twice that the electric signs had ceased during two or three days, and that they were then presented in opposite directions, that is to say, the capsules had exhibited vitreous electricity, and had continued to exhibit it, with a very weak intensity, during the rest of the vegetation."

Sect. V.—On the Electricity of Living Animals.

When we consider the structure of organized bodies endowed with life and motion, we should naturally expect, of living from the phenomena described in the preceding section, animals, that electricity would be developed in the chemical pro-

Phenomena and Laws.

During the processes of digestion and assimilation, for example, in which both solid and fluid bodies are changing their form, and in the process of respiration, in which the atmospheric air is decomposed, electricity cannot fail to be developed in greater or less intensity.

Another source of electricity in animal bodies is no doubt the friction between the clothing and the skin; and the electricity thus generated will be more or less intense, according to the state of the atmosphere, the nature of the clothes, and the constitution and habits of the individual.

But, independent of the electrical phenomena which arise from these causes, we find in certain fishes a regular system of electrical organs, by which they either defend themselves from the attacks of their enemies, or seize the prey which nature has provided for their use. The curious phenomena which have been observed relative to these subjects will be described under separate heads.

Arr. I. On the Electricity of the Human Body.

Long before electricity had become a science, electrical phenomena had been distinctly observed. Cardan relates that sparks were emitted from the hair of a Carmelite monk whenever it was stroked backwards; and Faber mentions a young woman from whose hair sparks of fire always fell when it was combed. Cassandra Buri, a Veronese lady, often terrified her maid-servants by brilliant sparks, and a crackling noise, which were emitted when her body was rubbed, or even touched slightly, by a linen cloth. Antonio Ciampi, a bookseller at Pisa, emitted sparks from his back and arms with a crackling noise, whenever he pulled off a narrow shirt and a piece of cloth which he wore upon his breast.

Gesner relates that in Germany, where heated stoves prevailed, it was exceedingly common to observe crackling flames issue from the shirts of persons who had been previously warming themselves at a stove.

The experiments of Mr. Seymour on the electricity of silk stockings that had been worn, which we have already detailed, correspond with the preceding facts; and there are few individuals who have not observed similar electrical phenomena in changing different parts of their dress.

That the electrical effects exhibited in the human body are, generally speaking, produced by the friction of the clothes against the skin, has been proved by the experiments of Saussure, Landriani, the Abbé Bertholon, and M. Volta. M. Saussure examined the electricity of his own body by means of Volta's electrometer and a condenser, and he never could discover any electricity in it when he was perfectly naked, when his clothes were cold, or when he was in a state of perspiration. In other states of his body and dress the electricity which did manifest itself was sometimes positive and at other times negative, without any apparent cause for these variations. When he bent his body forwards and raised himself suddenly, the balls of the electrometer diverged to a considerable distance, and then collapsed; but if he drew away his hand when the balls were thus divergent, they continued in this state of divergency, and exhibited positive electricity. Saussure observed also that the motion produced by respiration is of itself sufficient to produce a small quantity of electricity; for when he remained on the insulating stool in a state of the most perfect repose that a living being could observe, distinct indications of electricity were manifested when he laid his hand for some time on one of Volta's condensers.

The most complete series of experiments on the electricity of the human body were made by M. J. J. Hemmer of Mannheim. He insulated himself upon a board supported by glass feet, and then touched for about half a minute a condenser. The condenser was then applied to Saussure's improved electrometer, and, by means of a glass tube excited by woollen cloth, he examined the nature of the electricity. The following are the results of experiments which he made upon himself on the 21st of February 1786, and which he has repeated upon persons in every state of body and mind, and under every variety of dress and temperature.

1. The electricity of the human body is common to all men. It was found in thirty persons of all ages and sexes; but it varied in strength in different individuals, and was positive in some and negative in others.

2. The intensity and character of the electricity often varies in the same person. In 2422 experiments M. Hemmer found it 1232 times positive, 771 times negative, and 399 times imperceptible. Out of 94 experiments made upon his maid-servant, it was 17 times positive, 33 times negative, and 44 times imperceptible.

3. The electricity of the body is naturally positive; for when it is subject to no violent exertion this is always its character. Out of 356 experiments made upon himself when sitting at rest, and when the natural heat of his body was not disturbed, his electricity was 322 times positive, 14 times negative, and 10 times imperceptible.

4. The natural positive electricity of the body is changed into negative by cold, or is greatly diminished. Out of 62 experiments made upon himself when he came from a temperature of 32° of Fahrenheit, his electricity was 38 times negative, 15 times positive, and 7 times imperceptible.

5. The natural positive electricity of the body is changed into negative by lassitude. Out of 16 times that he walked backwards and forwards in his apartment, or was otherwise employed, he found the electricity only once weakly positive, 10 times negative, and 5 times imperceptible. In 32 experiments made when he was standing at rest, the electricity was 2 times weakly positive and 30 times imperceptible.

6. The natural positive electricity of the body is changed into negative by sudden, speedy, and violent motion.

It is obvious from these experiments, that the human body possesses no electrical organs over which the will exercises any control, and that its electricity depends on the chemical and physical changes which are taking place either in its interior or upon its surface.

It has been supposed that the remarkable phenomena of spontaneous combustion in the human body are somehow or other connected with its electrical state; but we possess no accurate data by which the truth of this opinion can be tried.

Some very interesting discoveries have been recently made respecting the electricity of animals. It has been of M. Matteucci and M. Dubois Remond, that there are electrical currents in the frog and in all other animals, whether cold or warm blooded. According to Matteucci, the current is more feeble in proportion to the rank which the animal occupies in the scale of animals. We have witnessed the fine experiments which he performed at York in 1844, by severing the lower halves of the thighs of a certain number of living frogs, and inserting the knee of the one into the central muscles of the second, and so on. A voltaic pile was thus formed of six or eight elements, which was capable of deflecting the needle of a galvanometer, or producing convulsions in an electroscopic frog. The direction of the voltaic current was from the interior to the exterior of the muscle. M. Matteucci also showed that there was a specific voltaic current in the frog, which is directed from the feet to the head, and is detected only in that animal.

The existence of electrical currents in the human body has been proved by M. Dubois Remond by means of an instrument of extremely sensitive galvanoscope. This instrument consisted of a coil of wire 16,752 feet or 3½ miles in length, which made 24,160 turns on the frame upon which it was wound. Its diameter was only 0.0055 of an inch. Two plates of homogeneous platinum, fixed one to each end of the galvanoscope, were immersed in two vessels containing salt water. When the two corresponding fingers of each hand are plunged into these vessels, the needle of the galvanoscope deviates a little from its place; but as it does not follow any law, M. Dubois Remond ascribes it to something heterogeneous in the skin of the finger, for when there is an abrasion of the skin or a slight wound in one of the fingers, the deviation is greater than usual. When the needle has returned to its place of rest at zero, the operator, dipping his finger into the vessel, forcibly stiffens or contracts all the muscles of one of his arms, and immediately the needle begins to move through a space of 30°, indicating an inverse current of electricity, or one which moves from the hand to the shoulder. The greatest effect is generally produced by the strongest persons, but sometimes no effect is produced by particular individuals.

This beautiful instrument has been improved and rendered more sensitive by Mr Rutter of Brighton, who has described it in his work on Human Electricity. Upon a circular stand is fixed a pillar, on the top of which is a moveable bracket with a collar and adjusting screws. An oval-shaped compact coil of insulated copper wire is suspended from the bracket by a fibre of raw silk. The ends of this wire are soldered to small binding screws at each extremity of the coil, by which contact is made with cups containing mercury. The coil or helix moves with scarcely any friction, as its terminals consist of fine silver wire, which dips into mercury. One of M. Logeman of Haerlem's horse-shoe magnets, constructed by the process of M. Elias of Haerlem, is supported horizontally with its poles as near as possible to the sides of the helix without touching them. Wires proceed from the cups of mercury and beneath the stand, and terminate in plates of platinum. A brass index with an engraved dial plate is fixed on a plate at the top of the helix. On a separate bench or table are placed two basins containing about 3 quarts of pure spring or distilled water.

In using the apparatus, the platinum plates are placed in these basins, and when the index points at 0° the operator places his hands in the basins so as to be well covered with water, and not to touch the plates of platinum. After the deviations of the needle noticed by M. Remond have subsided, clench one hand, firmly contracting the muscles of that hand and arm to the shoulder, keeping those of the left hand and arm relaxed. The index will move 4° or 6°. When the other hand is clenched similarly, the index will move in the opposite direction to 4° or 6° on the other side of 0°. By continuing this process the deviation of the Phenomena needle may amount to 14° or 16°. Mr Rutter has found that children of both sexes can deflect the needle with as much force as adults.

By forcibly extending the fingers of the hand in place of clenching it, and at the same time contracting the muscles of the hand and arm, Mr Rutter has found that the needle is deflected in a direction opposite to that when the hand is clenched. The force, however, is not so great as when the hands are clenched. In order to increase the sensibility of his galvanoscope he occasionally uses an electromagnet.

Arr. 2. On the Electricity of the Raia Torpedo.

The remarkable property of giving an electrical shock Electricity possessed by this fish was known in the time of Aristotle of the torpedo, and Pliny, and has been distinctly described by Appian, pedo, Redi, Reaumur, Kempfer, and Bancroft, successively described the phenomena which it exhibited; and Lorenzini, so early as 1678, published good engravings of the electrical organs of the torpedo.

The first person, however, who made accurate experiments on the torpedo was Mr Walsh. He confirmed the remarkable observation of Kempfer, that the shock could Walsh be evaded if the person who touched the animal held in his breath at the time. Mr Walsh made two series of experiments on this fish, one when it was placed in the air, and the other in the water. In the first series he placed a living torpedo upon a table covered with a wet napkin, round which stood five persons who were insulated. Having suspended from the ceiling by strings two brass wires, each thirteen feet long, one of them was made to communicate by one extremity with the wet napkin, while its other extremity was plunged in a basin of water placed upon a second table, on which other four basins of water stood. The first of the five insulated persons plunged a finger of one hand in the basin in which the above-mentioned wire was placed, and a finger of the other hand into the second basin. The second person put a finger of one hand in this second basin, and a finger of the other in a third basin, and so on till the five persons formed a communication with each other by the water in the basins. The end of the second wire was plunged in the last basin, and Mr Walsh having taken the other end of this wire in his hand, touched the back of the torpedo, when all the five persons experienced a shock which differed only in force from that of the Leyden jar. The shock seldom extended beyond the touching finger, and out of 200 only one reached above the elbow. When the torpedo was insulated, it gave forty or fifty shocks to insulated persons, without any diminution of its force. Mr Walsh found that the shock was communicable through iron wires and other conductors, but not through air, glass, and other electrics; and he was never able either to produce a shock, or move the pith balls of an electrometer. In the series of experiments in water, Mr Walsh held a large and powerful torpedo in both hands by its electric organs, and after plunging it about a foot under water, he raised it suddenly to the same height in air. The instant the lower surface of the fish touched the water in descending, he received a violent shock, and the instant the same surface quitted the water in ascending, he experienced a still more violent shock. A writhing of the fish accompanied both these shocks, particularly the last. The intensity of the shock under water was scarcely one-fourth of that at the surface, and not much more than one-fourth of those given in the air. The number of shocks in a minute was about twenty, generally two and always one when he was wholly in the air, and sometimes two when he was below water. When the finger of one hand touched

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1 See Mr Rutter's Human Electricity, chap. vii., Lond. 1854. Dr Ingenhouse, who repeated and confirmed these experiments, says that the sensation of the shocks is the same as if a great number of very small electrical bottles were discharged very quickly through the hand. M. Spallanzani found the shocks strongest when the fish was laid upon a plate of glass. When the animal was dying the shocks were not given at intervals, but resembled a continual battery of small shocks. The battery continued seven minutes, and in this time he experienced 316 shocks. Spallanzani also found that the fetus gave perceptible shocks like the full-grown fish.

In the year 1805, MM. Humboldt and Gay Lussac examined the properties of the torpedo at Naples, but they do not seem to have added much to the observations made by Mr Walsh. They found that a person accustomed to electric shocks could with some difficulty support the shock of a vigorous torpedo fourteen inches long; that before each shock there is a convulsive movement of the pectoral fins; that the animal must be irritated previous to the shock; that the shock may be felt when a single finger is applied to a single surface of the electric organ; that an insulated person will not receive a shock if he touches the fish with a key or any other conducting body; and that the least injury done to the brain of the fish prevents its electrical action.

At the request of Mr Walsh, Dr Hunter, the celebrated anatomist, examined the electrical organs of a torpedo about eighteen inches long, twelve broad, and two thick. These organs are placed on each side of the cranium and gills, reaching from thence to the semicircular cartilages of each great fin, and extending in length from the anterior extremity of the animal to the transverse cartilage which divides the thorax from the abdomen. Within these limits the organs occupy all the space between the skin of the upper and outer surfaces. This description will be understood from fig. I of Plate CCXXVI., which represents a female torpedo, the skin B having been flayed from the under surface of the fish, to show the electric organs A. The nostrils, in the form of a crescent, are shown at c, and the mouth, having a crescent form, opposite to the nostrils, at d. The mouth is furnished with several rows of small hooked teeth. The bronchial apertures are shown at E, five being on each side; F is the place of the heart, gggg the place of the anterior transverse cartilages, hh the exterior margin of the great lateral fin, i its inner margin on the confines of the electrical organ, f the abdomen, mm the place of the posterior transverse cartilage, which is single, united with the spine, and sustains the smaller lateral fins nnnn on each side; O is the anus, and P the fin of the tail.

Each organ is about five inches long, and about three inches broad at the anterior end, and half an inch at the posterior extremity. Each organ consists wholly of perpendicular columns reaching from the upper to the under surface of the body, and varying in their lengths according to the thickness of the parts of the body where they are placed. The longest column is about one and a half inch, the shortest about one-fourth of an inch, and their diameter about two-tenths of an inch. The figures of the columns are irregular hexagons or pentagons, and sometimes have the appearance of being quadrangular or cylindrical. The number of columns in the fish examined by Dr Hunter was 470 in each organ; but in a very large fish four and a half feet long, and weighing seventy-three pounds, the number was 1182 in each organ. The number of partitions in a column one inch long was 150. The nerves inserted into each electric organ arise by three very large trunks from the lateral and posterior part of the brain; and when they have entered the organs they ramify in every direction between the columns, and send in small branches on each partition, where they are lost. Dr Hunter remarks that there is no part of any animal with which he is acquainted, however strong and constant its natural action, which has so great a proportion of nerves; and he hence concludes that, if it be probable that these nerves are not necessary for the purposes of sensation or action, they are subservient to the formation, collection, or management of the electric fluid.

M. Geoffroy de St Hilaire has more recently examined the torpedo. He analysed the fluid in the cells of the hexagonal columns, and found it to consist of albumen and gelatine; and, what is very curious, he discovered organs analogous to those of the torpedo in other species of the same genus Raia, which do not possess any electrical power.

Some useful observations were made upon the torpedo of Observa-the Cape of Good Hope in 1812 by Mr John T. Todd. The torpedos of this locality are never more than eight, nor less than five inches in length, and never more than five, nor less than three and a half inches in breadth. The columns of their electrical organs were larger and less numerous in proportion than those described by Hunter, and they appeared to be of a cylindrical form. The shocks of these torpedos were never sensible above the shoulder, and seldom above the elbow joint. The electrical discharge was generally accompanied by an evident muscular action, as shown by an apparent swelling of the superior surface of the electrical organs. From a great variety of experiments, which we have not room to enumerate, Mr Todd drew the following conclusions:

1. That the electrical discharge is a vital action dependent on the life of the animal. 2. That the action of the electrical organ is entirely voluntary. 3. That frequent action of them is injurious to its life, and, if continued, deprives the animal of it. 4. That when the nerves and the organs are cut, the torpedo loses the power of giving a shock, though it appears more vivacious, and lives longer, than those in which this change has not been produced, and in which the electrical power is exerted. 5. That the possession of one organ only is sufficient to produce the shock. 6. That the perfect state of all the nerves of the electrical organs is not necessary to the production of the shock. 7. That (as was shown by Dr Hunter) a more intimate relation exists between the nervous system and electrical organs of the torpedo, both as to structure and functions, than between the same and any organs of any animal with which we are acquainted.

In 1816 Mr Todd made another series of experiments at La Rochelle, principally with the view of determining whether the torpedo possessed any voluntary power over the electrical organs, either in exciting or interrupting their action, except through the nerves of these organs. Shocks were given by the torpedo even when one half of each electrical organ was removed; and also when an incision was made extending round the circumference of both organs, so as to leave no attachment between these organs and the animal except the nerves. When the large lateral cartilages were removed, and a large portion of the surfaces of the electrical organs denuded, two distinct shocks were received; but the fish being much injured, soon died. During these experiments, Mr Todd observed how powerfully the action of the electrical organs was excited by the cutting of the scalp; and on one occasion, when he pressed on the electrical organ with his left hand, and held the scalpel wet in the other while cutting the electrical organ, he received a distinct shock in the right hand through the scalp. He observed also that all the nerves of the electrical organs arise from the medulla oblongata, notwithstanding the long course which three of them are obliged to follow.

Mr Todd informs us that the torpedo called la tremble, which occurs on the coast between the Loire and the Garonne, is eaten by the poorer inhabitants, who carefully avoid the electrical organs, which are supposed to possess some disagreeable properties.

In 1814 and 1815, when Sir H. Davy was on the shores of the Mediterranean, he was desirous of ascertaining whether or not the electricity of the torpedo possessed the chemical and magnetic powers of that agent. In both these trials he could neither decompose water, nor influence a highly delicate magnetic electrometer; and he seems disposed to infer that there is a stronger analogy between the common and animal electricity than between common and voltaic electricity, and that it is probable that animal electricity will be found to be of a distinctive and peculiar kind.

This eminent chemist intended to pursue these inquiries, but his ill health prevented him; and in his latest illness he requested his brother, Dr John Davy, to carry on the investigation. Dr Davy accordingly pursued the inquiry at Malta, and succeeded in obtaining several important results. He placed a needle perfectly free from magnetism within a fine copper spiral wire one and a half inch long and one-tenth of an inch in diameter, containing about 180 convolutions, and weighing about four and a half grains. By the electricity of a torpedo about six inches long, he succeeded in communicating distinct magnetism to this needle; and he repeated the experiment with the same success with fishes of different sizes. Dr Davy likewise succeeded in throwing into violent motion the needle of a magnetic multiplier. With every fish he tried he obtained decisive results, and he met with no instance of a fish which had the power of magnetizing a needle in the spiral wire failing to move the needle in the multiplier, though he met with more than one example of a fish whose electricity was equal to the latter effect and not to the former. Dr Davy, however, failed in obtaining any igniting power, or the faintest spark, by means of the torpedo. He also found that air was not impermeable to the electricity of the torpedo; but he never could exhibit any influence on the electrical, or any indications of attraction and repulsion in air. Dr Davy's experiments on the chemical agency of this species of electricity were highly satisfactory. He decomposed strong solutions of common salt, nitrate of silver, and superacetate of lead, and he inferred that the under surface of the organ corresponds to the zinc, and the upper surface to the copper extremity of the voltaic battery. In the deviation of the needle in the multiplier produced by the torpedo, the action of its under surface corresponded with the zinc plate, and that of the upper surface with the action of the copper plate. In like manner, the extremity of a needle that received polarity from a torpedo when placed in a spiral wire, had southern polarity when it was nearest the under surface of the fish, and the other extremity of course northern. In one experiment Dr Davy connected the spiral with the multiplier, and having charged the former with eight needles, a single discharge from an active fish moved the needle in the multiplier powerfully, and converted all the needles into magnets, each of them as strong if one only had been used.

Dr Davy's next object was to ascertain "the exact nature of the substance of the electrical organs, or the peculiar structure of which they are composed." The electrical organs when wet weighed only 302 grains; and when completely dried by sixteen hours exposure to the boiling heat of water, they weighed only twenty-two grains. They appeared to him to consist of 728 of matter not evaporable at 212°, and of 92-72 water. When the electrical organs are immersed in boiling water, they suddenly contract in all their dimensions, and the columns, from pentagonal, which they generally are, become circular. The electricity of a small voltaic trough, the shock of which was just perceptible, distinctly affected the voluntary muscles of the live torpedo, but did not in the least affect the electrical organs. Their substances appeared to be neither sensitive nor contractile by the application of other stimulants; and hence he infers that these organs "are not muscular, but columns formed of tendinous and nervous fibres, distended by a thin gelatinous fluid." Dr Davy never could observe satisfactorily in the fresh fish the horizontal partitions which Dr Hunter had counted. After describing more fully and accurately than Dr Hunter the distribution of the three great trunks of the nervous system, Dr Davy describes the mucous system, which forms a conspicuous part of the anatomical structure of the fish. It consists of several clusters and chains of glands, distributed chiefly around the electrical organs, at different depths beneath the cuts, and of strong transparent vessels of various lengths and sizes opening externally in the skin for the purpose of pouring out the thick mucus secreted by the glands, and destined for lubricating the surface. This system, which was not noticed by Dr Hunter, was described, but imperfectly, by Lorenzini. Dr Davy thinks that this system may not only be aided by, but also aid the secretion of the mucus. In comparing the phenomena of the torpedo with those of other kinds of electricity, Dr Davy notices the following points of difference:—“Compared with voltaic electricity, its effect on the multiplier is feeble; its power of decomposing water and metallic solutions is inconsiderable; but its power of giving a shock is great, and so also is its power of magnetizing iron. Compared with common electricity, it has a power of affecting the multiplier, which, under ordinary circumstances, common electricity does not exhibit; its chemical effects are more distinct; its power of magnetizing iron and giving a shock appears very similar; its power of passing through air is infinitely less as is also (if it possess it at all) its power of producing heat and light.”

These differences have been explained in different ways of Cavendish by different authors. Mr Cavendish endeavoured to account for them on the principles of common electricity. Mr Nicholson did the same with much ingenuity. Volta at first supposed that the superposition of the different cells in the columns, formed of substances some of which excite electricity by contact, while others transmit it, corresponds to that of the metallic and moist conductors of which the pile is composed; but he afterwards showed to Sir H. Davy another form of the pile, which he thought fulfilled the conditions of the organs of the torpedo; a pile of which the fluid substance was a very imperfect conductor, such as honey, or a strong saccharine extract, which required a certain time to be charged, and which, though it did not decompose water, communicated nevertheless weak shocks when charged. MM. Humboldt and Gay Lussac were more inclined to compare the action of the torpedo to a chain of small Leyden phials, like Cavendish, than to the voltaic pile. In order to explain why no spark is given by the torpedo, Mr Cavendish proved by experiment that the distance through which the spark flies is inversely (or rather in a greater proportion) as the square root of the number of jars; and hence the torpedo may contain sufficient electricity to give a shock, without being able to make it pass through such a space of air as is requisite for the production of the spark. He accounted also for the absence of every appearance of attraction and repulsion, from the known fact that the shock of a battery so weakly electrified as to be incapable of passing through a chain, which is the case with the electricity of the torpedo, is not capable of producing any divergency in the pith balls of an electrometer. Mr Cavendish corroborated these views by constructing an artificial torpedo of thick leather, connected with glass tubes and wires, and covered with a piece of sheep-skin leather, which was an exact imitation of the real torpedo. The battery was composed of forty-nine jars of very thin glass, and contained about seventy-six feet of coated surface.

Humboldt has enumerated the following species of the torpedo which are electrical:—Torpedo narke, Risso; Torpedo unimaculata; Torpedo marmorata; Torpedo Galvanii.

It is very difficult to preserve the torpedo for experiments. Dr Davy could not keep them alive for twelve or fifteen days; and M. Matteucci could not preserve one out of 116 above three days, though the greatest attention was paid to them.

Art. 3.—On the Electricity of the Gymnotus electricus.

The electrical cel of Surinam, or Gymnotus electricus, possesses electrical organs different from those of the torpedo, and exhibits different electrical properties. Its common size is about three feet in length; though Dr Bancroft was told that some have been seen in the Surinam river upwards of twenty feet long, and whose shock proved immediately fatal.

Richer was the first person who made known in Europe the electrical properties of this fish; and experiments have been since made upon it by various naturalists. It is from the observations, however, of Dr Williamson of Philadelphia, Dr Garden of Charlestown, and Mr Walsh, that our knowledge of its properties is derived; and these may be summed up in the following manner:—

1. When the gymnus is touched by the hand, a shock is felt in the fingers, and often as far up as the wrist and elbow; and when it is touched with an iron rod twelve inches long, the shock is felt in the finger and thumb.

2. If the cel is provoked by one person, the hand of another person held in the water will experience a small shock.

3. When the cel was touched and provoked with one hand, and the other held in the water at a small distance, a shock passed through both arms; and the same effect was produced when the hand held a wet stick in the water; and when the same experiment was made by eight or ten persons who joined hands, a shock was also experienced.

4. When the first of eight persons pinched the tail, while the last touched the head, they all experienced a severe shock.

5. The shock of the cel was found to pass through those substances which are conductors, and to be stopped by those which are non-conductors, of common electricity.

6. An insulated person electrified, exhibited no marks of electricity; and pith balls refused to diverge either when suspended over the cel's back, or touched by an insulated person when he received the shock.

7. Dr Williamson succeeded in making the electricity of the cel pass through a small space of air, and exhibit the electric spark when the fish was in the open air; but the spark is not visible when the fish is placed in water.

In the preceding experiments the gymnus was in a large vessel, supported by pieces of dry timber about three feet above the floor. A small hole having been bored in the vessel, a person who held his finger in the stream of water which flowed from it experienced a shock when the cel was irritated.

Dr Williamson threw a cat fish into the same vessel with the gymnus, and in a short time it gave the cat fish a shock, and caused it to turn up its belly and remain motionless.

Experiments on the gymnus have more recently been made by M. Fahlberg of Stockholm, and by MM. Humboldt and Bonpland. The Swedish philosopher succeeded in obtaining an electric spark from the cel when placed in the air, by interrupting the conducting chain by two gold leaves pasted upon glass, and a line distant from each other; but he never could discover any phenomenon of attraction or repulsion, though he employed very delicate electrometers, and caused very strong shocks to pass through them.

While MM. Humboldt and Bonpland were in South Of Hum-America, where the little streams, and even the basins of boldt and stagnant water, are filled with electrical cels, they enjoyed Bonpland, the finest opportunities of studying the phenomena of their electrical action. Having imprudently placed both his feet on a fresh gymnus, Humboldt experienced a more dreadful shock than he ever received from a Leyden phial, and which left a violent pain in his knees, and in almost every joint, during the rest of the day. When both he and M. Bonpland held a fish, the one by the head or by the middle of the body, and the other by the tail, and, standing on the ground, did not join hands, one of them received shocks which the other did not feel; and hence they concluded that the cel could direct its strokes where it chose, or towards the point where it was most strongly irritated, sometimes discharging them from the whole surface of its body, and sometimes from one point only.

The gymnus that had been rendered extremely tame during their voyage from Surinam to Stockholm were made to fast a long time, and when fishes were put into the tub they killed them at a distance, the electrical stroke passing through a very thick stratum of water. A fresh-caught gymnus was placed by Humboldt beside little tortoises and frogs, which, ignorant of their danger, placed themselves upon its back. The frogs did not receive the shock till they touched the body of the eel. When they recovered they leapt out of the tub. Humboldt remarks that this gymnus was not yet sufficiently tamed to attack and devour frogs.

Upon cutting a very vigorous fish through the middle of the body, Humboldt observed that the fore part alone gave shocks. The shocks, however, are equally strong in whatever part of the body the fish is touched, though it is most disposed to dart them forth when the pectoral fins, the electrical organ, the lips, the eyes, or the gills are pinched. Humboldt remarks that no person has ever perceived a spark issue from the body of the fish itself. He irritated it for a long time during the night, at Calabozzo, in perfect darkness, without observing any luminous appearance.

The method of fishing the electrical cels by horses, as described by Humboldt, is too interesting to be omitted in fishing the popular article. The Indians having brought about thirty gymnus wild horses, forced them to enter a pool of muddy water, surrounded with fir trees. "The extraordinary noise caused by the horses' hoofs makes the fish issue from the mud, and excites them to combat. These yellowish and livid cels, resembling large aquatic serpents, swim on the surface of the water, and crowd under the bellies of the horses and mules. A contest between animals of so different an organization furnishes a very striking spectacle. The Indians, provided with harpoons and long slender reeds, surround the pool closely, and some climb upon the trees, the branches of which extend horizontally over the surface of the water. By their wild cries, and the length of their reeds, they prevent the horses from running away, and reaching the banks of the pool. The cels, stunned by the noise, defend themselves by the repeated discharge of their electric batteries. During a long time they seem to prove victorious. Several horses sink beneath the violence of the invisible shocks which they receive from all sides in organs the most essential to life, and, stunned by the force and frequency of the shocks, disappear under the water. Others, panting, with mane erect, and haggard eyes expressing anguish, raise themselves, and endeavour to flee from the storm by which they are overtaken. They are driven back by the Indians into the middle of the water; but a small number succeed in eluding the active vigilance of the fishermen. These regain the shore, stumbling at every step, and stretch themselves on the sand exhausted with fatigue, and their limbs benumbed by the electric shock of the gymnus.

"In less than five minutes two horses were drowned. The cel being five feet long, and pressing itself against the belly of the horses, makes a discharge along the whole extent of its electric organ. It attacks at once the heart, the intestines, and the plexus coeliacus of the abdominal nerves. It is natural that the effect felt by the horses should be more powerful than that produced upon man by the touch of the same fish at only one of its extremities. The horses are probably not killed, but only stunned. They are drowned from the impossibility of rising amid the prolonged struggle between the other horses and the cels.

"We had little doubt that the fishing would terminate by killing successively all the animals engaged; but by degrees the impetuosity of this unequal combat diminished, and the wearied gymnus dispersed. They require a long rest and abundant nourishment to repair what they have lost of galvanic force. The mules and horses appear less frightened; their manes are no longer bristled, and their eyes express less dread. The gymnus approach timidly the edge of the marsh, where they are taken by means of small harpoons fastened to long cords. When the cords are very dry the Indians feel no shock in raising the fish into the air. In a few minutes we obtained five large cels, the greater part of which were but slightly wounded. Some were taken by the same means towards the evening."

The gymnus is the largest of the electrical fishes. A fish of three feet ten inches long, obtained by Humboldt, weighed twelve pounds. The transverse diameter of the body was three inches five lines. The gymnus of the Cano de Bera are of a fine olive-green colour. The under part of the head is yellow mingled with red. Two rows of small yellow spots are placed symmetrically along the back, from the head to the end of the tail. Every spot contains an excretory aperture, which keeps the skin of the animal covered with a mucous matter, which, as Volta has proved, conducts electricity twenty or thirty times better than pure water.

Dr Hunter examined, with his usual skill, the electrical organs of this fish; and in fig. 2 we have copied his engraving of it, in which the skin is removed to show the structure. In this figure A represents the lower surface of the head; C, the cavity of the belly; B, the anus; E, the back, where the skin remains; GG, the fin along the lower edge of the fish; EE, the lateral muscles of this fin, removed and laid back with the skin to expose the small organs; L, part of the muscle left in its place; FF, the large electrical organ; HHH, the small electrical organs; mmm, the substance which separates the two organs; and n, the place where this substance is removed. These organs occupy nearly one-half of the part of the flesh in which they are placed, and form more than one-third of the whole fish. There are two pairs of electrical organs of different sizes, and placed on different sides; the large one F occupies the whole of the lower and lateral part of the fish, constituting the thickness of its fore part, and extending from the abdomen to near one end of the tail, where it terminates nearly in a point. The two organs are separated at the upper part by the muscles of the back, at the lower part by the middle partition, and by the air bag at the middle part. The lesser organ stretches along the lower edge of the fish, and nearly as far as the other, terminating almost insensibly near the end of the tail. The two small organs are separated from each other by the middle muscle, and by the bones in which the fins are articulated. The large organ may be seen by merely removing the skin, which adheres to it by a loose cellular membrane; but in order to see the small organ, the long row of small muscles which move the fin must be removed. The electrical organs consist of two parts, viz. flat partitions of septa, and thin plates or membranes intersecting them transversely. The septa are thin parallel membranes stretching in the direction of the fish's length, and as broad as the semidiameter of the animal's body. The septa vary in length, some of them being as long as the whole body. In a fish two feet four inches long, the distance of the septa was nearly half an inch; and in the broadest part of the organ, which was one and a quarter inch, there were thirty-four septa. In the small organ the septa have a somewhat serpentine direction. They are only the fiftieth of an inch distant, and there are fourteen septa in the breadth of the organ, which is half an inch. The very thin plates which intersect the septa have their breadth equal to the distance between any two septa. There is a regular series of these plates from one end of any two septa to the other end, 240 of them occupying a single inch.

A number of interesting results were obtained in 1838 Dr Faraday with a gymnus by Dr Faraday. The fish was caught in day's eye March 1838. It was brought to the Adelaide Gallery on the 15th of August, but did not feed from the time of its capture up to the 19th of October, when it killed and eat four small fish. It subsequently ate one gudgeon, carp, or perch daily. Dr Faraday obtained from the electricity of this gymnus the electric spark, chemical decomposition, and the evolution of heat. It deflected the needle of a galvanometer, and he made magnets with it. He failed, however, in producing the attraction of gold leaves. The shock of the gymnus was most powerful when one hand was placed on the body near the head, and the other near the tail. It resembled "that of a large Leyden battery, charged to a low degree, or that of a good voltaic battery of perhaps one hundred or more pairs of plates, of which the circuit is completed for a moment only." Dr Faraday concluded from his experiments that "a single medium discharge of the fish is at least equal to the electricity of a Leyden battery of fifteen jars, containing 3500 square inches of glass, coated on both sides and charged to its highest degree;" and, great as this force is, he frequently experienced it to give two and even three shocks with scarcely a sensible interval of time between them. When the fish wills the shock, the anterior parts are positive, and the posterior parts negative.

The following results were obtained by Dr Faraday, and will be understood from the annexed diagram, where AB is the tub containing the animal, supported upon dry wooden logs. It was 46 inches in diameter, with a depth of water in it of 3½ inches. The numbers show the places where the hands were put; those across the fish implying that the hands touched it: the letters A, B, C are three different experimenters, A being the person who excited the fish to action.

When one hand was in the water a weak shock was felt only in it, whatever part of the fish it touched, and only in the part immersed. When both hands were in the water at the same parts of the fish, a weak shock was felt only in the parts immersed.

When both hands were at 1, 3, or 4, 6, or 3, 6, strong

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1 Humboldt notices it as remarkable that no electrical fish is covered with scales. shocks extended up the arms and even to the breast. The shock was strongest at 1, 8, and perceptible at 8, 9.

When B's hands were at 10, 11, four inches from the fish, whilst A excited it with a glass rod, B received a powerful shock. When A was at 4, 6, B at 10, 11, C at 16, 17, and D at 18, 19, they all received shocks at the same time, A and B strongly, and C and D feebly.

When B had both hands at 10, 11, or at 14, 15, whilst A had but one hand at 1, or 3, or 6, the former got a strong shock, and the latter a weak one.

When A's hands were at 3, 5, B's at 14, 15, and C's at 16, 17, A's shock was the most powerful, B's the next, and C's the feeblest.

When A excited the fish with his hands at 8, 9, whilst B was at 10, 11, B had a stronger shock than A.

When A excited the fish with one hand at 3, B with both hands at 10, 11 (or along), and C had the hands at 12, 13 (or across), A felt a pricking shock in the immersed hand only, B a strong shock up the arms, and C but a slight effect in the immersed parts.

Dr Faraday concludes from all the experiments he has made that "all the water and all the conducting matter around the fish, through which a discharge circuit can in any way be completed, is filled at the moment with circulating electric power." This state, he adds, "might be easily represented generally in a diagram by drawing the lines of inductive action upon it. In the case of a gymnus surrounded equally in all directions by water, these would resemble generally in disposition the magnetic curves of a magnet having the same straight or curved shape as the animal, that is, provided he in such cases employed, as may be expected, his four electric organs at once."

The subject of electric fishes has been more recently studied by M. Matteucci of Pisa. The following are some of the results which he has obtained:

1. When any part of the body of an electric fish is irritated, the irritation is transmitted by the nerves to the fourth lobe of the brain, and then only the electric discharge takes place.

2. An electric discharge is obtained by irritating a very small part of a prism of the electric organ of the torpedo.

3. The electric organ of the fish is a nervous fibril in contact with a small cell filled with albumen; and as this cell gives an electric shock when subjected to nervous action, the two opposite electricities must be separated to be instantaneously reunited.

4. Each prism of these electric organs is a pile of elementary organs upon each of which a nervous filament is spread normally to the axis of the pile, analogous to a cylinder of cast-iron inclosed in a helix of metallic wire, and traversed by the electric current.

5. The sum of the electric currents given by the different slices of the organ of a torpedo is approximately equal to the current given by the entire organ. In the gymnus the strongest discharge is that which is obtained by including the entire length of the animal within the circuit.

6. The electric shock increases with the vital action of the torpedo. The shock is increased by raising a little the temperature of the water, and diminished by hindering respiration or circulation. It is diminished also after a number of shocks. It is increased by rest, and it becomes more powerful than usual when the fish is excited by nux vomica.

ART. 4. On the Electricity of the Silurus electricus.

The Silurus electricus, of which we have given a drawing in fig. 3, is a fish about twenty inches long, which is found in the Senegal, the Niger, and the Nile. It is eaten by the Egyptians, who dress its flesh, and salt its skin as an aphrodisiac medicine. The shock of this fish is distinctly felt when it is laid on one hand, and touched by an iron rod six feet long held in the other. Its electrical organs, according to M. Geoffroy, are much less complicated than those of other electrical fishes. They lie immediately below the skin, and stretch all round the body of the animal. Their substance is a reticulated mass, the meshes of which are clearly visible, and these cells are filled, like those of other electrical fishes, with an albuminous gelatinous matter. The nerves distributed over the electric organs proceed from the brain, and the two nerves of the eighth pair have a direction and nature peculiar to this species.

ART. 5. On the Electricity of the Tetraodon electricus.

In the cavities of the coral rocks in Johanna, one of the Tetraodon Canary islands, Lieutenant Paterson discovered the Tetraodon electricus, which he found to possess the properties of other electrical fishes. It has a long projecting mouth, and is seven inches long and two and a half broad. The colour of its back is brown, of its belly sea-green, of its sides yellow, of its fins and tail sandy-green. Its body is covered with red, green, and bright white spots. It has large eyes, and its iris is red, tinged with yellow on its outer edges. It is found also in the American seas.

Lieutenant Paterson found this fish in water whose temperature was 56° or 60° Fahrenheit; and having caught two of them in a linen bag, he had no sooner taken one of them in his hand than he received so severe a shock that he was obliged to let it go. He carried the two fishes to the camp, and though one of them died, and the other was in a state of great debility, he was able to obtain the evidence of the surgeon and the adjutant in favour of his discovery. The former having held it between his hands, received a distinct electrical shock, and the latter received a shock by merely touching the fish on its back with his finger.

ART. 6. On the Electricity of the Trichurus electricus.

This fish, which we believe is the Trichurus indicus of Shaw, inhabits the Indian seas, and has been found to possess the power of giving an electrical shock. It has a pointed snout, and belongs to the family Tencoides, of the order Acantopterygii.

Other electrical fishes have been met with, but the descriptions given of them do not enable us to determine whether or not they are the same as those which we have described in this section. Mr Maxwell, in his observations on Congo and Louango, mentions his having found at sea an electrical fish, which made the sailor who took it exclaim "that the devil was in the fish." When examining it attentively, Mr Maxwell found that his astonishment arose from his having received an electrical shock. Before each shock the skin on his back and sides became very tense. It was like a cod, and weighed thirty pounds. He gave it to the natives to eat, and they praised it much. No electrical fish of such a size has, so far as we know, been found, and it is highly probable that it is a new species.

SECT. VI.—On the Electricity of the Atmosphere.

There is perhaps no branch of electricity so highly interesting as that which treats of the electricity of the atmosphere, whether we consider it in reference to ourselves, as beings exposed to its tranquil as well as to its disturbed influence, or in reference to the grandeur and beauty of the phenomena which it exhibits. The methods which have been adopted for examining the electricity of the atmosphere consist in elevating long vertical rods, the summits of which collect the electricity, or in extending insulated wires in a horizontal direction, or in sending up kites into the higher regions.

M. le Monnier, the Abbé Mazzea, Mr Kinnerley, Beccaria, Saussure, Ronyne, Cavallo, Read, Crosse, Ronalds, Schubler, &c., have made numerous experiments on the electricity of the atmosphere in its ordinary state. Le Monnier discovered that there was always more or less electricity in the atmosphere; that there was a regular diurnal period in which the electricity increased from sunrise, when it was scarcely perceptible, till three or four o'clock in the afternoon, when it reached its maximum; and that it again diminished till the fall of the dew, when it again increased, and subsequently diminished, till midnight, when it became insensible.

M. Beccaria found that the electricity of the air was always perceptible in a clear sky and calm weather. In rainy weather, without lightning, it always appeared a short time before the rain fell, and during its actual fall, but disappeared soon after the rain had ceased.

Saussure made many important observations on this subject. He found that the electricity of the air was very strong at nine o'clock in the morning; that it gradually diminished till six o'clock p.m., when the first minimum took place; that it afterwards increased to eight o'clock p.m., when the second maximum took place. It then diminished again with some irregularities till six a.m., when it reached its second minimum. It then increased again till eleven o'clock in the evening, when it again became a maximum. The electricity of the atmosphere has therefore a daily period, like the sea, increasing and decreasing twice in twenty-four hours. It, generally speaking, reaches its maximum intensity a few hours after sunrise and sunset, and descends again to its minimum before the rising and setting of that luminary. Saussure also observed that the electricity of the air is strongest during fogs, unless when they change into rain. Saussure likewise found that the electricity of clear weather is always positive; and the opinion of Volta is therefore highly probable, that the electricity of the atmosphere is essentially positive, and that the negative electricity which appears in rain, snow, and storms, is derived from more elevated clouds, which are electrified negatively by the discharge of a portion of their electricity into the earth or other clouds, in the same manner as an electrometer acquires negative electricity when it is touched at the instant that the air is electrified positively.

These results were confirmed by subsequent observers, whose observations we have not room more particularly to notice; but we shall make no apology for giving some account of the more recent and valuable observations of Mr Crosse and Mr Ronalds. Mr Crosse's experiments were made with an insulated copper wire, extending originally a mile and a quarter in length, and supported upon two masts from 100 to 110 feet high. The wire was one-sixteenth of an inch thick. It was subsequently shortened to 1800 feet in consequence of its being exposed to depredations. From the observations made with this apparatus, which was in use eighteen months, Mr Crosse deduced the following conclusions.

1. The electricity of the atmosphere in its ordinary state is invariably positive. It is always most copious during the night. It increases at sunrise, diminishes towards noon, increases again towards sunset, and again diminishes to its nocturnal minimum.

2. The electrical state of the wire is disturbed by fogs, rain, hail, snow, and sleet. It becomes negative when they first come on. It frequently changes to positive, increasing gradually in strength, and then decreasing, a change from positive to negative occurring every three or four minutes.

3. The approach of a charged cloud at first sometimes produces positive and sometimes negative electricity. Its intensity increases and then diminishes and vanishes, being succeeded by the opposite electricity, which increases to a higher maximum, and then diminishes and disappears, and is again followed by the electricity which first appeared. In general the electricity increases at every repetition, till sparks issue in a copious stream from the conductor to the receiving hall, sometimes with interruptions, and again returning with fresh energy. When this happens, a powerful stream of air issues from the wire and the connecting apparatus. An explosive stream of electricity rushes from the one hall to the other at every flash of lightning, and a brilliant light is thrown upon surrounding objects. When the lightning increases, it is wise to let it pass into the ground.

4. The wire is almost as strongly electrified during a driving fog and a smart rain as during a thunder storm, and the electricity passes into opposite states in a similar manner.

5. A very feeble degree of positive electricity occurs in cloudy weather. When rain falls it changes to negative, and again becomes positive when the shower is over.

The following table contains a list of the different states of the air in which its electricity appears, those at the top of the list being those in which it is most powerful.

1. Regular thunder clouds. 2. Driving fog with small rain. 3. A fall of snow, or a brisk hall storm. 4. A smart shower in a hot day. 5. A smart shower in a cold day. 6. Hot weather after some wet days. 7. Wet weather after some dry days. 8. Clear frosty weather. 9. Clear warm summer weather. 10. A sky obscured by clouds. 11. Mackerel or mottled sky. 12. Saltry weather with light hazy clouds. 13. A cold damp night. 14. Weather during north-east winds, with a sensation of dryness and cold not shown by the thermometer.

By means of an electrical apparatus, founded on a new method of electrical insulation, Mr Ronalds made some interesting observations on Vesuvius at the time of moderate eruptions, and another series at Palermo during the prevalence of a sirocco. The rod of the electrometer was placed perpendicularly on the highest pinnacle of Mount Vesuvius, on the north side of the great crater, and about five hundred yards distant from it, a ravine being interposed. The following were the results:

1. The electricity was always positive. 2. The intensity of it increased as the sun rose, unless on Vesuvius it was affected by the explosions of the volcano. Very frequent variations took place in the intensity, sometimes accompanying changes of the wind, sometimes following explosions from the crater, sometimes attending the approach of vapour from an aqueous fumerole, when the intensity of the electricity was always increased, and sometimes occurring without any apparent cause. 3. The black fumes from the old crater diminished the intensity more frequently than the white fumes, and very rarely increased it. Mr Ronalds supposes that the black fumes may be in a negative state; and that the white fumes, consisting principally of aqueous vapour, sulphuric and muriatic acids, and sulphur, may, when these vapours are condensed, and when the sulphur sublimes in the air, be brought into a positive state; and that these two states of the two fumes may sometimes act separately on the electrometer, or sometimes wholly and sometimes partially neutralize each other, either by induction or position, or by a discharge from the one to the other.

The observations of Mr Ronalds on the electricity of the atmosphere during a dry sirocco were made on the roof of Page's hotel, in Palermo. The electricity was always positive, the straw electrometer of Volta varying from five to twenty-one degrees. The electrical phenomena were diametrically opposite to those of the ordinary state of the atmosphere in serene weather; as the electric tension increases almost progressively from sunrise till the hottest part of the day, viz., about three o'clock p.m., when it gradually declined until sunset.

In the arctic regions in 1819-20, there were no sensible indications of electricity "in the summer months, when the clouds become more dense and frequent, and even when a slight shower of rain falls."

A series of most interesting observations have been made by Professor Schubler of Tubingen, on the electricity accompanying the condensation of aqueous vapours in the atmosphere, as affected by the direction of the winds. They were carried on during thirty months, between January 1805 and August 1811. The first series was made at Ellvanguen, during sixteen months, from January 1805 to April 1806; and the second at Stuttgart, during fourteen months, from June 1810 to August 1811. Schubler. Ellvanguen is situated 1331 feet above the sea, in 48° 57' 25" of N. Lat., and Stuttgart at 847 feet, in N. Lat. 48° 46' 32". Professor Schubler observed no fewer than four hundred and twelve atmospherical precipitations. He used the straw electrometer of Volta, and a simple condenser; and in storms he never pushed his observations beyond the 600th degree of the instrument.

The following table contains the results of these observations.

| Direction of the Winds corresponding to the Observations | Number of observed Precipitations, classified according to the Nature of their Electricity | Ratio of the Number of Positive and Negative Precipitations | Mean Intensity of each of the two Electricities | Mean Intensity of the Electricity without considering its Nature | Total Number of Precipitations observed | |----------------------------------------------------------|--------------------------------------------------------------------------------|-------------------------------------------------------------|---------------------------------------------------------------|---------------------------------------------------------------|----------------------------------------| | North | Positive: 12, Negative: 11 | Positive: 131, Negative: 99 | Positive: 116 | Positive: 116 | 23 | | North-east | Positive: 11, Negative: 12 | Positive: 105, Negative: 132 | Positive: 120 | Positive: 120 | 23 | | East | Positive: 3, Negative: 5 | Positive: 15, Negative: 13 | Positive: 8 | Positive: 8 | 8 | | South-east | Positive: 4, Negative: 7 | Positive: 19, Negative: 10 | Positive: 13 | Positive: 13 | 11 | | South | Positive: 5, Negative: 13 | Positive: 26, Negative: 23 | Positive: 18 | Positive: 18 | 18 | | South-west | Positive: 23, Negative: 65 | Positive: 68, Negative: 33 | Positive: 93 | Positive: 93 | 93 | | West | Positive: 73, Negative: 106 | Positive: 75, Negative: 39 | Positive: 179 | Positive: 179 | 179 | | North-west | Positive: 25, Negative: 32 | Positive: 31, Negative: 46 | Positive: 57 | Positive: 57 | 57 | | The three north winds, N.W.—N.—N.E. | Positive: 48, Negative: 55 | Positive: 74, Negative: 75 | Positive: 103 | Positive: 103 | 103 | | The three south winds, S.E.—S.—S.W. | Positive: 37, Negative: 85 | Positive: 57, Negative: 26 | Positive: 122 | Positive: 122 | 122 | | The three west winds, S.W.—W.—N.W. | Positive: 126, Negative: 203 | Positive: 57, Negative: 38 | Positive: 329 | Positive: 329 | 329 | | The three east winds, N.E.—E.—S.E. | Positive: 18, Negative: 24 | Positive: 71, Negative: 72 | Positive: 42 | Positive: 42 | 42 | | All the winds | Positive: 161, Negative: 251 | Positive: 69, Negative: 43 | Positive: 412 | Positive: 412 | 412 |

From these observations Professor Schubler draws the following conclusions:

1. The ratio of the positive to the negative precipitations follows a regular variation, setting out from the north or south wind, and proceeding either by the east or west winds. 2. By a north wind, the positive precipitations are a little more frequent than the negative ones; by a south wind, the negative precipitations are more than double the positive ones. 3. The negative precipitations, by the three south winds, viz., south-east, south, and south-west, are double those by the three north winds, viz., north-west, north, and north-east, the ratio being 114 to 230. 4. The east and west winds hold a mean in this respect. The former, however, approach more to those of the north, and the latter to those of the south, the electricity being often negative by the three west winds than by the three east winds in the ratio of 161 to 133. 5. The electricity of all the observed precipitations is often negative than positive in the ratio of 155 to 100. 6. The mean intensity of the positive electricity is, on the contrary, more considerable than that of the negative in the ratio of 69 to 43. 7. The intensity of the electricity, abstraction being made of its nature, is the strongest by the three north winds, particularly the north-east and north. 8. The electricity is at an average the weakest by the three south winds. Its intensity by these three winds in the ratio of 39 to 75 weaker than by the three north winds. 9. By the three east winds the electricity is in the ratio of 72 to 48 stronger than by the three west winds. 10. The mean intensity of the electricity of all the precipitations, whether positive or negative, observed in all directions of the winds, is almost the same as that of the electricity of the precipitations observed during the west winds alone. 11. During the north and east winds the opposite electricities appear most distinctly, and almost with equal intensities. The west winds, and particularly the south, exhibit, on the contrary, a more feeble electricity, but a greater number of negative precipitations. 12. The greatest number of electrical precipitations takes place during west winds, and the least during east winds. The mean direction of the wind during the whole of the precipitations is 86° 9', making use of the formula of Lambert, in which the south is marked by 0°, the west by 90°, the north by 180°, and the east by 270°. The number 86° 9' corresponds with the west with four degrees of declination to the south-west.

With respect to the cause of the phenomena now described, Professor Schubler is of opinion, that at the moment of the precipitation of the vapours in our atmosphere, positive electricity is at first developed, and the negative appears to arise most frequently from the influence of the former. The precipitations which first take place during storms, or passing rains and snows, are commonly positive, and are soon followed with negative ones of nearly equal intensity.

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1 Considering the quantity of the two electricities as made up of their intensity and the number of times that either of them is observed, the ratio of the quantities of positive and negative electricity observed will be 690 or 688, nearly that of equality. tensity. This alternation often happens several times, during which the drops of rain, hailstones, sleet, and snow, continually vary in their size, density, and continuity. At last the electricity, growing weaker and weaker, ends by remaining negative; and sometimes after the storm a rain falls with negative electricity.

It is, however, not uncommon to see regular and continuous rains negative from their commencement, and during whole days. This fact, together with the feeble intensity of this kind of electricity, seems to favour the opinion that it is often owing to the partial evaporation experienced by the drops of rain during their fall. In confirmation of this he adduces the fact of the negative electricity of the fine aqueous dust at the foot of cascades, which is sometimes so strong in large waterfalls as to make the electrometer diverge more than 100 degrees.

This explanation, Professor Schubler alleges, agrees also with the great frequency of negative rains in south winds, and of positive ones in north winds. A current of warmer air, and consequently more light and more elevated, in the first case ought to facilitate the evaporation of drops of rain during their fall; whilst, by the colder north wind, and consequently more heavy and nearer the surface of the earth, the clouds have in general a lower position, and the evaporation of the drops of rain is less easy, and almost nothing.

From these observations it also follows that we must not infer the negative state of the cloud from the negative electricity of the rain which falls from it; for it may happen that rain coming from clouds slightly positive may become negative by the partial evaporation of the falling drops.

M. Schubler also remarks, that the great intensity of the electricity, and the distinct manner in which the two electrical principles alternately predominate during north and east winds, seem to arise chiefly from the dryness of the air during their continuance; to which we must add the situation of the clouds brought by these winds near the surface of the earth, the electricity of which may then naturally exert a more sensible influence upon our instruments.

The positive electricity of the atmosphere was found by Saussure to increase in intensity in proportion to the height at which it was collected. When MM. Gay Lussac and Biot ascended in a balloon, they collected atmospheric electricity from the clouds below them, by suspending a wire about 160 feet long from the balloon, and stretching it with a ball of metal. The electricity collected at the upper end of this thread was very perceptible in their electroscope; and when it was examined with a stick of sealing-wax it was found to be resinous or negative, although the weather was perfectly serene. This result, though apparently inconsistent with the observations of Saussure and others, has been shown by M. Biot to be perfectly reconcilable with them. In fig. 4, Plate CCXXVI., let WW be the wire, let us call A the stratum of atmosphere through which the wire passes, B the stratum above this, and C the stratum below it; and let us suppose, what is true, that the atmosphere has positive electricity, which increases with the height. The positive electricity in the superior stratum A will attract the negative electricity of the wire WW with a force equal to + P, and will repel the positive electricity of WW with a force equal to + N. The positive electricity in the lower stratum C will do the very same, but in an opposite direction, and with an inferior degree of force, viz. + p and + n, since the electricity increases with the altitude. Hence it follows that the negative electricity of the wire will be attracted towards its upper end by an excess of force equal to P - p, and the positive electricity will be repelled to its lower end with an excess of force N - n.

To MM. Gay Lussac and Biot, therefore, who observed the electricity of the wire at its upper end, the electricity should be negative; and to M. Saussure and others, who observed it at its lower end, it should be positive.

Upon the same principle, M. Biot explains a very interesting experiment made by M. Hermann. A very sensible electroscope with gold leaves is fixed at a certain height in the atmosphere when the weather is clear, and it there gives no perceptible indications of electricity. A metallic wire, or any other conductor, placed horizontally at the end of an insulating rod, is then placed and kept a short time in a stratum of air a few feet only above the electroscope. It is afterwards quickly brought down so as to touch the electroscope, and the gold leaves diverge with vitreous or positive electricity; but if, on the contrary, the insulated wire is placed and kept a short time in a stratum of air below the electroscope, and is then quickly raised and made to touch the electroscope, the leaves will diverge with negative electricity. In order to explain these opposite explained results, we must consider that the insulated conductor is by Biot charged at each time with the degree of electricity which belongs to the stratum in which it is placed. When it is carried rapidly, therefore, so that its state is not quite destroyed by the contact of the molecules of air among which it is placed, it will communicate this state to the electroscope. If it comes from above, it will carry to it an excess of positive electricity; if it comes from below, it will carry to it a defect of the same electricity, or an excess of negative electricity. "In general," says M. Biot, "let + E be the quantity of free vitreous electricity which the insulated conductor ought to possess, in order to be in a state of electrical equilibrium in the stratum of air where the electroscope is placed, so that whilst it has + E, the molecules of air of this stratum neither give nor take anything from it. Let it now be carried to a superior stratum, where it takes E + δE, δE indicating the small excess of vitreous electricity which it has there taken. If we then bring it back quickly into the stratum of the electroscope, it will have + δE too much, and it will communicate this excess to all bodies that touch it. It will communicate it also to the electroscope if it touches it promptly, and, until the latter has lost by the contact of the air this excess which it has imparted, its leaves will diverge vitreously. On the contrary, when the insulated conductor returns from the lower region, it has E - δE of vitreous electricity. If we make it touch the electroscope, the latter will partake of its state. Then the quantity of vitreous electricity which it will possess can no longer be in equilibrium with the influence of the mass of the surrounding air, and its natural fluid will be decomposed. But the excess of vitreous fluid which will result from this cannot cause the gold leaves to diverge, because its repulsive force will be wholly employed in compensating that of the exterior electricity E. The repulsive force, then, of the resinous electricity will alone be exerted, because nothing compensates it; and the gold leaves will diverge in virtue of this electricity, until it has been carried away and neutralized by the immediate and successive contact of the molecules of air. Experiments of this kind present the unique case of an indefinite medium, which is air, of which all the molecules are individually charged with an excess of electricity of the same kind, adhering to their surface; so that the entire mass of the medium is found penetrated with it in a proportion which varies with the altitude. Consequently the different particles of this medium can only be at rest from the mutual compensation of their repulsive

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1 M. Delarive is of opinion, not only that the evaporation thus occasioned must be very feeble, considering that the air is charged and almost always saturated with humidity; but that if it did take place, it could not generate electricity, as M. Pouillet has shown that the conversion of pure water into vapour produces no electricity. He is disposed to seek for the cause of the negative electricity of rain either in the mechanical action of the air on the falling drops, or in the sudden change of temperature which they experience. forces combined with their gravity; and the same condition is also applicable to conducting bodies which are immersed in it. Thus, for all these bodies the electrical equilibrium cannot take place when their natural electricities are completely neutralized, but only when they possess an excess of either electricity which corresponds to the stratum where they are found, an excess which is vitreous in the atmosphere when it is pure. If they possess a greater excess of this same electricity, they will act only in virtue of this excess upon each other, and upon all the molecules of the surrounding air. They ought, therefore, to repel one another mutually. If, on the contrary, the excess of electricity which they possess is less than that which they would naturally take in the stratum where we place them, the mass of the medium will act upon each of them in virtue of this difference, and their natural electricities will be decomposed as far as is necessary to supply what they want of the electricity of the medium. In virtue of this addition they will repel the medium as much as the medium repels them, and will experience no more action from it. But they will act upon each other with the excess of opposite electricity which they have acquired; and if the medium is an indefinite fluid composed of particles susceptible of electrifying itself by contact, this excess will gradually dissipate in space. Many curious experiments would be necessary to establish the laws of electrical equilibrium, under circumstances sufficiently different from those which we have been generally accustomed to consider.

The electricity of clouds was noticed by some of the earliest writers on the electricity of the atmosphere. Canton observed that certain clouds were charged with positive and others with negative electricity; and he noticed that the electricity indicated by his apparatus often changed five or six times in half an hour. This fact was confirmed, as we have seen, by the observations of Mr Crosse. These irregularities, however, remained unexplained till Mr Luke Howard distinctly proved that the electricity at the circumference of a nimbus is negative, while that of the centre is positive; and he suggested it as an interesting subject of inquiry to ascertain if the negative electricity was descending and the positive ascending. Mr J. Foggo undertook this inquiry, and in 1823 he erected a conductor armed with a smoking match, and erected from a south window. On the 12th of March 1824 there was a brisk wind from the north-west, with frequent showers all around. About three p.m. large dense clouds, which discharged heavy showers of hail, passed over the zenith. Between the showers the electricity was always positive, and the leaves of the electrometer showed their maximum divergency. So powerful indeed was the electrical state of the air, that by rubbing the outside of the glass of a detached electrometer with soft leather, the leaves opened more than forty degrees. During the showers, or when the clouds were over head, though no precipitation took place, the electricity was invariably positive, and so strong that sparks could at any time be drawn by the finger from the conducting wire. Mr Foggo likewise ascertained that by taking hold of the wire he could at pleasure intercept the electric fluid from reaching the instrument, so that the charge must have been received from the atmosphere or cloud. When the edge or the circumference of the cloud was nearly over the conductor, the electricity became negative, and appeared to be fully as strong as when it was positive. Mr Foggo, however, now found that it could not be intercepted as formerly by taking hold of the wire, or by touching it with a pointed steel rod. Hence he concluded that the electricity was not proceeding from the cloud as before, but was given off by the earth to the cloud. When the steel point was presented to the instrument, the divergence was so much increased as to endanger the gold leaf, and sparks were heard to pass rapidly between the point and the electrometer, while sharp pricks were experienced when the finger was brought near the brass cap.

The subject of the electricity of clouds has been lately studied by M. Quetelet, who has obtained some very interesting results. According to his observations, very great mistakes have been committed in the classification of showers of rain into positive and negative. During the same shower we may have positive or negative electricity according to the time when we make the observation. When the rain is falling, the electricity is very powerful; but if we collect it when there is an inversion of the sign, it may be nothing next to nothing. When the air is clear, the upper strata are positively electrified in reference to the lower, the intensity increasing upwards. In order to understand what takes place when a storm cloud passes above any place, let

\[ \text{ABCDE} \]

be the ground in a neutral state, and \( A'B'C'D'E' \) a stratum of the atmosphere positively electrified when no cloud is there; and \( A''B''C''D''E'' \) the stratum above it, also electrified positively, but in a much higher degree. If a cloud \( B'C'D' \) arrives surcharged with positive electricity, its charge will be unequally distributed; it will be strongest in the upper part, and the cloud will be enveloped with strata of air which are relatively negative, and the more so as the electricity of the cloud is more powerful. To an observer placed at \( A \), the electroscope above the ground will give signs of positive electricity. These indications will grow feebler as the cloud approaches; then the electricity disappears and becomes negative, which will continue even during the commencement of its passage. The electricity will then gradually diminish, then disappear, and resume its positive state when the rain commences. It will return to its first value, after having passed through the same phases, when the cloud is sufficiently distant to have no influence. The sphere of activity of a cloud is sometimes so great as to extend to several leagues. It is not uncommon to observe clouds in the horizon indicating their presence by signs of negative electricity.

M. Quetelet explains more minutely what takes place when the cloud is positively surcharged, and the rain falls. The descending shower will carry to the earth the electricity of the cloud, and with more abundance as the rain is heavy. While only a few drops have fallen, they tend only to paralyze in part the effects of the negative atmosphere which encircles the cloud, and which acts on the electrometer, which, if now observed, might lead us to believe that the rain is negative. The change of the signs of the electricity is to a certain extent gradual. In a heavy shower it is almost always instantaneous, and the passage through zero cannot be observed. In this state of matters all the observers below the cloud, and where it rains most strongly, ought to find the electricity positive. On the margin of the region where they are placed, the electrometer will mark zero, and then it will indicate negative electricity more or less powerful. The annexed figure will show by the signs + 0 — the indications of the electrometer at the surface of

the earth on the passage of the storm cloud, or more or less in its vicinity. When the cloud is very low, so as to touch the ground, the electrometer will indicate the same electricity as that of the cloud—a fact shown in fogs, which always have an intense positive electricity; but when the cloud touches the ground it ought to lose rapidly its electricity.

When very high positive clouds give only a few drops of water, the negative atmosphere which surrounds them cannot extend its action to the earth, especially if its electricity is feeble.

When the cloud encounters mountains, it moves towards them more quickly in proportion to the negative electricity of their summits, and adheres to them like moderate conductors, abandoning successively its electricity. M. Quetelet informs us that M. Palmieri has obtained similar results, with this difference, that he seems to go too far in denying the existence of clouds charged with negative electricity, and limiting the period during which positive electricity is observed to the time during which the rain falls. M. Quetelet has frequently observed the rain falling when the electrometer indicated negative electricity.

Such are the general electrical phenomena of the atmosphere during its ordinary changes; but they appear with new splendour, and at once rouse the interest of the philosopher and the dread of the vulgar, when they are exhibited in the terrific grandeur of thunder and lightning. We have already seen that various writers had pointed out the identity of lightning and the electric spark; and though Franklin has obtained the special honour of having been the first who brought down fire from heaven,

Arripuit fulmen coelo, sceptrumque tyrannis,

yet he was no more the first who snatched the thunderbolt from heaven, than he was the first who wrested the sceptre from kings.

When Franklin called the attention of philosophers to the various points of resemblance between lightning and the electric spark, he conceived the idea of collecting the electricity of the atmosphere by means of pointed conductors, and of thus preserving buildings from its explosions. One of the first philosophers who endeavoured to verify these views was M. Dalibard, who, at the instigation of Buffon, erected an atmospherical conductor at Marly le Ville, about six leagues from Paris. An iron rod, forty feet long, an inch in diameter, and pointed at its upper end, was erected in a garden upon three large poles, and insulated by silken strings, and a stool with glass feet. In M. Dalibard's absence a thunder-storm appeared on the 10th May 1752, between two and three p.m., and M. Coiffier, who had the charge of the apparatus, drew sparks with a crackling noise from the lower end of it. Having called M. Raulet, the curate of the parish, this gentleman continued for some time, and in the presence of many of his parishioners, to draw large sparks of bluish fire from the conductor. A few days afterwards, on the 18th May, M. Delors drew similar sparks from a rod ninety-nine feet high, erected in Paris. The strongest of them were drawn at the distance of nine lines, and the conductor afforded sparks even when the cloud had moved at least two leagues from above the place of observation. On the 19th day Buffon obtained, at Montbar, similar evidence of the identity of electricity and lightning.

In our history of electricity we have already given an account of the observations made with the apparatus by which M. Franklin, in the month of June 1752, obtained sparks of electricity from the atmosphere during a thunder-storm. Attempts were everywhere made to repeat this remarkable observation; and the most successful of these was that of M. Romas, who, according to a decision of the Academy of Sciences, had invented the electrical kite more than a year before it was employed by Dr Franklin. The kite constructed by M. Romas was seven feet five inches high, three cal kites feet in its greatest width, and with a surface of eighteen square feet. The string was a cord wrapped round with copper wire. On the 7th June 1753 this kite was elevated to the height of 550 feet, by means of a string 700 feet long, and inclined 45° nearly. A silk cord three feet and a half long was fixed to its extremity, and suspended a large stone to govern the motion of the kite. A tube of white iron, about a foot long and an inch in diameter, was placed near the junction of the string and the silk cord, as a conductor, from which the sparks were to be drawn. From this conductor the spectators drew sparks with their fingers, keys, canes, and swords; and M. Romas having presented his knuckle, received a shock which struck him in the elbows, shoulders, breast, knees, and ankles. Seven or eight persons joined hands, and the shock struck the feet even of the fifth person. The storm now increased, and black clouds gathered in the zenith. At the distance of six inches sparks two inches long were obtained by a discharging rod. The electricity continuing to increase, flashes of fireabout a foot long, three inches wide, and three lines in diameter, were frequently received, and the noise of them was audible at the distance of 500 feet. At this time he felt the sensation of a spider's web on his face when he was five feet from the string. The kite was now 650 feet high, and the wind blowing strong from the east, when M. Romas saw on the ground, about three feet from the white-iron tube, three straws dancing up and down below it. One straw was twelve, another five, and the third four inches long. The electricity having increased still more, the longest straw was attracted by the tube, accompanied with three loud sounds, which some compared to the crack of a postilion's whip, and others to that of a large pot of earthenware dashed in pieces on a pavement. This crash was heard even in the centre of the town, and the accompanying flash had the form of a spindle eight inches long and four or five lines in diameter. The long straw followed the string of the kite, and was seen moving with great rapidity even at the distance of ninety or a hundred yards, now attracted and now repelled by the string, each attraction being attended with long plates of fire and constant explosions. A phosphoric smell was distinctly felt. A permanent cylinder of light, about three or four inches in diameter, surrounded the string.

M. Romas again raised his kite on the 16th August, and though the storm was not severe, yet in an hour he obtained thirty beams of fire, nine at ten feet long, and about an inch thick, each accompanied by a noise like that of a pistol. When the glass of his discharging-rod was two feet long, he was able to conduct beams of fire six or seven feet long as easily as he had done those of seven or eight It is obvious, from the preceding facts, that the well-known phenomenon of thunder and lightning is entirely an electrical one, the lightning being the electric spark, and the thunder the sound which accompanies it prolonged by successive echoes from among the clouds. That the clouds are capable of reflecting sound was determined by direct observation on the sound of cannon, made by Messrs Arago, Matthieu, and Prony. They observed that in a perfectly serene sky the explosions of their guns were always single and sharp, whereas when the sky was overcast, or when a cloud came in sight and covered any considerable portion of the horizon, the sound of the gun was attended by a long-continued roll like thunder; and sometimes a double sound was heard from a single shot. Sir John Herschel, however, has pointed out another cause for the rolling of thunder, as well as for its sudden and capricious bursts and variations of intensity. "To understand this cause," says he, "we must premise that, ceteris paribus, the estimated intensity of a sound will be proportional to the quantity of it (if we may so express ourselves) which reaches the ear in a given time. Two blows, equally loud, at precisely the same distance from the ear, will sound as one of double the intensity; an hundred struck in an instant of time will sound as one blow a hundred times more intense than if they followed in such slow succession that the ear could appreciate them singly."

Now let us conceive two equal flashes of lightning, each four miles long, both beginning at points equidistant from the auditor, but the one running out in a straight line directly away from him, the other describing an arc of a circle having him in its centre. Since the velocity of electricity is incomparably greater than that of sound, the thunder may be regarded as originating at one and the same instant in every point of the course of either flash. But it will reach the ear under very different circumstances in the two cases. In that of the circular flash, the sound from every point will arrive at the same instant, and affect the ear as a single explosion of stunning loudness. In that of the rectilinear flash, on the other hand, the sound from the nearest point will arrive sooner than from those at a greater distance; and those from different points will arrive in succession, occupying altogether a time equal to that required by sound to run over four miles, or about twenty seconds. Thus the same amount of sound is in the latter case distributed uniformly over twenty seconds of time, which in the former arrives at a single burst; of course it will have the effect of a long roar, diminishing in intensity as it comes from a greater and greater distance. If the flash be inclined in direction, the sound will reach the ear more compactly (i.e. in shorter time from its commencement), and proportionally more intense. If (as is almost always the case) the flash be zigzag, and composed of broken rectilinear and curvilinear portions, some concave, some convex to the ear; and if, especially, the principal trunk separates into many branches, each breaking its own way through the air, and each becoming a separate source of thunder, all the varieties of that awful sound are easily accounted for.

The distance of the point in the atmosphere where the lightning is generated, may be readily computed by multiplying 1090 by the number of seconds which elapse between the flash and the first stroke of thunder. The product will give in feet the distance required.

The general phenomenon of thunder and lightning occurs during the passage of electricity between two clouds oppositely electrified, or one of which has an inferior charge of the same kind of electricity; but it appears in its most appalling form when the accumulated electricity of the clouds descends to the earth, shivering the strongest oak in its passage, rending the thickest walls, setting fire to houses, or stacks, or forests, and instantly destroying animal life, when the frail tenement of man or of beast happens to obstruct its path, or afford to it a more easy transit. Sometimes, however, the thunderbolt passes from the earth to the clouds, and in this case it is called the ascending thunderbolt. The Marquis Massé was the first who observed this curious phenomenon. He distinctly saw during a storm the lightning issue from the ground with a loud noise. The Abbé Lionni and M. Seguin of Nismes saw the lightning rise in the form of a flame six feet high, followed by a loud noise.

One of the most interesting cases of the ascending bolt has been recorded by John Williams, Esq. It took place upon the hills above the village of Great Malvern, on the 1st of July 1826. A party had taken refuge from the storm in a circular building roofed with sheet iron, and one of the ladies on entering the hut expressed her alarm lest the lightning should be attracted by the iron roof. They had scarcely entered their retreat, and were about to partake of some refreshment, when a violent storm of thunder and lightning came on from the west. About forty-five minutes past two, a gentleman who stood at the eastern entrance saw a ball of fire which seemed to him moving on the surface of the ground. It instantly entered the hut, forcing him several paces forwards from the doorway. On his recovering from the shock, he found his sisters on the floor of the hut, fainting, as he imagined, from terror. Two of the ladies had died instantly; another lady, and the rest of the party, were much injured. The explosion which followed the flash of lightning was said by the inhabitants of the village to have been terrific. Mr Williams, who immediately examined the hut, found a large crack in the west side of the building, which passed upwards from near the ground to the frame of a small window, above which the iron roof was a little indented. Mr Williams conceived it to be quite clear, from the place of the fragments of stone and other appearances, that the clouds were negatively electrified during this storm.

Various electrical phenomena of a very interesting kind have been observed by travellers when ascending lofty mountains. In 1767, MM. Saussure, Picot, and Jailabert, on mount Etna during a storm of thunder and lightning accompanied Etna; by a heavy fall of snow. One of the party felt his hair moving, and upon raising his hand to his head a buzzing sound issued from his fingers. The rest of the party experienced the same sensations, and by moving their hands and fingers they produced a variety of musical sounds, audible at the distance of forty feet. On the 27th of June 1825, Dr Hooker and a party of botanists witnessed effects like those described, during a fall of snow on Ben Nevis when on Ben there was no thunder-storm. The snow fell very heavily Nevis for nearly two hours. Soon after it began, a hissing sound was heard everywhere around them, and continued about an hour and a half. It seemed to proceed from every point in the vicinity; and on arriving at the cairn on the summit of the mountain, they could almost determine the stones from which the electricity issued. The hair of several of the party exhibited, when touched, the usual electrical phenomena.

A very remarkable phenomenon of the same kind was observed by General Pollock, when in the command of a division of our Indian army and stationed at a fort about forty miles from the Khyber Pass, where the soil is an extended plain of sand. About the end of April 1842, when there was not a cloud in the sky, the air was so charged with electricity that the musket, with a fixed bayonet, of a European soldier on duty became so electrical as to emit a succession of sparks from the barrel when any conducting body touched it. General Pollock drew from it with his knuckle several powerful sparks. The barrel of the musket was insulated by the stock, which was made of wood from the Sipoo tree.

Before quitting the subject of lightning we must submit to our readers a brief account of the remarkable observations made by M. Fusinieri on the ponderable substances transported by lightning, and which it deposits in a permanent state on the bodies which obstruct its passage. When we consider the magnitude of the scale on which the great electrical machine of our atmosphere enables us to study its effects, it appears strange that so little attention has been paid to those interesting phenomena which accompany the electric stroke. M. Fusinieri is the only person who has made this an object of special investigation; and the results to which he has been led possess, as might have been expected, a very peculiar interest. The following are the general results which he obtained: Lightning contains, like the common electric spark, matter in a state of extreme division, and in a state of ignition and combustion. In the matter deposited by lightning on houses and on trees which have been struck by it, he has found iron, sulphur, and carbon. Lightning divides and subdivides itself indefinitely into sparks, which end in being not much larger than those of ordinary machines; and each of these sparks contains ponderable substances in the state of extreme division already mentioned. The lightning deposits the substances with which it is charged while it passes through them, and while it breaks hard bodies; and it deposits them on the surface by which it enters the body, as well as on that by which it escapes, and also on the surfaces of fracture. When the resistance to its passage is not great, it leaves no perceptible deposit; and the quantity of matter deposited increases, and is proportional to the difficulty with which the lightning traverses the body. At the same time that lightning deposits the matter which it contains, it takes up new matter from the combustible bodies, such as iron, charcoal, &c., through which it passes. The deposited matter tends always to expand itself in thin films on the surface which receives it, and it does this most readily on surfaces that are smooth and free from all asperities.

In examining the traces left by lightning when it fell at Vicenza in 1829, and at Padua in 1831, M. Fusinieri made the following observations: It deposited, on the surface of a wall by which it entered the house, a thin layer of pulverulent matter, of a brown colour at its centre, and yellowish and much less deep at its margin. When this matter was collected and carefully examined, it proved to be iron in different degrees of oxidation. Upon some stones which the lightning had detached from the wall there was found a stratum the fifth part of an inch thick, and of a brownish colour, which seemed to have undergone a species of fusion. This stratum was sulphuret of iron, which gradually changed into a sulphate of the same metal. M. Fusinieri indeed had previously found small crystals of sulphuret of iron upon an iron rod which the lightning had struck, and also upon a stone to which it had passed from the iron. The position of these crystals indicated that they had been formed in the middle of the passage of the lightning; a fact which he considered as proving that the electric matter could transport sulphur across metal itself. When the lightning escaped from the wall, it deposited upon the wood a dust composed of small aggregated grains, which had all the qualities of ferruginous matter. In pursuing the passage of the lightning, it was found to have divided itself into a great number of sparks more or less voluminous upon the windows, formed of pieces of rectangular glass united in a leaden frame. The traces left on the glass and on the lead were very slight, and there were only a few marks on the glass very near its contact with the lead. The traces on the lead were small cavities, round which there had been a fusion of the metal, means of Some of these cavities passed through the whole thickness of the lead, and their diameters varied with the size of the sparks that had produced them. In general, each cavity of any size was surrounded with several smaller cavities, which seemed to prove that each discharge was accompanied by smaller electric sparks disseminated around it. Besides these cavities, the lightning had spread on the surface of the metal a stratum of pulverulent matter, which adhered so strongly to the lead that none of it could be detached without removing at the same time a portion of the metal. Each large cavity was the centre of one of these strata, which appeared to be composed of globules of lead in the central part, and ferruginous dust on the margin. The glass, though an insulating body, was, as we have mentioned, marked also by the lightning. The origin of the thin strata formed on its surface was at those points where it had been in contact with the lead; but they extended much beyond this, and were composed at first of a powdery matter, sometimes blackish and sometimes whitish; and beyond this they terminated in continuous and diaphanous laminae, which reflected the colours of thin plates. The central and pulverulent portion was lead; the exterior portion, and the thinnest, appeared to be iron more or less oxidated. On one occasion one of these thin plates was formed of an extremely thin stratum of metallic iron not oxidated. M. Fusinieri had formerly succeeded in diffusing metals in thin plates upon mercury by the common electric spark; and he considers the fact, that the same phenomenon takes place on glass as on mercury, as demonstrating that the effect is not owing to a molecular attraction of the surfaces, but solely by the property of expansion which is possessed in a state of fusion by those substances which are transported by the lightning. This property belongs in an especial manner to combustible bodies, particularly to metals, though these last do not all enjoy it in the same degree. Iron, for example, is more expandable than lead, as is demonstrated by the thin films which are deposited by electrical discharges.

M. Fusinieri next proceeds to describe the traces of iron, &c., which lightning deposits upon trees. By means of chemical re-agents and the magnetic needle he had previously determined that traces of iron had been left by lightning on two poplars and a pear-tree which it had struck; and he also found traces of sulphur at the extremity of the roots of a poplar tree, at which the lightning had escaped. These observations were confirmed subsequently by many others. A poplar having been struck at Casale, near Vicenza, on the 14th May 1829, M. Fusinieri found that the part of the trunk deprived of its bark was covered with small black spots, which he regarded as produced by the sparks already mentioned which had been disseminated by the electric current at the instant its bark was carried away. The bark itself must have been reduced into extremely small parts, and immediately consumed, for not a vestige of it could be found. It would appear also that the lightning had carried away a part of the wood which it decomposed, such as the carbon, while the rest was volatilized. Traces of sulphur were found at the foot of the tree; and the lightning having insinuated itself between the bark and albumen of the roots, there was felt, by removing the former, a strong odour of sulphuretted hydrogen, similar to, though more powerful than, that which the traces of sulphur had left upon the ground. The roots torn asunder by the lightning were impregnated with a

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1 Sir W. Snow Harris's Rediscovery of Electricity, p. 177, 3d edit., Lond. 1853. moist and brownish matter, which was extraneous, but which had penetrated into their organic tissue with the lightning which conveyed it. This matter exhaled the same fetid odour as the surrounding earth, especially that portion of the earth which, from being in contact with the roots, was impregnated with the same brownish matter. In penetrating farther into the earth, it was found traversed by serpentine furrows, covered with a cinereous matter, the odour of which was the same as that which was exhaled by the other traces of lightning. The serpentine form of these furrows clearly indicated the tendency of the lightning to disseminate itself. All these substances and deposits were carefully collected and examined by M. Fusinieri.

In a pear-tree which had been struck with lightning in 1827, M. Fusinieri discovered very remarkable effects. Though its trunk, three feet in diameter, was torn into four parts throughout its whole length, no foreign matter nor odour could be perceived either in its roots or in the earth. At the places where the branches joined the trunk, the substance of the pear-tree was altered to the depth of several lines. It had acquired an acid taste and a reddish colour. It exhaled while burning a penetrating and peculiar odour, and it continued to burn without flame till it was completely consumed. The matter of the lightning had therefore penetrated the tissue of the wood, and there presented traces of iron.

M. Fusinieri has collected and detailed many interesting observations respecting the substances deposited by lightning on the various parts of houses which have been struck by it; but we regret that our limits will not permit us to pursue any farther this most important subject.

These and many other facts seem to prove that iron exists in the air and in clouds; and it is well known that the same metal mixed with manganese, nitrous salts, and organic substances, is found in rain water. M. Fusinieri is of opinion that the iron has been drawn from the earth, and chiefly from mountains, where the mines are most frequent, and where storms commonly begin to form. The colouring matter of snow and rain, and the existence of meteoric stones, prove the existence in our atmosphere of dry and ferruginous vapours, the molecules of which are more or less rarefied or condensed according to the causes which may generate them. The fact that meteoric stones fall during the prevalence of storms and other electric phenomena, and especially the fact that hailstones have sometimes a nucleus of small pieces of sulphurite of iron, appear to M. Fusinieri to afford the true origin of these remarkable bodies. It has been already proved also, that electricity does transport matter; and when we consider, as Ampère has shown, that magnetic currents surround our globe, that matter in an extreme state of subdivision spontaneously expands itself, that radiating heat, like electricity, transports ponderable substances, we may obtain a very simple explanation of the origin of meteoric stones. As the temperature of the surface of the globe is not high enough to detach from it the material bodies which exist in the atmosphere, M. Fusinieri concludes that we ought to attribute this action to other causes, which are yet to be discovered, rather than deny a fact so completely demonstrated.

The effects produced by lightning may be divided into mechanical, physical, and chemical. The mechanical effects of lightning are very powerful. Trees have not only been cleft and crushed to pieces, and their branches thrown to a great distance, but in some storms the sap of the tree has been drawn into steam and the dried trunk of the tree split up, as it were, into bundles of fibres like lucifer matches. When spires and elevated buildings are struck, large masses of stone are displaced or thrown down; and when houses have been injured, the articles of furniture are displaced, and even fastenings torn from the walls. M. Pouillet mentions a case, without giving the authority, where a brick wall several toises long was torn up from its foundation and carried in one piece to the distance of several toises. Effects so powerful as these are produced, as Pouillet has shown, by the sudden and simultaneous decomposition of the natural fluids of the body, which is seized with such violence that the arrangement ordinarily produced by the laws of equilibrium has not time to be effected, and the bodies are thus impelled by forces incomparably greater than those which could be resisted by the air.

The physical effects of lightning are generally limited to the production of such a degree of heat as is sufficient to set fire to the roofs of houses where there is straw or dry wood, and to the fusion of metallic bodies, such as bell wires. In some cases traces of carbonization have been found on trees struck by lightning. Effects still more powerful were observed by Dr Withering on the 3rd September 1789. A man with a staff in his hand was killed when standing beneath an oak struck with lightning. The electric fluid which passed along the staff excavated a hole five inches deep and two and a half in diameter. The hole itself contained some roots of burnt grass, but upon subsequently turning up the ground it was found to be blackened to the depth of ten inches, and two inches below this the quartzy earth presented distinct traces of fusion. Among the specimens presented to the Royal Society by Dr Withering were a siliceous stone, one of the angles of which was completely fused, and a lump of sand agglutinated by the heat, in a hollow of which the siliceous matter had run, when melted, along the cavity, and formed a globular portion at the bottom.

By the same powerful agency are produced what are called fulminary tubes, which have been found in beds of sand in Cumberland, Silesia, Eastern Prussia, and near Bahia in Brazil. These tubes are in general about two inches in diameter externally, over two-tenths in their interior diameter, and about ten or twelve yards long. They are produced by the passage of the electric fluid through the sand, the particles of which are melted and agglutinated by its heat. Dr Fieger, who has described many of these tubes from different localities, has observed that at a certain depth below these plains of sand there are little portions of water, and he ascribes the tubes as produced by the passage of the electric fluid from the surface of the ground to these portions of fluid where it is neutralized. M. Hachette conceived the idea of imitating these tubes by using a strong electrical battery; and he and M. Savart and M. Beaudant having placed a quantity of pounded glass produced in a hole made in a brick, and having caused the electrical artificially discharged by the battery to pass through the pounded glass, they succeeded in forming tubes exactly similar to those formed by atmospherical electricity. One of those which they made was an inch long, its external diameter varying from one-sixteenth to one-eighth of an inch, and its internal diameter being the fiftieth of an inch. In another experiment, where a little chloride of sodium was mixed with the pounded glass, the length of the tube was an inch and a fifth, and of uniform diameter. Its mean external diameter was one fifth of an inch, and its internal diameter one twentieth of an inch. When they used powder of felspar or pounded quartz, they could not succeed in making the tubes.

The repeated discharges of lightning against the summits of lofty mountains have partially fused the hard rocks of which they are composed. Saussure has observed these effects on the summit of Mont Blanc composed of schistose

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1 The returning stroke of lightning, when it passes from mountains or places containing iron and other metals, must necessarily carry along with it these substances in a state of extreme subdivision. See p. 584. amphibole, Ramond on the Pic du Midi formed of micaeous schists, and on the highest point of the volcano of Toluco MM. Humboldt and Bonpland observed more than two square inches of the rock vitrified, and in several places cavities in the interior of which there was a vitreous crust.

One of the most remarkable effects of lightning which we have seen was produced in a hay-stack which was struck with lightning in the parish of Dun, near Montrose. The stack was perforated as with a red-hot bolt from top to bottom, and at the end of the perforation where the electric fluid entered the ground, there was found a vitreous mass about the size of an infant's hand formed by the siliceous mass which exists in the hay. This specimen was in our possession for some years, and was presented to a public museum.

In order to give some idea of the tremendous agency of lightning, we shall give an abridged account of the disaster which befell the village of Chateauneuf-les-Moustiers, in the department of the Lower Alps, on the 11th July 1819, as communicated to the Academy of Sciences by M. Trancolye. About half-past two o'clock on Sunday, the curate of Moustiers accompanied a procession to church to install a new rector. The weather was fine, with only a few large clouds in the sky. While a young man was chanting the epistle there was heard three claps of thunder with hardly any interval. The missal was carried away from the hands of the young man, and dashed in pieces. He himself felt as if his body was grasped tightly by the flame, which seized him by the neck. By an involuntary movement this young man, who had at first cried loudly, shut his mouth, and being thrown down was rolled upon the persons in the church, who were all thrown on the ground, and forced out at the door. Upon coming to himself again he returned to the curate, whom he found suffocated and insensible. He raised him up, put out the fire on his surplice, and restored him to life by means of vinegar about two hours after he had been stunned. The electric fluid had struck the upper part of the gold lace of his robe, ran down his arm, carried off one of his shoes to the very end of the church, broke his metallic buckle, and broke the chair on which he sat. His arms were all scarred with the lightning, and the wound did not heal for two months. A child was driven some paces from its mother's arms. Every person had their limbs paralysed. All the women were hideously dishevelled. The church was filled with a black and thick smoke, and objects were visible only by the flames of dresses set on fire by the lightning. Three persons were killed, and eighty-two wounded. The officiating priest was not struck, probably from his wearing a silk gown. All the dogs were killed on the spot where they lay. A person at a distance from the village saw three masses of fire descend upon it. The lightning first struck the cross of the spire, which was thrown to the distance of eighteen yards. An excavation of nearly two feet was made in the church, and continued under the pavement of the street, and another went beneath a stable in which five sheep and a horse were found dead.

Among the phenomena of atmospherical electricity, one of the most interesting is the production of hailstones, particularly those of an enormous size. The connection between the formation of hail and an atmosphere highly charged with electricity has been long ago recognised; but our almost total ignorance of the subject may be inferred from the character of the hypotheses which have been framed to account for the production of hail. Volta supposes that a small globule of snow becomes a hailstone, gradually increasing in size by being kept in a state of reciprocating motion between two clouds charged with opposite kinds of electricity, until the gravity of the constantly increasing mass overpowers the electrical force, or till the electricity of the clouds is spent by their mutual action. M. Matteucci has justly objected to this strange hypothesis, that it is difficult to conceive how a hailstone nearly two pounds in weight could be formed by such a process. He denies that the clouds possess an electric force sufficient to produce such an effect; and, admitting that such a force does exist, he maintains that the electricity of the clouds would be directly discharged from the one to the other, hall. M. Matteucci conceives that the hailstones are produced instantaneously, and that they fall completely formed. He considers that there can be only two epochs in their formation, viz. the production of the snowy nucleus; and, secondly, that of the icy crust which covers it. When a cloud has its temperature greatly reduced, it is easy to conceive its surface covered with small flocks of snow; and if an electrical discharge should in this case pass through it, it would give rise to hail, by obliging the cooled vesicles to condense round each snowy nucleus. It is this shock, he observes, which is necessary to destroy the inertia of the particles, which ought to unite to each other, as seen in the experiment of the congelation of water with the cryophorus of Wollaston. M. Matteucci was led to these views by studying the hailstorm which took place at Tussi on the 24th July 1832. About six o'clock A.M., after a brilliant sun, the whitish and scattered clouds were seen suddenly to unite and to form a thick mass scarcely detached from the horizon, and which covered the country with a thick darkness, that continued uninterruptedly by the effects of strong electrical discharges. An impetuous north-west wind soon rose, and was followed by copious rain mixed with hail. This storm, which lasted about fifteen minutes, was followed by a lucid interval, after which there fell a thick snow, which ceased and began again several times. "I do not wish," says M. Matteucci, "to cite any facts which might appear fabulous; but it appears certain that a hailstone was found which weighed fourteen pounds, and that another in falling upon a house forced its way through the roof; that trees from three to six centimeters in diameter were destroyed; that oxen were wounded, and that several walls were overthrown or rent by the force of the hail. I state as certain, the fact that hailstones collected a few instants after their fall still weighed a pound and a half. M. Pouillet assures us that he can himself certify that hailstones have fallen half a pound in weight. I can certify that they have fallen three times that weight."

In consequence of the demonstrated connection between Paragrele, hailstones and a certain electrical state of our atmosphere, M. Lapostolle, professor of physics at Amiens, proposed to protect vineyards and other cultivated grounds from the destructive effects of hail, by erecting wooden poles twenty-five feet high, for the purpose of carrying off the atmospherical electricity. The use of these hailrods has extended itself over France, Switzerland, Germany, and Italy; and it is not easy to resist the evidence that has been collected of their efficacy, notwithstanding the opposition that they have met with from many scientific individuals. Each pole is supposed to protect a circle of a hundred or a hundred and thirty feet, in the centre of which they are placed. Rods of metal being too expensive, they are made of wood, in a way which will be described in a subsequent part of this article. Each rod does not cost more than a few shillings, and the practice is to take them up after having put them under cover with the other rural implements, and replace them at the vernal equinox.

Another phenomenon, which is either formed by atmospherical electricity or connected with it, is the water-spout, a meteor of rare occurrence, and often most destructive in its effects. That distinct electrical phenomena are developed during the continuance of certain water-spouts cannot be doubted; but the electricity has in these cases been supposed to be a secondary phenomenon, produced by the motion of the air. This view of the subject has received some support from the researches of M. le Comte Xavier de Maistre, who has succeeded in imitating the principal phe- nomena of water-sprouts, and even the co-existing ascending and descending currents, by the mechanical circular motion of a liquid; and it is to this mechanism of the water-sprout that the electrical phenomena are ascribed. The lower parts of the atmosphere, and those above the clouds are brought to the same point by the two interior and opposite currents of the sprout, and strata of air charged with vapour, and often with different electricities, are thus brought into union, and produce the electrical phenomena in question.

The well-known phenomenon called sheet or summer lightning has recently been examined by M. Matteucci of Bologna. This ingenious author considers it to be proved that there is an accumulation of one of the two electricities at the surface of the earth; and he ascribes this electricity either to evaporation or to the analogous causes which M. Pouillet has substituted for it, or to chemical actions which are constantly going on in the interior of the earth. In order that this electricity may not escape and pass immediately into the mass of the globe, as soon as it is developed, it is necessary that the ground in which it is accumulated may not be a conductor, either from its own nature, or in consequence of the evaporation which dries it. It is also chiefly in high and insulated places rather than in the plains, above rocks rather than above forests, in summer rather than in winter, in the middle of the day rather than in the night, that these stormy clouds show themselves, whose formation cannot be well accounted for but by the influence of the electricity which the ground retains. To what other cause, he asks, can we attribute those clouds which are sometimes suddenly formed on the flanks of mountains, and afterwards rise into the air, without any variation of temperature, any change of barometrical pressure, or any other apparent modification in the state of the atmosphere? In applying these principles to the explanation of summer lightning, he considers the electricity of the earth's surface to be detained there by the desiccation of the ground, which renders it an insulator. At the moment of sunset, and during the night, the vapours which are thus condensed by cooling near the ground form a conducting stratum which serves to re-establish by degrees the electrical equilibrium between the atmosphere and the earth, which are charged with opposite electricities. It is chiefly in the plains, he conceives, that we ought to observe sheet lightning; and it ought to last a much longer time, because on elevated and insulated places the flow of electricity accumulated during the day will be much more rapid, on account of their form and position in the middle of an atmosphere more rare, more cold, and consequently more highly charged with vapours. These electrical discharges between the ground and the atmosphere may, according to our author, take place with much force, and produce even violent effects, especially if the ground and the atmosphere are too much dried; and he supposes that some earthquakes, and particularly those which take place after great droughts, may be owing to this cause. This supposition explains, in a satisfactory manner, the process employed by the ancients, and often with success, to protect from earthquakes those places which are subject to them, and which are particularly those where the nature of the ground renders the accumulation of electricity easy, and its escape difficult. This process consisted in sinking into the ground, even to a considerable depth, long bars of iron, which, according to the explanation given above, ought to facilitate the establishment of the electric equilibrium, by establishing a metallic communication between the interior of the ground and the surface, which, by its insulating faculty, retains its electricity.

Among the atmospheric meteors generated by electricity the aurora borealis holds a distinguished place. The phenomena which it exhibits have already been fully described under another article (the Aurora Borealis), but it belongs to our present subject to treat briefly of its electrical origin. The crackling and hissing noise of electricity passing from one place to another has been distinctly heard really in this country by Mr Nairne and M. Cavallo, and we can ourselves bear testimony to the same fact. In the north of Europe the sound accompanying the northern lights is an universally admitted fact, and proves beyond a doubt that, in certain auroras at least, the atmosphere is highly charged with electricity. Mr Trevethan learned when he was in Faroe that the peculiar smell which accompanies electrical discharges was distinctly felt during a brilliant aurora; and in the year 1821 Sir David Brewster had the good fortune to observe, at Belleville, in Inverness-shire, an aurora, the phenomenon of which were actually combined with those of a thunder-storm. This case is so remarkable, and so instructive, that we shall give the description of it in his own words: "On the evening of the 29th August, about half-past nine o'clock p.m., when there was not a breath of wind, and when the thermometer stood at 63°, the noise of very distant thunder was heard towards the south; sheets of very brilliant lightning illuminated the sky, issuing in general from a small black cloud near the horizon. I was surprised, however, to observe, that, with the exception of a few thin black clouds, which were rendered visible by the lightning, the greater part of the sky was covered with shining masses, like those which form the aurora borealis. The stars were easily seen through this luminous matter, which was arranged in irregular masses separated by clear intervals, but having a tendency to assume the appearance of irradiations diverging from the cloud whence the lightning appeared to issue. When the lightning flashed, it was propagated in a particular manner along these masses of light; but, what was very singular, the luminous patches were constantly in a tremulous or undulating motion during the intervals of the flashes of lightning. They shifted their place and changed their form exactly like the light which appears in many of the varieties of the aurora borealis. As the luminous clouds now described did not appear in the northern part of the horizon, and were distinctly related in their position and form to the thunder-cloud from which the lightning emanated, we are entitled to refer the two classes of phenomena to the peculiar electrical condition of the atmosphere, and to suppose that the phenomena of the aurora borealis may have an analogous origin. It seems now to be clearly proved that auroras exist not only at great heights in our atmosphere, such as from 62 and 105 miles, the lowest as given by Cavendish and Dalton, to 500 and even 1000 miles, as measured by other observers; but that they appear even close to the earth, in the lowest region of the atmosphere, is clearly established by a decisive observation of Captain Parry's. In the first of these cases it would be in vain to look for electrical indications, when the meteor is so far beyond the sphere of our electrosopes and the reach of hearing; but, in the latter case, we may reasonably expect not only to observe the peculiar electric state of the atmosphere, but also to hear the sound which invariably accompanies the passage of the fluid. This view of the subject reconciles the apparently contradictory observations which have been made on the aurora; and the connection of the phenomenon with the magnetic meridian, as well as its influence in certain cases on the magnetic needle, present no difficulty since the recent discoveries respecting the connection between electricity and magnetism.

That the other luminous phenomena of the atmosphere Fire-balls have their origin in its electricity cannot be doubted. Fireballs or globes of fire have been observed at altitudes from 30 to 100 miles, and moving with velocities varying from 5 to 33 miles in a second. These balls of light sometimes leave behind them a luminous track after they have vanished. Sometimes they explode into globes of a smaller size; sometimes they are dispersed into divided sparks; and at other times they are accompanied with showers of meteoric stones. Falling or shooting stars are only the same phenomena on a smaller scale; they appear at all seasons, but most frequently during the prevalence of the northern lights, and generally in the lower regions of the atmosphere.

The prismatic columns of light which were observed by Mr Fisher and others in the arctic regions have obviously an electrical origin. "On the afternoon of the 25th October Mr Fisher observed at Winter Harbour two vertical columns of prismatic colours, about 15° on each side of the sun, which was below the horizon; they were about 5° long, and their lower end touched the horizon; they continued for about an hour, from noon to one o'clock. Similar columns were observed two or three times, and about the same time the aurora appeared."

The fire of St Elmo, or Castor and Pollux, is a brilliant light which frequently appears on the summits of ships' masts, on the points of bayonets, on the tops of spears, and on the tips of the ears of horses. It is obviously nothing more than the electricity discharging itself either from or into pointed bodies. Its connection with the electrical state of the atmosphere is obvious from the following account of the phenomenon as given by Lord Napier, who saw it in the Mediterranean in June 1818. "About nine, when the ship was becalmed, the darkness became intense, and was rendered still more sensible by the yellow fire that gleamed upon the horizon to the south, and associated by the deep-toned thunder which rolled at intervals in the mountains, accompanied by repeated flashes of that forked lightning whose eccentric course and dire effects set all description at defiance. By half-past nine the sails were got aloft to furl the top-gallant sails and reef the top-sails, in preparation for the threatening storm. When retiring to rest, a sudden cry of St Elmo and St Ann was heard from those aloft and fore and aft the deck. On observing the appearance of the masts, the main top-gallant-mast head, from the truck, for three feet downwards, was completely enveloped in a blaze of pale phosphoric light flitting and creeping round the surface of the mast. The fore and mizen top-gallant-mast heads exhibited a similar appearance. This lambent flame preserved its intensity for the space of eight or ten minutes, and then it gradually became fainter, till it diminished at the end of half an hour."

An interesting case of the fire of St Elmo, in which the electricity first settled on the most prominent metallic body, and then on the bodies next in conducting power, is described in the memoirs of the Count de Forbin. "In the night," says he, "it became extremely dark, and it thundered and lightened fearfully. As we were threatened with the ship being torn to pieces, I ordered the sails to be taken in. We saw from different parts of the ship above thirty St Helmo's fires; amongst the rest was one upon the top of the vane of the mainmast, more than a foot and a half in height. I ordered one of the sailors to take the vane down; but scarcely had he taken the vane from its place when the fire fixed itself upon the top of the mainmast, from which it was impossible to remove it."

Sometimes the electricity of the atmosphere shows itself at the yard-arms and mast-heads of vessels, in the form of balls of fire. Captain Clavering of the Griper experienced a severe gale, which lasted three days without intermission, when about 100 miles to the west of the Florid of Drontheim. This gale was remarkable for the small amount of the effect produced on the barometer, on its approach, during its continuance, or after its cessation; and Captain Clavering was induced, from this and other causes, to ascribe it to a disturbed state of electricity in the atmosphere. It was accompanied with very vivid lightning, which is particularly unusual in high latitudes during winter, and by the frequent appearance and continuance for several minutes of balls of fire at the yard-arms and mast-heads. Of these no fewer than eight were counted at one time. This phenomenon is obviously an interesting variety of the fire of St Elmo.

The observations which have been detailed in the preceding section place it beyond a doubt that the electricity effects generated in our atmosphere is identical with that which is developed by friction. Philosophers, however, have sought to establish their similarity as chemical agents. M. Bonjol, for example, has decomposed water by means of the electricity of the air collected by an insulated pointed rod, in stormy states of the atmosphere; and the late unfortunate Mr Alexander Barry, who lost his life in the cause of science, succeeded in August 1824 in decomposing a solution of sulphate of soda coloured with syrup of violets. Bubbles of hydrogen appeared in the tube connected with the string of the electrical kite, while bubbles of oxygen appeared in the tube connected with the ground. In about ten minutes the blue liquid in the first tube became green from the separation of the soda, while the sulphuric acid, by passing to the pole in the other tube, changed its contents as usual into red. See page 597.

CHAP. III.—ON THE CHANGES PRODUCED BY ELECTRICITY ON ORGANIC AND INORGANIC BODIES.

That electricity is a powerful agent in the material world, Changes has long been the opinion of those who have studied its produced effects. We have clearly seen that it performs a distinguished part in the economy of our atmosphere; but there is reason to believe that its agency is still more general, and that it exercises an influence almost universal over the laws of inorganic matter, as well as over the functions of organic life. Our knowledge, however, on these subjects is but in its infancy; and though the following sections will present to the reader many interesting and important phenomena, he will not fail to deduce from them the conclusion, that a wide field of discovery is yet unexplored, and that there is no branch of science more likely to reward the diligence of the young philosopher than that which treats of the agency of the electric fluid in animal and vegetable life, its effects upon inorganic matter, and its connection with the imponderable agents of light and heat. The general effects of electrical action may be comprehended under the following heads:

1. On the mechanical changes produced by electricity on inorganic bodies. 2. On the chemical changes produced by electricity on inorganic bodies. 3. On the changes produced by electricity on phosphorescent bodies. 4. On the changes produced by electricity on odoriferous bodies. 5. On the magnetic effects of electricity. 6. On the effects of electricity on animal bodies. 7. On the effects of electricity on vegetable bodies.

SECT. I.—On the Mechanical Changes produced by Electricity on Inorganic Bodies.

Although we know nothing of the real nature of the electric principle, yet, from its properties and effects, it has been found convenient to speak of it as a fluid. Its action upon bodies which either obstruct its motion, or afford it a ready passage, renders its analogy with a fluid still more striking, and we are thus enabled to comprehend phenomena which it would otherwise be more difficult to understand. A canal with a smooth bottom and sides may be considered as a good conductor of the aqueous fluid, and a river with a rocky bed and a tortuous course may be regarded as a bad conductor. Small quantities of water turned into each of these conductors will find its way by a slow movement, without injuring the surfaces over which it flows, just as a small or a large wire will carry off small quantities of electricity without suffering any mechanical change. But when the current of water is deep and strong, it will overcome its obstructions, burst its barriers, and destroy the channel which at first confined it; while the same current running with the same velocity in a smoother bed will make its way without producing any change upon the materials over which it runs; in the same manner as a small metallic wire will sometimes be expanded and sometimes burst in pieces when it transmits with difficulty an electric discharge, whereas the same discharge will find an unobstructed passage through a wire of still greater diameter.

The influence of electricity in expanding solid bodies was discovered by Dr Priestley during his experiments on the effects of explosion through metallic substances, when he found that a chain was actually shortened after the charge of a battery had been sent through it. A length of chain of exactly twenty-eight inches, after having transmitted a charge of sixty-four square feet of coated glass, was shortened one-fourth of an inch, or \( \frac{1}{24} \) part of the whole.

Mr Nairne found that a piece of hard drawn iron wire, ten inches long and \( \frac{1}{8} \) th of an inch in diameter, after receiving many times in succession a discharge of twenty-six feet of coated glass (or nine jars), was shortened \( \frac{3}{8} \) ths of an inch, or \( \frac{1}{24} \) d of an inch, by such discharge. Its length was examined after the sixth, ninth, and fifteenth discharges. The total contraction of the wire was fully one inch and one-tenth, or one-ninth of the whole length.

Mr Brooke obtained a contraction still higher than this, by passing a charge of nine bottles or sixteen feet of coated surface nine times in succession through a steel wire twelve inches long and \( \frac{1}{8} \) th of an inch in diameter. The wire was shortened one inch and a half, or one-eighth of its whole length.

If the wire, however, through which the shock is passed has suspended to it a weight so as to stretch it considerably, the wire will be increased in length, in place of being diminished. This effect is by no means inconsistent with those already described. The heat, which, as we shall afterwards see, is always evolved during the transmission of an electric charge, produces a softness short of fluidity, which allows the extending force to overcome the absolute tenacity of the wire.

It is very obvious that the contraction of the wire in one dimension in these experiments was owing to its expansion in a direction at right angles to the length, in the same manner as a piece of caoutchouc extended in one direction is shortened in the other. Mr Nairne indeed observed that the wire had increased in thickness; but though he used a pair of scales which turned with one-eighth of a grain, he could not observe any change in the weight of the wire.

The same phenomenon takes place in fluid metals. If, for example, we fill a capillary or thermometer tube with mercury, and transmit through the mercurial column an electrical charge, the metal will suffer such a degree of expansion as to burst the tube to pieces.

When the body which transmits the shock has a less conducting power than metals, the tendency to expand will of course be still greater. If a little water, for example, is placed in a glass tube, and a shock passed through the water, the tube will burst by the expansion of the fluid; and the experiment will succeed even if a common drinking glass is filled with water and substituted for the tube.

Beccaria placed a drop of water in the centre of a solid glass-ball, and burst the ball by transmitting a shock through the fluid drop. This experiment was beautifully varied by the Italian philosopher, who constructed a small mortar, and having put a ball into it, he placed behind the ball a drop of water, so as to lie between the two wires which passed through the side of the mortar. When an electric charge was sent through the two wires, the expansion sustained by the water drove out the ball with great velocity. M. Lullin gave the ball a still greater impulsion by substituting a drop of oil for the drop of water.

Even when the conductor is air, a violent expansive effect is created during the transmission of the electric shock. This effect is well shown by fitting a cork cap into an ivory mortar having a cavity an inch deep and half an inch wide. When a shock is sent through the wire in this cavity, it is expanded so suddenly as to drive out the cork with great violence.

The mechanical effect of electricity is well shown in the following experiment:—Upon the surface AB of a dry piece of wood, paste a strip of tinfoil about 18 inches long and half an inch wide, and having cut out with a sharp knife small crosses at a, b, and c, place four wafers on each, as shown by the little circles in the figure. Place also wafers at c and d. If a strong electrical charge from a Leyden phial is passed from A to B, all the small waters at the crosses will be violently thrown off, while those at c and d will remain in their place.

When the electric shock is made to pass through solid bodies which are imperfect conductors, such as wood, stone, sugar, and glass, they will be broken by the expansion which is produced. In the case of glass not very thick, it will be broken into innumerable pieces. When the glass is so thick as to resist the shock, it is marked with vivid prismatic colours, which Mr Henley supposes to be thin laminae of the glass separated from one another by the shock.

The expansive effect created by the shock is finely exhibited by dipping a clean brass chain in melted rosin, and laying it upon paper. If the charge of thirty-two square feet of coated glass is sent through it, the resinous coating will be driven off from every part of the chain, which will be entirely cleared of it.

When a clean uncotted brass chain is laid upon a plate of glass, and a charge of thirty-two square feet passed through it, the glass will be marked in every part of its surface where it was touched by the chain, every marking having the width and colour of the link. The metal could be scraped off the glass at the outside of the mark, but it was actually driven in other places into the pores of the glass. Dr Priestley, who made this interesting experiment, produced a similar effect upon glass with a silver chain, and small pieces of other metals.

The reader who has perused with attention our chapter Diffusion on Electric Light, will recognise in these experiments the origin of those beautiful results which have been obtained by Fusinieri, by passing the electric shock from a metallic ball to a polished metallic surface; and the diffusion of solid bodies into metallic vapour, as it may be called, is finely illustrated in the following experiments. Take three strips of window glass, each about three inches long and one wide, and having placed two narrow strips of gold leaf or leaf brass between them, so that the ends of the gold leaf project a little beyond the glass, transmit the charge of a large Leyden jar through the gold leaf. The gold leaf will be found to

Phenomena and Laws.

Diffusion of metals into vapour.

be melted by the shock, and driven into the surface of the glass. The outer plates of glass are generally broken in this experiment, and the middle one, which frequently remains entire, has an indelible metallic stain upon each of its surfaces. This stain is obviously the metallic vapour of the gold driven into the pores of the glass.

The dispersion of gold or silver into a metallic vapour may be exhibited in another manner. Let a strip of gold or silver leaf; or Dutch metal, be fixed with gum to the surface of a piece of paper, and be placed in such a manner between the forceps of a universal discharger, that a strong electrical charge may be passed through it. The metallic strip will entirely disappear, in consequence of having been dispersed into a vapour or powder, part of which remains in a state of oxidation on the paper, which, from this cause, receives a greenish-brown tinge.

The metallic colours thus obtained have been employed for impressing ornamental figures upon paper or silk. In order to do this, trace the outline of the figures on thick drawing paper, and having cut it out as in stencil plates, place it on the silk or paper intended to be ornamented. When a gold leaf is laid upon it, and a card above the gold leaf, the whole is placed in a press or beneath a weight, and an electrical charge sent through it; the metallic stain is limited to the portion of the drawing paper that is cut away, and consequently any outline figure may be readily impressed upon the ground employed to receive it.

Dr Franklin was the first person who impressed metallic stains upon glass by electrical discharges. Fine gold communicated a reddish stain and silver a greenish one, and the metallic vapour, when driven into the pores of glass, was able to resist the action of the strongest aqua regia.

Sect. II.—On the Chemical Changes produced by Electricity on Inorganic Bodies.

Chemical changes produced by electricity.

The effects of electricity as a chemical agent are strikingly displayed in its power of evolving heat, and consequently of inflaming and fusing bodies, and its power of promoting chemical composition and decomposition. The influence of electricity in producing combustion may be shown by several beautiful experiments.

Exp. 1. To light a candle by Electricity. Having wrapped some loose cotton wool round the extremity of a long brass pin or wire, roll the cotton in the powder of white or yellow rosin. Bring the naked end of the wire into contact with the external coating, while the cotton end is applied to the brass knob of a charged jar, and the rosin and cotton will be instantly inflamed.

By dipping the cotton in oil of turpentine, and using a large jar, the cotton may be inflamed in a similar manner; and its inflammation will be promoted by strewing upon it some fine brass dust.

Exp. 2. To light a candle in another way. Thrust a wire up through the middle of the candle to within a short distance of the wick, and having connected the outside coating of a charged jar, by means of a chain, with the lower end of the wire, touch the wick with the knob of the jar, and the candle will be lighted.

Exp. 3. To inflame Phosphorus, &c. Having placed powdered phosphorus, rosin, or camphor, on some cotton wool, and wrapped it round one of the knobs of a discharging rod, apply the knob thus covered to the ball of a charged jar, and the naked knob to the external coating of the jar, and the powder will be set on fire.

Powdered rosin floating on water may be set on fire by transmitting a charge over the surface of the water between two points.

Exp. 4. To inflame alcohol or ether. The alcohol or ether being placed in an insulated metallic cup, electrify the cup, and upon taking a spark from the cup either with the knuckle or any other conductor, the fluid will be set on fire.

If ether is placed in a thin stratum upon the surface of water, in a clean wine glass, a spark taken from the surface will inflame the ether when the water is connected with the prime conductor.

Exp. 5. To inflame gunpowder. Upon the end of an insulated metallic wire fix a small cartridge, and when the cartridge is presented to the knob of a charged jar, the powder which it contains will be exploded.

Exp. 6. To exhibit the heat evolved by electricity. Take a wooden rod, for example one of red fir, about one inch thick and ten inches long, and place it between the hall of the conductor and the conducting wire; put the ball of a thermometer in a hole bored in the wood, and in a few minutes the mercury will rise to about 112°. Van Marum, who made this experiment, found that in three minutes the mercury rose from 61° to 88°, and in five minutes to 112°.

The evolution of heat by electricity is finely shown by Sir William Snow Harris's instrument constructed by Sir William Snow Harris. Mr Children and other philosophers have Harris's instrument constructed by Sir William Snow Harris, who deduced from a variety of facts that the heat evolved by a thermoelectric wire while transmitting an electric charge, is in inverse ratio of its conducting power; and hence Sir William was desirous of measuring the relative degrees of the evolutions so evolved by various metals and alloys in a gaseous medium such as air, and thus to discover their precise relations as conductors of electricity. The instrument which he used for this purpose is represented in Plate CCXXVII. Plate fig. 1. A glass tube CDA, whose bore is regular, and CCXXVII somewhat less than one-tenth of an inch, has one of its extremities DA bent upwards and outwards for about two inches, and is united by welding to a spherical reservoir A, containing a coloured fluid. This tube is fixed to a correctly divided scale E, supported by a suitable base; and the zero of the scale is at o, on a level with the coloured fluid in the reservoir A. Above the reservoir A is screwed air-tight, by means of brass caps closely cemented, a glass ball B, three inches in diameter; and through this ball a metallic wire m n, varying from 1/10th to 1/4th of an inch in diameter, may be passed air-tight by means of small flanges of brass m n, fig. 2, cemented in and round two holes drilled through the ends, each flange having a small projecting shoulder to receive the wire, and, upon which are screwed two brass balls a b, so as to render the whole air-tight. In order to fix the wire, the brass parts are made quite clean internally, and the wire being passed directly through them, is gently stretched, and then compressed in the holes by small pegs of tough wood, so as to insure a good contact. The pegs and the wire are allowed to project a little, to enable the observer to substitute different wires expeditiously. When an electrical explosion of sufficient force is now made to traverse the wire m n in the ball B, the heat which it evolves will be made evident by the ascent of the coloured fluid along the scale E.

Sir William now submitted to examination equal wires of different metals; and in order to insure the transmission of equal and similar explosions through each of them, he

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1 This fluid may consist of rectified alcohol, one part distilled water, three parts coloured tincture of cochineal, with a little sulphuric acid to make the whole sour.

2 This instrument, which was invented by Sir Snow Harris in 1820, and described in the Transactions of the Plymouth Institution for 1825, has been appropriated by some foreign writers as their invention, and has been confounded by M. Fred. Ries with Kinnersley's gas thermometer, shown in Plate CCXXX., fig. 1, to which it has no relation. adopted the following contrivance. Two equal brass balls were fixed at a given distance, as in Lane's discharging electrometer. One of them, which was insulated, was placed in immediate connection with the positive side of the battery, while the other was connected with the negative side; the metallic wire to be examined forming part of the circuit. This last connection was made by means of two fixed copper wires inserted into the balls on each side of the glass, and made perfect at the points of junction. When the charge therefore of the battery was sufficiently intense to pass the given interval, the discharge took place through the wire in the ball. Sir W. Harris used a battery of five jars, each containing five square feet of coated surface. They were placed on a metallic base communicating with the negative conductor, and were charged by means of long copper rods projecting immediately from the bottom of each jar. The machine employed was a plate one, with a disc of glass three feet in diameter.

The results which Sir William obtained from an extensive series of experiments are given in the following table:

| Metals | Effects | |-------|--------| | Copper | 6 | | Silver | 6 | | Gold | 9 | | Zinc | 18 | | Platinum | 30 | | Iron | 30 | | Tin | 35 | | Lead | 72 | | Brass | 18 |

Alloys: - Gold 1 part, copper 1 part: 20 - Gold 3 parts, copper 1 part: 25 - Gold 1 part, copper 3 parts: 15 - Copper 1 part, silver 1 part: 6 - Copper 1 part, silver 3 parts: 15 - Copper 3 parts, silver 1 part: 6 - Gold 1 part, silver 1 part: 20 - Gold 1 part, silver 3 parts: 15 - Gold 3 parts, silver 1 part: 25 - Tin 1 part, lead 1 part: 54 - Tin 3 parts, lead 1 part: 45 - Tin 1 part, lead 3 parts: 63 - Tin 1 part, zinc 1 part: 27 - Tin 3 parts, zinc 1 part: 32 - Copper 8 parts, tin 1 part: 18

Considering the heat to be in the inverse ratio of the conducting power, it appears from this table, 1st. That the heats evolved from silver and copper are alike, and also those from iron and platinum, and from zinc and brass, while the heats evolved from lead and tin, from zinc and gold, and from brass and gold, are as 2:1. 2dly. That silver and copper being regarded as the best conductors, from being the least heated by the explosion, the conducting power of gold to copper will be as: 2:3 Zinc or brass to copper or silver: 1:3 Platinum or iron to copper or silver: 1:5 Tin to copper or silver: 1:9 Lead to copper or silver: 1:12

3dly. That the conducting power of an alloy of gold and copper, or gold and silver, is less than either metal separately; and that the difference in the conducting power increases with the quantity of the inferior conductor alloyed; and that tin and lead in alloy have a conducting power equal to the mean of their two separate conducting powers. And, 4thly, that copper alloyed with an eighth part of its weight of tin becomes as much heated by an electrical explosion as iron.

Taking the heat of lead in the preceding table as unity, and the conducting power of the several metals named as being in a simple inverse ratio of the heat evolved; then the order and relative value of the given metals as conductors of common electricity will be as follows:

| Metals | Conducting power | |-------|------------------| | Lead | 1 | | Tin | 2 | | Platinum | 2.4 | | Iron | 2.4 | | Zinc | 4 | | Gold | 8 | | Copper | 12 | | Silver | 12 |

It appears from numerous experiments made with the Snow aid of this instrument, that the heating power of the electrical discharge on metals increases as the square of the experiment quantity of electricity discharged through them, without any ments on relation to the intensity of the charge of the battery, as indicated by the common electroscope or the extent of coating by electric glass on which the given quantity is accumulated; so that city, the heating effect of the same quantity is always the same under the same conditions of circuit—a general law of quantity fully confirmed by Faraday in the course of his fine magneto-electrical researches. We may hence, with the data in the preceding table, easily deduce the relative quantities of electricity which the respective metals would transmit under the same elevation of temperature, and which would be nearly as follows—taking lead as unity:

| Metals | Quantity | |-------|----------| | Lead | 1 | | Tin | 1.4 | | Platinum | 1.55 | | Iron | 1.55 | | Zinc | 2 | | Gold | 2.8 | | Copper | 3.5 | | Silver | 3.5 |

Viewing the question of conducting power in relation to the quantity of electricity which given metals can transmit under the same elevation of temperature, we should say that zinc had twice the conducting power of lead; gold twice the conducting power of tin, and so on. The relative numerical value of the metals as conductors of electricity will hence greatly depend on the particular way in which the measure of conducting power is considered. Thus it appears that the quantity which raised the temperature of lead, for example, 72° of the scale, only raised copper 6°, being in the ratio of 1:12. Taking these numbers as a measure of the resistance of the respective metals, and supposing the conducting power to be as the resistance inversely, then it would follow that copper has 12 times the conducting power of lead for any given quantity of electricity transmitted; it does not however follow from this, that copper can transmit 12 times the quantity of electricity under the same degree of heat,—for the resistance is found to increase with the heat evolved, and that is, with electricity of high tension such as lightning, as the square of the quantity of charge transmitted, so that it would only require about 3.5 times the quantity to raise the temperature of copper to that of lead transmitting a unit of quantity, all other things being the same.

In a further communication made to the Royal Society of Edinburgh in December 1831, the author has added considerably to our information upon this interesting question. Having adapted his instrument to the purposes of voltaic electricity, he proceeds to examine the heat excited in metallic wires by permanent electrical currents passed through them under a great variety of conditions. The following are some of the most striking of these experiments.

On exposing fine wires of different metals to electrical currents varying in intensity, he found that in certain cases the instrument ceased to be materially affected by the increased charge, the fine wire passing through the air thermometer ball not having sufficient conducting power to transmit the whole of the current, whilst in substituting wires of higher conducting power the fluid ascended the scale with great rapidity, a result identical with that arrived at by Sir H. Davy, who states that in a battery where the quantity of electricity was very great and the intensity low, a foot of wire of platinum was scarcely heated whilst the same length of silver wire became red hot. It was therefore only when wires were employed of sufficient conduct-

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1 Phil. Trans., for 1821. ing power that we obtain indications proportionate to the increased electrical current, still no greater effect than was directly proportional to the increased power could be obtained, under any circumstances, as in the ordinary electrical discharges from the Leyden battery; a result depending probably on some peculiar condition of the species of electrical action.

In order to exemplify the effects of heat in diminishing the conduction, about six inches of the circuit transmitting the current through the instrument was made up of platinum wire of about the 8th of an inch in diameter, and to this part of the circuit heat and cooling liquids were alternately applied; the result was that the fluid invariably descended the scale on the application of heat to the circuit wire, and ascended when that wire was cooled by evaporation, thus clearly evincing a diminished and increased power of transmission.

With a view of determining the relative conducting power of various metals, the given metal, the subject of experiment, was drawn into a wire of a given length and diameter, and caused to transmit a given current accumulation upon the wire passing through the ball of the electrometer. In order to avoid any error arising from elevation of temperature, the current wire was placed in a cool fluid medium. The facility with which the given metal transmitted the current electricity became, in this case, approximately measured by the indications of the instrument. The order of the metals as conductors of this kind of electricity were found to be as follows:

| Silver | Platinum | |--------|----------| | Copper | Lead | | Zinc | Antimony | | Gold | Fluid Mercury | | Tin | Bismuth | | Iron | |

This order, with the exception of tin, is nearly the same as that already given by the ordinary electrical discharge: it was found liable to exceptions in placing the wires within the hall, as done in the preceding experiments, from the circumstances already explained—viz. that in certain cases the best conductors may appear to be the most heated.

Great care appears to have been taken in the manipulation of the experiments, and all the sources of anomaly or error liable to vitiate the result fully traced in operating upon fluid mercury, antimony, and bismuth. These metals were cast into small bars, and compared with zinc, tin, and lead, similarly heated.

Many beautiful experiments have been made by different philosophers, on the fusion of metallic wires. Mr Knersley, in the presence of Dr Franklin, transmitted the charge of a case of bottles through a fine iron wire. After first appearing red hot, the wire was melted into spherical drops like small shot. With a battery of thirty-two square feet, Dr Priestley melted into globules wires even so large as the seventieth of an inch in diameter, sometimes placing them in tubes of glass, and sometimes in paper. Mr Brooke, Van Marum, Baron Kienmayer, Mr Cuthbertson, and Mr Singer, made many successful experiments on the fusion of wires by electricity, and succeeded in melting them in considerable lengths; but they have given us no sufficient data for determining any of the relations of this kind of electrical action, although we find the experiments accompanied with many important and interesting observations.

Mr Singer found that the power of any coated surface to melt wires varies with the thickness of the jars; which confirms the conclusion of Mr Cavendish, that the quantity of electricity necessary to charge different jars of the same extent of coated surface is inversely as the thickness of the jars. "The effects of gradually increasing the power of the charge," says Mr Singer, "when wires of the same length and diameter are employed, are very remarkable.

If the wire be iron or steel, its colour is first changed to yellow, then (by an increased charge) blue, by a further increase it becomes red hot, then red hot and fused into balls; if we continue to increase the charge, it becomes red hot and drops into balls, then disperses in a shower of balls, and lastly disappears with a bright flash, producing an apparent smoke, which, if collected, is a very fine powder, weighing more than the metal employed, and consisting of it and a portion of the oxygen of the atmosphere, with which it has combined."

In the course of these experiments Van Marum observed the curious fact, that when a charge of 225 square feet of coated surface was transmitted through fifty feet of iron wire, the jars were not entirely discharged, and the residual charge was capable of melting two feet of the same wire.

The only general deductions from these experiments are by Mr Singer, who observes "that the action of electricity on wires increases in the ratio of the increased power;" because he found "that two jars charged to any degree will melt four times the length of wire melted by one jar." Mr Singer, however, found the law to vary with different jars; which would necessarily be the case so long as no quantitative measurement of the actual amount of electricity employed could be referred to. Of this, the common electrometers formerly employed show little or nothing.

With the view of determining the relative fusibility of different metals, Van Marum applied the same electrical charge to wires of different metals drawn to the same diameter. The following were the results with wires the 32d of an inch in diameter:

| Metals | Length of Wire Fused | |-------|---------------------| | Lead | 129 inches | | Tin | 129 | | Iron | 5 | | Gold | 3 | | Silver| | | Brass | | | Copper| |

Hence he concludes that lead and tin are the worst metals for conductors, and copper, brass, and silver, the best.

Although these experiments are by no means wanting in scientific interest, they are still manifestly deficient in absolute quantitative measurement; a defect which renders them more or less indefinite and inconclusive. By means of the unit jar and other quantitative processes employed by Sir W. Snow Harris in this class of experiments, we can now determine the actual quantity of electricity in action, and estimate the relations subsisting between the indications of the intensity electrometer, and the conditions under which the charge is accumulated, whether on one or more electrical jars, or on thick glass or thin. We can estimate and measure the heating effect in relation to the quantity discharged, and determine the law of its operation, and all this in easy and intelligible terms. By means of such measurements it is now fully proved that the heating effect of a given electrical discharge is altogether independent of the indications of an intensity electrometer, and of the extent of coated glass upon which the given quantity has been collected, whether upon one, two, or more jars, or upon thick glass or thin. A given measured quantity of electricity may exhibit, under these varying conditions, all sorts and degrees of intensity, as indicated by an attractive or repulsive electrometer—such as those of Brookes and Cuthbertson; and yet at the instant of the discharge through a metallic wire the heating effect is always the same, and this heating effect is invariably as the square of the quantity of electricity discharged through the wire. For a given extent of coated glass taken as a constant, the quantity accumulated will be always as the square root of the attractive or repulsive force of the intensity electrometer, or commonly the indications of the intensity electrometer will be always as the square of the accumulation, all other things being When Mr Singer made the explosion over glass, he found that a portion of the metal appeared immediately under the wire in an oxidated state, while the oxidated portion produced round the other a figure of some width. The figures are in this way more beautiful, though less permanent, than when they are produced upon paper.

The oxidating power of common electricity is finely exhibited in the following experiment, given by Dr Wollaston's experiments. Having coloured a card in a strong infusion of limes, a current of electric sparks was passed along it by means of two fine gold points, which touched the card at the distance of an inch from each other. After a very few turns of the machine, and when the card was nearly dry, a redness at the place of the positive wire was distinctly manifest to the naked eye; and when the experiment was repeated with the negative wire on the same spot, it was restored to its original blue colour.

The metallic oxides may be revived, or restored to the Revival of metallic state, by means of electricity. Beccaria, who discovered this property, revived the oxide of zinc, and produced quicksilver from cinnabar by exploding a jar between two pieces of the calces. The following method of making this interesting experiment is given by Mr Singer. Introduce into a glass tube some oxide of tin, so that the oxide may cover about half an inch of the lower internal surface of the tube when it is laid horizontally. Place the tube on the table of Henley's discharger, and introduce the pointed wires into its opposite ends, that the oxide may lie between them. When several strong charges have been sent through the tube, a part of the tube will soon be stained with metallic tin, which has been revived by the transmitted electricity. The charge of a very moderate-sized jar is sufficient to revive the mercury and sulphur which composes vermillion.

The oxidating power of negative electricity is well illustrated by the following elegant experiment of Dr Wollaston's. Having coated with wax about two or three inches of negative of the middle of a fine silver wire, the hundred and twentieth of an inch in diameter, he cut the wire through in the middle of the wax, so as to expose a section of it. The two coated extremities of the divided wire were plunged in a solution of sulphate of copper, placed in an electric circuit between the two conductors, and sparks taken at the distance of one-tenth of an inch were passed through the solution. After a hundred turns of the machine, the wire communicating with the negative conductor had a precipitate formed upon its surface, which by burning was clearly copper, whereas there was no such coating upon the other wire. The direction of the electric current being reversed, the order of the phenomena was reversed, and the copper was shortly re-dissolved by the aid of the oxidating power of common electricity, and a similar precipitate formed upon the opposite wire. Dr Wollaston obtained similar results from gold wires and a solution of corrosive sublimate.

The influence of electricity in effecting chemical composition and decomposition forms one of the most interesting departments of electrical and chemical science. The most valuable researches which have been made on this subject were carried on by means of the voltaic battery, and must necessarily be detailed under another article; but the decomposition. coveries which were made through the agency of the electrical machine fall to be recorded in the present section.

One of the earliest experiments on the influence of electricity as a powerful chemical agent, was made by Mr Warltire, who fired a mixture of atmospheric air and hydrogen gas by means of electricity in a close copper vessel containing about three pints. Although no air could escape by the explosion, yet a loss of two grains was perceived in every experiment. When the vessel which contained the gases was clean and dry, a dewy moisture was found adhering to the inside of the vessel. Guided by this indication, Dr Priestley entered upon the subject. Having placed a blue solution of water and litmus in a glass tube, he transmitted through it a current of electrical sparks from a brass wire. In two or three minutes the blue liquor became red, particularly at the part where the sparks entered, and the air inclosed in the tube was diminished. The appearance of an acid having been formed at the expense of the air confined in the tube, induced Dr Priestley to place the tube in the receiver of an air-pump, so that by gradually exhausting the air, the part of it inclosed in the tube expanded and pushed out the discoloured liquor. Upon again admitting the air, a new portion of the litmus solution was introduced, while the confined air remained the same as before, and resumed the space which it had occupied after the passage of the electric current. After this the electrical sparks were no longer able to alter the colour of the solution, or to cause any decrease in the volume of the confined air.

In passing a current of electric sparks through olive oil, turpentine, oil of mint, and ether, Dr Priestley found that an inflammable gas was evolved.

In his experiments on the gases Dr Priestley was more successful in transmitting the spark through ammoniacal gas; he found that two hundred shocks passed through a given quantity of the gas produced an increase of volume equal to one-fourth of the whole. Upon subsequently admitting water, the original quantity operated upon was absorbed, and the remaining gas, equivalent to the expansion effected by the electric shocks, was found highly inflammable.

Dr Priestley likewise passed an electrical current, consisting of slight shocks continued for about an hour, through an inch of carbuncle acid gas confined in a glass tube one-tenth of an inch in diameter, when, upon admitting the water, one-fourth part only was absorbed. In a similar manner Dr Priestley succeeded in decomposing carburetted hydrogen, the charcoal being deposited in a pulverulent form on the interior of the tube. When a succession of electric sparks from a moderate-sized conductor during the space of five minutes had failed in effecting this decomposition, he found that two shocks of a jar, each of which might be produced in less than a quarter of a minute with the same machine in the same state, were sufficient to cover the whole of the inside of the tube with the black carbonsaceous matter. Dr Priestley remarked in these experiments that no shock, however powerful, would decompose the gas, if the quantity operated upon were great.

The power of electricity as a chemical agent was studied with peculiar success by the Honourable Mr Cavendish. In the year 1781 he fired 500,000 measures of hydrogen with about two and a half times that quantity of atmospheric air, and having by this means obtained a hundred and thirty-five grains of pure water, he was led to the conclusion, previously indicated by Mr Watt, that water was composed of two gases, viz., oxygen and hydrogen. In pursuing these inquiries Mr Cavendish made use of the apparatus shown in fig. 6 of Plate CCXXVI. CCXXXVI. The air to be examined was confined in a bent glass tube A, filled with mercury, and having its ends immersed each in a vessel of the same fluid. The air to be electrified was introduced by a piece of glass tube ABC, fig. 7. In order to use this apparatus, the tube ABC being filled with mercury, is introduced as in fig. 7, with its bent extremity uppermost, into the vessel containing the gas, and standing in the pneumatic trough. In this part of the process the orifice at C is stopped by a finger, by withdrawing which a little mercury will descend through C, and an equal volume of the gas will enter at the end A. When the gas has been admitted in sufficient quantity into the tube ABC, it is withdrawn and reversed, the end C, which is placed uppermost, dish remaining carefully closed. The extremity A, which fits the end of the tube in fig. 6, is introduced beneath the mercury in either of the glasses, and by withdrawing the finger from the upper end C of the transferring tube, the mercury will descend, and the gas will be forced into the tube A, fig. 6. The orifice of the transferring tube should not be greater than that of a common thermometer tube.

In order to introduce portions of air successively during the same experiment, Mr Cavendish used a tube AB of a small bore (see fig. 8), a bulb C, and a tube DE, having a bore larger than that of A.B. This apparatus having been first filled with mercury, the bulb C and tube A.B are filled with the gas, by introducing the end A beneath the inverted jar, upon the shelf of the pneumatic trough, and then drawing the mercury from the leg D by means of a syphon. The aperture A being closed, the apparatus is weighed. The extremity A, fig. 8, is then fitted into the end of the tube A, fig. 6. When it is required to force air out of this apparatus into the tube, a wooden cylinder with a tight fitting is thrust down into the tube ED, an additional quantity of mercury being occasionally poured in at E to supply the place of what is forced into the bulb C. When the experiment is completed the apparatus is again weighed. The increase of weight is due to the mercury introduced, and its volume is equal to that of the air transferred to the tube A, fig. 6. The bore of the tube A was generally one-tenth of an inch in diameter, and the aerial column in the bend of the tube from one-half to three-fourths of an inch.

In transmitting the electric spark through this tube, Mr Cavendish, instead of making one end of it communicate with a conductor, placed an insulated ball at such a distance from the conductor as to receive a spark from it, and made a communication between that ball and the mercury in one of the glasses, the mercury in the other glass communicating freely with the ground.

In transmitting the electric spark through common air in contact with a blue aqueous solution of litmus, a red tint was produced in the solution. When lime-water was inclosed in the tube instead of litmus, and sparks transmitted till there was no farther diminution in the volume of the included air, no cloudiness appeared in the lime-water, and the diminution of volume, amounting to one-third of the original bulk of the air, exceeded the diminution from deoxidation alone, which would have been only one-fifth.

When this experiment was repeated with some impure oxygen gas, a considerable diminution of volume was produced, but there was no cloudiness in the lime-water, and none could be perceived by adding to it a little carbonic acid gas; a small portion of caustic ammonia, however, produced a brown precipitate. Hence it is obvious that the lime-water was saturated with some acid formed in the process.

Having inclosed in the tube some of the same impure oxygen in contact with soap leys, the diminution of volume proceeded faster than with the lime-water, the greater strength of the alkaline lixivium acting as a more powerful absorbent of the acid which was generated.

When pure oxygen or pure azote was used, no absorption took place; but when five volumes of pure oxygen were mixed with three of common air, the absorption was almost total; and as common air contains about one part of oxygen and four of azote, the mixture of five parts of oxygen and three of common air was equivalent to seven parts of oxygen and three of azote.

Mr Cavendish now supplied the interior of the tube with a little alkaline ley, and having introduced a mixture of seven parts of oxygen and three of azote, he transmitted a current of electric sparks, admitting fresh gas as the volume of air diminished. When the diminution ceased, a little pure oxygen, and afterwards a little common air, were added, in order to see if the absorption ceased from any want of a proper proportion in the two elements. As this was not the case, the soap leys were separated from the mercury, and were found to have become perfectly neutral, from their having no effect on the colour of litmus. When the leys were evaporated a dry nitrate of potash was obtained.

By repeating this experiment on a more extended scale, Mr Cavendish demonstrated that the soap leys had been converted into a solution of nitre, and therefore established the great truth that nitric acid had been formed during the process, and that nitric acid is a compound of oxygen and azote.

By means of the great Teylerian electrical machine at Haerlem, Van Marum, Van Troostwyk, &c., made many experiments on the chemical agency of electricity. The apparatus which they used, shown in Plate CCXXXVI. fig. 9, consists of a tube of glass D E, twelve inches long and a quarter of an inch in diameter, hermetically sealed, and having a gold or platinum wire D d an inch and a half long fixed at D. Another platina wire E e was carried up from the open end of the tube E to e, within one eighth of an inch of the end d of the upper wire. The tube D E having been filled with distilled water, the open end of it E was immersed in a vessel V containing quicksilver, and the upper end D of the wire Dd was brought into contact with the insulated brass ball C, placed at a little distance from A B, the prime conductor of the electrical machine. The lower wire Ee, immersed in quicksilver, communicated with a chain VG connected with the outer coating of a Leyden jar containing about a hundred and forty-four square inches of coated glass, and having its ball M in contact with the prime conductor A B. When the electrical discharges were sent through the distilled water, the gas was disengaged as long as the ball C was in contact with the conductor; but upon increasing its distance, a position was found where the gas was disengaged, and ascended to the top of the tube.

The evolution of the gas continued till it reached nearly the lower extremity of the upper wire, and then a discharge caused the whole gas to disappear, its place being supplied by water. With this apparatus the Dutch philosophers made the following experiments.

Oxygen gas from red precipitate had its original volume diminished one twentieth, and the properties of what remained were not changed.

Nitrous gas had its volume diminished to less than one half. There were no red fumes when it was mixed with atmospheric air, neither was there any condensation. It would not support combustion, and it lost its usual smell. A kind of powder covered the surface of the mercury, consisting of a new combination formed from the mercury.

Hydrogen gas, obtained from sulphuric acid and iron, suffered no diminution. Owing probably to some admixture of common air, it gave a slight redness to tincture of turnsole.

Oleficent gas from sulphuric acid and alcohol had its original volume tripled, and in some degree lost its inflammability.

Sulphurous acid gas, from sulphuric acid and charcoal, had only one eighth of its volume absorbed by water. Black spots were formed on the inside of the glass receiver. It had little smell, and extinguished a candle.

Muriatic acid gas experienced a considerable diminution of volume, but the remainder was readily absorbed by water. The electric sparks would not pass through more than two inches and a quarter of this gas.

Carbonic acid gas from sulphuric acid and chalk had its volume increased a little, and was made less absorbable by water.

Ammoniacal gas had its volume at first almost doubled, and then experienced a slight diminution. It became incapable of being absorbed by water, and exploded by the contact of flame.

Fluoric acid gas experienced no perceptible change.

Atmospheric air gave a slight redness to tincture of turnsole, and at the same time became sensibly deoxygenated. Others. The diminution of volume was $\frac{1}{20}$ths, the mean of three experiments; and of the same air not electrified $\frac{1}{30}$ths, the mean of three experiments.

The Dutch philosophers made many other experiments which we have not space to describe, and in 1789 they succeeded in repeating the experiment of Cavendish on the decomposition of water.

Hitherto a powerful apparatus was deemed necessary for Dr Wollaston's experiments on the decomposition of water, and a succession of his own experiments on the decomposition of water was deemed indispensable. Dr Wollaston, however, considering that the decomposition must depend upon a proper proportion between the quantity of water and the decomposing force, conceived the idea of reducing the surface of communication between the air and the metal which conveyed the electricity.

"Having procured," says he, "a small wire of fine gold, and given it as fine a point as I could, I inserted it into a capillary glass tube; and after heating the tube so as to make it adhere to the point, and cover it in every part, I gradually ground it down till, with a pocket lens, I could discern that the point of the gold was exposed.

"The success of this method exceeded my expectations. I coated several wires in the same manner, and found that when sparks from the conductors before mentioned were made to pass through water by means of a point so guarded, a spark passing to the distance of one eighth of an inch would decompose water when the point exposed did not exceed one seven hundredth of an inch in diameter. With another point, which I estimated at $\frac{1}{15000}$th, a succession of sparks one twentieth of an inch in length afforded a current of small bubbles of air.

"I have since found that the same apparatus will decompose water with a wire one fortieth of an inch diameter, coated in the manner before described, if the spark from the prime conductor passes to the distance of four tenths of an inch of air.

"In order to try how far the strength of the electric spark might be reduced by proportional diminution of the extremity of the wire, I passed a solution of gold through a capillary tube, and, by heating the tube, expelled the acid. There remained a thin film of gold lining the inner surface of the tube, which, by melting the tube, was converted into a very fine thread of gold through the substance of the glass.

"When the extremity of this thread was made the medium of communication through water, I found that the mere current of electricity would occasion a stream of very small bubbles to rise from the extremity of the gold, although the wire by which it communicated with the positive or negative conductor was placed in absolute contact with them. Hence it appears that decomposition of water may take place by common electricity as well as by the electric pile, although no discernible sparks are produced. The appearance of two currents of air may also be imitated, by occasioning the electricity to pass by fine points of communication on both sides of the water; but in fact the resemblance is not complete, for in every way in which I have tried it, I observed that each wire gave both oxygen and hydrogen gas, instead of their being formed separately, as by the electric pile.

"I am inclined to attribute the difference in this respect to the greater intensity with which it is necessary to employ..." common electricity; for, that positive and negative electricity so created have each the same chemical power as they are observed to have in the electric pile, may be ascertained by other means."

The preceding experiment, which is only an elegant repetition of one formerly made by Dr Pearson and the Dutch philosophers, has excited much attention, and cannot be regarded as any proof of the identity of ordinary and voltaic electricity; Dr Faraday justly remarks that it should never be quoted as establishing true electro-chemical decomposition, because the law which regulates the transference and final place of the evolved bodies has no influence here. The water is decomposed at the two poles by an independent action, and the oxygen and hydrogen evolved are the elements of the water existing at the wires the instant before. The substitution of the finger for one of the points will not interfere with the action of the other. But although Dr Wollaston did not decompose water in any way analogous to that of the pile, yet Dr Faraday seems to have succeeded in doing it by the same apparatus; but when he considered that he had obtained the true effect, the gas evolved was so small in quantity that he could not ascertain whether or not oxygen was emitted at the one wire and hydrogen at the other, as ought to have been the case.

The inability of Dr Wollaston's apparatus to exhibit in an unquestionable manner true electro-chemical decomposition being thus obvious, Dr Faraday devised the following ingenious arrangement for effecting chemical decomposition by ordinary electricity, and by means of it he effected true electro-chemical decompositions, perfectly identical with those produced by voltaic electricity. The plate machine which he used had its glass disc fifty inches in diameter. It had two sets of rubbers. The prime conductor consisted of two brass cylinders, connected by a third, the whole length being twelve feet, and the surface in contact with air was 1422 inches. When well excited, one revolution of the plate gives ten or twelve sparks, each an inch long; and sparks or flashes from ten to fourteen inches long may easily be drawn from the conductors. When moderately worked, each turn of the machine is made in four fifths of a second. The electric battery consists of fifteen equal jars, each twenty-three inches in circumference, and coated eight inches upward from the bottom, so as to contain 184 inches of glass each, coated on both sides, independent of the bottoms, which are thicker glass, and contain each about fifty square inches.

In order to carry off instantaneously electricity of the feeblest tension, Dr Faraday formed what he calls a discharging train. This discharging train consisted in connecting a sufficiently thick wire metallically, first with the metallic gas pipes of the house, then with the metal pipes of the public gas works of London, and lastly with the metallic water-pipes of London. This arrangement was so effectual that the electricity even of a single voltaic trough was instantly carried off; and this was essential to the success of many of his experiments.

The arrangement for applying the apparatus now described to chemical decomposition is shown in Plate CCXXVII. fig. 3. Two pieces of tinfoil a, b are placed upon a glass plate raised above a piece of white paper to prevent the interference of shadows. One of these pieces a, is connected by an insulated wire c, or by a wire and wet string, with the electric machine, and the other piece b, by a wire g, with the discharging train or the negative conductor. Two pieces of fine platinum wire must then be provided, bent as in fig. 4, so that the part d shall be nearly upright, whilst the whole rests on the three points e, f, p. By this means we can obtain at pleasure surfaces of contact as minute as possible; the connection can be discontinued or removed in a moment, and the substances which are acted upon can be readily examined. With this apparatus Dr Faraday obtained the following results.

1. Having made a coarse line on the glass plate with a solution of sulphate of copper, the ends p and n of the platinum wires were put into it, the foil a being connected by a wire and wet string with the positive conductor of the machine, so that no sparks passed. After twenty turns of Dr Faraday's apparatus there was so much copper precipitated on the day's apparatus that it looked like copper wire, no apparent change having taken place at n.

2. A large drop of a mixture of equal parts of muriatic acid and water coloured a deep blue by sulphate of indigo was placed on the glass, so that the ends p and n were plunged in opposite sides of it; one turn of the machine evolved sufficient chlorine to exhibit bleaching effects round p. Twenty revolutions produced no effect at n, but there was so much chlorine got free at p, that when the drop was stirred the whole became colourless.

3. Having mingled a solution of iodide of potassium with starch, the ends p and n were immersed in a drop of it as before; on turning the machine, iodine was evolved at p, but not at n.

Dr Faraday improved his apparatus still further by wetting a piece of filtering paper in the solution to be examined, and placing it on the glass beneath the points p, n. The paper will retain the substance evolved at the point of evolution; its whiteness will render visible the least change of colour, and will allow the point of contact between it and the wires p, n to be contracted to the utmost degree. Dr Faraday found a piece of paper moistened in the solution of iodide of potassium and starch, or of the iodide alone, to be with certain precautions a most admirable test of electro-chemical action; and when it is placed and acted upon in the manner already described, it will exhibit the evolution of iodine at p by half a turn only of the machine. He found, indeed, that with these adjustments, and the use of iodide of potassium on paper, chemical action is sometimes a more delicate test of electrical currents than the most delicate galvanometer.

A piece of litmus paper wetted in a solution of muriate or sulphate of soda was quickly reddened at p; and a similar piece wetted in muriatic acid was soon bleached at p, no similar effects taking place at n.

A piece of turmeric paper wetted in a solution of sulphate of soda was reddened at n by two or three turns of the machine, and by twenty or thirty turns abundance of alkali was evolved at the same place. By turning the paper round so that the spot came under p, and working the machine, the alkali soon disappeared, the place became yellow, and a brown alkaline spot appeared in the new part under n.

Dr Faraday next combined a piece of turmeric paper with a piece of litmus, wetting both with a solution of sulphate of soda. The paper was placed so that p was on the litmus and n on the turmeric paper. By a few turns of the machine, acid was evolved at p and alkali at n, as in galvanic decomposition. These various decompositions were equally effected whether the electricity passed to the foil a from the machine through water or wire only, by contact with the conductor, or by sparks there, provided the sparks were not so large as to cause the electricity to pass in sparks from p to n, or towards n.

Dr Faraday's final experiment deserves peculiar notice, as affording a case in which there is the most perfect analogy between the effects of ordinary and voltaic electricity. Three compound pieces of litmus and turmeric paper, when wetted by a solution of sulphate of soda, were disposed on a plate of glass as shown in fig. 5. The wire m was connected with the prime conductor, t with the discharging train, and the wires r and s connected the moistened pieces fig. 5. of paper, each wire resting on three points, one of the points, at r and s, being on the glass, and the others on the papers, the ends p, p, p resting on the litmus and n, n, n on the tur- When the machine had been worked for a short time, acid was evolved at all the poles \( p, p, p \), by which the electricity entered the solution; and alkali at the other poles \( n, n, n \), by which the electricity left the solution.

The precaution above referred to, in using the iodide of potassium as a test of electro-chemical action, is that no sparks should be passed in any part of the current, and no increase of intensity allowed by which the electricity may be induced to pass between the platina wires and the moistened papers, otherwise than by conduction; for if the electricity burst through the air, a different effect is produced. The litmus paper is in this case reddened by the spark, and iodine will be evolved from paper moistened by iodide of potassium. This effect is owing to the formation of nitric acid by the oxygen and nitrogen of the air. The acid thus formed reddens the litmus paper, or prevents the exhibition of alkali in the turmeric paper, or evolves iodine from the iodide of potassium. We have thus a simple and elegant method of illustrating Mr Cavendish's experiment of forming nitric acid from the atmosphere.

M. Bonjol of Geneva was among the first who decomposed water by common electricity. The electricity was obtained from an insulated lightning rod; and the decomposition is said to have proceeded continuously and rapidly even when the electricity of the atmosphere was by no means powerful. M. Bonjol also decomposed potash and chloride of silver, by passing the sparks of an ordinary machine through these substances placed in narrow tubes. Dr Faraday justly regards these as not cases of true electro-chemical decomposition, but as analogous to that of Dr Wollaston's apparatus, arising either from a very high temperature acting upon minute portions of matter, or perhaps connected with the results produced in air by the passage of the spark.

One of the most remarkable decompositions, however, which has been obtained previous to Dr Faraday's experiments, is that of the late Dr Barry. This experiment is given as a proof of the chemical action of atmospheric electricity; but, as Dr Faraday has shown, it possesses a much greater interest if confirmed. The following is his own account of it:—“In August 1824 I elevated the kite in an atmosphere favourable to the exhibition of its phenomena. It was raised from an apparatus firmly fixed in the earth, and was insulated by a glass pillar. The usual shocks were felt on touching the string, which simple fact I am induced to mention from the circumstance of no electrometer having been employed. The portion of string let out, with a double gilt thread passed through it, was about 500 yards. I then made the connection shown in fig. 6, where the straight glass tubes, A, B, having platina wires passed from above half down their axes, and standing in their respective glass cups C, D, were filled with a solution of sulphate of soda coloured with syrup of violets connected also with each other by the bent glass tube E, likewise filled with the above solution in the usual manner. A portion of gilt thread d was then brought from the tube at A, and united to the kite-string K, whilst a similar thread b was carried from B to the earth. Bubbles of hydrogen in A, and of oxygen in B, soon appeared. In about ten minutes the blue liquid in A became green from the separation of the soda, whilst the sulphuric acid, by passing to the pole in the tube B, changed its contents, as usual, red.”

The effect now described as produced by atmospheric electricity was never produced by common electricity. Dr Wollaston and other philosophers could not obtain the gases in separate vessels, and Dr Faraday kept his powerful machine in action for a quarter of an hour, during which 700 revolutions were made, without producing any sensible effect, although the shocks that it could then have given must have been more numerous and powerful than could have been taken with any chance of safety from the kite-string of Mr Barry. Dr Faraday thinks “it just possible that the air which was passing by the kite and string, being in an electrical state sufficient to produce the ‘usual shocks’ only, could still, when the electricity was drawn off below, renew the charge, and so continue the current. The string was 1500 feet long, and contained two double threads; but when the enormous quantity which must have been thus collected is considered, the explanation seems very doubtful.” Dr Faraday therefore considers Mr Barry’s experiment as a very important one to repeat and verify; and he remarks, that if it is confirmed, it will be the first recorded case of the true electro-chemical decomposition of water by common electricity, and will supply a form of electrical current which is exactly intermediate, both in point of quantity and intensity, between those of the common electrical machine and the voltaic pile.

The effects of electricity on mixed and compound gases Effects of have been given by Mr Singer in his work on electricity. These effects are regarded by Mr Singer as mechanical, on gases, and as arising from the momentary agitation into which the various media are thrown by the action of the spark, which tends to promote a new arrangement of parts. This theory, of which Mr Singer himself not only saw but has stated the difficulties, cannot now be maintained with any show of reason; and there can be no doubt that the effects in question arise from a molecular polarity related to the two poles of the electric circuit, or to the two kinds of electricity which exist in nature.

It has been asserted, but not from any extensive series Influence of accurate experiments, that putrefaction and fermentation of electri- are promoted by electricity. M. Achard of Berlin, con-sidering that, in animals killed by lightning, the process of putrefaction advances with great rapidity, cut a piece of beef into three parts, and electrified one piece positively for ten hours, another negatively during the same time, while the third was not electrified at all. On the fourth day the electrified pieces had an intolerably fetid smell, while the un-electrified piece had only begun to smell a little. The same result was obtained with a piece of boiled veal. M. Achard also killed several birds by electrical shocks, and having killed others by sticking a needle through their heads, he placed them all under similar circumstances. The birds killed by electricity became putrid much sooner than the rest. The influence of electricity upon fermentation was studied and fer-also by M. Achard. A handful of rye brought into a state mentation of fermentation for the purpose of being distilled, was sepa-rated into two portions, one of which was electrified and the other not. Five hours afterwards the vinous fermentation had ceased in the electrified portion, but did not cease in the other portion till after the lapse of eight hours.

The influence of electricity upon colours is a subject of Influence peculiar interest, and cannot fail to prove a rich field of dis-e of electri-covery to those who may enter upon it with ardour. That city on electricity does alter the colour of particular bodies is un-doubted; but whether it produces a real chemical change on these bodies, or merely a transient change in their power of absorbing specific rays of the spectrum, remains to be determined. The few experiments made by Cavallo are extremely vague. He found that vermillion, carmine, verdigris, white and red lead, had their colour altered by the electric shock; and that the colours of orpiment, gamboge, sage-green, red ink, ultramarine, Prussian blue, and of a few other compounds, were not altered. The eye, however, is no judge of a real change of colour. It can judge only of the general result of the change, without indicating the nature of the change or changes that have taken place. A body, too, which may have lost or gained the power of absorbing definite rays of the spectrum, may often appear to have suffered no change at all, provided that the sum of the changes is a colour similar to the original colour of the body.

In producing these effects, namely, putrefaction, fermentation, and change of colour, the electric fluid may act, not by any virtue of its own, but by the intermediate of those ponderable substances which the spark carries along with it, and consequently leaves in bodies.

**Sect. III.—On the Changes produced by Electricity on Phosphorescent Bodies.**

Although the phenomena of phosphorescence have been lately much studied, yet philosophers are very little acquainted with its cause, whether it is developed by the light of the sun, the action of heat, or the transmission of electricity.

Almost all bodies may be rendered luminous during the transmission of an electric discharge through their substance; but unless this luminosity continues after the discharge is over, the body cannot be said to have been rendered phosphorescent by electricity. In his numerous experiments on this subject, Mr Skrimshire invariably kept his eyes closed till the sound of the discharge had been heard, and therefore the light which he then saw was not the light of the electric spark, modified by its transmission through the body, but was a real phosphorescence, which continued after the original cause of it was withdrawn.

The substances which he submitted to examination, including minerals of all classes, were placed on a horizontal brass plate, fixed to the ball of the prime conductor, and he then tried to obtain a spark from the body by means of a common discharger. The body was next placed upon a table, and the charge of a Leyden phial passed over it, at the distance of about a quarter or half an inch above its surface; and, as a last trial, the charge of the jar was made to traverse its surface by resting the points of the discharging rod at an inch or more distant from each other, upon the specimens under examination.

As many bodies possess the property of becoming phosphorescent by heat, the phosphorescence produced in the preceding experiments may in general be ascribed to the heat which accompanies an electrical discharge. But although the heat thus produced may be either the sole or an auxiliary cause of the phosphorescence which is excited in bodies which are known to be phosphorescent by common heat, yet it is obvious from other experiments, that electricity exercises a specific influence upon that peculiar structure or condition of bodies which causes them to give out light when heated.

M. Dessaignes seems to have been the first person who established a relation between electricity and phosphorescence. He found that the metallic powders, such as those of zinc and antimony, which are the most phosphorescent, lose their luminous qualities in a damp state of the atmosphere. Even in dry weather antimony loses its power of phosphorescing if it be rubbed in a metallic mortar; whereas, in an insulated mortar the light is very much increased. Glass pounded in dry weather becomes much more luminous than when it is pounded in a damp state of the air. It loses this property entirely in wet linen; but being as it were self-insulated, it is not deprived of its phosphorescence, like antimony, by being pounded in a mortar, which is a conductor of electricity. In order to make adularia phosphoresce briskly it must be pounded in an insulating or insulated mortar, and the handle of the pestle should likewise be insulated. M. Dessaignes also found that if glass be calcined till its phosphorescence is diminished, it resumes that property by being exposed on an insulated support, and subjected to a few electrical discharges, or to a current of electrical matter. Other substances which have lost their phosphorescence by calcination resume it when electrified; but our author remarked that electricity does not restore the phosphorescence of those substances which have been deprived of it by the light of the sun.

In pursuing this branch of the subject M. Dessaignes found that those phosphorescent substances which are imperfect conductors of electricity, are susceptible of receiving the luminous property from the action of the sun's rays; that non-conductors will not thus phosphoresce at all, or at least very imperfectly; and that most conductors give out no light whatever. Orpiment, and some of the oxides of arsenic, tin, zinc, and lead, are exceptions to this remark, and also the muriate of tin, and the sulphate and phosphate of lead. Non-conductors, and conductors which refuse to phosphoresce after feeble electrical discharges, become luminous after strong ones; while imperfect conductors that phosphoresce after weak electrical discharges give no light whatever when the discharges are much increased. In support of the analogy between electricity and phosphorescence, M. Dessaignes remarks that phosphorescence is affected by the presence of points. Fluor spar, which has the asperities produced by fracture, phosphoresces readily, while the entire and smooth crystal remains dark. The same is true of calcareous spar, adularia, apatite, emerald, and common salt. If both sides of the glass be rough, it phosphoresces throughout; but if one side be rough and the other polished, it only shines when the rough surface receives the heat. It is a fact still more curious, that when Iceland crystal with smooth faces is exposed to the solar rays, it acquires very little phosphorescence; whereas, if but one of the faces is roughened and exposed to the sun, it readily becomes luminous. In like manner, arragonite becomes luminous when a fractured face is exposed to the sun, but acquires very little light when the smooth natural surface is exposed to it. M. Dessaignes likewise maintains that nearly all the bodies that are susceptible of phosphorescence by friction become luminous by heat, by electricity and by exposure to light. The general view which our author takes of these phenomena is, that phosphorescence is produced by a particular fluid, which is set in motion by light, by heat, by electricity, and by friction, and that it is dissipated by overheating or too long exposure to light.

The influence of electricity upon phosphorescence and the colours of certain bodies has been examined by Mr Pearssall, who has amply confirmed the general result that bodies which have lost their phosphorescent property by calcination acquire it again when an electrical discharge is passed through them. Having submitted a piece of chlorophane to a powerful heat, it gave out a strong phosphorescent light of a pale violet colour; but the specimen decrepitated so much during its calcination that a piece of sufficient size to be electrified could not be preserved. He therefore placed the calcined fragments in a glass tube, and sent through them three electrical discharges, the effect of which was the emission of a deep violet light. He then heated the fragments upon platinum, and they emitted a phosphoric light of different colours. Some of the fragments appeared green, others yellow, and all of them finished by emitting a deep violet light. These colours are evidently distinct from those of the natural mineral, for a portion of the latter heated at the same time produced only a feeble violet colour. Another portion of the same specimen, calcined but not electrified, emitted no light when heated.

A specimen of chlorophane, whose phosphorescence had been destroyed by an intense heat, was exposed to the solar rays for two days without any of its phosphorescent quality being restored. A single electrical discharge, however, restored its phosphorescence, which increased in the ratio of the number and the intensity of the shocks it received. The green light emitted by the action of heat was more deep and of longer continuance after three, six, or even twelve discharges, than after one. Mr Pearsall obtained the same results with apatite and some diamonds; but electricity produced no effect in developing phosphorescence by heat in amethyst, sapphire, ruby, garnets, and other mineral substances which he tried.

In the course of his experiments Mr Pearsall observed the curious fact, that the specimens of fluor spar, though colourless in their natural state, received a bluish tint when electrified, and the acquired phosphorescence was proportional to the depth of the tint. When a number of fragments were used, the larger fragments were of a blue colour, and emitted a blue light when heated, whereas the smaller fragments emitted only a pale yellow light. Mr Pearsall thinks it probable that the phosphorescent property is communicated by electricity only to the surface, which he considers as explaining the fact that fragments of different dimensions emit differently coloured lights.

In resuming the investigation of this subject, Mr Pearsall found, that bodies not naturally phosphorescent, such as statuary marble in its natural or calcined state, ivory when its carbonaceous part was removed, calcined mother-of-pearl, calcined oyster shells, calcined pectenules, egg shells, and lime, were not only rendered phosphorescent by heat after being strongly electrified, but acquired this property with a beauty, a variety, and an intensity of colour superior to those which occur in specimens that possess a natural phosphorescence. We regret that our limits will not permit us to give in detail a second series of experiments which Mr Pearsall performed with twelve different varieties of fluor spar, all of which gave distinct phosphorescence previous to their being electrified; but the general result of them may be thus expressed. When the natural spars emit by heat a light of different colours, the electric action produces only one of them; but when the mineral yields only one natural colour by heat, this is replaced, when electricity is applied, by a phosphorescence of various colours, among which the primitive tint does not appear. As the colours change with the number of electrical discharges, Mr Pearsall found the following to be the order of progression. The specimen was a green fluor.

1 discharge, pale purple light when heated. 2 ... pale green, changing into purple. 3 ... the same colours, more intense and durable. 4 ... purple, with increased intensity. 5 ... green, brighter and deeper. 10 ... green bright; fine and more durable purple. 20 ... deep and more durable colours. 40 ... very rich colours, the purple at last inclining to red. 100 ... green colour, highly brilliant, and becoming yellowish. The purple had now a superb thin. 160 ... an intense light nearly white, followed with a brilliant green light, then with a durable purple, and then with a yellow accompanied with violet tints.

This specimen had been successively heated and electrified nearly fifteen times, and had suffered no deterioration in its phosphorescent property.

Mr Pearsall next shows that the property communicated by electricity was preserved even for three months, when the specimen was kept in the dark. Out of twelve fragments, two had completely lost their acquired phosphorescence by exposure to the sun for twenty-one days, five had nearly lost it, and six had experienced a modification in their colours by this exposure.

Mr Pearsall now examined the influence of electricity on the natural phosphorescence of bodies, and he found that an augmentation of intensity was produced, of which it is difficult to give an idea. Specimens of fluor whose pyro-phosphorescence was feeble or uncertain, were raised to the rank of highly phosphorescent bodies, and some of them even rivalled the Siberian fluor. At the end of fifty days some of these specimens still preserved the excess of phosphorescence which had been communicated to them, while others continued to exhibit the same order of colours.

Mr Pearsall has brought forward several experiments to prove that the phosphorescence of bodies, and the modifications it experiences, depend on their structure and mechanical condition. Phosphate of lime, for example, which in the form of apatite has an intense natural phosphorescence, has none when aggregated from a precipitation of it in a solution of muriatic acid, nor when obtained from powdered or calcined apatite. A calculus of phosphate of lime, however, gave green, yellow, and orange light when heated after having been calcined and exposed to twenty electrical discharges. Mr Pearsall also several times observed that the power of phosphorescence returned after it had disappeared.

With the view of showing that the phosphorescence was not owing to any radiating matter which was carried along with the sparks, Mr Pearsall inclosed coloured chlorophane in glass tubes hermetically sealed, and found them phosphorescent after 225 discharges. He found voltaic electricity capable of producing phosphorescence in some cases and not in others; so that it differs greatly from common electricity in this property.

In explaining the preceding phenomena, Mr Pearsall considers the intense electricity of the Leyden jar as altering the structure upon which phosphorescence depends, by the vibratory motion which it communicates, and which allows the particles to take a new arrangement. When the body has had a new structure communicated to it by the vibrations or shocks of each electrical discharge, the action of heat is supposed by our author to permit the body to return to its primitive structure; and he conceives that the vibrations of the atoms during these changes of structure may produce light.

Sect. IV.—On the Changes produced by Electricity on Odoriferous Bodies.

It was discovered by M. Libri of Florence that electricity exercises a curious influence over odoriferous bodies, on odoriferous bodies. Having caused a continued current of electricity to traverse a piece of camphor, the odour of this substance became more and more feeble, and at last entirely disappeared. When the camphor has suffered this change, and is withdrawn from all electrical influence, and put in communication with the ground, it will remain without odour for some time, but it will afterwards resume its former properties slowly and gradually. M. Libri seems to have obtained a similar result with other odoriferous bodies; but he has not, so far as we know, given any more particular account of his researches.

Sect. V.—On the Magnetic Effects of Electricity.

During almost every period of the history of electricity, magnetic philosophers have pointed out strong resemblances between effects of the phenomena which it exhibits and those of magnetism. Some of the most striking points of resemblance were, that each consisted, as it were, of two powers or directions of powers, of an opposite nature, and subjected to similar laws of attraction and repulsion; that the action of magnetism has a great analogy with that of electricity; that the distribution of the forces in an electrified body differs very little from that of the forces in a magnet; and that the pyro-electrical tourmaline has the strongest resemblance to an artificial magnet.

These views were powerfully confirmed by the fact, often observed, that magnetism was communicated to bodies by a stroke of lightning, and that the compass needles of ships have had their polarity changed by a similar cause. The ship Dover was struck by lightning in the Atlantic on the 9th January 1749; and in four compasses on board, one of which was in a brass box, and the other three in wooden boxes, all the needles had lost their virtue. At first their polarity seemed to have been nearly reversed, but after a little while they moved about in every direction, and were of no use. Mr Gowin Knight, having examined one of these compasses, observed that the outward case was joined together by pieces of iron wire, sixteen of which were found in the sides of the box and ten in the bottom. By applying a small needle to each of these wires, Mr Knight found that they were all strongly magnetic, particularly those which had joined the sides.

Another very remarkable case occurred on board the New York Packet, in its voyage from America to Liverpool in 1827; and as a very accurate description of it was communicated by the Rev. Mr Scoresby to the British Association at York in 1831, we shall lay before our readers his own abstract of it. "Soon after the commencement of the voyage, this vessel encountered a severe thunderstorm, and received a stroke of lightning, which shattered the masts in several parts, and started some of the exterior planks of the bends. This was in the morning before day-light. The weather continuing unsettled, and the air in a highly electric state, with water-spouts in various directions around, the captain, fearing another explosion from the highly charged atmosphere, put up a lightning conductor which he had on board. In the afternoon of the same day the ship was a second time struck, but preserved by the conductor, though the iron of which it was composed was destroyed, and fell in melted globules upon the deck. No lives were lost, though some of the crew received heavy shocks; whilst one person, an invalid passenger, derived essential benefit from the electric discharge. Mr Scoresby had an opportunity of examining the vessel immediately on her arrival in Liverpool, when, on investigating the condition of the iron on board, he found almost every article capable of permanent magnetism, with sensible polarity. Table-knives and forks were capable of lifting needles or small nails, and one knife sustained a travelling-trunk key. Most of the watches on board suffered by the magnetic influence, especially those which were under the pillows of their owners in bed. These were all stopped, and on examination were found so highly magnetic that portions of the steel-work were capable of suspension by each other, in a chain of three or four pieces. Of one of these pieces (the cap-spring) Mr Scoresby made a pocket compass, which was exhibited when his communication to the association was made, and was observed to be in all respects a delicate and perfect instrument."

In enumerating the points of analogy between lightning and electricity, Dr Franklin remarks that they have both the power, not merely of reversing the poles of magnets, but of completely destroying their magnetism. By discharging four large jars through a common sewing needle, he communicated to it such a degree of magnetism, that it placed itself on the plane of the magnetic meridian when it was made to float on water. If at the time of receiving the discharge the needle lay east and west, the end at which the discharge entered pointed north; but if the needle lay north and south, the end which lay to the north continued to point to the north, at whatever end the discharge entered. He found also that the magnetic intensity developed in a needle was a maximum when the needle lay north and south, and a minimum when it lay east and west, at the time of receiving the electrical discharge. If the charge of a large jar or battery is transmitted through a steel wire perpendicular to the horizon, it will be permanently magnetized, and the lower end, at the time of the discharge, will afterwards turn to the north when it is made to traverse in a horizontal plane. If we now replace the wire in its vertical position, the end which was formerly the lowest being now the highest, and again transmit the discharge, the polarity of the needle will either be completely destroyed, or the poles will be reversed.

It has been found also that the polarity of a natural magnet may be completely destroyed by transmitting through it the charge of a battery.

In repeating the experiments of Franklin, Beccaria discovered that lightning always communicates the magnetic effects of polarity to bodies containing iron, and he observed this phenomenon even in common bricks that had been struck by lightning. Guided by the observations which he made on the polarity of such bodies, he was able to trace the directions which the lightning had taken in passing through them.

A series of elaborate experiments were made by Van Marum, on the magnetic effects of electricity. He employed a battery of 135 jars, containing 130 square feet of coated surface, and he transmitted the powerful charges which it yielded through watch-spring needles from three to six inches long, and also through steel bars nine inches long, between a quarter and half an inch broad, and nearly a line thick.

In this way he found that when the needle or bar was placed horizontally in the plane of the magnetic meridian, its north end acquired north polarity, and its south end south polarity, in whatever direction it received the discharge. When the bars possessed some degree of polarity before receiving the shock, it was either diminished or reversed after receiving it. When the needle or bar received the shock in a vertical position, its lower end became the north pole whether it had been previously magnetic or not. Generally speaking, the degree of magnetism which was communicated was as strong in a horizontal as in a vertical position. When the needle was placed in the magnetic equator, and received the discharge longitudinally or along its axis, it received no magnetism whatever; but when the shock was passed through its width, or at right angles to its axis, the needle received a considerable degree of magnetism, the end which pointed to the west becoming the north pole, and that which pointed to the east the south pole.

When the charge was so powerful as to render the needle hot, no sensible polarity was communicated to it.

Such was the state of our knowledge respecting the connection between electricity and magnetism, when Professor of electromagnetism, or Magneto-electricity. The fundamental fact which Mr Oersted discovered may be thus expressed.

When a wire conducting electricity is placed parallel to a magnetic needle properly suspended, the needle will deviate from its original or natural direction. This deviation follows a regular law.

1. If the needle is above the conducting wire, and the positive electricity goes from right to left, the north end of the needle will be moved from the observer.

2. If the needle is below the wire, and the positive electricity passes as before, the north end of the needle will be moved towards the observer.

3. If the needle is in the same horizontal plane with the wire, and is between the observer and the wire, the north end of it will be elevated.

4. If the needle is similarly placed on the opposite side, the north end of it will be depressed. In these two experiments the needle must be very near the wire.

From these simple facts Mr Oersted concludes, that the magnetic action of the electrical current has a circular motion round the wire which conducts it. This law will be understood by inspecting Plate CCXXVI. fig. 10, where, if fig. 10. AB is the conducting wire or the direction of the positive electricity, the circle ed ef will be the plane in which the magnetical circulation takes place. The small arrows show the direction of the austral or polar magnetism, the sharp ends or heads of the arrows indicating the direction in which the austral magnetism, and consequently the north end of the needle is repelled, and the boreal or north-polar magnetism is attracted; while the opposite ends of the arrows indicate the direction in which the boreal magnetism, and consequently the south end of the needle is repelled, and the opposite magnetism attracted.

The preceding discovery was made with the electricity of the galvanic battery, but it is equally true when a strong current is obtained from the common electrical machine. An electric spark sent along a conducting wire passes too quickly to move the needle, and a current produced by the electrical machine does not appear to contain a sufficient quantity of electricity to act upon the needle, or rather to show its action. If the electrical effect of the current, however, is multiplied, its action upon the needle becomes apparent. In order to do this we must use, as Dr Colladon first did with success, Schweigger's multiplier, which is shown in fig. 11, where ABCDE is the wire which conducts the electrical current, bent several times, and covered with three folds of silk for the purpose of insulation. The needle NS is then inclosed within the coils of the wire, and the effect of the current upon it is obviously quadrupled by the four coils of the wire which surround it. The coils should be as near to each other as possible; and as they can be repeated a great number of times, the multiplication of the effect is almost unlimited. The needle is suspended by a single fibre of silk, and the sensibility of the instrument may be increased by using a magnet for the purpose of diminishing the directive power of the needle. When Dr Colladon brought the two ends of the wire of this apparatus to the two conductors of an electrical battery of 4000 square inches, so as to make the discharge go a little way through the air before it entered the wire, a current of sufficient strength and of some duration was obtained, which produced a considerable deviation in the needle. Dr Colladon also obtained a deviation of several degrees with this multiplier, by means of the electrical current obtained from an electrical machine.

These interesting experiments of M. Colladon have been amply confirmed and beautifully extended by Dr Faraday. Although MM. Arago, Ampere, and Savary had witnessed a successful repetition of M. Colladon's experiments, yet the conclusions to which they led were doubted by some and denied by others. Dr Faraday was therefore induced to repeat them with great care. He employed for this purpose the electrical machine, battery, and discharging train already described (see page 596).

The galvanometer which he used was sometimes a single one, consisting of sixteen or eighteen convolutions of copper wire covered with silk, and sometimes a double one, consisting of two independent coils, each containing sixteen feet of silked copper wire. The glass jar which covered the galvanometer and supported the needle was coated inside and outside with tinfoil, the upper part (left uncoated for the purpose of examining the motions of the needle) was covered with a frame of wire-work with numerous sharp projecting points. When this frame and the two coatings were connected with the discharging train, an insulated point or ball connected with the machine in its most active state, could be brought within an inch of any part of the galvanometer, without the inclosed needle being affected by any ordinary electrical attraction or repulsion.

Dr Faraday expected, by means of the retarding power of bad conductors, to obtain from ordinary electricity the powers of voltaic electricity. After the connections were properly made, a battery charged positively by about forty turns of the machine was discharged through the galvanometer, when the needle immediately moved. By repeating this experiment when the needle was vibrating, its vibrations were extended to above forty degrees on each side of the line of rest: on reversing the galvanometer the needle was equally well deflected in the opposite direction, the deflections being in the same direction as if a voltaic current had passed through the galvanometer, the positive surface of the battery coinciding with the positive end of the voltaic apparatus. Similar effects were obtained by taking the electrical current directly from the prime conductor, and dispensing with the battery altogether. When the electricity, too, was passed through an exhausted receiver to imitate the aurora borealis, and then through the galvanometer to the earth, it was equally efficacious in deflecting the needle.

From these and other experiments, Dr Faraday concludes that a current of common electricity, whether transmitted through rarified air, water, brine, acids, and other imperfect conductors, or through wire, or by means of points in common air, is still able to deflect the needle (the only thing necessary being to allow time for its action), and is just as magnetic as a voltaic current.

As it is by the galvanic battery, however, that this subject has been studied, we cannot pursue it any farther at present, and must refer our readers to the articles already mentioned, in which a full view of this new science will be given.

Sect. VI.—On the Effects of Electricity upon Animal Bodies.

The influence of electricity on the human frame, whether it is administered in small quantities so as to excite and surprise us, or in the more powerful and awful form of a stroke of lightning, must be well known to the least informed of our readers.

When any part of the body receives an electric shock, a disagreeable sensation is felt in the place; and, according to Dr Robison, it is sharper when taken from a long wire than from a large body. When the human frame forms part of the electric circuit, or when the charge of a Leyden phial is made to enter the body at one hand and pass out of it at the other, a violent concussion or shock is felt along the line of its passage across the breast and through the arms. This electrical shock, and the involuntary motion which accompanies it, arises no doubt from the obstructions which an imperfect conductor like the human body, composed of fluids and solids of different conducting powers, presents to the free passage of the electric fluid. If the charge is increased, the patient through whom it passes falls down under its influence, and suffers a temporary suspension of vital action; and if it is increased to a still greater degree, it will produce instantaneous death. This case is frequently exemplified when persons are killed by lightning; and a very remarkable instance of the laceration of the human body occurred several years ago which could have arisen only from an obstruction to the free passage of the fluid. The case to which we refer presents us with a most singular variety of action exhibited by the lightning in passing through animal bodies; and it is so interesting, and so well described by Mr B. Boddington, the father of the gentleman who was struck with the lightning, that we shall present our readers with an abstract of it.

On the 13th of April 1832, Mr and Mrs T. F. Boddington left Tenbury, occupying the hind barouche seat of their effects past-chariot, the servants being in the inside. About half-past three o'clock, with the sun shining, and a serene sky, they observed a dark cloud to arise in the direction of their route. Soon after a clap of distant thunder was heard, but no lightning was seen. A few drops of rain having begun to fall, Mr Boddington put up an umbrella, and, after giving it to his wife, he put up another, and when he was in the act of extending the latter, a flash of lightning struck them both senseless, threw the horses on the ground, and cast the post-boy at a distance. One of the servants, after recovering from his alarm, looked out of the window, and saw the head of Mr Boddington hanging over the seat, and apparently lifeless. Jumping from the carriage, he raised his master's head, and found his clothes on fire, while Mrs Boddington was standing up tearing off her bonnet and shawls. She had neither seen the flash nor heard the thunder, but felt a sense of suffocation, and was putting off her things to obtain air. She and the servant then proceeded to extinguish the fire, which was still consuming her husband's dress. The lightning, passing down through the umbrella, penetrated through the bonnet into Mrs Boddington's neck, and zigzagged along the skin of her neck to the steel busk of her stays, leaving a painful but not a deep wound, and affecting the hearing of the left ear. From the lower end of the busk the lightning pierced through all the garments down to her thighs, where it made wounds on both; but the one on the left was so deep and so near the femoral artery, that the astonishment is she escaped with her life, the hemorrhage being very great. None of her clothes were burnt, notwithstanding their inflammable nature, nor did any of her wounds present the appearance of burns. Mr Boddington, after remaining insensible for ten minutes, revived, and felt a pain all over him. The main force of the shock passed down the handle of the umbrella to his left arm, though a portion of it made a hole through the brim of his hat, and burnt off all the hair that was below it, along with his eye-brows and eye-lashes. The fragments of the burnt parts falling into the eyes, deprived him nearly of sight for two or three days. The electric stream shattered his left hand, melted his gold shirt-buttons, and tore the clothes in a most extraordinary manner, forcing parts of them, with the buttons, to a distance, and inflicting a deep wound under their position on the wrist. The arm was laid bare to the elbow, a severe wound was made in his body, and every article of his dress torn away as if by gunpowder. It then passed to the iron of the seat, wounding his back, the whole of which was literally flayed. The horse rode by the postilion was killed. A very striking difference was observed in the wounds of Mr and Mrs Boddington. Hers were fractures of the flesh. His, on the contrary, whether deep or shallow, were all burns, and had a white and blistered appearance. No wound was visible on the dead horse, excepting an indentation on the head where the fluid entered, discolouring the spine in its passage.

For the purpose of determining in what manner death is produced by a powerful electric discharge, Van Marum sent the electric shock through eels one and a half and three and a half feet long. The smaller eels were instantly killed when the shock was sent through their whole body; but when the charge was only sent through individual parts, these parts only lost their irritability, while the rest retained it. When the shock went through the upper and fore part of the head of the large eels, the under jaw, as well as the muscles of the neck and belly, and even the lower part of the body, preserved their irritability, while the parts which conveyed the charge had totally lost it. When smaller shocks were sent through warm-blooded animals, similar effects were observed; and hence it has been inferred that the circulation of the blood cannot take place when such an effect has been produced, and that the suspension or destruction of life must arise from this cause. When the shock does not affect the large arteries the animal may recover, provided that the spinal marrow and the cerebellum are not injured.

Various experiments have been made by Mr Morgan and others, with the view of ascertaining the influence of electricity on the animal functions. Mr Morgan found that if the diaphragm forms part of the circuit between the inside and outside coating of a jar containing two square feet, the lungs will make a sudden effort, followed by a loud shout. When a small charge is similarly applied, a violent fit of laughter is always produced, even on the gravest persons. A strong charge transmitted through the diaphragm is frequently accompanied by tears and sighs, and sometimes by fainting. When a strong charge is sent through the spine of a person standing, he will frequently either drop on his knees, or fall prostrate on the floor. A strong charge having been transmitted accidentally through Mr Singer's head, he felt the sensation of a violent but universal blow, which was followed by transient indistinctness of vision and loss of memory, but no permanent injury was received. When the charge of a battery is sent through the head of a bird, its optic nerve is always injured or destroyed; and when a smarter shock is given to a larger animal, a tremor and depression, with a general prostration of strength, is produced.

Mr Cavendish observed that the sensible shock depended more on the quantity than on the intensity of the charge, double degree of intensity with only half the quantity invariably producing a less powerful shock. According to Volta, only a little more electricity is necessary to produce an equal shock from a larger surface. A surface, for example, 16 times as large required only an elevation of the electrometer to one-tenth of the number of degrees. Dr Robison informs us that the shock obtained from a small bison charge given to a large surface, yields a less unpleasant shock than a large charge given to a small surface. As these observations, however, depend upon individual feelings, and as it is known that different persons are affected in very different ways with the same degree of electricity, they may not be generally correct.

The influence of electricity on the pulse has been examined by different authors, though with some variety of result. M. Trembley found that the arterial pulse was on the quenched in persons electrified. M. Boze was of the same opinion; but the Abbé Nollet could not discover any increase in the rapidity of the circulation of various animals which he electrified. Cavallo, on the contrary, informs us that an experienced medical electrician assured him that, "in a diseased state of the body, an obvious acceleration of the pulse was observed to result from the application of electricity."

In the experiments made by M. Nollet, his attention was directed to other points besides the state of the pulse. His experiments were made with birds, cats, and the human subject; and having selected and carefully weighed pairs of the animals, he communicated to them a current of electricity for some hours, when they were again weighed. The loss sustained was ascribed to perspiration. The general result was, that the animal which was electrified was always found to be lighter than the one which was not. The persons who submitted to these experiments suffered no inconvenience from them. They experienced a slight degree of exhaustion, and an increase of appetite, but none of them found themselves sensibly warmer.

In order to settle these questions respecting the influence of electricity on the pulse and on insensible perspiration, Van Marum selected eleven persons, and repeated the experiment four times upon each, with negative as well as with positive electricity. They were placed in a room so remote from the machine that they could not hear the noise which was made in working it. They were placed on insulating stools, and their pulse was felt and carefully counted both when the machine was in motion and at rest. The general result was, that no decided acceleration was observed, a few additional beats having taken place in some cases. In general, however, there was a great irregularity in the pulse.

The next experiment of Van Marum was a very interesting one. He placed a boy eight years old in one scale of a delicate balance, which scale was insulated by means of a silk cord. The boy being connected with the conductor, the balance was brought to a state of exact equilibrium. Having determined that the boy, previous to being electrified, lost 280 grains in an hour, he electrified him, and found that the loss was 295. In another experiment the boy lost 330 grains before, and 310 after being electrified. A girl seven and a half years old lost 180 grains before, and 165 after being electrified. A boy eight and a half years old lost 430 grains before, and 290 after being electrified. A boy nine years old lost 170 before, and 240 after being electrified. As this boy had remained very quiet during the experiment, the increase was ascribed to electricity, and the experiment was carefully repeated. He now lost 550 grains before, and 390, 330, 270, 550, and 420 after being electrified. Hence it appeared that the insensible perspiration had rather decreased than augmented.

The powerful influence of electricity on the human frame led the more sober part of the medical profession to view it as a valuable auxiliary in the healing art, while those who were more sanguine regarded it as an universal medicine, which might be resorted to in every form of disease. Charlatans of every degree found the electrical machine a lucrative article of trade; and there were not wanting well-meaning enthusiasts who contributed to prolong the reign of medical electricity.

But though electricity has not yet taken up a position in the healing art, there can be no doubt that in various disorders its application has been found advantageous, and that patients have, in a particular class of diseases, experienced instantaneous relief.

The machine used for medical purposes should have sufficient power to yield a continued current of strong sparks. The diameter of the plate in a plate machine should be about twenty inches, and that of a cylinder about ten or twelve inches. The only apparatus necessary is a jar fitted up with Lane's electrometer (see Plate CCXXIX, fig. 16), and a pair of directors, each consisting of a glass handle surmounted by a brass cap, with a wire a few inches long, carrying a ball at its extremity. A wooden point is sometimes substituted for this ball. When it is required to pass a shock through any part of the body, the directors are applied at the opposite extremities of the part, one director being connected by a wire with the inside coating, and the other with the outside coating of the jar, or, what is the same thing, with the receiving ball of Lane's electrometer, previously placed at such a distance from the ball of the jar as to yield a charge of the proper magnitude. When sparks are to be administered, it is done with the director and brass ball; but when the organ is very delicate, such as the eye, a stream of electricity is thrown upon it from the wooden point, held at the distance of about half an inch. An insulating stool, capable of holding a chair for the patient, is also necessary. In certain cases a brass plate, communicating with the inside of the jar, is placed in the bottom of the chair, so as to apply itself to the lower part of the body, when the electricity is required to pass through the abdomen or adjacent parts.

Sect. VII.—On the Effects of Electricity upon Vegetable Bodies.

It has been distinctly shown by Priestley, Ingenhouz, and Sennebier, but especially by Theodore Saussure, that the various parts of plants act upon atmospheric air; that they insensibly disengage a large quantity of carbonic acid Apparatus at the expense of the oxygen; and that, owing to some combination within the plant, they sometimes exhale pure oxy-Effects of gen. Now, as all carbonic acid has vitreous electricity, this electricity exhalation of the acid from the plants ought to furnish an on veg- abundant supply of it to the atmosphere. M. Pouillet, of whose researches we have already given an abstract, has placed this truth beyond a doubt.

From this fact alone we might reasonably infer that electricity performs an important function in the phenomena of vegetation; but so little attention has been paid to this subject, that we have some hesitation in laying before our readers the very imperfect and unsatisfactory experiments which have been recorded. The best experiments, indeed, have entirely a negative character; and the general result of them is given when we say that electricity appears to have no decided efficacy as a stimulus to vegetable life.

The recent discoveries, however, which have been made on endosmose and exosmose by M. Dutrochet, render it extremely probable that an electrical action is the cause of the ascent of the sap in plants; but as M. Poisson has ascribed these curious facts to capillary action, and other philosophers to other causes, we must wait for further experiments before we can treat this subject as a branch of electricity.

PART II.

DESCRIPTION OF ELECTRICAL APPARATUS.

In the preceding part of this treatise we have already had occasion to refer to several pieces of electrical apparatus, particularly to two or three varieties of the best machines for generating electricity by friction. Notwithstanding this slight anticipation, however, we must resume the subject at some length, on account of its great importance in a popular and practical view of the science.

The various kinds of electrical apparatus may be classed under the four following heads:

1. Instruments for generating and collecting electricity. 2. Instruments for accumulating electricity. 3. Instruments for indicating the presence of electricity, and measuring its quantity. 4. Instruments for miscellaneous purposes.

CHAP. I.—DESCRIPTION OF INSTRUMENTS FOR GENERATING AND COLLECTING ELECTRICITY.

The instruments which belong to this chapter are, elec- Instru- trical machines, atmospherical conductors, and electrophori- ments for generating electricity.

Sect. I.—Description of Electrical Machines.

The simplest of all pieces of apparatus for generating electricity is a tube or rod of glass, which, when rubbed with a piece of woollen cloth, will yield as much electricity as will charge a jar in a short time. In consequence, however, of the labour which attends this operation, it has been usual to turn a sphere or cylinder of glass round an axis by a simple winch, or by a double wheel and band, for the purpose of generating electricity rapidly, and without fatigue to the operator.

We have already exhibited two of these machines in Plate CCXXII. figs. 1, 2, 3, 6, & 7, and described their general construction. It is easy to modify this construction in various ways;—and for particular purposes and particular classes of experiments particular forms of the machine may be most convenient: But as the philosopher is best capable of introducing such modifications for his own use, we shall not occupy our pages with the descriptions of electrical machines which have sprung more from the fancy and caprice of individuals than from the wants of the science. There can be no doubt that the plate-glass machine is the most commodious and the most powerful form of the electrical machine. But whether the glass has the form of a plate or a cylinder, it should be of that kind which has the least quantity of alkali in its composition. Bohemian glass, and glass pretty old, is preferred to modern glass. It has also been stated that a glass plate will derive great electrical power by a considerable exposure to the sun's rays.

We have already described, and given representations of very excellent plate-glass machines in Plate CCXXII., figs. 1, 2, and 7, and in Plate CCXXXIII., figs. 1–5, of the last of which we shall give a fuller description; but we have reserved to the present chapter the description of the best form of the electrical machine with which we are acquainted, and which we owe to the ingenuity of Sir Snow Harris, F.R.S., Plymouth.

1. Description of Sir W. Snow Harris's Electrical Machine.

This machine, which is shown in perspective in Plate CCXXVII., fig. 7, consists of a circular disc of plate glass ZZ, three feet in diameter, mounted on a horizontal axis, resting on two horizontal supports of mahogany. These supports are themselves sustained by four vertical mahogany columns, fixed upon a firm frame as a base. To the lower side of this frame are fixed four legs M, N, O, P, upon which the whole machine rests; and these legs again rest upon another steady frame RST, furnished with rollers, so as to move it easily into any required position, and likewise with three levelling screws, RST, for placing it horizontally. By these means the machine may be so adjusted and fixed that the axis of the plate of glass, which has a free motion backwards and forwards in the holes in which it turns, may not tend more to one side than to the other, and occasion an unequal action on the rubbers. The rubbers, which are four in number, are insulated on pillars of glass A, B, one placed at each extremity of the horizontal diameter A, B, of the plate. The positive conductor CBD projects in a vertical position in front of the plate ZZ, while the negative conductor passes in a curvilinear direction behind, and connects the rubbers of each side.

The plate of glass is turned by an insulated handle, immediately in front of which is placed a short index, which is fixed to the axis, and which moves over a graduated circle L, attached to the horizontal part of the frame, and through the centre of which the axis passes. In this manner the number of revolutions of the plate may be accurately registered.

In order to strengthen the centre of the plate, two smaller plates are cemented to each side by varnish; and a small stop is inserted into the axis, to prevent the pressure from increasing beyond a certain point.

When the machine is used for ordinary purposes, the conductors shown in fig. 7 are employed; but when it is employed to accumulate electricity, the conductors should have the smallest extent possible, and, excepting at the receiving points, where they collect the electricity from the edge of the silk flaps about H, H, they should be covered with sealing wax. In this case the positive conductor is formed of small straight tubes, as shown in fig. 8, and its extremities terminate in balls of varnished wood, through the substance of which the metallic communications pass.

2. Description of Van Marum's Electrifying Machine.

This machine, to which we have made a brief reference in Sect. III., Chap. II., Part I., is represented in elevation and in section in figs. 1 and 2 of Plate CCXXXIII. The plate of glass AB, which is thirty-one inches in diameter, is sustained by a single pillar E, at the upper extremity of which are two similar brass collars I, I, one of which is shown separately in fig. 4. The horizontal axis MN rests upon these collars, and this axis carries a counterweight L, in order to balance the plate of glass and its appendages, and thus equalize the friction on the collars. The rubbers, which cannot be seen in the section, fig. 2, are shown at m, n, fig. 1. The pair at m is attached to the ball O, and supported by the glass pillar e; and in like manner the pair at n is attached to the ball P, and supported by the glass pillar f. A horizontal section of the rubbers and balls is shown separately in fig. 3. A semicircle of brass CD is attached to an axis g that turns on the ball G, resting on the pillar F, so as to give the conductor CGD a motion round that axis. Collectors six inches long and two and a half in diameter are placed at C and D, to collect the electricity from the revolving plate AB. At the outer end of the axis g is a copper tube HH, terminating at its lower end in a ball H, and its upper end in a smaller ball h, two inches in diameter, which, screwing into G, will fix the tube HH in any position round g. An arch of brass wire cld, half an inch in diameter, is fixed to the end of the bearing piece K, and moves round I into any given azimuth, so as to be placed, as in fig. 1, opposite the rubbers m, n, or at right angles to them. In like manner, the conductor CGD can be placed either horizontally, so that the collectors C and D may be opposite the rubbers m and n, or vertically, as shown in fig. 1. By this apparatus it is easy to produce either positive or negative electricity. In the position of the conductor shown in fig. 1, where CGD is at right angles to the rubbers, and where the rubbers are connected with the ground by the arch cld, and by the wire KK, fig. 2, the conductor G will give positive electricity; but when we wish negative electricity, the conductor CGD is placed horizontally, with its collectors C, D opposite the rubbers, and the arch cld is placed vertically, so as to insulate the rubbers.

A mahogany cap T covers the metallic caps of the supports, in order to insulate them more perfectly. A hollow ring of mahogany, V X, is, for the same reason, made to cover the metallic socket into which the support is inserted. In fig. 3, a, b, a, b, are four pieces of gum-lac. In figs. 1 and 2, W is the handle by which the machine is wrought.

3. Description of Hare's Electrical Machine.

This machine, which we have previously noticed, differs Hare's from those generally made, in having its glass plate horizontal-electrical tal; and it is considered by its inventor, Professor Hare of Philadelphia, as giving negative electricity in a way preferable to that in which it is obtained in Van Marum's machine.

The glass plate MN, thirty-four inches in diameter, is supported on an upright iron bar PR, about an inch in diameter, and covered by a stout glass cylinder, sixteen inches high and four and a half inches in diameter, open only at the base, through which the bar is introduced so as to form its axis. At the top of the bar PR is a block of wood turned to fit the cavity at the apex of the cylinder, and cemented therein. The external apex of the cylinder, is fixed by cement into the brass cap which carries the plate. The glass cylinder, which is liable to no strain, effectually insulates the plate from the iron axis PR. The brass cap seen at P is surmounted by a screw and flange, which, with the aid of a corresponding nut and discs of cork, keeps the plate firm. The wheel W, driven by a handle, communicates by means of a band with another wheel about twenty inches in diameter, placed on the iron axis RS.

"Nearly the same mode of insulation and support," says Dr Hare, "which is used for the plate is used in the case of the conductors. They consist severally of arched tubes of brass (ABC, DEF), of about an inch and a quarter in diameter, which pass over the plate from one side of it to the other, so as to be at right angles to, and at a due distance..." Electrical from each other. They are terminated by brass balls and caps, which last are cemented on glass cylinders of the same dimensions nearly as that which supports the plate. The glass cylinders are suspended upon wooden axes, surmounted by plugs of cork turned accurately to fit the space which they occupy. The cylinders are kept steady below by bosses of wood which surround them. In this way the conductors are effectually insulated, while the principal strain is borne by the wooden axes.

The collectors are shown at MN in connection with the positive conductor ABC, and the rubbers are shown between P and the balls D and F in connection with the negative conductor DEF. The advantage of this form of the machine over that of Van Marum is, that the two conductors are permanently fixed in their places, and that positive and negative electricity can be at any time obtained without any change in the machine. Dr Hare considers the band as of advantage in preventing the plate from being cracked by any hasty effort to put it in motion when it adheres to the cushions, as it often does. Dr Hare uses a winch on the other side of the wheel, so that two persons, or one with both hands, may drive it.

The great expense of large cylinders and plates of glass, and their liability to injury, have induced artists to construct electrical machines of different substances. M. Walckiere de St Amand of Brussels constructed a machine of extraordinary power, which consisted of a web of varnished silk twenty-five feet long and five feet wide, revolving upon two wooden cylinders covered with woollen serge. During the revolutions of the cylinders, the silk moves between two cushions, each seven feet long and two inches in diameter, covered by cat's skin or hare's skin, and moveable so as to vary the friction. The machine was driven by four men, and it had so great power that it gave sparks fifteen inches long, and nobody durst take a spark from it but with the shoulder and elbow.

Dr Ingenhousz constructed machines with discs of pasteboard four feet in diameter, and soaked in copal or amber varnish dissolved in linseed oil. They were covered with the same varnish, and were mounted upon an axis or flat board, three inches broad, and covered with flannel or hare's skin, being placed between each two discs, so as to act as a rubber. Sparks one and even two feet long were given out by the front disc when the knuckle was presented to it.

Wooden discs, and cylinder discs of gum-lac partly immersed in mercury, which acted on the rubber, and stretched varnished ribbons, have been all used in the construction of electrifying machines, but it would be an unprofitable task to describe them.

4. General Observations on the Construction and Use of the Electrical Machine.

Although, in fine dry weather, and in a warm and dry place, a good electrical machine may be brought into an excellent state of action merely by wiping it with a warm linen cloth and afterwards with a silk handkerchief, yet in a different state of the atmosphere, and in humid apartments, every precaution is necessary to insure the vigorous and steady action of the machine. By turning the machine before a fire, or placing it in a current of heated air, or, as Dr Faraday suggests, by placing it over a sand-bath or a hot iron plate whose temperature does not exceed 212 degrees, the different parts of the machine will be thoroughly dried and heated without affecting the cements.

We have already described (see page 536) the improvement of Mr Ronalds, who heats the inside of the machine, &c., by a spirit-lamp. Dr Faraday recommends the heating of a cylinder machine by placing a chemical Argand lamp with a low flame beneath the cylinder, and to support a plate of metal nearly six inches square about an inch above the chimney of the lamp. This plate, by being heated, varies the air above it, and produces a large moderately heated current, which encircles the cylinder and thoroughly warms it. Care must be taken not to heat the cylinder in spots, but to bring it, and especially the insulating parts, to an uniform temperature, which shall never be sufficient to melt the cement which is used in any part of it.

The state of the rubbers requires particular attention. They must be carefully freed from dust, and supplied with a soft and uniform coating of amalgam, which should always be rubbed in a mortar with tallow previous to being used. Large spots of amalgam should be removed from the cylinder or plate, either by the nail or a piece of wood. Dr Faraday remarks that a few spots of amalgam rather increase than diminish the activity of the machine, and that the silk which proceeds from the rubber is better when impregnated with amalgam than when free from it. Dr Faraday adds that it is often useful to hold a piece of silk, with some amalgam adhering to it, against the revolving plate or cylinder, and also to rub the surface of amalgam on the rubber with the same amalgamated silk. When the machine is thus put into good action, and the prime conductor removed, it should discharge a continued series of brushes from the edge of the silk, and abundance of sparks flying round the glass.

Armstrong's Hydro-Electric Machine.

This powerful electrical machine, the history of which we have already given, is represented in the annexed figure, where AB is a steam-boiler made of the usual material, 61 feet long, and 3½ broad, or of a less size if required. It is supported on four or six strong glass pillars by which it is insulated. The steam generated by the heat of the furnace F, escapes from the common steam pipe by a stopcock S; and issues through a great number of bent iron tubes a, b, c, terminated in jets or adjutages of box-wood. A conductor N insulated by a glass rod R projects from the boiler, in order to collect from it the excited electricity, and another conductor P is placed in front of the jets to carry off the opposite electricity of the steam, and prevent it from neutralizing that of the boiler. The tubes are often inclosed in a box containing water for the purpose of cooling the tubes, which are kept wet, not by contact with the water, but by cotton threads which carry it up to them by capillary attraction. In this way minute drops of water are pro- Electrical duced, which being carried away by the steam produce the electricity by their friction upon the wooden adjutages. According to Dr Faraday, the steam performs no other part than that of making the particles of water rub upon the wood through which they pass. A magnificent hydro-electric machine has been erected at the Faculty of Sciences in Paris, with no fewer than 80 jets. Its enormous sparks succeed one another so rapidly as to form continuous and brilliant jets about a foot in length, and some inches in breadth.

Sect. II.—Description of the Electrophorus.

This ingenious instrument, which was invented by the celebrated Volta, is shown in Plate CCXXVII. fig. 9. It consists of a circular metallic disc A, or a plate of wood covered with tinfoil, having an insulating handle of glass screwed into a nut E, made of wood or brass. The plate A is called the upper conductor, or cover. The next plate B, called the resinous plate, consists of a plate half an inch thick, composed of equal parts of shell-lac, common resin, and Venice turpentine, poured when hot upon a marble or stone table. The next plate is a metallic one C, called the lower conductor, or sole, which may be either separate or not from the resinous plate which rests upon it. The edge of the first plate A must be pretty thick, and made smooth and round. The following is the method of generating electricity with this apparatus.

The cover A being held in the left hand, rub the upper surface of the resinous plate B with a piece of dry fur, or whip it with a fox's tail or stripe of cat's skin. It will thus be excited negatively. Place the upper conductor above the resinous plate, and while it is there touch it with the finger, and then raise it by its glass handle. It will exhibit signs of positive electricity, and will yield a spark either to the knuckle or to the knob of a Leyden jar. If the cover A is again placed upon B, and, after being touched, again raised, it will give another spark, and twenty of these sparks will charge a Leyden jar of a moderate size. If the upper conductor A is not touched by the finger when placed upon B, it will exhibit, when raised, very faint, if any, traces of electricity. Now, as the resinous plate B continues, without any new excitation, to charge the upper conductor A, it is manifest that its electric condition is not destroyed by the contact and removal of A; and as it is necessary to connect the upper conductor with the ground, by touching it previous to its being raised, it is obvious that the electricity acquired by A is derived from its contact with B.

In order to explain the theory of the electrophorus, let us insulate the lower conductor C, by placing it on a glass stand, as in fig. 10, and let this conductor communicate with the pith balls of an electroscope. As soon as the upper surface of the cake B is excited, the pith balls will diverge with negative electricity. The negative electricity developed by the excitation of the upper surface has decomposed the natural electricity of C, by attracting the positive part and repelling the resinous part into the electroscope where it is indicated. If we now touch the conductor C, its negative electricity is carried off, and the positive undergoes no diminution; but, owing to the escape of the negative portion, the balls will collapse. If we now make the upper conductor A approach to B, and rest upon it, touching it at the same time with the finger, so as to connect it with the ground, the positive electricity of the cake B will decompose the natural electricity of A, repelling its negative electricity to the earth through the finger, and attracting its positive portion to its lower surface. This positive electricity of A attracting the negative electricity of the surface of B, and repelling the vitreous electricity of C, thus doubly tends to diminish the force by which this positive electricity is rendered latent or detained. Some of it, therefore, will be set free, and the pith balls will diverge with positive electricity, the divergence increasing as the conductor A comes nearer and nearer to the plate B. But as the positive electricity of the lower conductor has a tendency to repel the positive electricity with which we wish to charge the upper conductor A, we must cause the lower conductor C to communicate with the earth, as in fig. 9. By this means the fig. 9. electricity of C is reduced to its natural state, and the electricity of the upper surface of the cake B renders latent the maximum quantity of positive electricity on the upper conductor B.

Although the air produces a gradual dissipation of the electricities which are not rendered latent in an excited electrophorus, yet a well-constructed electrophorus will remain for months in full energy.

M. Biot has ingeniously applied the principle of the Lichtenberg figures to explain what have been called the figures of Lichtenberg. If, when the electrophorus is charged, we electrical raise the conductor A, and replace it on the cake B, by figures making it rest obliquely and upon its edge, then its positive electricity, accumulating itself wholly in the part which touches B, will become much stronger. It will escape from A, and will completely neutralize the negative electricity of the places towards which it goes, and after some contacts thus repeated upon different parts of the cake B, it will be all discharged. Hence we may deduce the following curious experiment:—Instead of bringing back upon the negative electricity the positive which it has developed by its influence, carry it to another resinous cake B', in its natural state. It will likewise attach itself to the surface of this cake, which will become positively electrified, and be capable in its turn of developing by its influence negative electricity. When the second cake B' is thus charged, place upon its surface a disc of metal. We shall then have an electrophorus of an opposite kind to the first; and if this last is used to charge a third cake B", the latter will have negative electricity; and in this way we may have any number of cakes, which will be electrified positively and negatively alternately. By this process we may electrify each surface only in certain parts, by attaching to the conductor A a rod and metallic button. If we then touch the resinous cake with this button, the electricity will be carried wholly to the point of contact. These points may be so chosen as to form the outlines of any regular or picturesque figures. In order to render these forms or pictures visible, we have only to strew on the surface of the resinous cake some light powder formed by a non-conducting substance, such as pounded resin or sulphur. The small particles of the resin, for example, will attach themselves only to the electrified spots; so that, by inverting the plate, all the rest will fall down by their own weight. These small particles affect regular and different arrangements, according to the nature of the electricity which makes them adhere; so that, by forming figures with the two electricities in different parts of the same plate, we obtain at the same time two sorts of figures. Lichtenberg's method of making these figures visible is exceedingly beautiful. Having triturated sulphur and tinminium or red lead together in a mortar, so as to have a mixture of a yellow and red powder, he traced his figures on the resinous cake with the knob of a jar charged with vitreous electricity, and repeated them with the knob of a jar charged with resinous electricity. The compound powder being now projected, either with a powder puff or by means of a pair of bellows, upon the cake, the particles of sulphur which are electrified positively by trituration will attach themselves to the negatively electrified spots, while the negatively electrified particles of red lead will adhere to the positively electrified spots, so as to form a series of red and yellow figures when the cake has been inverted, and the rest of the powder has fallen from it. Many beautiful variations of this experiment have been devised; Electrical and Mr Bennet has shown how to make the figures permanent, by transferring them to paper.

When this experiment was first made, some German philosophers observed that the powder of rosin had sometimes a progressive motion which was not regular, and a new theory was the consequence of this. It was found, however, that they were very small insects of the genus acarus which happened to be in the powder, and which walked over the surface of the plate.

When well made and properly used, the electrophorus is a very powerful and useful instrument. Dr Klincock of Prague has shown, that if we transfer alternately the upper conductor from one resinous cake to another, and touch it after it is placed on the cakes, both cakes continually acquire more and more electricity, so that the upper conductor returns from either plate quite overcharged; and Leyden jars may be so strongly charged by them as to burst by the charge. The conductor returns from one plate charged with positive, and from the other charged with negative electricity.

M. Cavallo informs us that an electrophorus made of sealing-wax spread upon a thick plate of glass six inches in diameter was capable, when once excited, of charging a Leyden jar several times in succession, and so strongly as to perforate a card with the discharge. The upper conductor, when separated from the plate, was sometimes so strongly electrified that it darted strong flashes to the table upon which the electric plate was laid, and even into the air.

2. Mr J. Phillips' Modification of the Electrophorus.

As the contact of the operator's finger is of no other use than to connect the upper conductor with the earth, Mr John Phillips of York conceived the ingenious idea of producing the same effect by a momentary contact between the upper and under conductors. In effecting this he adopted three methods. The first consisted in raising a brass wire and ball from the lower conductor above the edge of the resinous cake, so that the edge of the upper conductor, or a brass ball upon it, may be brought in contact with it. This method answered very well with small instruments, in which the upper conductor can be easily directed to any particular point of the sole. In the second mode he fixed a narrow strip of tinfoil across the whole diameter of the resinous surface, so as to join the metallic sole or lower conductor. This construction answers perfectly, and is particularly suitable to large circles, whose upper conductors will infallibly touch some point of the metallic strip. The third method is to perforate the resinous disc quite through at the centre, and at any other point, and to insert in these perforations brass wires with their smoothest tops level with the resinous surface.

These three methods are represented in fig. 11, where a represents the ball in the first method, b the slip of tinfoil in the second, and c, c, e the conducting wires in the third and best method.

"On two of the largest electrophori," says Mr Phillips, "which I have made, both the second and third methods have been tried with equal success, but I much prefer the latter construction. The largest instrument has a cast-iron basis 20½ inches diameter, resinous surface 19½ inches, cover 16½ inches. The resinous composition was made according to the directions in Mr Faraday's work on Chemical Manipulation. The cover is made of a plate of thin copper, strengthened at the edge by a thick brass wire, from which three radial brass wires pass to the upper part of a central brass tube. In consequence of the angle they thus form with the plane of the plate, they act as pretty strong braces to maintain its figure, and the whole is very light. This central brass tube receives a cylindrical piece of wood, into which the insulating glass handle covered with sealing-wax is screwed by its wooden foot.

"With ordinary excitation this instrument will yield loud flashing sparks two inches long or more, and speedily charge considerable jars. The cover can be easily charged and discharged fifty or a hundred times in a minute, by merely setting it down and lifting it up as fast as the operator chooses, or the hand can work. In charging a jar or plate, I placed one knob of the connecting rod near the insulated surface of the jar or plate, and the other some inches above the cover; then the cover being alternately lifted up and set down, the jar is very quickly charged.

"One instrument nine inches in diameter, which I have made from the second plan above described, has very often surprised me by its remarkable power of retaining electrical excitation.

"The following example is worthy of notice. Early in September 1832 this instrument was removed from a house in York, where it had been for some time laid by, and brought to my present residence, distant one third of a mile. It was placed on a shelf on my book-cases, where it remained untouched until the 23rd March 1833, and was then taken down covered with dust. It was found to be in a state of feeble excitement, so as to give sparks, visible in the daylight, nearly one fourth of an inch long."

3. On Dr Faraday's Improvements on the Construction of the Electrophorus.

As the electrophorus is an excellent substitute for an improved electrical machine in the laboratory of the chemist, from its merit on being capable, when in good order, of inflaming the greater number of explosive mixtures operated upon in eudiometers, Dr Faraday has given the following simple and ingenious methods of constructing this instrument:

He recommends the cover to be made of a piece of flat deal board one third or one half of an inch thick. This board is to be covered with pasted tinfoil laid on smoothly, particularly at the edges, and having all asperities rubbed down. The smoothest and flattest side being reserved for the lowest, a piece of glass tube seven or eight inches long is to be fixed on the centre of the other side for a handle; and towards the edge, on the same side, there should be fixed a piece of thick wire, about two inches long, bent outwards, and carrying a smooth metal ball at its upper end.

In order to make the resinous plate, a sheet of tinfoil one or one and a half inch wider than the cover is laid smoothly in the bottom of a flat dish, so that its edges may rise up all round, or in the inside of a hoop. Shell-lac, common resin, and Venice turpentine, in equal proportions, are then to be melted together in a metallic vessel, and kept in a state of fusion from 230 to 240 degrees of Fahrenheit, till the vapour has ceased to evolve, and the fluid is quiet. When it has thickened by cooling, it must then be poured quickly, to avoid the formation of bubbles, upon the tinfoil, so as to form above it a cake one third or one half of an inch thick. The tinfoil should then be trimmed round its edge, and the cake should rest upon or be attached, by its tinfoiled side, to a board, to serve as a base and prevent it from injury. Dr Faraday observes that the cover, instead of a board, may be a plate of tin turned up round a thick wire, so that no sharp edge or angle may be presented outwards; and that for the resinous plate may be substituted a sheet of thin crown glass, having for its metallic base a sheet of tinfoil pasted to it. He adds also, that a large plate of mica without fissures, and coated in the same manner with tinfoil on one side, makes an excellent electrophorus. When glass, however, is employed, it must be well warmed at first, and kept warm during the experiments. The glass should be excited by being rubbed with a piece of silk with some Electrical amalgam spread upon it. It should be passed briskly over its surface backward and forward, and finally slid quickly off at its edge, so as not to rest upon any one point of the glass, lest it should discharge that portion of its surface.

To return, however, to the use of the electrophorus first described. The resinous plate, when warm and dry, should be placed horizontally on its board, with the tinfoil below, and connected by a wire or chain with the ground, or with a discharging train when it can be obtained. See page 596. A piece of warm flannel, doubled up loosely into a roll about ten inches long, is to be held in the hand by one end; and the other end, being swung round in an inclined direction with a quick motion of the wrist, should strike the surface of the plate obliquely each time it passes, so as to produce an effect between that of a rub and a blow. When the whole surface of the warm resinous cake has been thus struck, it will be excited to a considerable degree. The cover of the electrophorus, being previously warmed, must now be lifted by its glass handle and placed on the middle of the resinous cake; and if the knob or metallic ball of the cover be now touched, a spark will pass from it to the finger. The cover is next to be lifted by its handle in a horizontal direction; and when it is two or three inches above the plate, the knob upon it is again to be touched by the finger or a ball, when a spark stronger than the first will be obtained. The cover being again put down on the plate, a third spark will pass between the knob and the knuckle. The cover being again lifted as formerly, a spark as strong as the second may be taken from it. By repeating this process, similar effects may be obtained for a long time. The sparks which are taken by the knuckle after putting the cover down are negative, and those which are taken after lifting it up are positive. Hence we charge a jar either positively or negatively, according as we take the spark when the cover is up or down. In order to obtain strong positive sparks, the cover, when on the resinous plate, must be touched with the finger, which must be removed before the cover is lifted up; and to obtain the strongest negative sparks, the cover when raised should have all its electricity carried off by the hand or some other conducting body before it is again placed on the plate. As the cover ought to be in a state of good insulation, the handle should be made of sealing-wax and gum-lac, or if made of glass, it should be varnished with sealing-wax dissolved in alcohol.

Sect. III.—Description of Conductors for bringing down Electricity from the Atmosphere.

Various means have been adopted for collecting the free electricity of the atmosphere, either for the purposes of experimental investigation, or in order to defend buildings and ships from lightning. The apparatus for the first of these purposes is essentially different from that which is used for the last.

1. Electrical Kites.

When the lower atmosphere is charged with electricity, it is not difficult to collect it for the purposes of experiment; but in ordinary states of the air, or when the free electricity exists at some height above the earth, it is necessary to bring it down by means of a kite. For this purpose a schoolboy's kite is sufficient. It is only necessary to twist a copper wire round the hempen string. Dr Franklin covered the frame of his kite with a thin silk handkerchief, in order that it might the better sustain the violence of a thunder-storm. In order to compensate for this additional weight, he made the framework of two strips of cedar wood in the form of a cross. The string of the kite terminates towards the observer in a silk string or cord, which insulates the kite and its conducting string; and in order to protect the observer still farther, a safety chain has been sometimes suspended from the extremity of the conducting string, so as to reach the ground and carry off the electricity in case of its becoming too powerful.

Mr Cuthbertson sometimes found it necessary to use three kites all connected together. On one occasion when he could collect no electricity from the atmosphere with a kite having a string 500 feet long, he succeeded in obtaining it by adding other two kites, each of which had strings of the same length. Mr Cuthbertson likewise employed an apparatus for raising his kites, in which the strings were lengthened or shortened by coiling them round a drum.

2. Exploring Conductors.

One of the simplest instruments for collecting atmospheric electricity is the hand-exploring rod used by Mr Read. It was of the same material, length, and thickness as a common fishing-rod, and had small wire twisted round it from one end to another. Standing on an insulating hand-stool, he raised the rod in a vertical position, and after a ploring minute or two be touched with his other hand an electrometer, which indicated the nature and intensity of the electricity brought down. When the electricity thus obtained was very weak, he placed on the rod a lighted torch, keeping it as far up the rod as the strength of his arm would permit; and he always found that the flame attracted the electricity more powerfully than the end of the rod.

Mr Read, however, found it necessary to use a fixed conductor or thunder-rod; and we have shown in Plate CCXXVII. fig. 12, the apparatus which he used in experiments on the electricity of the atmosphere, of which we have already given some account. The principal part of it is a wooden rod AA, twenty feet long, one inch in diameter at the top, and two at the bottom. Into the lower end of it is cemented a solid glass pillar B, coated with wax, and twenty-two inches long. This pillar rests on a wooden pedestal C, carried by a bracket D. At thirteen inches above D, the rod passes through a glass tube F, coated with wax, and supported by a strong arm of wood E. A lining of cork lies between the rod A and the tube F, to prevent the latter from being broken when the rod is bent by the wind. Several sharp-pointed wires G stand out from the top of the tube. Two of them are of copper, about one-eighth of an inch thick, one of them being twisted round the rod to the right, and the other to the left, as shown in the figure, so as to reach the brass collar at the top of the lower funnel H, to which they are soldered. The use of the two funnels H, H is to defend the glass rods B, F from the weather. Through a hole in the wall at I passes a glass tube coated with sealing-wax, through which a strong brass wire passes from the rod at M into the room. At the end of the tube this wire passes through a brass ball L, two inches in diameter; and, after proceeding a little farther, it suspends from its extremity a pith-ball electrometer K, about twelve inches from the wall. A bell N, carried by a strong wire, is placed two inches from the brass ball L, three tenths of an inch in diameter, suspended from the nail O. The bell N, which has a metallic communication R with the moist ground, is rung by the ball L. Jars and other pieces of apparatus are placed when wanted upon the small shelf P; and all this part of the apparatus is protected from the weather by being inclosed in a wooden box.

M. Cavallo's apparatus, called an atmospherical collector, merits a description here, on account of its simplicity and ingenuity. A common jointed fishing-rod AB, fig. 13, has its smallest joint replaced by a slender glass tube C, coated with sealing-wax. From a cork D at its outer end is suspended a pith-ball electrometer. A piece of string AHGI, Electrical is fixed to the end A of the rod, and supported at the point Apparatus G by a piece of twine FG. When a pin at the end I of the cord is pushed into the cork D, the electrometer is insulated; but it is insulated for the purposes of observation in the following manner:—The pin being fixed in the cork D, and the rod held by the hand at A, it is held out of one of the highest windows, at an angle of about 50° or 60° to the horizon, and kept there for a few seconds. The cord is then pulled at H, so as to disengage the pin from the cork D, and the string drops into the dotted position KL, leaving the electrometer insulated and electrified in a state opposite to that of the atmosphere.

3. On Lightning Conductors.

We have already seen that electricity is from various causes generated and set free in our atmosphere, and that individual clouds and masses of clouds are often highly charged with electricity, and insulated by the surrounding air. The earth and the sea are good conductors of electricity; and, generally speaking, their natural electricity is undisturbed. The attraction, therefore, of the electricity of the clouds for the opposite electricity in the earth or the sea, may become so powerful as to break through the resisting medium which intervenes. If the clouds are above a mountain or rising ground, this discharge of electric matter into the earth is attended with no danger. The effects have sometimes been traced in the fusion of portions of the rocks which crown these exposed summits. If a tree stands in the stratum of air through which the cloud discharges itself, the lightning passes through it, cleaving it, dissipating its sap into a visible vapour, and sometimes splitting the lower part of it into fibres like lucifer matches. If a house obstructs its path, the electricity descends through its walls, seeking the quickest and easiest passage to the earth. It will follow bell-wires, iron rods, damp walls, and gilded pictures, and find out any matter, whether organized or unorganized, living or dead, which is placed near its path and is capable of advancing it on its rapid and breathless errand to the earth. If a living animal grazing, or a human being walking, in an open field, intervenes between the overcharged cloud and the ground, the one or the other will become the chosen path of this irresistible foe. If a ship floats under an electrified canopy of vapour, it has less chance of escape than the tree, the house, or the living being.

The only terms upon which we can meet this relentless enemy, is a humble admission of its supreme and irresistible power, and a resolution to give it the freest and the fullest passport. We must supply it, in short, with a railway of metal, the only species of road upon which it can travel with a suitable speed, and a harmless intention. The moment it ceases to find a conducting body, it begins its devastation among imperfect or non-conducting substances, till it again gets into a safe and easy path.

The common practice of using cylindrical rods of copper or iron for conducting the lightning to the ground affords to buildings a very imperfect security. It has been supposed that such rods would influence the discharge from a cloud at a distance equal to two-thirds of the height of the rod, and therefore give protection to that extent. This, however, is not the case, for buildings have been often struck in one place when there was a conductor not very far from the spot that was struck; and, in the case of ships, the foremast has been frequently struck when a lightning chain was applied at the mainmast.

In securing buildings, one or more capacious channels of conduction should be applied systematically to certain parts of them along the walls, terminating above in solid projecting points, and below by two or more branches under the surface of the earth. These main channels should consist of the best metallic conductors. Copper is to be preferred, and they may either consist of stout copper tubing, from an inch to 2 inches in diameter, and not less than 1/8th of an inch in thickness; or otherwise, of copper plates, varying from 2 to 4 inches in width, and of a similar thickness. These may be carried down either within or without the building, as circumstances may require. When copper tubing is employed, the joints must be well secured over solid plugs by screw joints, and be firmly pinned to them. When plates are used, it would be desirable to place them in two layers, one over the other, so as to admit of the continuous portions of the one covering the joints of the other. The plates should be firmly united at the joints, and should be 1/8th to 1/4th of an inch in thickness. These lines of conduction should be secured immediately against the masonry, and not be placed at any distance from the building, or pass through rings of glass or other insulators, as is sometimes erroneously done. It is to be here remembered, that the materials of which a building consist are already conductors of electricity to a very considerable extent, and will of themselves allow large quantities of atmospheric electricity a fair passage to the earth. The object in applying metals along the walls is to complete the conducting power of the general mass up to the point required for a full transmission of a shock of lightning without intermediate explosion, and therefore the closer the conductor is applied to the walls the better. The notion of keeping the electrical discharge out of the building by insulating the conductor from its walls is evidently very futile, and can only arise out of a false view of the nature of the electrical discharge, which (as we have already seen experimentally) is determined to the surface of the earth in a path of least resistance, which the conductor itself supplies. We can not therefore imagine that the electrical agency will leave a good capacious conductor, immediately in its line of action, and in which the resistance is a minimum, to move in a bad conducting circuit out of that line, in which the resistance is a maximum. But if we were to admit that such an effect were possible, even then it is not to be supposed that a small mass of bad conducting matter, such as a small ring of glass or a little pitch, could arrest such a terrible agency in its onward course. An agency which can shiver immense oak trees, split solid rocks asunder, and break down half a mile thick of air, would scarcely be arrested by an insignificant mass of glass or pitch. It has been the custom in some instances to place balls of glass on the pinnacles of towers and on the masts of ships, under the impression that glass is a repeller of lightning, but it is evident that glass is no more a repeller of lightning than the air itself. The spire of Christ Church at Doncaster had a ball of glass placed on it, with a view to its security from lightning. In November 1836, however, it was struck by the electrical discharge, and the spire totally destroyed.

The main conducting channels being complete to the earth and applied in selected positions in the way alluded to, we should be careful to link into connection with them, so far as possible, all the vicinal detached masses of metal in the building, either by branches or immediate contact; and in this way bring the general mass into that passive electrical state it would have as regards the electrical discharge, supposing the whole structure were metallic throughout, or at least as nearly as may be, so that in case of lightning striking with fury on any point of the building, the explosive action may vanish, and the discharge have unlimited room for expansion in all directions upon the surface of the earth to which it is always determined.

The necessity of providing against the destructive agency Lightning of lightning on ship board has been long felt. The continuous and fixed metallic conductors available for build- Electrical fittings were not however equally applicable in ships, in consequence of the masts, the only parts to which they could be well attached, being subject to motion in a variety of ways, to frequent elongation and contraction, and to the necessity which frequently arises for removing the higher portions of the mast altogether. It was hence proposed to substitute long flexible chains, or links of copper about the size of a goose quill. These being supplied to ships packed in boxes, were hoisted up to the truck as occasion required, were stepped along the rigging, and allowed to hang overboard in the sea. Of late years copper wire rope has been somewhat extensively applied in this way as rigging, and there is little doubt but that these temporary forms of lightning conductors for ships may occasionally transmit very heavy shocks of lightning. Unfortunately, however, the many trying and variable circumstances under which a ship is placed—the liability of damage to the ropes and rigging; and the complication of changes to which the whole is subject, has, generally speaking, baffled all these attempts to secure ships against the electrical discharge; so that notwithstanding that ships have been commonly supplied with such lines of conduction, the amount of destruction in the royal navy and in the merchant service has been very frightful, as appears by parliamentary papers, printed by the House of Commons in December 1864. Hence the necessity of some more capacious and permanent security, and which from time to time has been repeatedly called for. Mr Singer, in his excellent work on electricity, suggested the possibility of joining continuous rods applied to the masts by flexible spiral wires, observing at the same time, "that the conductors employed to protect ships from lightning are by no means so efficient as requisite, that they are kept packed in boxes, so that, partly from inattention, and partly from prejudice, they frequently remain in the ship's hold unemployed altogether." But besides these mechanical objections, such temporary forms of conductors are not always secure on account of their small capacity. The Hotel des Invalides at Paris having a conductor of twisted wire ropes was struck by lightning in June 1839, the wire was knocked in pieces and the building damaged. The same thing happened in H. M. ship "Hazard," struck by lightning at Sarawak, Borneo, in June 1846. This vessel had a conductor of copper wire rope—the wires were twisted in pieces, torn from the ship's side; the electrical discharge divided between the mainmast and rope, a portion of it shivered the spars, and went into the hull. The commission appointed by the Board of Admiralty in 1858 under the countenance of the House of Commons, to inquire and report on the best form of lightning conductors for H. M. ships gave, in their report, many examples of the insufficiency of temporary conductors applied as rigging, to meet the exigencies of the case, and concurred fully in the opinion advanced by every practical seaman, that if lightning conductors be applied on shipboard, they should be applied under a capacious and permanent form, so as to render them secure and independent of the officers and crew of the ship.

About the year 1820, Sir W. Snow Harris having devoted himself to the investigation of the best means of guarding the royal navy from lightning, and the general laws of atmospheric electrical discharge, proposed to the Board of Admiralty a new method of capacious electrical conductors for ships, to be permanently fixed and systematically applied along the masts, and generally throughout the hull, so as to render the vessel secure from lightning at all times and under all circumstances, without the officers and crew of the ship incurring any responsibility whatever for due attention to their application or care in handling them under various trying circumstances, as in the case of the ordinary chains, Apparatus, and not unfrequently to their great peril and annoyance. Adopting the broad general principles we have already laid down as the true theoretical ones for lightning conductors, the inventor came to the conclusion that if a ship were perfectly conducting in all its structure, no damage could arise to it when struck by lightning; since at the instant of the electrical discharge falling on any point aloft, the explosive action would vanish, and would be converted into a comparatively quiescent current action with unlimited room of expansion upon the surface of the sea in all directions, that is upon one of the surfaces to which the discharge is determined.

In order to afford a ship effectual protection from lightning, Sir W. Snow Harris conceived it to be essential that Snow Harris's ship's conductor be as continuous and direct as possible from the highest points to the sea; that they be permanently fixed in the masts throughout their whole extent, so as to allow one part of the mast to move upon another; and, if any part of the mast should be accidentally or wilfully removed along with the conductor attached to it, that the remaining portion of the conductor should still be perfect, and capable of transmitting an electrical discharge into the sea. To accomplish these objects, a sort of double conductor should be formed, consisting of two laminae of sheet copper, placed one above the other, so that the extremities of the laminae of one layer should be opposite the middle of the laminae of the other layer. These laminae are each about four feet long, from six inches to one and a half broad; the thickness of the under layer being one eighth, and of the upper layer one sixteenth, of an inch. The copper bands thus formed are fixed in a fine dove-tailed groove in the aft sides of the different masts, and are secured in their place by wrought copper nails, so as to form a smooth surface, the nails being driven at each side, so as to be about four inches apart. Before inserting the conductor, the groove should be painted over with white lead, and must be deep enough to allow the copper to lie a little beneath the surface of the wood. "The metallic line," says Sir S. Harris, "thus constructed, will then pass downward from the copper spindle at the mast-head, along the aft sides of the royal-mast and top-gallant-mast, being connected in its course with the copper about the sheave-holes. A copper lining in the aft side of the cap through which the top-mast slides now takes up the connection, and continues it over the cap to the aft side of the top-mast, and so on as before, to the step of the mast; here it meets a thick wide copper lining, turned round the step, under the heel of the mast, and resting on a similar layer of copper fixed to the keelson; this last is connected with some of the keelson bolts, and with three perpendicular bolts of copper, of two inches diameter, which are driven into the main keel upon three transverse or horizontal bolts, brought into immediate contact with the copper expanded over the bottom. The laminae of copper are turned over the respective mast-heads, and secured about an inch or more down on the opposite side; the cap which corresponds is prepared in a somewhat similar way, the copper being continued from the lining in the aft part of the round hole over the cap, into the fore part of the square one, when it is turned down and secured as before, so that when the cap is in its place the contact is complete. In this way we have, under all circumstances, a continuous metallic line from the highest points to the sea, which will transmit the electric matter directly through the keel, being the line of least resistance."

1. Comptes Rendus, 1839. 2. "Since the mizen-mast does not step on the keelson, it will be necessary to have a metallic communication at the step of the mast, with the perpendicular stanchion immediately under, and so on to the keelson as before, or otherwise carry the conductor out at the sides of the vessel." This metallic line is shown in Plate CCXXVII. figs. 14, 15, 16, by the dotted line ABCD; and it will be seen that any elongation or contraction of the masts, as in figs. 14 and 15, or the removal of either of them, as in fig. 16, which brings them into a new position, will in no way disturb the continuity of the line ABCD, which evidently remains the same, and is therefore, under these different circumstances, the shortest and best conducting line between the mast-head at D and the sea at S. When the sliding masts are struck, a part of the conducting line necessarily remains below the cap and top; but as this is quite out of the circuit, it will not at all influence the passage of the electric fluid along the shorter line. For this invention Sir William Snow Harris has received from the government the sum of L5,000, a very inadequate reward for so great a service.

CHAP. II.—DESCRIPTION OF INSTRUMENTS FOR ACCUMULATING, CONDENSING, AND MULTIPLYING ELECTRICITY.

The instruments which have been employed for the purposes of accumulating, condensing, doubling, and multiplying electricity, may be divided into four classes:

1. Jars and batteries. 2. Condensers. 3. Doublers. 4. Multipliers.

SECT. I.—On the Construction and Action of Jars and Batteries.

By means of the prime conductor of an excited electrical machine, we can obtain electricity in sufficient quantity and intensity for many important researches; but when we wish to accumulate it in great quantities, and to obtain a powerful charge, it is necessary to employ the Leyden phial or jar; and by increasing the number of jars, and uniting them together, we can accumulate electricity to an unlimited extent.

An electrical jar, in its best form, is shown in Plate CCXXVIII. fig. 1, where AB is a glass jar, having its lower end CDEB coated both on the outside and the inside with tinfoil, which is made to adhere to the glass by means of gum water. The jar should have no cover, as it generally has, and the charge should be conveyed to the bottom of the jar by a copper tube FGH, three eighths of an inch in diameter. This tube terminates in a ball, F, of baked wood, and is kept in its place by a convenient foot, firmly cemented to the bottom of the jar, which is previously covered with a circle of pasted paper, leaving a central portion of the coating free for the perfect contact of the charging rod FGH, which passes through the centre of the foot, as shown by the dotted lines in the figure. When the jars are either employed singly, or united so as to form a battery, they should be placed on a conducting base, supported by short columns of glass, or some other insulating substance, such as rosin or brimstone, so that the whole can be insulated when necessary.

In order to allow the jars to be charged and discharged with precision, Sir W. Snow Harris connects them with what he calls two centres of action, A, B, shown in fig. 2. The first of these, A, consists of a brass ball, which slides with friction on a metallic rod AD, so as to admit of its being placed at any required height. This ball has a number of holes perforated in its circumference, to receive the points of the rod or rods which connect it with the jar or jars. The rod AB which supports this ball may be either insulated on a separate foot, and connected with the prime conductor, or it may be inserted directly into it. The second centre of action consists of a larger ball of metal, B, attached to a firm foot, and placed on the same conducting base with the jar, so as to be perfectly connected with it. Electrical When the first centre of action, A, requires to have a separate insulation, the insulating glass rod is screwed immediately into the lower ball B, and sustains the metallic rod above described, by the intervention of a ball of baked wood, D, the opposite end of the rod terminating in a similar hall, C, through the substance of which the conducting communication with the machine passes when it is placed on a separate foot. All the metallic connection should be covered with sealing-wax, except at the points of junction, and the wooden balls and different insulations should be carefully varnished.

A battery constructed in this manner, and containing six jars, is shown in fig. 3, A and B being the two centres of battery action, and C and D the two balls of baked wood, as shown in fig. 2. The communication with the prime conductor is made by a wire CE passing through the hall C, and the jars communicate with the centre of action A by means of wires entering the hall A, as shown in the figure.

In order to charge the jar shown in fig. 1, it is only necessary to make the copper tube FG communicate with the prime conductor of the electrifying machine by means of a wire passing through F. It was formerly the custom to make the copper rod HG terminate above in a brass hall at F; and when this was the case the jar could be charged by bringing the ball F near the conductor, or by holding the jar by the outside coating, and bringing the brass knob close to the conductor.

When the jar is fully charged, it may be discharged by holding the outside coating in one hand, and touching with the other the copper tube FG, or the hall F if it is a brass one; but in this case the person will receive a shock, the electrical charge passing into his body. The jar may be discharged without receiving a shock, by a very simple instrument called a discharging rod, shown in fig. 4. It consists of two bent wires BC, BD, having a brass hall C and ing rod, D at each end, and uniting at B, where they are fastened at their common junction into a glass handle AB. The operator takes hold of the glass handle, and placing the lower knob D on the outside coating of the jar, and the upper knob C in contact with the copper wire FG, or the brass hall at F, if there is one, the discharge takes place with a loud snap the instant that the knob C touches F.

A more convenient form of discharging rod is shown in fig. 5, where the two halls C, D, and the branches CE, DE, correspond to the halls C, D of the branches CB, DB, in fig. 4; but in place of attaching one insulating glass handle to the joint E, a separate glass handle, viz. A and B, is attached to each branch. By this means, by taking the handle A in one hand, and B in the other, we can open the halls C, D to the required distance without touching the metallic branches CE, DE, and also with greater facility and certainty.

If the jar is connected with the piece of apparatus BC, fig. 2, so that the centre of action A communicates with the internal coating of the jar, and the centre of action B with the external coating, then the jar will be discharged by making the knob D of the discharging rod touch B, while the knob C touches A. In like manner the whole battery in fig. 3 may be discharged by making the knob D of the discharging rod touch B, while the knob C touches A.

A general instrument for discharging jars and batteries, Henley's invented by Mr Henley, has been much used, particularly universal in the deflagration of metals by electricity. It is shown in discharge fig. 6, where A and B are insulating glass pillars, cemented into a wooden stand. A brass cap with a horizontal and vertical motion is fixed on the top of each of these pillars;

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1 See the Edinburgh Review, Oct. 1844, vol. lxxxi. p. 442. 2 The method of constructing the jars and batteries here described is that used by Sir William Snow Harris, and described in his valuable paper on the Laws of Electrical Accumulations. Electrical and at the top of this joint is a spring tube, through which the handles D or C can be slid backwards or forwards. These handles are made of strong brass wire, terminating at one end in a ball, or point, or a pair of forceps, and at the other in a solid glass rod for an insulating handle. A small wooden table F, about five inches in diameter, has a slip of ivory glued into its upper surface, and may be raised or depressed in its socket by the screw-nut G. Sometimes a small mahogany press accompanies the instrument. It consists of two boards, which can be pressed together by two nuts, and is put into the socket in place of the table F, when it is necessary to fix or hold steady the body through which the discharge is to be passed. The body to receive the charge must either be laid on the table or fixed in the press, or held between the balls, points, or forceps. The two sides of the jar or battery are then connected with the two brass caps at the tops of the pillars A, B; and, by means of the insulating handles, the distance is regulated through which the charge has to pass.

This instrument was originally constructed without any insulating handles; and the wires, and the handles C and D, were thick brass wires, terminating on one side in a ball or point, and on the other in a ring of brass, with which the connection with the jar or battery was formed.

Although it has proved advantageous to use jars for receiving and accumulating electricity, yet this form of the recipient is by no means essential. The very same effects is obtained if a plate of glass is coated on both sides within an inch of its edges; for a jar may be considered as a plate of glass rolled up into a cylindrical form. Hence a battery may be composed of a number of coated plates of glass; and it was actually with one of this kind, consisting of eleven panes of window glass, that Dr Franklin performed most of his experiments.

When one of these panes of glass is fitted up as in the annexed figure, it is called the sparkling pane, or the magic picture, and is a good illustration of the Leyden jar. A sheet of tinfoil, TT, is attached to its two surfaces, leaving a space of two or three inches between the edges of the tinfoil and the edges of the plate of glass, both fixed in a frame. When one of the sheets is electrified, and the other is put in communication with the ground, a bright spark is obtained by making the two sheets communicate by means of a discharging rod. A Leyden jar or phial is therefore nothing more than a pane turned into a cylinder, the coating on the inside of the jar representing one side of the pane, and the outside coating the other.

If, instead of the sheets of tinfoil, we cover the space which they occupy with metal filings stuck upon the glass by gum, and electrify one side as before, the discharge produced by the discharging rod will produce an infinite number of brilliant sparks passing from each metallic particle to its neighbours, and occasioning the sparking from which the name is derived.

Dr Franklin was the first person who explained the principle upon which the action of the Leyden jar depends. He began by examining the electricities of the inside and outside coatings. A cork ball suspended by a silk thread was attracted by the outside and repelled by the inside coating. When the jar was charged with the opposite electricity, the ball was repelled by the outside and attracted by the inside coating. Hence it follows that the outside and inside coatings of a Leyden jar are charged with opposite electricities.

When the inside coating was charged from the prime conductor, its electricity was positive, while that of the outside was negative; but when the outside of the jar was charged from the same conductor, the outside was positively electrified, and the inside negatively, and the charge was as strong as before.

In order to show that the negative electricity of the one coating was equal to the positive electricity of the other, Dr Franklin hung a small linen thread near the outside coating of a charged jar, and every time that he touched the knob or wire of the jar the thread was attracted by the coating; the electricity taken from the inside by the finger being equal to what was drawn in on the outside by the thread. He then repeated this experiment when the jar was placed upon an insulating stand, and he found that at every successive contact a portion of the electricity of the outside became free, and the linen thread sprang to the outside coating to receive and carry off the superfluous electricity.

The equality of the two electricities is still more clearly evinced in the fine experiment of Professor Richman, shown experimentally in fig. 7. Having coated with tinfoil the opposite sides of a plate of glass AB, to within two inches of its edges, the equality of glass was then placed vertically, and a linen thread was suspended to the upper part of each of the two coatings, electricity. When the plate was not charged, the two threads hung down parallel to each other, and touched the tinfoil; but when the plate of glass was charged, the threads were repelled from the glass, and formed equal angles with it on both sides. When any conducting body, such as the finger, was brought near one coating, the thread on that side sank, and formed a less angle with the glass, while the thread on the other side rose to a greater angle, the augmentation of the angle on one side being exactly equal to its diminution on the other. When the finger touched one coating, the corresponding thread fell down entirely, and the thread on the other side rose to a double elevation, so that the angle formed by the two threads was a constant quantity, depending on the intensity of the charge communicated to the plate.

The next point of inquiry to which Dr Franklin applied himself, was to ascertain where the two electricities resided, experimentally, and what was the function performed by each coating. Having charged a jar, and placed it on an insulating stand, Fig. 1, he took out the ball F, and rod FH, fig. 1, and found that they did not contain any electricity. He then touched the outside coating with one hand, and putting the finger of another hand into the mouth of the bottle, he received a shock as powerful as if the ball and rod FH had been in their place. He next put into the phial some clean water, which, being a conductor, answers the same purpose as tinfoil, and having charged the jar, and poured out the water into an insulated bottle, he found that it would not give the shock. Upon filling the phial with fresh water, and without giving any new charge to the jar, he received a shock as at first, which clearly proved to him that the electricity resided in the glass. This important truth may be clearly established in the following manner:—Take a cylindrical jar, and let the outside and inside coatings of tinfoil be nicely fitted, and applied to the surface without any cement. When this jar is charged in the usual manner, place it on an insulating stand or a glass plate, and holding it by the uncovered part, lift out the interior cylinder of tinfoil without injuring its shape, and then lift the glass cylinder out of its exterior coating. If we now touch the outside of the glass cylinder with one hand, and at the same time the inside with the other, we shall obtain no perceptible shock. In like manner, no shock will be experienced by touching the outside coating of tinfoil with one hand, and the inside with the other, nor by touching either separately. But if the two coatings are again replaced on the glass cylinder, the one on the outside and the other on the inside, a shock will be obtained in the usual way. Hence it follows, as Dr Franklin concluded from the same experiment in another Electrical form, that the electricity is accumulated on the surface of the glass, and that the metallic coatings, or other conducting substances, which are placed in contact with both sides of the glass, perform only the function of forming a perfect communication between every point of the external with every point of the internal surface of the glass at the instant of the discharge.

In order to explain the theory of the Leyden jar, let us place a jar AB uncharged upon an insulating stand, and make its outside coating B communicate with a pair of insulated pith balls m, n, as shown in fig. 8. From the prime conductor convey a few sparks of positive electricity to the jar by the knob F, the pith balls m, n will diverge with positive electricity, owing to the decomposition of the natural electricity of the external coating. If we now touch the pith balls, the positive electricity which made them diverge will escape, and they will fall into their natural vertical position. But we have not thus removed all the positive electricity which was communicated to the interior coating. A portion of it has become fixed or latent, or disguised or dissimulated, which can only happen from the influence of a portion of resinous electricity. If we now touch, indeed, the brass ball F, the portion of positive electricity which remains free in the inside coating will cause the pith balls communicating with the outside coating to diverge with negative electricity.

If, when the pith balls are divergent with positive electricity supplied from the conductor, we touch them so as to allow it to escape, the repulsive force which it exerted on them in the interior coating will cease, and the ball F and the interior coating will be capable of receiving an additional quantity from the conductor. The pith balls will again diverge with positive electricity; and if this be removed, the ball F will again be able to receive a farther supply from the conductor, so as to make the pith balls again diverge. The interior coating will be receiving more and more positive electricity, till its repulsive power becomes so great as to resist the introduction of any more. The jar is now charged, and will give a shock in the usual manner, or may be discharged by the discharging rod. Hence we may conclude that the positive electricity introduced into the inside coating of the jar decomposes the natural electricities of the outside, drives away from it the positive and fixes the negative electricity, which, by its reciprocal attraction, fixes also a part of it in its turn.

From the principles above established, it may be shown that a given quantity of electricity from the prime conductor may be made to charge two or more jars, almost as powerfully as if the whole quantity was communicated to one jar only. The jars being placed as in fig. 9, the electricity from the prime conductor is conveyed by a chain A to the ball B of the first jar. The ball F of the second jar has a similar connection with the coating of the first, and by a third chain the coating of the second jar is connected with the earth. When the inner coating of the first jar receives positive electricity, the outer coating has its natural electricity decomposed; the negative portion is fixed by the influence of the positive electricity within, and the positive portion is repelled to the interior of the second jar, where it does the very same thing which was done by the same electricity within the first jar. The positive electricity set free at the exterior of the second jar is repelled to the earth, and any requisite portion of negative electricity is conveyed to the outer coating of the same jar, in order to fix by its influence the positive electricity which arrives at the interior of the jar. If we now remove all the connections, the two jars will have received their full charge; and the same will take place with any number of jars similarly arranged for the purpose.

As the accumulation of electricity in a jar depends upon the mutual attraction of the two electricities, and as this force varies inversely as the square of the distance of the molecules, the intensity of charge which any jar can receive should increase with the thinness of the glass which separates the two fluids. Mr Cavendish inferred from his researches, that the intensity of the charge was inversely as the thickness of the glass; but we cannot avail ourselves of this principle in practice, as a certain thickness of glass is necessary to the due strength of the jar; and it has been proved by experiment, that when the glass is thin, the mutual attraction of the two electricities has been capable of forcing a passage through the glass itself. The common glass jars, therefore, are as thin as they can be made with perfect safety. Mr Brooke always placed a layer of paper between the tinfoil and the glass, for the purpose of enabling the jar to contain a charge of greater intensity.

A very remarkable apparatus, similar to a Leyden jar, is made by a long wire insulated with gutta percha, and placed in water. We owe this beautiful experiment to Dr Faraday, who made it with a wire 140 miles long, one end of which communicated with a pile of 360 elements of zinc and copper, charged with acidulated water. The pile was perfectly insulated, and communicated with the ground by its second extremity. The insulation of the wire was so perfect that the remaining current produced only a deviation of 5° in the galvanometer. When the communication of the pile with the long wire was cut off, the following phenomena were observed. Upon touching with the finger either extremity of the long wire, a powerful sensation was felt, which was repeated after a great number of successive touches of the finger. The sensation was felt even after an interval of five minutes. If, instead of discharging the electricity with the finger, the wire is connected with a galvanometer, a very great deviation is produced, and is still perceptible half an hour after the pile is removed from the wire. All this is easily explained. The wire is a Leyden phial! The copper wire is the inside coating, the gutta percha is the glass, and the water the external coating; and when one end of the wire communicates with a source of electricity, it becomes charged. When the wire is placed in air the phenomena disappear. In one of Mr Faraday's experiments the wire was 1'6 millimetre in diameter, and the insulating layer of gutta percha 2'5 millimetres. Hence the interior coating of this species of Leyden jar had a surface of nearly 770 square metres, and the exterior coating a surface of 3050 square metres.

Before concluding this section, we shall describe a pretty Cavalli's little instrument invented by M. Cavalli, called the self-charging jar. Having procured a glass tube eighteen inches long and one and a half inch in diameter, coat one half of the inside of it with tinfoil, and close the aperture of the coated end with a cork, through which there passes a wire touching the inner coating, and terminating in a brass ball fixed at the uncoated end of the tube. If we now hold the uncoated part of the tube in one hand, and rub the outside of the coated part with the other, and after every three or four strokes touch with the rubbing hand the brass knob or ball, the hand will communicate to it a spark, and the inside coating will thus be gradually charged. If we now grasp the outside of the coated end with one hand, and with the other touch the brass ball, we shall discharge the tube and receive a shock.

If we insulate a metallic tubular or solid rod rounded at the lateral extremities, and about an inch or less in diameter, and explosion place it against the external coating of a charged jar, and then place another metallic cylinder of five or six feet long within about half an inch of its opposite extremity, it will be found that at the instant of discharging the jar by means of a common discharging rod and a chain resting on the table near the outside of the jar, a small spark will be ob- served to pass between the insulated conductor, touching the outer coating of the jar, and the conductor near its opposite extremity. Or if we connect a piece of metallic chain any how with the outer coating, and discharge the jar through another circuit independent of the chain, then at the instant of the discharge the links of the chain will appear luminous, showing that some kind of electrical action has been produced in it. This phenomenon has been called "Lateral Explosion," and as it has been sometimes associated with the passage of lightning along lightning conductors, it will be requisite to see in what this kind of electrical action consists. The most critical analysis as yet made of this phenomenon appears in a paper by Sir Snow Harris in the London and Edinburgh Philosophical Magazine for December 1839, to which we must refer the reader.

Sect. II.—On the Construction of Condensers of Electricity.

An apparatus for condensing electricity in a conducting body was the undoubted invention of M. Epiphanus, who also gave the true theory of its action. Volta, however, had the merit of first applying it to an electrometer for indicating small quantities of electricity.

The condenser shown in fig. 10 consists of two separate parts, the first of which is a metallic disc B, supported by a metallic stand BD, and the second is a similar disc A, having a glass handle C rising from its tube, and a small metallic pin P projecting from its circumference. The upper surface of the plate B and the lower surface of A are covered with a thin film of a non-conducting substance, such as varnished silk, rosin, or glass. If it is now wished to condense any feeble electricity from any body, as, for example, from a feebly electrified conductor, bring the metallic pin P into contact with the body or electrified conductor; and while it is in contact let the metallic disc B be brought close under it, as in the figure, the varnished surface of A resting on the varnished surface of B. In this state withdraw the whole from the prime conductor; and having removed the plate B, apply the plate A to two suspended pith balls, which will separate to a very considerable angle in consequence of the electricity having been condensed by the contact of the disc B. That this is the case may be readily proved by applying A to the pith ball before the disc B was joined to A, when their divergence will be greatly less than before. The explanation of this is very simple. The positive electricity, for example, conveyed by the prime conductor to the plate A decomposes the natural electricity of B. The positive portion of B is repelled to the earth by the similar electricity in A, while the negative portion is attracted to the upper surface of B by the opposite electricity in A. In this position it is capable of attracting to the inner surface of A an additional quantity of the free electricity in the prime conductor; and this additional quantity will in its new position produce a farther decomposition of the natural electricity of C. All these effects will take place simultaneously till an equilibrium is established between the free positive electricity supplied to A by the prime conductor, and the negative electricity which the attractive force of this electricity can draw from the earth.

It is manifest, from these observations, that the principle of the condenser is exactly the same as that of the Leyden jar. The upper disc A which receives the electricity corresponds with the inner coating of the jar, the under disc C with the outer coating, and the film or films of rosin, &c., with the glass of the jar.

Volta's condensing electrometer of Volta is shown in fig. 11, where CAB is the condenser above described. From the lower side of the plate B are suspended, by two metallic wires, two perfectly even and straight straws m, n, and on the mouth of the bottle DEFG is fixed the disc B, so that the two straws hang freely in the axis of the neck of the bottle. A graduated circle seen below mn is pasted on the outside of the bottle, to estimate the angular separation of the straws, which affords a mean of the electricity condensed in the manner already described.

Mr Cuthbertson's condenser, shown in fig. 12, consists of Cuthbertson's two flat circular plates of brass, A, B, about six inches in diameter. The receiving plate A is supported by a glass pillar, firmly fixed to a wooden stand, while the condensing plate B is sustained by a brass pillar, but so as to move round a joint at its lower end, in order that it may be thrown back into the dotted position shown in the figure. When the plates stand parallel and vertical, the receiving one A is connected by a wire with the body whose electricity is to be condensed. In this state it is allowed to continue for a short time, when the wire is removed and the plate B thrown back into the dotted position. The electricity will then be found condensed in the plate A.

When this instrument is applied to an electrometer, as in fig. 13, it forms an excellent condensing electrometer; and Fig. 13. the effect may be greatly increased by uniting Cuthbertson's condenser with that shown in fig. 13. This may be done by merely uniting the moveable plate of the former to the fixed plate A of the latter by a small brass pin.

Nicholson's spinning condenser, which is a very ingenious Nicholson's instrument, is shown in fig. 14, where A is a metallic vase, which revolves about a steel axis EK, whose pivot K runs in the adjustable socket C at the bottom of the stand H. A circular, disc of glass D, one and a half inch in diameter and two-tenths thick, is fixed to the vase A, and revolves along with it, while a similar plate E is fixed on the top of the stand H. These two discs are shown separately in fig. 15. In the edge of the plate E are drilled two holes to receive metallic hooks F, G, and into the edge of the upper plate D are cemented two small tails of the flattened wire used in making silver lace. These tails are bent down so as to strike the hooks F, G during their revolution, without touching the rest of the apparatus. The two adjacent faces of the glass discs are coated with segments of tinfoil, as shown in fig. 15; and they may be set at any distance by means of the screw C. Each tail communicates with the tinfoil coating of D; the hook F communicates with that of E, but the hook G is insulated so as to communicate only with the electrified body. The coating of E communicates with the earth by means of the stand H.

If the vase A, the plate D, and the axis EK, are now set a spinning by the action of the finger and thumb applied at T, one of the tails will strike the hook G, and receive through it from the electrified body some of its electricity, which it will convey to D, which will thus assume the electric state of the body. The tail which has struck G proceeding onwards, will after half a revolution touch F, and will convey the free electricity received at G to the two coatings, which with the hook F constitute one insulated mass. The tail advances, acquires more electricity from G, deposits it at F, and thus condenses it, on the principle of the common condenser, till it is capable of affecting the pith balls at F. The instrument constructed by Mr Nicholson was five inches high, and condensed very small degrees of electricity.

Sect. III.—On the Construction of Electrical Doublers.

This class of instruments operate by continually doubling small quantities of electricity till the common electrometer is capable of indicating its presence and qualities.

The doubler invented by Mr Bennet consists of three Bennet's plates, A, B, and C, fig. 16. The plate A, which is of brass, doubler. Electrical has an insulating handle rising from its centre; the plate B, which is also of brass, has a similar handle fixed in its circumference. The third plate, C, also of brass, is placed on Bennet's gold-leaf electrometer. The under side of A, the upper side of C, and both sides of B, are varnished. The body whose electricity it is required to double is brought into contact with the under side of C, which rests on the cap of the electrometer, while B is touched with the finger of the other hand. The communication with the electrified body being broken off, B is lifted up by its glass handle. If the electrometer leaves do not diverge, A is placed by its handle upon B, thus lifted up; and A being now touched by stretching a finger over the juncture of its insulating handle and immediately withdrawing it, A is separated from B. In this situation two of the plates have obviously nearly equal quantities of one kind of electricity, while the third plate has the opposite kind. The plate A is then made to touch the under surface of C, resting on the electrometer, and at the same time C is covered with B. The plate B is now touched by the finger as A was; and removing A, and withdrawing the finger from B, and lifting it up from C, the electricity is doubled. By repeating this operation ten or twenty times, which may be done in forty seconds, the electricity will, by continual duplication, be augmented 500,000 times. When sparks are required, C must rest on an insulating stand in place of the electrometer.

It was found by Mr Bennet, Cavallo, and others, that the doubler became strongly electrified even when no electricity was communicated to it. To remove this evil, M. Cavallo used three plates without varnish, and he placed them on insulating stands, so as to have a vertical direction, and to stand within one-eighth of an inch of each other, the plates of air being a substitute for the varnish. The method of doubling is exactly the same as before. Dr Rohson adopted the same idea, but he kept his plates horizontal, making them rest on each other by three small spherules of glass or sealing-wax. Notwithstanding these precautions, however, electricity was still produced.

In order to perform the operation of doubling with more rapidity, Dr Darwin proposed the moveable doubler, or one in which the plates could be moved by wheel-work into their proper positions. Dr Nicholson improved upon this idea by producing the whole effect with the simple revolution of a wheel.

This revolving doubler, as it has been called, is represented in fig. 17. It consists of two fixed plates of brass A, C, two inches in diameter, insulated separately, and placed in the same plane, so that a revolving plate B may pass near them without touching. A brass ball D is fixed on the end of the axis which carries B, and is loaded within at one side so as to counterpoise the plate B, and allow it to rest in any position. The axis PN, and the axes that join the three plates with the brass axis NO, which passes through the brass piece M, by which the plates A and C are supported, are made of varnished glass. One end of this axis carries the ball D, and the other is connected with a rod of glass NP, upon which the handle L is fixed, and also the piece GH insulated separately. The pins E, F rise from the back of the plates A, C, at equal distances from the axis. The arm K is parallel to GH, and the ends of both are armed with pieces of harpsichord wire, so as to touch the pins E, F in certain points of their revolution. A pin I is fixed on M to intercept a small wire proceeding from the revolving plate B. These wires are so bent that, when B is opposite to D, GH connects the two fixed plates, A, C, while the wire and pin at I connect the ball D and plate B. On the other hand, when B is opposite C, D is connected with C by the contact of F with the wire at K, the plates A, B being then entirely unconnected with any other part of the instrument. In all other positions the three plates Electrical and the ball D will have no connection with each other. The operation of this instrument is thus described by Mr Nicholson: "When the plates A and B are opposite to each other, the two fixed plates A and C may be considered as one mass, and the revolving plate B, together with the ball D, will constitute another mass. All the experiments yet made concur to prove that these two masses will not possess the same electric state; but that, with respect to each other, their electricities will be plus and minus. These plates would be simple, and without any compensation, if the masses were remote from each other; but as that is not the case, a part of the redundant electricity will take the form of a charge in the opposed plates A and B. From other experiments, I find that the effect of the compensation on plates opposed to each other at the distance of one-fortieth part of an inch is such that they require, to produce a given intensity, at least a hundred times the quantity of electricity that would have produced it in either singly and apart. The redundant electricities in the masses under consideration will therefore be unequally distributed; the plate A will have about ninety-nine parts, and the plate C one; and for the same reason the revolving plate B will have ninety-nine parts of the opposite electricity, and the ball D one. The rotation, by destroying the contacts, preserves this unequal distribution, and carries B from A to C, at the same time that the tail K connects the hall with the plate C. In this situation the electricity in B acts upon that in C, and produces the contrary state by virtue of the communication between C and the ball; which last must therefore acquire an electricity of the same kind with that of the revolving plate. But the rotation again destroys the contact, and restores B to its first situation opposite A. Here, if we attend to the effect of the whole revolution, we shall find that the electric states of the respective masses have been greatly increased; for the ninety-nine parts in A and B remain, and the one part of electricity in C has been increased so as nearly to compensate ninety-nine parts of the opposite electricity in the revolving plate B, while the communication produced an equal mutation in the electricity of the ball. A second rotation will of course produce a proportional augmentation of these increased quantities, and a continuance of turning will soon bring the intensities to their maximum, which is limited by an explosion between the plates."

An ingenious instrument, called a pendulum doubler, has Ronalds' been recently constructed and described by Mr Ronalds, pendulum. Having found it necessary to keep a telegraphic wire constantly electrified with a very small source of electricity, he converted the bob of a pendulum into the centre plate of a doubler, and he found the instrument thus modified not only useful for that purpose, but also for that class of experiments, such as those on vegetation and animal life, which require a constant supply of small quantities of electricity to repair the loss occasioned by unavoidable defective insulation, either in the glass which is used, or in the surrounding atmosphere. This improvement on the doubler is shown in fig. 18, where A and B are the two fixed plates, about 4 inches in diameter, supported by glass pillars; C is the bob carried by the pendulum rod D, and insulated by the piece of glass e. The form of the bob C is that of a plano-convex lens, with its interior filled with lead; f is a small cylinder connected to C with screws, which also adjust the plane of C parallel to the plane of vibration; g is another insulating glass rod, carrying the bent wire h, the left end of which lies nearly in the same vertical plane as the end of the wire m, the right end being nearly in the same plane as the end of the wire n. A wire, i, rises perpendicularly from C; and another, k, perpendicular to the plane of vibration, is fixed into the brass cup at the end of the pendulum rod. A wire, Electrical I, is screwed into the edge of the plate B, and the long wire Apparatus m fixed on the lower edge of B, so as to approach within a small distance of A, where it is bent at right angles, and then projects in a plane perpendicular to that of vibration. Another wire, n, is fixed into the edge of A, so as to bend and project similarly; but n projects farther than m, that the right side of the bow h may pass the end of m without touching it. A wire, o, is fixed at right angles into the base of the instrument.

When the bob C is exactly opposite A, the insulated wire h touches simultaneously the ends of the wires m and n, and establishes a communication between A and B, while at the same time the wire i, by touching o, forms a communication between C and the ground. Now, if a quantity of positive electricity, for example = 1, is given to A or B when the centres of A and C are opposite each other, that quantity will be nearly all condensed on A, and C will acquire negative electricity nearly = 1.

"If C," says Mr Ronalds, "be now allowed to begin its vibrations, the connection of A and B with each other will be instantly broken, as also that of C with the earth, and they will be all insulated, and all retaining the electric states which they possessed before the connections were broken (i.e., A will be positive nearly = 1, B negative nearly = 1, and C positive almost 0).

"When C has arrived opposite B, the uninsulated wire k will touch the wire L, and thus place B in connection with the earth; therefore C, by virtue of its negative charge, will induce a positive charge in it nearly = 1.

"When C arrives a second time opposite A, all the former connections will be re-established, and the charge of B will (by means of the wire m) be nearly all condensed on and added to the original charge of A, making a tension nearly = 2 of positive electricity, which tension will induce a tension of nearly = 2 of negative electricity on C.

"And so the charges in A and C would go on, nearly doubling at each vibration of the pendulum, until their tensions would arrive at such a point as to cause a spark to pass between them.

"But P is a Leyden jar furnished with a Lane's discharging electrometer q; a connection is established by means of a small chain between it and A; and the distance between the two balls r and s is considerably less than that between A and C; therefore the spark will be given to the jar, and a spark will be continued to be given at the completion of almost every second vibration, until it is charged almost as highly as A is capable of being charged, or the sparks will continually supply the loss of electricity by any defect of insulation, either of the jar, or of any conducting body in connection with its interior coating within certain limits.

"The contacts of the wires do not impede the velocity of the vibrations, because they are made small enough to act as springs of a required force; but the electric attractions of the plates and bob do tend to do so. The pendulum is suspended by two springs, placed one at each extremity of a cross piece, to which the rod is attached, for the purpose of preventing the bob from being drawn, by their attractions, out of its assigned plane of vibration, as much as possible."

Sect. III.—Description of Instruments for Multiplying Electricity.

The electrical multiplier invented by M. Cavallo is shown in fig. 19, on a scale about one-third of its real size, and is chiefly useful in ascertaining the presence of a considerable quantity of electricity occupying an extended space. Its principal parts are four plates of brass A, B, C, D. The plates A, C are supported by two glass rods G, H, fixed in the wooden base RSQ. A similar plate B is supported by another glass rod I, cemented into the wooden lever LK, moving round a pivot K. The fourth plate D is supported by a metallic rod. By the lever KL the plate B can be moved from its position on the figure into the dotted position KX. The plate D is screwed at P into a piece of brass FP, which slides in a groove, so that D can be pulled out to any distance from C. At the corner Q is fixed a brass rod N, and O is a small bent wire fixed to the brass socket O on the back of B. When B is as near as possible to A, their distance being one-twentieth of an inch, this wire m touches the rod N, and forms a communication with the earth; when FP is pushed in as far as possible, the surfaces of C and D are one-twentieth of an inch distant. As the lever KL moves towards X, the end m of the wire mO quits N and insulates the plate B; and when the lever has the position KX, the wire m will touch the plate C, so as to put the insulated plates B into communication with each other.

If a body weakly electrified positively is now made to touch A, when A and B are placed together as in the figure, A will acquire a greater quantity of positive electricity from the presence of the uninsulated plate B, which will be negatively electrified. When KL comes into the position KX, so that B touches C by the wire mO, its negative electricity will pass almost wholly to C, owing to its proximity to D, which communicates with the ground. By a number of successive oscillations of the lever between the two positions KL and KX, this operation may be repeated till an accumulated charge of negative electricity has been fixed upon C. The plate D must now be drawn away from C by means of the slider FP, and if pith balls are presented to C they will diverge with negative electricity.

In our chapter on the chemical agencies of electricity, Schweigger have already described Schweigger's multiplier or galvanometer, which was used by M. Colladon in his experiments on the chemical action of ordinary electricity; and also the multiplier of Dr Faraday with a double helix, which he employed in his researches on the identity of the electricity of the machine with that of the pile. Various improvements have been made on the multiplier by M. Nobili, Professor Oersted, and others; but we must reserve our account of them for the articles Galvanism and Magneto-Electricity.

CHAP. III.—DESCRIPTION OF INSTRUMENTS FOR INDICATING THE PRESENCE OF ELECTRICITY, AND MEASURING ITS QUANTITY.

Instruments which are intended merely to indicate the indicators presence of electricity are called electrosopes, while those which are intended for measuring the quantity of electricity are called electrometers. The earliest electrometer which seems to have been employed was a pair of silk threads, which indicated the presence of small quantities of electricity by their divergence; and the Abbé Nollet even attempted to measure the quantity communicated to them, by determining the inclination of the two threads, from their shadow on a board. Mr Waitz improved the instrument by suspending small weights to the threads, and Mr Canton perfected it by substituting the finest linen threads for the silk ones, and by suspending from them a pair of small balls turned out of the dry pith of the elder.

Description of Cavallo's Electroscope.

M. Cavallo made this little instrument portable by fitting electro- Electrical B. When it is unloosed, the end B carrying the pith balls is screwed off, and the balls are put into the glass tube at A, which serves for a handle. This glass case is three inches long and three tenths of an inch wide, and half of it is coated with sealing-wax. A cork tapering at both ends is made to fit the mouth of the tube, and to one end of the cork are fixed two linen threads carrying two small cones of elder pith. The case of the electrometer at C incloses at one end a piece of amber for giving negative electricity, and at the other end a piece of ivory insulated upon a bit of amber for giving positive electricity, to the balls, when rubbed with a piece of woollen. All these instruments may be greatly improved by substituting for the pith of elder the pith of the sola, a tree which grows in the East Indies.

Description of Bennet's Gold-Leaf Electrometer.

One of the most useful electrometers is that which was invented by Mr Bennet, and called the gold-leaf electrometer. This instrument, which is shown in fig. 2, and a section of it in fig. 3, consists of a cylinder, ABED, with a broad brass cap, AB. In a hole, a, in the centre of the cap, is fixed a wedge of wood, on each side of which is fastened by a little varnish a smooth-edged strip of gold leaf, shown at m and n, about two inches long and a quarter of an inch broad. Two pieces of tinfoil, b, c, are pasted opposite each other, and within the cylinder, so as to rise a little higher than the ends of the gold leaves, and the lower ends of these pieces of foil are in contact with the brass stand DEF which sustains the instrument. The inside of the cap AB, and the upper part of the glass cylinder, are sometimes coated with wax. A pointed wire, C, is used to collect the electricity at the atmosphere. In using this instrument, the cap AB is turned round till the surfaces of the gold leaves are parallel to those of the pieces of tinfoil. When no electricity is present the two gold leaves hang in contact in the axis of the cylinder; but if a fully electrified body is made to touch the cap AB, the gold leaves m, n will diverge as in the figure, and their lower ends will strike the pieces of tinfoil b, c, which will convey the electricity to the ground.

Mr Nicholson has proposed to substitute two flat radii of brass in place of the tinfoil, and by moving them to and from the gold leaves with a micrometer screw, to make the instrument more sensible, and at the same time obtain a kind of measure of its quantity.

Singer's Improved Electrometer.

Although insulation may be procured by coating glass insulators with wax, yet, as Mr Singer observes, this affords only a temporary defence, as moisture is eventually precipitated upon them; and in removing this it is almost impossible to avoid exciting the surface of the wax, and disturbing delicate experiments by the electricity which is thus generated. To remove this evil Mr Singer proposes to inclose the insulator in a narrow channel, as the moist air in contact with it would be then limited in quantity, and little disposed to motion. In applying this principle to the improvement of Bennet's electrometer, the insulation is effected by a glass tube four inches long and one-fourth of an inch internal diameter, coated out and in with sealing-wax, and having a brass wire five inches long and one-sixteenth or one-twelfth of an inch thick to pass through its axis, so as to be perfectly free from contact with any part of the tube, in the middle of which it is fixed with a plug of silk, which keeps it concentric with the internal diameter of the tube. This arrangement is shown in figs. 4, 5, where A is a brass cap screwed upon the upper part of the wire, which prevents the atmosphere from having free contact with the outside of the tube B, and defends at the same time its inside from dust. To the lower end of the wire the gold leaves are fastened, and the glass tube passes through the centre of the usual cap of the electrometer, and is cemented in it near the middle of its length, as may be seen by the dotted lines which represent the cap. "When this construction," says Mr Singer, "is considered, it will be evident that the insulation of the wire, and consequently of the gold leaves, will be preserved until the inside as well as the outside of the glass tube is coated with moisture; but so effectually does the arrangement preclude this, that some of those electrometers that were constructed in 1810, and have never yet (1814) been warmed or wiped, have still apparently the same insulating power as at first." The electrometer constructed upon the preceding principles is shown complete in fig. 4.

Dr Faraday recommends strongly the use of this electrometer; but having found from repeated experience that day's indications are not in general well understood by those who have occasion to use it, he has given a very valuable description of the kind of charge which it receives under different circumstances, and the precautions which are necessary in interpreting its indications. As this description would lose its value by any abstract or alteration, we shall make no apology for giving it in his own words, especially as it is applicable to many other analogous instruments.

"If an insulated portion of conducting matter, as a brass ball at the end of a glass handle or silken thread, be electrified, and then placed in contact with the cap of the electrometer, the cap and leaves will immediately partake of the electricity of the ball, and the leaves will diverge. If the charge in the ball be of considerable intensity, the leaves will be torn to pieces by their mutual repulsion, and the attraction of the sides of the glass jar; but if the intensity be small, the leaves will diverge moderately, so as not to touch the glass, and the degree of divergence will be in some proportion to the intensity of the charge communicated. The appearances will be the same whether the electricity communicated be positive or negative.

"The circumstances will be different if the body brought in contact with the electrometer is an electrified portion of what is usually called non-conducting matter; if, for instance, it be a stick of sealing-wax rubbed with flannel instead of a metallic ball. If highly electrified, this will cause the same disturbance and appearance in the leaves during its approach as the ball; if moderately electrified, it will, when in contact with the cap, cause the usual appearance of divergence in the leaves, but upon removing it, the leaves, instead of remaining diverged, will either collapse, or remain very slightly, and frequently uncertainly, electrified. This is a consequence of the non-conducting power of the wax; and the method of transferring electricity to the electrometer in such a case is, to draw the excited parts of the wax over the edge of the cap; small portions will be communicated, and the electrometer will be left electrified similarly to the wax. Such a process is, however, very uncertain; for if the electricity of the wax be weak, the friction of the substance against the electrometer cap will sometimes generate an electricity stronger than that previously existing on the surface of the wax, and the electrometer will become charged, not by the previous electricity of the wax, but by that produced during its friction against the cap.

"This difficulty may, however, be avoided in most circumstances, simply by bringing the electrified non-conductor into contact with the cap, and retaining it there during the experiment; for the electricity which in this way is made by induction to exist in the leaves, and causes their divergence, is the same as that which would exist over..."

Electrical the whole of the cap and leaves, if the electricity of the Apparatus wax could be transferred to them.

Such are the circumstances relating to the charge of the electrometer, by bodies brought into contact with it, and communicating to it part of the electricity they previously possessed. As before mentioned, when highly electrified, they cannot be so applied to the instrument without tearing the leaves to pieces; but they may then, when held at a distance, be made to diverge the leaves by induction, and even to communicate a charge to the instrument, and thus enable it to exhibit divergencies when the inducing electrified body is removed. The effects thus produced by induction are the same in kind, and nearly in extent, whether the electrified body be a mass of conducting or non-conducting matter, so that in this respect the metallic ball and the stick of wax are equal; the only difference being in the kind of electricity produced, which, with bodies charged positively, is the reverse of that occasioned by such as are charged negatively.

When an electrified substance is placed at such a distance from the cap of the electrometer as to occasion considerable divergence, and is retained there for a few minutes, the divergence of the leaves will generally diminish, and the more rapidly as the instrument becomes cold or the glass damp, as the leaves are ragged, or any part of the cap angular and pointed.

On removing gradually the electrified substance to such a distance that it can no longer affect the instrument, it will be found that the leaves will collapse at first, and afterwards expand again more or less, according as they had lost more or less of their first divergence.

This ultimate divergence of the leaves will be due to a charge of electricity in the instrument, of the opposite kind to that of the inducing or approximated body.

If no effect of this kind takes place, and there be no diminution of the first divergence, nor any ultimate change, then the insulation and goodness of the electrometer is proved by a powerful test. This being ascertained, then, if whilst the electrified body is in the neighbourhood, and the leaves diverged, the cap be touched by the hand, or any other conducting substance communicating with the earth, the divergence of the leaves will instantly cease. In this state of the instrument, if the communication be broken so as to leave the cap and leaves insulated, they will still remain collapsed; but if the inducing electrified body be now removed from the situation in which it at first caused the divergence, the leaves will immediately diverge, and the electrometer become charged with electricity of the opposite kind to that of the inducing body. The degree of charge thus given to the instrument will be in proportion to the degree of divergence induced in the leaves before they were made to collapse by the touch of the finger.

In the case in which a weakly electrified non-conducting substance was directed to be laid on the cap of the electrometer, to occasion a divergence by electricity like its own, it may be observed that, if, during the experiment, the cap be touched by the fingers, and the electrified body afterwards removed, the leaves will first collapse, and then diverge with opposite electricity, although at the commencement of the experiment they were diverged with the same electricity as that of the body to be examined. If, therefore, the electricity of an excited body is to be examined, the leaves of the electrometer are in the first place to be diverged. This may be done with the same electricity, by bringing the body, if weakly electrified, into contact with the cap, leaving it there if of non-conducting matter, or removing it after contact if of conducting matter; or, if strongly electrified, by approaching it so near as to cause a sufficient divergence of the leaves, and retaining it there until the conclusion of the experiment. On other occasions however with strongly excited bodies, it may be convenient, either because of their size or other circumstances, to communicate a charge of the opposite kind, in the manner described; then upon determining what that kind is, in the manner to be immediately described, the electricity of the originally electrified body will of course be known to be opposite to it.

The tests of the kind of electricity by which the leaves are diverged are of the following nature. A stick of sealing-wax rubbed with warm flannel becomes negatively electrified; a tube of warm glass rubbed with a dry silk handkerchief, or, better still, with a piece of silk having a little amalgam upon it, becomes positively electrified, both these excitations being so strong as to make the leaves of an uncharged electrometer diverge whilst the wax or glass is at a considerable distance. If one of these excited substances be brought near the cap of an electrometer already diverged, it will either cause the divergence to increase or diminish. The divergence will increase if due to electricity of the same kind as that of the body approached, but will diminish if of the opposite kind; so that the electricity of the body approached being known, that of the electrometer will also be known, and consequently that of the excited body which had originally caused its divergence. The sealing-wax for instance is rendered negative by flannel; being approached to a diverged electrometer it may cause the leaves to collapse; the conclusion to be drawn is, that the electrometer leaves were in a positive state; being approached to another diverged electrometer it may increase the divergence, in which case it will indicate that the leaves of the electrometer were in a negative state. An excited rod of glass brought to these electrometers would make the first diverge still more, and would cause the second to collapse, in both cases indicating the same states as the wax.

Some precaution is required with respect to the manner in which these excited rods are to be applied. The electrometer being diverged, the wax or glass is to be excited at such a distance as to have no influence over the instrument; the most strongly excited part of the wax or glass is then to be gradually approached to the cap, the hand and all other unnecessary conducting bodies being kept out of the way as much as possible, or at least not moved in the neighbourhood of the electrometer during the experiment. As soon as the rod begins to affect the leaves (even though the distance be two or three feet), the effect must be watched, and then their collapse or further divergence will become evident immediately on moving the rod a little way to or from the instrument.

It is this first effect that indicates the kind of electricity in the electrometer, and not any stronger one; for although, if the repulsion be increased from the first, no approach will cause a collapse to take place except the actual discharge of the leaves against the sides of the glass, yet when collapse is the first effect, it may soon be completed, and repulsion afterwards occasioned from a too near approach of the strongly excited test-tube. It is, therefore, the first visible effect that occurs, as the test-rod is made to approach from a distance that indicates the nature of the electricity; and when this effect is observed, the rod should not be brought nearer, so as permanently to disturb the state of the electrometer, but should be removed to a distance, and again approached, for the purpose of repeating and verifying the preceding observation.

It is to be understood, that the approach of the test-rod, though it affects the divergence, causes no permanent change of the electricity in the instrument, unless it be brought much too near, and cause considerable disturbance of the leaves. The electrometer will remain, after a good experiment, in the same state as at first.

When the body to be examined is so strongly electrified... Electrical field that it may not be brought near to the electrometer, but has been placed at such a distance as to affect it, and left there to cause a proper divergence, then its place should not be directly over but rather on one side the cap, that the test-tube, when applied, may be brought towards the instrument on the other side; the originally electrified body, and the test-tube, being retained in directions as widely apart as they conveniently can be.

In order to protect his most sensitive gold-leaf electroscope from the influence of electricity to which they may be exposed, Dr Faraday covers them with nets of linen or cotton with loose meshes, so that when they are placed in the neighbourhood of powerful electrical machines in action, the electricity never reaches them, being wholly taken up by the net which surrounds them.

Saussure's Electrometer.

The electrometer by which Saussure made the observations on the electricity of the atmosphere is shown in figs. 6, 7. It consists of a glass vessel, ACB, of a bell shape, and so wide that the balls g, g, when at their maximum divergence, cannot reach the strips of tinfoil h, h, h, h pasted within the glass. The pith balls, which are spherical, should not be above half a line in diameter, and should be suspended by the finest silver wires, moving freely in nicely-rounded holes. Four pieces of tinfoil are used, each internal piece having a corresponding one on the outside; and the bottom of the instrument is made of metal, and round it there is a graduated scale for measuring the divergence of the balls.

In order to collect much electricity from the atmosphere, the instrument has a pointed wire, one and a half or two feet long, which unscrews in three or four pieces; and in order to preserve its insulation, a small umbrella is screwed on the top of the instrument, see fig. 7. On other occasions he connected with a hook at H a fine metallic wire fifty or sixty feet long, at the end of which was a three or four ounce ball of lead, which he threw to the height of forty or fifty feet, in order to bring down the electricity of the atmosphere.

By dividing between two equal and similar bodies the electricity contained in one, and carrying on the subdivision progressively downwards, M. Saussure determined the relation between the divergence of the balls g, g, and the force of the electricity which acted upon them. The results which he thus obtained are given in the following table:

| Distance of Balls from a fourth of a line | Relative Forces of Electricity | |------------------------------------------|-------------------------------| | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | | 5 | 5 | | 6 | 6 | | 7 | 8 | | 8 | 10 | | 9 | 12 | | 10 | 14 | | 11 | 17 | | 12 | 20 |

In order to use this instrument, place it in open ground, free from trees and houses, and having screwed the conductor on the top of the electrometer, lay hold of it by its base, and place it so that the base and conductor may touch the ground at the same time; then raise it to the height of the eye, and observe on the scale the number of fourths of a line that the balls have diverged; then lower it till the balls almost touch each other, and measure the distance of the top of the conductor from the ground; this distance is the height at which the electricity of the air begins to become sensible. If the balls still diverge, the other parts of the conductor should be unscrewed, and it will then be seen at what height the electricity becomes sensible.

Hare's Single-Leaf Electrometer.

As the divergency of the gold leaves is increased by the Hare's proximity of the strips of tinfoil, Dr Hare, of the university single-leaf of Pennsylvania, conceives that the leaves are separated by electromagnet attraction, and not by repulsion; and he was thus led to construct an electrometer with a single leaf, as shown in fig. 8. A brass ball one fourth of an inch in diameter is so situated that it may be made to touch the leaf, or retire from it to the distance of an inch, by means of a screw which supports it. It is obvious that this instrument is not only more simple than the double-leaved electrometer, but less liable to be destroyed by accident; and Dr Hare informs us that it is exceedingly sensible, and that it has enabled him to detect the electricity produced by one contact between a copper and zinc disc, each six inches in diameter.

Henley's Quadrant Electrometer.

This useful instrument is represented in fig. 9. It consists of a semicircle of ivory, C, fixed to the side of a stand, quadrant AB, about seven inches high, rounded and smoothed in all its parts. The lower quadrant of the semicircle is divided into 90°, and a thin piece of cane ab is suspended at the centre m of the semicircle, carrying a pith ball b. When fig. 9, the electricity to be measured is communicated to the instrument, the ball is repelled by the stem AB, and the angular elevation of the cane ab is a measure of the electrical force. This instrument may be screwed from its base B, and fixed on the end of the prime conductor, or on the summit of a Leyden phial. Mr Achard states from experiment that the quadrant should be divided according to a scale of arcs whose tangents are in arithmetical progression. It is most frequently used as an appendage to the prime conductor, for the purpose of measuring the state of action of the electrical machine.

In employing this instrument to show the progress of the charge of any jar or battery, Dr Faraday justly observes that it should be so placed that the moving index does not approach to any ball, wire, or surface charged similarly to itself, but on the contrary should recede from it. If it is therefore placed on the end of the conductor, the index should move outwards and away from the conductor, and not in a direction over it towards its more central parts; for the latter would interfere with the free indications of the electrometer, and in some cases would make it quite useless.

Brooke's Steelyard Electrometer.

This electrometer, which is represented in fig. 10, is calculated to measure the number of grains which the repulsive steelyard force of the accumulated electricity is capable of raising. Its base AB, about nine inches and a quarter in diameter, Fig. 10, adjusted horizontally by screws A, B, sustains an insulating pillar DD, upon which the electrometer rests. To the brass rod H are attached two tubes of copper, G, g, which have a motion round the rod, so as to be turned to a proper distance from the body whose electricity is to be measured. The tube G is screwed into a solid piece within the hall F, and moves in a vertical plane about an axis close to F. The halls I K are of copper, and hollow. The arm E, which moves round an axis in a vertical plane behind the dial-plate R, carries a ball C, which touches the ball L fixed on the top of the glass rod DD. If the arm E rises from a vertical to a horizontal position, or through 90°, the index R on the dial is made to move through a whole circle, or 360°. The apparatus NPH forms the communication between the electrometer and the body whose electricity is to be measured.

Let the body be electrified positively, then the electrometer will be similarly electrified. The balls I, K will repel each other, G will rise in a vertical plane, L will repel C, and EC will also rise in a vertical plane.

The apparatus F, G, g, I, K, is chiefly used for graduating the inner circle of the dial-plate. For this purpose a weight m moves along the rod G, till it forms an exact counterpoise to the weight of F. One end of the weight m will consequently be the zero of the scale. Let m be now shifted to n, near to the ball I, and determine by a pair of good scales the weight of the ball I, or rather the weight produced by shifting m to n. Divide the space mn into as many divisions as the grains now found, and subdivide it into halves and quarters. These divisions are now to be transferred to the inner circle of the dial-plate, by observing the position of the upper or shorter half of the index R when m stands at any number of grains in the scale mn. When the inner scale on the dial-plate is thus graduated, the arms G, g and balls I, K may be removed.

Cuthbertson's Balance Electrometer.

This electrometer, which is particularly useful for jars and batteries, is shown in fig. 11. It consists of a metallic rod CD, about thirteen inches long, terminated by balls C, D, and balanced on a knife edge, the ball b being constructed in such a manner as to permit the rod CD to move in a vertical plane. A bent tube of brass FG supports a similar ball G; and four inches below D is placed another insulated ball E, which communicates with a wire and chain with the outside of the jar or battery. If the rod AB be now connected with the prime conductor, or the inside of the jar, and this last be electrified, the ball E will attract D, because they are oppositely electrified, from being connected with opposite surfaces of the jar; and when this attractive force exceeds the weight at a with which the opposite arm is loaded, the arm bD will descend and give out its electricity to the ball E. In order to obtain a measure of the attractive force between D and E, and consequently of the intensity of the charge, the arm CD is divided experimentally into sixty parts or grains, which are indicated by one side of the moveable index a. A Henley's quadrant electrometer is placed at A, to indicate the progress of the charge, which is not shown by the balance electrometer.

Sir W. Snow Harris's Electroscope.

This very ingenious and beautiful instrument, invented and used by Sir W. Snow Harris, is represented in fig. 12. The following description of it we owe to the kindness of the inventor:

Fig. 12 represents an electroscope, which acts on the principle of electrical divergence. A small elliptical ring of metal, a, is attached obliquely to a small brass rod, ab, by the intervention of a short tube of brass at a; the rod ab terminates in a brass ball, b, and is insulated through the substance of the wooden ball m. Two arms of brass, r, r, are fixed vertically in opposite directions on the extremities of the long diameter of the ring, and terminate in small balls; and in the direction of the shorter diameter within the ring there is a delicate axis at a, set on extremely fine points. This axis carries, by means of short vertical arms, two light reeds of straw, an, an', terminating in balls of pith, Electrical and constituting a long index, corresponding in length to the fixed arms above mentioned. The index thus circumstanced is susceptible of an extremely minute force. Its tendency to the vertical position is regulated by small sliders of straw, s, s, moveable with sufficient friction on either side of the axis. To mark the angular position of the index in any given case, there is a narrow graduated ring of card, board, or ivory, of, placed behind it, the divisions being distinctly legible through sights cut in the reeds. This graduated circle is supported on a transverse rod of glass, ed, by the intervention of wooden caps, and is sustained by means of the brass ball e, in which the glass rod is fixed. The whole is supported on a long insulator of glass, A, by means of wooden caps terminating in spherical ends.

In the above arrangement, as is evident, the index will diverge from the fixed arms whenever an electrical charge is communicated to the ball b.

This instrument is occasionally placed out of the vertical position, at any required angle, by means of a joint at m; and all the insulating portions are carefully varnished with a solution of shell-lac in alcohol.

Sir W. Snow Harris's Electrometer.

The object of this electrometer, an account of which has Sir W. been kindly communicated to us by its inventor, is to measure directly the attractive force of an electrified body, in terms of a known standard of weight, estimated in degrees, on a graduated arch, xy, fig. 13. An insulated conductor, Plate f, is fixed on a varnished rod of glass, fg, resting, by the CCXXIX. intervention of a wooden ball on the extremity of a micro, fig. 13. meter screw, S, by the aid of which the conductor may be raised or depressed through given intervals to within the hundredth of an inch of any required point. A moveable and similar conductor, m, made of light wood, hollowed and gilded, is suspended immediately over the former from the periphery of a small brass wheel W, figs. 13 and 14, by means of a fine silver thread attached near its vertical arm, and passing from thence over its grooved circumference, as shown. This conductor, m, is counterpoised by a short cylinder of wood, pn, figs. 13, 14, suspended in a similar manner from the opposite side of the wheel by means of a silk thread, and resting partly in water contained in the glass vessel N, fig. 13.

The extremities of the axis of the wheel W, figs. 13, 14, are turned to extremely fine pivots, and rest on two large friction wheels, after the manner represented in the figures, by which great freedom of motion is obtained.

There is a fine index of light straw, Wc, attached to the extremity of a small steel needle, inserted diametrically through the circumference, which indicates on the graduated arc xy the force exerted between the conductors m, f. The weight of this index is accurately poised by a small globe of brass, n, fig. 14, moveable on a screw, cut in the opposite arm of the steel needle carrying the index.

The centre of the wheel W is accurately placed in the centre of the arc xy, which, with its radii of support, is made of varnished wood; the graduated scale being of card, board, or ivory. This arc is the sixth part of a circle, divided into 120 equal parts, 60 in the direction er, and 60 in the direction cy; the centre c being marked zero.

Fig. 14 represents the wheel W, with the suspended con- Fig. 14 ductor and counterpoise, the index and its balance-weight, together with the lines of suspension, passing freely over the circumference, and fixed at the point ir.

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1 Sir W. Snow Harris resorted to this method of employing friction rollers, as being more efficient than that in which the axis is allowed to rest in the angle formed between the peripheries of four smaller wheels. In this case it rolls fairly on a large circumference, and is prevented from passing off it on either side by the check-wheels, either of which, when acted on, opposes little or no resistance to motion. The various wheels above mentioned, with the graduated arc, are sustained on a projecting metallic rod, passing through a glass column B. The column is secured by means of the rod to a sort of double stand pp, fig. 13, supported on three levelling screws. The interval between the plates of this stand contains the glass vessel N and the micrometer screw S. The upper plate has a circular hole, p, through which the cylindrical counterpoise passes into the water n. The levelling screws serve to regulate the position of the counterpoise through the hole; so that when it hangs in it centrally, the whole is accurately adjusted.

The gravity of the suspended conductor m being in the above arrangement opposed by that of the counterpoise, it may be so far considered as existing in free space devoid of weight, and will therefore become very readily moved by any new force applied to it. It may consequently be caused to approach or to recede from the fixed conductor f, by the operation of forces acting in either of these directions; the motion, will, however, be speedily arrested by the cylindrical counterpoise n, which becoming either further immersed in, or otherwise raised in the water, furnishes, in the greater or less quantity of water displaced, a measure of the force. In this way the force may be estimated either in degrees or grains of actual weight; since the number of grains requisite to add to either side, in order to advance the index in either direction a given number of divisions, may be immediately found by experiment; and which, as the sections of the cylinder are all similar, will be found to increase or decrease with the degrees of the arc. Thus, if one grain advances the index in either direction five degrees, then two grains will advance it ten degrees, and so on.

In the application of this instrument to electrical inquiries, the force to be measured is first communicated to the fixed conductor f, a free communication being established between the suspended conductor m and the ground, or otherwise with the negative side of the jar or battery, should the attractive force be derived from this species of accumulation; this is readily effected through the brass work of the apparatus, in connection with the rod passing through the interior of the glass column B.

For the repulsive force we connect the conductor f as before, and suspend m by a silk thread; in which case it will, after being electrified similarly to f, recede from it; but this method of experiment is evidently more complicated than the former, and liable to fallacy. The distance between the conductors m f corresponding to a given force is easily ascertained by means of the degrees indicated on the arc x y. In the instrument above described, each degree corresponds to a variation of distance between the conductors equal to the '01 of an inch. If, therefore, at the commencement of any given experiment, we first bring the nearest points of the conductors m, f in contact, the index being in zero, and then depress the inferior conductor f a given distance, known by means of the micrometer screw s, then all subsequent distances may be readily determined between these points.

It is now only requisite to observe, that the interior of the cylindrical counterpoise is hollow, in order to weigh it accurately, and cause it to hang vertically in the water; and there is a small hemispherical cup, p, fixed on its stem for the reception of small adjusting weights, by which the position of the index at O of the scale is regulated with great nicety. With respect to the form of the conductors m, f, they are generally plain circular areas, backed by small cones, and are about two inches in diameter. Conductors of other forms, however, such as spheres and cylinders, may be occasionally used when the object is to experiment more particularly on bodies of peculiar forms.

Experiments with this instrument are remarkably clear, considering the subtle character of the principle we have to investigate. Thus, when the insulations are perfect, and the atmosphere dry, the index immediately exhibits the amount of the attractive force, and remains stationary for a much longer time than is required to note the result.

By varying the superficial dimensions of insulated conductors, and the quantity of electricity accumulated on them, we may, by the help of the above instrument, deduce many curious and important laws of electrical action. It is, however, first requisite to explain a method of charging simple conductors with comparative quantities of electricity; for without an accurate measure of quantity, little can be effected in almost any department of this branch of science.

Simple conductors may have comparative quantities of electricity disposed on them, by abstracting small sparks from an insulated charged jar, fig. 15, either immediately on the given substance, or, otherwise, on an insulated transfer plate, p, fig. A. An insulated jar, charged with a given Fig. A. accumulation, as estimated by the unit of measure, which will be presently described, is of singular importance in researches with simple conductors; for series of sparks may be obtained from it of such slow convergence, that many successive terms may be considered as equal. Thus, an insulated metallic disc, d, being placed in connection with the electrometer, fig. 13, or with the electroscope, fig. 14, was electrified many times in succession to precisely the same amount, by sparks drawn on an insulated plate from the positive coating; the negative side of the jar after each contact being restored to a neutral state. When a portion of the charge was abstracted so as to sensibly decrease the quantity in the jar, then a new point was arrived at, from which another series of sparks can be obtained, differing extremely little in quantity; and this process may be continued to a low point of accumulation on the jar.

The quantity given off by the positive coating is dependent on the dimensions of the abstracting conductor, and on the free state of the negative side of the jar. If it be free for each experiment, or be otherwise connected with a conductor of sufficiently large dimensions, it may be observed that a conductor of a double capacity receives a double quantity, a conductor of a treble capacity a treble quantity, and so on. The extent to which this process may be carried with a jar exposing about two square feet of coating is somewhat considerable. We only require in these experiments an extremely perfect insulation.

In disposing given quantities of electricity on simple conductors in this way, and investigating the attractive force by means of the electrometer, Sir W. Snow Harris arrived at the laws formerly explained. (See page 550.)

Lane's Discharging Electrometer.

This admirable instrument is shown in fig. 16. To the Lane's distem AB of a Leyden jar MN, is fixed a bent piece of charging glass BC, for the purpose of supporting and insulating the electrometer brass rod DE, which has two equal brass balls at its extremities. This rod moves through a spring tube at C, so that the brass ball D can be placed at different distances from the equal ball A, by which the jar is charged. The insulated ball D is connected through the metallic wire DE with the outside coating of the jar, by a wire EF. If we bring the ball D near to A, a small electrical charge conveyed to the jar MN will discharge itself from A to D, and

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1 The counterpoise should be free from grease or varnish of every sort, and should, previously to being used, be kept immersed in water; the insulation of the conductor f also should be made extremely dry, and occasionally warmed by a stick of burning charcoal. Electrical pass off to the ground by the wire EF. If the distance Apparatus AD is increased, the jar must be more highly charged before it discharges itself; so that the distance AD of the balls is a measure of the intensity of the charge at the time of its discharge. As long, therefore, as the jar has not discharged itself, we are sure that its charge is less than that which corresponds to the distance AD. The chief defect of this instrument arises from the occasional interposition of particles of dust or other light conducting materials between the balls, which occasions the charge to take place sooner than it otherwise would do. The arm of glass is sometimes fixed on the top of the charging rod, where a ball of wood is placed, and is bent downwards, so as to carry the balls D, E. In this case the jar is charged by another ball projecting from the charging rod towards D. This electrometer is sometimes fixed to the prime conductor, with and without a jar accompanying it.

Sir W. Snow Harris's Measuring Electrometer.

This elegant instrument, which we have had the advantage of seeing in operation, is an invaluable addition to our electrical apparatus. According to the law of electrical accumulation on coated jars, the quantity added to one side is always proportional to the quantity given off by the other, and reciprocally. Hence the amount of the accumulation may be always estimated by insulating the jar to be charged, and observing, by means of a discharging electrometer, the explosions of a small jar connected with the negative coating. This process is, however, complicated in its general application; but Sir W. Snow Harris has modified it in the following manner:—Let a small jar N, be furnished with a discharging electrometer, n, and inverted as in fig. 17, being supported by a brass rod, pq, inserted into the ball D of the prime conductor ABC. Then, as the electricity passes up the rod and accumulates on the inner coating, a similar quantity is given off from the outer coating, which may be made to pass from a ball at p. Now, when the small jar N has been charged to a given degree, an explosion or discharge takes place from m to n, and restores the equilibrium; and hence one measure of electricity is marked by the first explosion. When this has taken place, the jar is in the same state as at first; and hence by a repetition of the process we obtain the exact number of measures (or explosions) which pass from the unit jar N, and are finally accumulated on the jar J, or battery, into which the electricity passes from the ball p. This process of charging a battery from the outer coating of an exploding jar, instead of from the prime conductor, supersedes all electrometers, and is the best way of measuring quantity.

Volta's Flame Electrometer.

It was observed by Mr Bennet, that a lighted candle placed above the cap of his electrometer, and communicating with it, greatly increased the sensibility of the instrument; and it appears, from various experiments, that flame possesses the property of carrying off from bodies the electricity with which they are charged. M. Volta ingeniously availed himself of this principle in order to bring down to his electrometer the electricity of the atmosphere, the nature and intensity of which he was desirous of examining. This effect is produced by elevating above the atmospheric conductor a lighted match or torch.

Matteucci's Phosphorus Electrometer.

As the preceding instrument cannot be employed when there is the least wind or rain, and still less during a storm of hail or wind, M. Matteucci conceived the idea of constructing an electrometer depending on the strong conducting power of the vapour of phosphorus. He prepared rods Matteucci of this substance between the twenty-fifth and the fiftieth et's phosphorus of an inch in diameter, by melting the phosphorus under pressure water, and by blowing it while in a state of fusion through a tube of the requisite diameter. He afterwards made the rod of phosphorus project from the fiftieth to the seventy-fifth of an inch beyond the end of the tube. He then fixed the glass tube on a wooden pole, and he insulated the pole by fixing to its extremity a glass handle. The phosphorus communicated by its base with a metallic wire which descended along the pole, and which could be kept at a distance from the pole by some tubes of glass placed at regular distances. M. Matteucci kept the rod of phosphorus perfectly insulated in the time of rain, by means of a glass hood varnished on both its surfaces, and having its convexity turned upwards. The pole was composed of three or four rods, which were adjusted into one another; and the instrument thus constructed was found extremely useful in examining the electricity of the atmosphere.

Mr Ronalds' Improvements on Electrometers.

As threads, pith or cork balls, and even straws when very dry, lose some of their conducting power, Mr Ronalds prefers fine silver wire, and hard charcoal balls from boxwood, for electrometric purposes. The following is the method which he employs in making electrometers of this kind. The instrument which he uses is represented in fig. 18, where ABCD is a bow of steel wire with a hook at each end, fig. 18. When the charcoal ball has been threaded on the silver wire, and rings formed at each end, it is very gently stretched in this bow, by passing the hooks through the rings, and shoving it forward with the thumb placed against the end of the tongue near the handle, which tongue is thus made to open wider by pressing the screw E on each side. The screw E is then turned a little farther into the piece F, in order to fix it firmly. The fine wire is now placed cautiously upon a piece of iron, a little below a red heat, which will make it perfectly straight, when it may be taken from the bow, and suspended on one of the rings of the piece of brass, fig. 19. Fig. 20 shows Mr Ronalds' method of making gold-leaf electrometers.

Melloni's Electroscope.

When a conductor in a neutral state approaches an electrified conductor, it disguises or dissimulates a portion of its electric state, and in restoring to the disguised fluid its positive tension in proportion as the sensible fluid passes from it by dispersion, it prolongs the duration of the electric charge. We know also that this effect takes away from the contrary electricity developed by induction in the part nearest to the body introduced, and that the electricity homologous to that of the inducing body appears on the more distant portions, where it diffuses itself in proportions inversely as the radii of curvature.

A fortunate combination of these three data led M. Melloni to conceive it possible to construct an electroscope singularly sensible, and capable of preserving its electricity a much longer time than any instrument of the same kind. The effect of the instrument is such as he expected, and he regards it as one which will become very useful in all kinds of electrical researches.

Let A (fig. 13) be a metallic cup furnished with two filiform appendages D, D soldered to the opposite points of Electrical the upper margin of the cup, and communicating by a conductor which passes into the axis of a tube of glass having a ball or a metallic disc at E. A second metallic cup B (fig. 14), inverted, and a little smaller and much lighter than A, is fixed below a wire or very thin lever of metal CC, suspended by a silk fibre F. The cup B is then suspended in the manner shown in fig. 15, so as to be inclosed in A without touching it. A small metallic cylinder f is placed in the middle of the cup A, so as not to touch the inside of the cup B.

If we now electrify positively the ball or conductor E, the electricity will pass to the cup A, will act by induction on the cup B, will repel the positive and attract the negative electricity of the natural fluid in B, which will react upon the free fluid in A, disguising a certain quantity and abandoning the rest according to the laws which regulate the distribution of electricity in insulated conductors, so that the intensity of the action will depend on the curvature of the surfaces, and will be less strong on the sides of the cup than upon its appendages. The cup A will therefore contain a certain proportion of positive electricity disguised, that is accumulated without tension and without mobility, and its appendages D, D will possess a free electricity of the same kind increasing towards their extremities. The cup B and its lever CC will possess negative electricity disguised at the central part in respect to the cup A, of free positive electricity in the rest of the moveable system, that is upon the top plate of the inverted cup B, and on the lever above it. But this last species of electricity will be more powerful at the extremities of the lever, than in the middle part or on the top of the cup; 1. Because these extremities are the most distant points of the inductive action; and, 2. Because their radius of curvature is smaller than that of any other part.

As the lever CC thus possesses the same kind of electricity as the appendages DD, it will be energetically repelled if it is not precisely in the same azimuth with them, and after a few oscillations it will rest at a certain angle of deviation. The electric charge communicated to the fixed system will then begin to diminish. This diminution will be much slower than in ordinary electrometers.

In this electrometer the moveable part is always electrified by induction, and never by communication; the difference of form between the centre and the extremities of the fixed and moveable pieces makes the distribution of the moving forces the most favourable for the rotation of the index, and the inductive action of the central surfaces, disguising a portion of the electricity to give it by degrees the free state in proportion to the losses sustained, prolongs the duration of the charge received. The following are the dimensions of the instrument:

| Diameter of glass cage | Millim. | |-----------------------|--------| | Height of do | 11 | | Length of fibre of silk| 36 | | Distance between dial and glass plate | 3 | | Inner diameter of A | 21 | | Do. do. of B | 16 |

Sir W. Snow Harris's Bifilar Balance.

About the year 1831, Sir W. Snow Harris proposed a new method of obtaining a delicate reactive power applicable to the measurement of electrical and magnetic forces, and the movements of oscillating bars and other suspended masses.

If a magnetic or electrical needle, as p n, No. 1, fig. 16, or other body, G, No. 2, be suspended horizontally by two equal filaments of unspun silk a b, a' b', fixed parallel to each other, it is evident that its position of rest will be in the vertical plane passing through the two threads. Whenever, therefore, we turn the needle from this position about the imaginary axis e e' fig. 2, the filaments of suspension will be deflected from the vertical, so that the distance e e' will be somewhat less than before; hence arises a reactive force derived from the weight of the suspended needle or other body, the centre of gravity of which having been raised, Electrical will endeavour to descend and rest in its previous position— it will be in a similar condition to that of a body falling down a very small arc. If therefore the needle be freely abandoned to this reactive force, a vibratory motion will arise; by observing which, we may determine the laws of the reactive force imparted to the threads.

By carefully noting the rate of oscillation of a suspended cylinder of wood G, No. 2, about 2 inches high and 2 inches diameter, the following results were arrived at:—1. The time of an oscillation is as the square root of the length of the threads of suspension divided by their distance apart, without reference to the weight of the suspended mass. 2. The oscillations are isochronous at all angles. Applying the general formula \( n = \frac{Pd^2}{2gT^2} \) employed by Coulomb in his experiments on torsion; it is found that the force im- parted to the threads \( = n \), will vary with the squares of the distances between them divided by their lengths, and will be also as the weight of the suspended mass \( = P \), hence we have \( n \propto \frac{Pd^2}{l} \); and since the oscillations are isochronous, we may conclude that \( n \) is proportional to the angle, or per- haps more nearly the sine of the angle, of deflection of the threads, results which the inventor fully verified by ex- periment.

Upon these data the electrometer, termed by the inven- tor a bifilar balance, was designed. The instrument, as at first constructed, is somewhat elaborate in its form and con- trivances for estimating and observing rapidly electrical forces, to which end it is remarkably available. It will be found very fully described in the Philosophical Transactions for 1836. A more simple form of it, however, is that simi- lar to the torsion balance already described, fig. 1, Plate CCXXIV., substituting the bifilar suspension for the wire of Coulomb.

The two threads \( ab, a'b' \), No. 1, fig. 16, are secured above to a small cross wire \( aa' \), at the extremity of a verti- cal rod terminating in a micrometer and index \( M \); they are about 20 inches long, and are set at a quarter of an inch apart; the electrical needle \( pn \) is attached to the lower ex- tremities of the threads, in a similar way, and is constructed much in the same way as the needle of Coulomb's balance of torsion, except that the repelling bodies \( p, q \) are thin discs of gilded cork of about .5 of an inch diameter, and the ex- tremity \( n \) of the needle balancing the insulated disc \( p \), is formed into an index, and moves over a graduated quadrant \( mn \) within the glass cage, thereby indicating more accu- rately the degrees of repulsion of the electrified bodies \( p, q \). The suspension threads are further stretched by a vertical cylinder of brass \( g \), carrying a small stage \( o \) for receiving such small circular weights as may suit the purpose of the experimentalist. The lower portion of the cylinder \( g \) ter- minates in a fine needle, which plays freely in a hole drilled in a small elevated plate of brass beneath, as seen in the figure; and by which the whole is prevented from swing- ing out of a vertical position. The whole is mounted on a raised base of varnished wood \( B \), supported by three level- ing screws, and there is a pretty large opening in the floor at \( H \), for the purpose of introducing any small electrified body, and communicating a repulsive force to the discs \( p, q \). To avoid a collapse of the threads when the angle of de- flection is considerable, the threads pass through one or two stays of light cork; with this precaution, the force is strictly proportional to the sine of the angle of deflection up to 360°. The angle, however, for all practical purposes need not ex- ceed 60°. There is a small angular lever \( y \) moveable with friction through the floor of the glass cage, by which the Electrical needle is prevented from taking a violent swing when the Apparatus repulsion commences, and by which it may be eased off gradually. In the instrument thus constructed, a force so small as the \( \frac{1}{1000} \)th part of a grain for each degree may be measured: by increasing the length of the threads, diminish- ing their distance apart, or changing the weights on the stage \( o \), almost any very small fraction of a grain may be obtained and valued. This instrument may be used pretty much in the same way as Coulomb's balance of torsion be- fore described, fig. 1, Plate CCXXIV. The inventor is of opinion, that in experiments with this, and all similar instru- ments, the electricity should be equal on each of the repel- ling bodies \( p, q \); the force will then be, at a constant dis- tance, always as the square of the quantity of electricity, as clearly proved by Cavendish in his manuscript papers, and as he himself has fully shown for attractive forces. If one of the repelling bodies \( p \) be in a higher state of elec- trical charge than the other \( q \), then the result is very un- certain—the indications may, in some instances, be nearly as the quantity on one of the discs; but this is not always the case. It is clear, that by continually diminishing the quantity on one of the discs, we shall at length arrive at an attractive force; and to this, under every circumstance, the action of the discs on each other virtually tends, as he thinks he has fully shown.

When it is found desirable to examine the repulsive force in connection with electrified bodies placed externally to the glass cage, then the disc \( q \) is attached to a light con- ducting-wire, terminating above in a contact ball, and insu- lated through a varnished tube of glass in the hole \( h \), after the manner of the gold-leaf electrometer.

The electroscope of the Abbé Haüy, and the torsion electromètre of Coulomb, have been described in a preced- ing part of this treatise.

CHAP. IV.—ON MISCELLANEOUS ELECTRICAL INSTRUMENTS.

1. Sir W. Snow Harris's Electrical Balance or Scale Beam Electrometer.

In investigating the law of the attractive forces of elec- tricity accumulated in jars and batteries, Sir W. Snow Harris made use of the electrical balance shown in fig. 1. The Harris's beam \( mn \) of the balance, constructed in the usual manner, electrical is suspended from a projecting arm of brass, \( ca \), supported Plate by a vertical stand, \( abe \), consisting of a brass slider and socket, \( cXXX \), \( ab \), by which the balance can be moved up or down, and of fig. 1, a glass tube, \( bc \), with a ball of varnished wood, \( b \). A wire, pointed out by the line \( ef' \), passes through the tube \( abe \), and connects the beam with the negative coating of the jar. A hollow gilt conductor of wood, \( f \), is suspended by a metallic thread from one of the arms \( m \), and from the opposite arm \( n \) is hung by silk lines a light brass scale, \( d \). In this scale there is placed as much additional weight as will balance the conductor \( f \), and put the whole in a state of equilibrium. By means of an insulated conductor \( f' \), of the same dimen- sions as \( f \), and fixed directly under it, the attractive force of the electricity in the jar is made to act directly on the sus- pended conductor \( f \). The conductor \( f' \), which is connected with the positive coating, is so placed that it can be de- pressed from contact with the conductor \( f \), through given distances, by means of a cylindrical slide, \( r \), attached to it, which moves in the socket \( s \), and indicates its depression on an engraven scale, divided into twentieths of an inch. The socket \( s \) is supported on a glass pillar by means of a var-

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1 Edin. Trans. for 1834, Oscillations of the Horizontal Needle, sec. 29. 2 Phil. Trans. for 1836, p. 419. Electrical nished ball of baked wood, on which the socket is fixed, and through which the conductor \( f \) is connected with the positive coating. The whole balance can be raised or depressed through a small distance by the micrometer screw at \( e \).

From this description it is obvious that the attractive force acts directly between the conductors \( f \) and \( f' \), and can therefore be measured by weights placed in the scale \( d \). The scale rests on a small circular stand \( q \), which can be raised or depressed by the sliding brass rod and tube \( r \), to accommodate itself to the horizontal position of the beam, and to check any oscillation. The balance is fixed on an elliptical base, having three levelling screws.

The following experiment, made by Sir W. Snow Harris, will best explain the use and value of this balance.

Having connected the inside coating of a single jar containing five square feet with the conductor \( f \), and the outside coating with the wire \( ac \), the conductor \( f' \) was depressed through half an inch, and a weight of sixteen grains was placed in the scale; then, when five turns of the plate were completed (or, if the measuring electrometer is used, when explosions were conveyed to the jar), the attractive force between \( f \) and \( f' \) was sufficient to tip the beam. The accumulated electricity being discharged, the conductor \( f' \) was depressed through a second interval of half an inch, making the whole distance one inch; and four grains or one-fourth of the former weight being placed in the balance, the beam was again depressed with five turns of the plate, or explosions of the measuring electrometer. The accumulation being again discharged, and the conductor \( f' \) depressed through a third interval of half an inch, and one-ninth part of the first weight placed in the scale, the beam was again depressed with five turns of the plate, or explosions. Hence, as the distances in the first experiment were as two to one, and the weights four to one, and as in the second experiment the distances were three to one and the weights nine to one, we may infer that the attractive force between the conductors varied in the inverse ratio of the square of their distance.

2. Dr Ure's Detonating Eudiometer.

The electrical eudiometer is a simple instrument, for detonating or exploding gases by means of an electrical spark or shock. The common eudiometer is merely a short tube of glass closed at the upper end, and having two pieces of platinum wire passing through the glass near its upper end, so as nearly to meet at the axis of the tube. These wires communicate, the one with the inner and the other with the outer coating of a charged jar, so that when the discharge passes between the platinum points, it inflames the gas in the tube. As the gas subjected to the action of the spark is transferred to the tube over water or mercury, the lower or open extremity of the eudiometer must be kept in the water or mercury, in order to confine the gas. With the common eudiometer two persons are required, the one to manage the instrument and the other to manage the electrical machine; but Dr Ure has given it such a form that a single individual can perform all the operations with the greatest facility.

Dr Ure's instrument, shown in fig. 2, consists of a glass syphon, ABC, with a bore of from two-tenths to four-tenths of an inch. Its legs AB, CB are from six to nine inches long, and from one-fourth to half an inch apart. The open end A is slightly funnel-shaped, and the other, C, which is hermetically sealed, has two platinum wires, \( a, b \), inserted near it by the blowpipe. The outer end of the one wire is bent vertically upwards, and then horizontally so as nearly to touch the edge of the aperture A. The end of the other wire is formed into a little hook, to allow a small spherical buttton, \( d \), to be attached to it when the electrical spark is to be transmitted. The sealed leg CB is graduated by introducing in succession equal weights of mercury from a measure glass tube. Seven ounces troy and sixty-six grains occupy the space of a cubic inch, and thirty-four and a half grains represent a hundredth part of that volume.

The method of using this apparatus is shown in fig 3, Fig. 3. The whole syphon being filled with mercury or water, a convenient quantity of the gas to be examined, not exceeding one-sixth of the capacity of the tube, must then be introduced in the ordinary manner. The tube is then held upright by the hand, and the gas being transferred into the sealed leg CB, the mercury is brought to a level in both legs, either by the addition of a few drops, or by displacing a portion by pushing down a glass or wooden rod. The tube being grasped as in the figure, the thumb must be placed tightly over the aperture, so as to close it; and at the same time touch the wire next it. A spark from the conductor of the electrical machine is then made to enter the button \( d \), and after inflaming the gas, is conducted away by the thumb and hand of the operator, the tip of the finger feeling only a slight push or pressure. When two or more inches of air are left beneath the thumb, it acts as a recoil spring to restrain the violence of the explosion. When condensation of volume takes place, the finger feels pressed down to the orifice. On sliding it gradually to one side and admitting the air, the mercurial column in CB will rise above that in AB. More mercury must then be poured in till the equilibrium is restored, when without any reduction we may read off the resulting volume of gas. If the charge of a jar is to be transmitted through the wires, the thumb must not touch the wire when it closes the aperture. In this case the wire from the outside coating must be hooked on the eudiometer wire nearest the thumb, and then the knob or ball on the charging rod of the jar must be brought in contact with the button on the other wire, when the gas will be exploded.

3. Volta's Electrical Lamp.

As hydrogen gas is readily inflamed by a very small electrical spark, Volta conceived the idea of constructing a lamp for temporary purposes, such as that of obtaining a light at night, or in summer for the purpose of sealing letters, by employing the electrophorus to light the hydrogen. With this view a quantity of gas is put into a reservoir, and when subjected to the pressure of a column of water, it escapes from a small aperture by turning a stop-cock. Beneath this reservoir is placed an electrophorus in a box, and from the upper part of the box a wire passes through a glass tube to the small aperture. When the handle of the stopcock is opened to let out the gas, the cover of the electrophorus is raised by means of a silk cord connected with the handle of the stop-cock, and the spark from this cover is conveyed by this insulated wire to the stream of gas, which is instantly kindled, so as to allow a candle to be immediately lighted. From the smallness of the quantity of gas consumed, a light may be procured an hundred times from the same reservoir of gas. When the hydrogen gas is expended, it is troublesome to persons unaccustomed to chemical manipulations to replenish the reservoir with fresh gas. M. Gay-Lussac removed this defect by suspending a bar of zinc on the apparatus, so as to reproduce, by the action of diluted sulphuric acid upon it, as much gas as was exhausted.

Although a good electrophorus, when well excited, will retain its charge for many months, yet in general its improvement upon it. Electrical Apparatus has been so uncertain, especially in damp weather, that many persons have been obliged to lay aside the instrument.

Mr. Cutbush of Philadelphia found that he could produce a spark in the dampest weather when he warmed the electrophorus before exciting it with a fox's tail, and kept the electrophorus box as tight as possible. As the cock is apt to become loose and allow the gas to escape, Mr. Cutbush applied a mixture of tallow and finely pulverized plumbago to the cock; and, what is very curious, he found that the hydrogen gas prepared from zinc escapes much more readily than that procured from iron filings. He found that the former sometimes disappeared in twenty-four hours, while the latter often remained more than a week. The gas from iron filings is more impure than the other, from containing more or less carbon. With these precautions Mr. Cutbush found that the lamp of Volta seldom disappointed him in producing flame. He ascertained that one cubic inch of gas will light the taper at least ten times if the cock is quickly turned.

A hydrogen lamp acting by voltaic electricity in place of that of the electrophorus has been invented by Professor Jacob Green of Nassau Hall, and is quite independent of the state of the atmosphere. Its description, however, belongs to the subject of another article.

4. Ronalds' Electrograph.

M. Magellan had proposed to delineate the changes which take place in the electricity of the atmosphere, by a cylindrical and a plain electrograph. As our limits, however, will not permit us to describe these instruments, we shall content ourselves with giving a drawing and description of the more recent and useful electrograph invented by Mr. Ronalds. This instrument is shown in fig. 4, where AA is a box with a strong time-piece placed horizontally, and moved by the weight B, and CC a disc of baked mahogany eight inches in diameter, with an aperture of 2½ inches at D. The circumference of this disc, and also that of the perforation, are provided with edges or rims, and the outer broad rim is divided off and marked with hours and minutes like a common clock. The space between the two edges is almost filled with cement, composed of rosin, bees' wax, and lamp-black, and this part of the apparatus may be taken from the box at pleasure. A glass-tube EF, with brass caps, and covered inside and out with hard cement, screws by its lower end into the disc CC, while the upper end carries a small sheave, g. Within this tube EF a stem of glass is fixed by its lower end on the minute arbor of the time-piece, and a pivot attached to its upper extremity passes through F and the sheave g. This pivot carries the iron ball and cup h, into which is screwed the horizontal steel wire i, carrying the slider k, which moves with little friction along the wire. The piece k carries the vertical wire l, terminating below in a hook, upon which hook is hung a ring at one end of a short wire m, whose other end carries a small gold bead. A fine thread, n, is attached to k by one end, and by the other to the sheave g.

When the clock is going, its minute arbor carries round the arm k, and the effect of this is to coil the thread n round the fixed sheave g, and to make the piece k advance towards the ball h, so that the gold bead will trail upon the resinous disc CC, and describe a spiral upon it. If we now cause the little iron cap above h to communicate with a wire connected with any atmospherical conductor, the gold bead will electrify the resinous surface, so that when the plate is removed from the clock and powdered with pounded resin, or even dry hair powder, the spiral line will exhibit configurations varying in shape and in breadth according to the intensity and nature of the electricity which the resinous surface has received from the trailing bead. The Electrical times at which these phenomena take place will be shown by the dial-plate.

If this instrument is used for recording the phenomena of serene weather, dew, &c., the hour arbor should be used in place of the minute one; but if for those of a thunder-storm, hard shower of rain or hail, or snow, the minute arbor should be used. Mr. Ronalds adds, that he has sometimes found a more rapid motion necessary, which can be obtained by the addition of a third arbor; the glass tube EF, with its appendages, being transferred to the most suitable arbor, and the disc adjusted to a new centre. Sheaves larger and smaller than g will be requisite for different applications of this electrograph.

5. The Electrical Air Thermometer.

This instrument, invented by Mr. Kinnersley, is shown in fig. 5, where AB is a glass tube about ten inches long and two inches wide, having its ends closed by two air-tight brass caps, A and B. Through these caps slide two hooked wires, FG, EI, so that the small brass balls G, I, can be set at any distance, and an electrical spark passing between them may be made stronger or weaker as the occasion requires. Another small tube, HA, open at both ends, passes through a tube in the copper caps, and through this tube a sufficient quantity of mercury or water is introduced to fill the lower ends both of the wide tube AB and the narrow one HA. If an electrical charge is sent through the balls G, I when they are placed in contact, by connecting the books E, F with the outside and inside coating of a Leyden jar, no effect will be produced; but if the balls G, I are separated so that the charge passes in the form of a spark through the interposed air, the rarefied and displaced air will press on the surface of the mercury or water at the bottom of the tube AB, and raise it nearly to the top of the small tube HA. It will then sink after the explosion, and resume its former position.

6. Volta's Electrical Pistol.

A brass vessel of a pear shape, or of an ellipsoidal form, Volta's being perforated at its two ends, a glass tube of the same diameter as the perforation is inserted in one of them, so as to extend to the centre of the ellipsoid, and to project about four inches beyond the vertex. Through this tube there passes a metallic stem, which is furnished with a brass ball at its outer end, while its other extremity reaches beyond the inner end of the glass tube. A mixture of equal parts of hydrogen gas and atmospheric air having been introduced at the second aperture, this aperture is closed tightly with a cork. The operator now grasps the ellipsoid by its equator, and when a spark is taken by its brass ball from the prime conductor, the gaseous mixture will instantly be exploded, and drive out the cork with a smart explosion. In place of a mere perforation at the extremity of the ellipsoid, a barrel may be inserted, and by using a cylinder of cork as a wadding, a ball may be discharged from the pistol.

7. Ronalds' Electrical Pistol.

Another form of the electrical pistol is shown in fig. 9, Ronalds' at BCF, forming a part of Mr. Ronalds' electrical telegraph. The pistol has the form of a pear, and the brass rod and ball, in place of being a continuation of its axis, is inserted on one side, as shown at D. We have given a separate section of this pistol in fig. 6, where AB is the body of the pistol, which contains the inflammable gas, C the cork which is to be discharged, D the glass tube, G the brass...