In general, signifies the operations of a very subtile fluid, in most cases invisible, but which sometimes becomes the object of our sight and other senses, discovering itself to be one of the chief agents employed in producing the phenomena of nature.

Sect. I. Definitions of Terms used in the Science.

Before we can enter upon this science with propriety, even so far as to give an history of its rise and progress, it seems necessary to give some explanation of the terms made use of by writers on electricity, that the reader may not be embarrassed with words whose meaning he cannot perhaps easily comprehend.

1. The foundation of all that is known upon this subject, is the difference between electric bodies and such as are not. The former may generally be distinguished by their attracting and repelling light substances, which the latter cannot be made to do. The principal electric bodies are glass, amber, sealing-wax, gum-lac, sulphur, rosin, &c. They are often called non-conductors, or electrics per se.

2. The usual way in which the electric power of any body can be discovered, is by rubbing it with some soft substance, generally woollen, silk, or fur; and, according to the strength of the electric virtue, the former body will attract and repel light substances presented to it at a greater or less distance. If the virtue is very strong, the electric body will emit sparks, or even strong flashes of fire, to a considerable distance. In some cases electricity discovers itself by heating the body, or blowing air upon it; but in both these ways it is much weaker than that produced by rubbing. In whatever way this power is made to show itself, the substance possessed of it is said to be excited.

3. Conductors, called also non-electrics, are such substances as, though incapable of being excited, can yet in certain circumstances convey the electric power from one body to another, and that to any imaginable distance. The best conductors are metals of all kinds, charcoal, and water.

4. Electrics, we have already observed, are also called non-conductors; and this name they have from their power of stopping the communication of the electric virtue from one body to another. Thus, though any conductor be placed properly for receiving the virtue from an excited electric, none will pass to it if any electric substance be interposed; or, if the conductor be terminated by an electric, none will pass beyond the place where the electric substance begins.

5. Insulation is when a conducting substance is placed upon an electric, so that any power communicated to it cannot pass off. It must be remembered, however, that all this is to be understood with some degree of limitation; for there is no substance either a perfect electric, or a perfect conductor; the best conductors making a sensible resistance to the passage of the fluid through them when they are very long; and the most perfect electrics transmitting some of the fluid over, or through them. Indeed, though these two different kinds of substances seem to be so far removed from one another, they in reality approach to a surprising degree, insomuch that there are many substances which can be excited as electrics, and yet have a very considerable conducting power.

6. The effects of the electric fluid discovering themselves... Sect. II. ELECTRICITY.

History selves either by attraction and repulsion, or by emitting streams, or pencils as they are called, of blue light, are all clasped under the general word electricity; and any body to which that power of attraction and repulsion, &c., is communicated, is said to be electrified. If its virtue is inherent in itself, it is said to be excited.

7. Electricity is found to be of two kinds; the one called negative, and the other positive. It is uncertain in what the difference between these two consists. Dr Franklin is of opinion that the former consists in a superabundance of the fluid, or when more is thrown upon any substance than it can conveniently contain; the other, when a part of it is abstrated, and the body contains less than it naturally ought to do. Other theories suppose, that when the fluid is directed outwards from any substance, that substance will in all cases be electrified positively; and that when the fluid is either entering or has a tendency to enter into any substance, it will then be electrified negatively. This question will be discussed in the course of the treatise.—The most remarkable differences we can perceive between the positive and negative electricities are that they attract each other, though strongly repulsive of themselves; that is, two bodies positively electrified, or negatively electrified, repel each other; but one body positively electrified will attract another negatively so; and if the electricities are very strong, a spark will be observed between them at meeting. These electricities are produced naturally by exciting different substances, or by using a different rubber to the same substance. Thus, glass usually produces the positive electricity; but by using a certain kind of rubber, or altering the smoothness of its surface, it may be made to produce the negative kind. The two electricities are sometimes called the vitreous and resinous, as well as positive and negative.

Sect. II. History of Electricity.

Though it is certain that, ever since the creation of the world, the fluid we speak of hath had the same share in all the natural operations that it hath just now; yet the discovery of its action, and even of its existence, is, comparatively speaking, of a very late date. Thales the Milesian, who lived about 600 years before Christ, was the first that observed the electrical properties of amber. Of these, indeed, he knew no more than that this substance would attract light bodies when it was rubbed. For 300 years after his time, we hear nothing farther concerning this subject. Theophrastus then tells us, that the lyncurium (the same substance now called the tourmaline), has the property of attracting light bodies, as well as amber. From this time, there is a chasm in the history of electricity for no less than 1900 years. Indeed, it is scarce to be supposed that during this long interval any person applied himself to the investigation of the subject; as, for the greatest part of it, science of every kind was almost totally extinguished. The electrical properties of jet, however, and, according to Mr Bofe, of the agate, were some way or other discovered during the abovementioned period. But it was not till the beginning of the 17th century, that the subject of electricity became properly a distinct science, and the foundation was laid of those discoveries which have since taken place.

The first who can properly be called an electrician, was Dr William Gilbert, who, in the year 1600, wrote a book de Magnete, which contains a variety of electrical experiments. All these, however, considered only the attractive property of certain substances, which, from their agreement in this respect with amber (in Latin elelrum), were called electric. Dr Gilbert's merit consists in his having been at great pains to find out a number of such substances, and thus considerably enlarging the number of electrics.

Till the year 1670, it doth not appear that any farther discoveries were made; except some trifling additions to the catalogue of electrics. About this time, Mr Boyle applied himself to the study of electricity. He enlarged the catalogue of electrics; and found that their electric properties were increased by wiping and warming them before they were rubbed. He observed also, that all kinds of bodies were attracted promiscuously; and imagined that they were attracted in vacuo as well as in air. This last position, however, is denied by Mr Beccaria; and we shall afterwards show that Mr Boyle must necessarily have been mistaken. He also observed the electric light, though only in the influence of some diamonds.

Otto Guericke, however, who was contemporary with Mr Boyle, improved the science much farther. He made use of a sulphur globe, whirled on an axis much in the same way with our present glass globes. Thus Newton could excite a vastly greater power of electricity than any of his predecessors, and try all their experiments to much more advantage. He discovered electric repulsion; and not only saw the electric light more clearly than Mr Boyle, but heard the hissing sound with which it is emitted. He also made another remarkable discovery, but which has since been very generally overlooked; namely, that a feather, when repelled by an excited electric, always keeps the same face towards the body which repels it, as the moon does to the earth.

The next discovery of any moment was made by Sir Isaac Newton; who observed, that the electric attraction and repulsion penetrated through glass; and it is much to be regretted that this accurate philosopher did not apply himself to the study of electricity with greater assiduity.

In 1709, a treatise was written on electricity by Mr Haukbee; who not only far excelled all his predecessors and contemporaries, but also made some discoveries which well deserve the attention of the most expert electricians at this day. Besides a variety of new experiments made upon electric attraction and repulsion, as well as the light emitted by electric bodies; he found a method of rendering opaque bodies transparent by means of electricity. He lined more than half the inside of a glass globe with sealing-wax; and having exhausted the globe, he put it in motion; when applying his hand to excite it, he saw the shape and figure of all the parts of his hand distinctly and perfectly, on the concave surfaces of the wax within, just as if only pure glass without any wax at all had been interposed between his eye and his hand. The lining of wax, where it was spread the thinnest, would... but just allow the sight of a candle through it in the dark; but in some places the wax was at least an eighth part of an inch thick. Yet, even in these places, the light and figure of his hand were as distinguishable through it as anywhere else. The sealing-wax did not adhere to the glass in all places; but this made no difference with regard to the transparency. Pitch answered the purpose equally well with sealing-wax.

M. Haukbee also made a farther improvement, by using a glass globe, which acts much more powerfully than a sulphur one. After his death, however, not only the use of glass globes, but even the study of electricity itself, seems to have been pretty generally laid aside for some time. The reason of this was, that the recent discoveries of Sir Isaac Newton engrossed the attention of philosophers to such a degree, that they had no leisure for anything else. After the death of that great man, however, the science of electricity began to revive; and, in 1729, a capital discovery was made by Mr Stephen Grey. This was the distinction between conductors and non-conductors of electricity. As the discovery was entirely accidental, and attended with several curious circumstances, we shall here give some account of it. In the month of February, 1729, Mr Grey, after some fruitless attempts to excite an electric power in metals, recollected a suspicion he had for some time entertained, that as a glass tube, when excited in the dark, communicated its light to various bodies, it might at the same time possibly communicate to them an electricity; that is, a power of attracting light bodies; which, as yet, was all that was understood by the word electricity. For this purpose he provided himself with a glass tube, three feet five inches long, and near one inch and two-tenths in diameter. To each end was fitted a cork; to keep the dust out when the tube was not in use. His first experiments were made with a view to determine whether the tube would attract equally well with the ends shut as with them open. In this respect there was no difference; but he found that the corks attracted and repelled light substances as well, and rather better, than the tube itself. He then fixed an ivory ball upon a stalk of fir about four inches long, and thrusting the end of the stalk into one of the corks, he found the ball endowed with a strong attractive and repulsive virtue. This experiment he repeated in many different ways; fixing the ball upon long sticks, and upon pieces of brass and iron wire, always with the same success; but he constantly observed, that the ball at the end attracted more vigorously than that part of the wire nearest the tube.

The inconvenience of using long wires in this manner, put Mr Grey upon trying whether the ball might be suspended by a pack-thread with a loop on the tube, with equal success; and the event fully answered his expectation. Having thus suspended bodies of the greatest length he conveniently could, to his tube, he ascended a balcony 26 feet high, and fastening a string to his tube, found that the ball would attract light bodies on the ground below. This experiment succeeded in the greatest heights to which he could ascend; after which, he attempted to carry the electricity horizontally. His first attempt miscarried, because he suspended his line, which was intended to carry the electricity horizontally, by a pack-thread; and thus the fluid got off from it; but though Mr Grey knew this was the case, he could not at any time think of any method to prevent it.

On the 30th of June 1729, Mr Grey paid a visit to Mr Wheeler, in order to give him a specimen of his experiments; but told him of the unsuccessful attempt he had made to carry the electric fluid horizontally. Mr Wheeler proposed to suspend the conducting line by silk instead of pack-thread. For this advice he could give no reason, but that the silk thread was smaller than the other; however, with it they succeeded perfectly well. Their first experiment was in a matted gallery at Mr Wheeler's house, on the 2d of July 1729. About four feet from the end of the gallery they fastened a line across the place. The middle of this line was silk, the rest pack-thread. Over the silk part they laid one end of the conducting line, to which was fastened the ivory ball, and which hung down about nine feet below the line stretched across the gallery. The conducting line was 80½ feet in length, and the other end of it was fastened by a loop to the electric tube. Upon rubbing the tube, the ivory ball attracted and repelled light substances as the tube itself would have done. They next contrived to return the line, so that the whole length of it amounted to 147 feet; which also answered pretty well. But, suspecting that the attraction would be stronger without doubling or returning the line, they made use of one carried straight forward for 124 feet; and, as they expected, found the attraction in this manner stronger than when the line had been doubled. Thus they proceeded with their experiments; still adding more conducting line, till at last their silk string broke with the weight. This they endeavoured to supply, first with a small iron-wire, and then with a brass one. The result of these experiments, however, soon convinced them that the silk refused to conduct the electric fluid, not on account of its smallness, as they had supposed, but on account of some difference in the matter. The wires were smaller than the silk-thread, yet the electricity was effectually carried off by them. They had recourse, therefore, to thicker lines of silk; and thus conveyed the electric matter to the distance of 765 feet; nor did they perceive the virtue to be at all diminished by the distance to which it was carried.

This discovery of the non-conducting power of silk was quickly followed by a discovery of the same power in many other substances; and thus, in fact, the foundation of almost all the subsequent improvements in electricity was laid; though in the sciences, as well as in most others, few discoveries have been made by reasoning, but many by accident. Mr Grey continued to study electricity as long as he lived; and has given a set of experiments, of which Dr Priestley says, "It is not easy to know what to make of them." He discovered that he had discovered in all electric substances a perpetual attractive power, which required no external kind of excitation either by heating, rubbing, or any power in kind of attrition. He took 19 different substances, electrics, which were either rosin, gum-lac, shell-lac, bees-wax, sulphur, pitch, or two or three of these differently compounded. These he melted in a spherical iron ladle; except the sulphur, which was best done in a glass vessel. When these were taken out of the ladle, and their spherical surface hardened, he says they would. would not attract till the heat was abated, or till they came to a certain degree of warmth; that there was then a small attraction, which increased till the substance was cold, when it was very considerable. The manner in which he kept these substances in a state of attraction was, by wrapping them in any thing which would preserve them from the external air. At first, for the smaller bodies he used white paper, and for the larger ones white flannel; but afterwards, he found that black worsted stockings would do as well. When thus wrapped up, they were put into a large firm box, where they remained till he had occasion to use them. Thus prepared, they retained their attractive virtue for four months. These experiments are similar to some others lately made and published as new discoveries.

Some other experiments were made by Mr Grey, with regard to the attraction of electric bodies in vacuo; and in this he determined with Mr Boyle against the opinion of Mr Beccaria above mentioned. But the most remarkable experiments mentioned by Mr Grey, are his imitations of the planetary motions. "I have lately made (says he) several new experiments upon the projectile and pendulous motions of small bodies by electricity; by which small bodies may be made to move about large ones, either in circles or ellipses; and those either concentric or eccentric to the centre of the large body about which they move, so as to make many revolutions about them. And this motion will constantly be the same way that the planets move about the sun, viz. from the right hand to the left, or from west to east. But these little planets, if I may so call them, move much faster in their apogee than in the perigee parts of their orbits; which is directly contrary to the motion of the planets about the sun."

The manner in which these experiments were made, as delivered by him on his deathbed to Dr Mortimer, was as follows: "Place a small iron globe (said he), of an inch or an inch and a half in diameter, on the middle of a circular cake of rosin, seven or eight inches in diameter, gently excited; and then a light body suspended by a very fine thread, five or six inches long, held in the hand over the centre of the cake, will, of itself, begin to move in a circle round the iron globe, and constantly from west to east. If the globe is placed at any distance from the centre of the circular cake, it will describe an ellipse, which will have the same eccentricity as the distance of the globe from the centre of the cake. If the cake be of an elliptical form, and the iron globe be placed in the centre of it, the light body will describe an elliptical orbit of the same eccentricity with the form of the cake. If the globe be placed in or near one of the foci of the elliptical cake, the light body will move much swifter in the apogee than in the perigee of its orbit. If the iron globe is fixed on a pedestal an inch from the table, and a glass hoop, or a portion of a hollow glass cylinder excited, be placed round it, the light body will move as in the circumstances mentioned above, and with the same varieties." He said, moreover, that the light body would make the same revolutions, only smaller, round the iron globe placed on the bare table, without any electrical body to support it; but he acknowledged that he had not found the experiment succeed if the thread was supported by any thing but a human hand, though he imagined any other animal substance would have answered the purpose.

These experiments occasioned a great deal of speculation. Dr Mortimer was the only person who was able to repeat them with success, and he only when nobody but himself was present. It was therefore generally supposed that both he and Mr Grey had been deceived; but from some experiments to be related hereafter, it seems probable that the success of Mr Grey and Dr Mortimer was owing to their having performed their experiments with candle-light; and the failure of the others, to their having attempted them by daylight. Notwithstanding which, it is more than probable that Mr Grey has been deceived in a number of particulars; for no motion can be performed by an artificial excitation of the electric fluid, but what is attended with much irregularity.

Soon after Mr Grey's discovery of the difference between conductors and non-conductors of electricity, Mr Du Fay discovered the difference between positive and negative, or, as they were for some time called, by Mr Du Fay, viscous and refractive electricities. This discovery was quite accidental. It was made in consequence of his casually observing, that a piece of leaf-gold, repelled by an excited glass tube, and which he meant to chase about the room with a piece of excited gum copal, instead of being repelled by it as it was by the glass tube, it was eagerly attracted. The same was the case with sealing-wax, sulphur, rosin, and a number of other substances. He discovered also, that it was impossible to excite a tube in which the air was condensed.

In the year 1742, the use of glass globes was again introduced by Mr Boe, professor of philosophy at Wittenberg; though some attribute this to Christian Augustus Hansen, professor of mathematics at Leipzig. He added also a prime conductor, which consisted of a tube of iron or tin. It was at first supported by a man standing upon cakes of rosin; but afterwards suspended by silk lines horizontally before the globe. A bundle of thread was put into the end next to the globe, which not only prevented any injury to the glass, but rendered the electricity stronger.

The most remarkable discovery that hath yet been made in the science of electricity, was in the end of the year 1745, and beginning of 1746. This was the method of giving the electric shock, or the accumulation of the power of electricity in a vial. This had its name of the Leyden vial, from Mr Cunzeus, a native of Lyden, who exhibited it as he was repeating some experiments made by Messrs Mulchenbroek and Allamand, professors in the university of that city. He was not, however, the inventor. The merit of this discovery (if any merit can arise from a discovery made by accident) belongs to Mr Van Kleist, dean of the cathedral at Camin. On the 4th of November 1745, he sent the following account of it to Dr Leibekuhn at Berlin: "When a nail, or a piece of thick brass wire, &c. is put into a small apothecary's vial, and electrified, remarkable effects follow: but the vial must be very dry, or warm. I commonly rub it over before-hand with a finger, on which I put some pounded chalk. If a little mercury or a few drops of spirit of wine are put into it, the experiment succeeds the better. As soon as this phial and nail are removed from..." the electrifying glass, or the prime conductor to which it hath been exposed is taken away, it throws out a pencil of flame so long, that with this burning machine in my hand, I have taken above 60 steps in walking about my room. When it is electrified strongly, I can take it into another room, and there fire spirits of wine with it. If while it is electrifying, I put my finger, or a piece of gold which I hold in my hand, to the nail, I receive a shock which fluns my arms and shoulders.

"A tin tube, or a man placed upon electrics, is electrified much stronger by this means than in the common way. When I present this vial and nail to a tin tube, which I have 15 feet long, nothing but experience can make a person believe how strongly it is electrified. Two thin glasses have been broken by the shock of it."

Soon after this, a method of giving the shock was discovered in Holland by Mr Cuneus, in the following manner: M. Mulchenbrock and his friends, observing that electrified bodies exposed to the common atmosphere, which is always replete with conducting particles of various kinds, soon lost their electricity, and were capable of retaining but a small quantity of it; imagined, that, were the electrified bodies terminated on all sides by original electrics, they might be capable of receiving a stronger power and retaining it for a longer time. Glasses being the most convenient electric for this purpose, and water the most convenient non-electric, they first made these experiments with water in glass bottles: but no considerable discovery was made, till Mr Cuneus, happening to hold his glass vessel in one hand, and endeavouring to disengage it from the conductor with the other (when he imagined the water had received as much electricity as the machine could give it), was surprised with a sudden shock in his arms and breast, which he had not in the least expected.

The discovery of such a terrible effect of the electric power immediately raised the attention of all the philosophers in Europe. Many of them greatly exaggerated their accounts; either from a natural timidity, or their love of the marvellous. Mr Mulchenbrock, who tried the experiment with a very thin glass bowl, told Mr Reaumur in a letter wrote soon after the experiment, That he felt himself struck in his arms, shoulder, and breast, so that he lost his breath; and was two days before he recovered from the effects of the blow and the terror. He added, that he would not take a second shock for the whole kingdom of France. Mr Allamand, who made the experiment, with a common beer-glass, said, that he lost his breath for some moments; and then felt such an intense pain all along his right arm, that he was apprehensive of bad consequences, but it soon after went off without any inconvenience, &c.

Other philosophers, on the contrary, showed their heroism and magnanimity, by receiving a number of electric shocks as strong as they could possibly make them. Mr Boze above mentioned, wished that he might die by the electric shock, in order to furnish, by his death, an article for the memoirs of the academy of sciences at Paris. "But (adds Dr Priestley, from whom this account is taken), it is not given to every electrician to die in so glorious a manner as the justly envied Richman."

From the time of this discovery, electricity became the general subject of conversation. A great number of people all over Europe got their livelihood by going about and showing the phenomena of it; and, at the same time, the passion for the marvellous strongly discovered itself in some effects of electricity, pretended to be found out in Italy and Germany. It was affected by Signor Pivati at Venice, and after him by Verati at Bologna, Mr Blanchi at Turin, and Mr Winckler at Leipzig, that if odoriferous substances were confined in glass vessels, and the vessels excited, the odours and other medicinal virtues would transpire through the glass, infect the atmosphere of the conductor, and communicate the virtue to all persons in contact with it; also, that those substances, held in the hands of persons electrified, would communicate their virtues to them; so that the medicines might be made to operate without being taken into the stomach. They even pretended to have wrought many cures by the help of electricity applied in this way. To see the wonderful effects of these medicated tubes, as they were called, Mr Nollet travelled into Italy, where he visited all the gentlemen who had published any account of these experiments. But tho' he engaged them to repeat their experiments in his presence, and upon himself; and though he made it his business to get all the information he could concerning them; he returned fully convinced, that in no instance had odours been found to transpire through the pores of excited glass, and that no drugs had ever communicated their virtues to people who had only held them in their hands while they were electrified. He was convinced, however, that by continued electrification without drugs, several persons had found considerable relief in various disorders; particularly, that a paralytic person had been cured at Geneva, and that one who was deaf of an ear, another who had a violent pain in his head, and a woman with a disorder in her eyes, had been cured at Bologna; so that from this time we may date the introduction of electricity into the medicinal art. See Medicine-Index.

Another wonderful experiment was the beatification of Mr Boze; which other electricians, for a long time, endeavoured to repeat after him, but to no purpose. His description of this remarkable experiment was, that if, in electrifying, large globes were employed, and the electrified person stood upon large cakes of pitch, a lambent flame would by degrees arise from the pitch, and spread itself around his feet; that from thence it would be propagated to his knees and body, till at last it ascended to his head; that then, by continuing the electrification, the person's head would be surrounded by a glory such as is in some measure represented by painters in their ornamenting the heads of saints. Dr Watson took the utmost pains to repeat this experiment. He underwent the operation several times, and was supported during the time of it by solid electrics three feet high. Being electrified very strongly, he felt a kind of tingling on the skin of his head and in many other parts of his body. The sensation resembled what would arise from a vast number of insects crawling over him at the same time. He constantly observed the sensation to be the greatest in those parts of his body which were nearest to any non-electric; but no light appeared upon his head, though the experiment was several times made in the dark, and with some continuance. At last the Doc- tor wrote to Mr Boze himself, and his answer showed that the whole had been a trick. Mr Boze acknowledged that he had made use of a suit of armour, which was decked with many bullions of steel, some pointed like nails, others like wedges, and some pyramidal; and that when the electrification was very vigorous, the edges of the helmet would dart forth rays something like those which are painted on the heads of saints.

The identity of the electrical matter with lightning is a discovery that hath been of more practical use to mankind than any other. From almost the first discovery of the electric light, and the crackling with which it is emitted, a similarity between it and the phenomena of thunder and lightning had been observed. This is taken notice of by Dr Wall, one of the first who viewed the electric light in any perfect manner. The Abbé Nollet, Mr Winckler, and others, also enumerated many resemblances between the phenomena of electricity and those of thunder; but they did not think of any method by which their suppositions could be brought to the test of experience. This was first proposed by Dr Franklin in 1750. He had before discovered the effects of pointed bodies in drawing off the electric matter more powerfully than others. This was suggested to him by one Mr Thomas Hopkinson, who electrified an iron ball of three or four inches diameter with a needle fastened to it, expecting to draw a stronger spark from the point of it; but was surprised to find little or none. Dr Franklin, improving on this hint, supposed that pointed rods of iron, fixed in the air when the atmosphere was loaded with lightning, might draw from it the matter of the thunder-bolt, without noise or danger, into the body of the earth. His account of this supposition is given by himself in the following words. "The electric fluid is attracted by points. We do not know whether this property be in lightning; but since they agree in all the particulars in which we can already compare them, it is not improbable, that they agree likewise in this; let the experiment be made."

This supposition of Dr Franklin's was verified in 1752, and the discovery is perhaps the only one in the whole science that hath not been the result of accident. The most active persons were two French gentlemen, Messrs Dalibard and Delor. The former prepared his apparatus at Marly la Ville, situated five or six leagues from Paris; the other at his own house, on some of the highest ground in that capital. Mr Dalibard's machine consisted of an iron rod 40 feet long, the lower extremity of which was brought into a cistern-box, where the rain could not come; while on the outside it was fastened to three wooden posts by long silk strings defended from the rain. This machine happened to be the first that was favoured with a visit of the ethereal fire. Mr Dalibard himself was not at home; but, in his absence, he had entrusted the care of his apparatus to one Coissier, a joiner, who had served 14 years among the dragoons, and on whose courage and understanding he could depend. This artisan had all the necessary instructions given him, and was desired to call some of his neighbours, particularly the curate of the parish, whenever there should be any appearance of a thunder storm. At length the long expected event arrived. On Wednesday the 10th of May 1752, between two and three in the afternoon, Coissier heard a pretty loud clap of thunder. Immediately he ran to the machine, taking with him a phial furnished with a brafs wire; and presenting the wire to the end of the rod, a small spark issued from it with a snap like that which attends a spark from an electrified conductor. Stronger sparks were afterwards drawn in the presence of the curate and a number of other people. The curate's account of them was, that they were of a blue colour, an inch and an half in length, and smelled strongly of sulphur. In making them, he received a stroke on his arm a little below the elbow; but he could not tell whether it came from the brafs wire inserted into the phial, or from the bar. He did not attend to it at the time; but the pain continuing, he uncovered his arm when he went home in the presence of Coissier. A mark was perceived round it, such as might have been made by a blow with the wire on his naked skin.

About a month after this, Dr Franklin himself had an opportunity of verifying his own hypothesis. He was waiting for the erection of a spire in the city of Philadelphia, not imagining that a pointed rod of moderate height could answer the purpose. At last it occurred to him, that by means of a common kite he could have a readier access to the higher regions of the atmosphere than any other way whatever. Preparing, therefore, a large silk handkerchief and two cords sticks of a proper length on which to extend it, he took the opportunity of the first approaching thunderstorm to take a walk into a field where there was a shed convenient for his purpose. But dreading the ridicule which too commonly attends unsuccessful attempts in science, he communicated his intention to nobody but his son, who assisted him in raising the kite. A considerable time elapsed before there was any appearance of success. One very promising cloud had passed over the kite without any effect; when, just as he was beginning to despair, he observed some loose threads of the hempen string to stand erect and avoid one another, just as if they had been suspended by the conductor of a common electrical machine. On this he presented his knuckle to a key which was fastened to the string, and thus obtained a very evident electric spark. Others succeeded even before the string was wet; but when the rain had begun to descend, he collected electric fire pretty copiously. He had afterwards an insulated iron rod to draw the lightning into his house; and performed almost every experiment with real lightning that had before been done with the artificial representations of it by electrical machines.

Thus a new field was opened for philosophers; but it was soon found, that experiments of this kind were not always to be made without danger. This very year, 1752, the Abbé Nollet published some cautioning to those who tried experiments on lightning. He had been informed by letters from Florence and Bologna, that some people there had received violent shocks while they drew sparks from an iron bar electrified by thunder. One of his correspondents informed him, that once, as he was endeavouring to fasten a small chain with a copper ball at one of its extremities to a great chain which communicated with the bar at the top of the building, there came a flash of lightning which he did not see, but which affected the chain with a noise like that of wild-fire. The observer instantly stantly received such a shock, that the ball fell out of his hands, and he was struck backwards four or five paces.

The greatest instance of the danger of these experiments, however, was the death of Mr Richman professor at Petersburgh above-mentioned. This happened on the 6th of August 1753, as he was making experiments on lightning drawn into his own room. He had provided himself with an instrument for measuring the quantity of electricity communicated to his apparatus; and as he stood with his head inclined to it, Mr Solokow an engraver, who was near him, observed a globe of blue fire, as big as his fist, jump from the instrument, which was about a foot distant, to Mr Richman's head. The professor was instantly dead, and Mr Solokow was also much hurt. The latter, however, could give no particular account of the way in which he was affected; for, at the time the professor was struck, there arose a sort of steam or vapour, which entirely benumbed him, and made him sink down to the ground, so that he could not even remember to have heard the clap of thunder, which was a very loud one. The globe of fire was attended with an explosion like that of a pistol; the instrument for measuring the electricity (called by the professor an electrical gnomon), was broken to pieces, and the fragments thrown about the room. Upon examining the effects of the lightning in the professor's chamber, they found the door-case half split through, and the door torn off and thrown into the room. They opened a vein in the body twice, but no blood followed; after which, they endeavoured to recover life by violent friction, but in vain: upon turning the corpse with the face downwards during the rubbing, an inconsiderable quantity of blood ran out of the mouth. There appeared a red spot on the forehead, from which spurted some drops of blood through the pores, without wounding the skin. The shoe belonging to the left foot was burnt open, and uncovering the foot at that part, they found a blue mark; from whence it was concluded, that the electric matter having entered at the head, made its way out again at that foot. Upon the body, particularly on the left side, were several red and blue spots resembling leather shrunk by being burnt. Many more also became visible over the whole body, and particularly over the back. That upon the forehead changed to a brownish red, but the hair of the head was not singed. In the place where the shoe was unripped, the flocking was entire; as was the coat everywhere, the waistcoat only being singed on the foreflap where it joined the hinder: but there appeared on the back of Mr Solokow's coat long narrow streaks, as if red-hot wires had burned off the nap, and which could not well be accounted for.

When the professor's body was opened next day, the cranium was very entire, having neither fissure nor contra-fissure; the brain was found; but the transparent pellicles of the wind-pipe were excessively tender, and easily rent. There was some extravasated blood in it, as also in the cavities below the lungs. Those of the breast were quite sound; but those towards the back of a brownish black colour, and filled with more of the blood above mentioned. The throat, the glands, and the small intestines, were all inflamed. The singed leather-coloured spots penetrated the skin only. In 48 hours the body was so much corrupted that they could scarce get it into a coffin.

Since the discovery of the identity of lightning and the electric matter, long rods of iron or other metal used for have been made use of with a view to protect buildings preserving from the danger of strokes of lightning. A considerable dispute has been carried on whether these rods ought to be pointed or not; but a committee of the royal society have very lately determined it in favour of the former.

For some time, the science of electricity seems to have been at a stand. Numberless improvements indeed have been made upon what was before discovered, but scarce anything new hath been added. The only thing which can properly be reckoned a new discovery is that of the electrophorus by Signor Volta an Italian, which on many accounts may be reckoned the most surprising machine hitherto invented.

Sect. III. Of the Apparatus necessary for exciting Electricity, and communicating it to other Bodies, &c.

The instruments most in use for this purpose are those called electrical machines, of which there have been so many different forms, that it would be tedious and difficult to give only a very short description of them all. We shall therefore first lay down the most necessary rules for constructing electrical machines in general; and then give a particular description of those machines which are most generally useful, and contain all the improvements hitherto made.

§ 1. Of the Construction of Electrical Machines.

The principal parts of the machine are the electric, Cavalli's moving engine, and the prime conductor, i.e., an Electricity inflated conductor, which immediately receives the electricity from the excited electric.

Formerly, different kinds of electrics were used, as what substances were also various, as globes, cylinders, spheroids, &c. Their forms likewise are various, as globes, cylinders, spheroids, &c. The reason of this variety was, in the first place, that it was not then ascertained what substance acted most powerfully; and secondly, in order to produce a positive or negative electricity at pleasure. At present smooth glass only is used; for when the machine has an inflated rubber, the operator may produce positive or negative electricity at his pleasure, without changing the electric. In regard to the form of the glass, those commonly used at present are globes and cylinders. The most convenient size for a globe, is from nine to twelve inches diameter. They are made with one neck, which is cemented to a strong brass cap in order to adapt them to a proper frame. The best cement for electrical purposes is made with two parts of rosin, two of bees-wax, and one of the powder of red ochre. These ingredients are melted, and mixed together in any vessel over the fire; and afterwards kept for use. This kind of cement sticks very fast; and is much preferable to rosin only, as it is not so brittle, and at the same time inflates equally well. The cylinders are made with two necks; they are used to the greatest advantage without any axis; and their common size is from four inches diameter and eight inches long, to twelve... twelve inches diameter and two feet long, which are perhaps as large as the workmen can conveniently make them. The glass generally used is the best flint; though it is not absolutely determined which kind of metal is the best for electrical globes or cylinders. The thickness of the glass seems immaterial, but perhaps the thinnest is preferable. It has often happened, that glass globes and cylinders, in the act of whirling, have burst in innumerable pieces with great violence, and with some danger to the bystanders. Those accidents are supposed to happen when the globes or cylinders, after being blown, are suddenly cooled. It will therefore be necessary to enjoin the workmen to let them pass gradually from the heat of the glasshouse to the atmospheric temperature.

It has been long questioned, whether a coating of some electric substance, as rosin, turpentine, &c. on the inside surface of the glass, has any effect to increase its electrical power; but now it seems pretty well determined, that if it does not increase the power of a good glass globe or cylinder, at least it does considerably improve a bad one.

The most approved composition for lining glass globes or cylinders, is made with four parts of Venice turpentine, one part of rosin, and one part of beeswax. This composition must be boiled for about two hours over a gentle fire, and stirred very often; afterwards it is left to cool, and reserved for use. When a globe or cylinder is to be lined with this mixture, a sufficient quantity of it is to be broken into small pieces, and introduced into the glass; then, by holding the glass near the fire, the mixture is melted, and equally spread over all its internal surface to about the thickness of a sixpence. In this operation, care must be taken that the glass be made hot gradually, and be continually turned, so that it may be heated equally in all parts, otherwise it is apt to break in the operation.

In respect to the engine which is to give motion to the electric, multiplying wheels have been generally used, which, properly adapted, might give the electric a quick motion, while they are conveniently turned by a winch. The usual method is, to fix a wheel on one side of the frame of the machine, which is turned by a winch, and has a groove round its circumference. Upon the brass cap of the neck of the glass globe, or one of the necks of the cylinder, a pulley is fixed, whose diameter is about the third or fourth part of the diameter of the wheel; then a string or trap is put over the wheel and the pulley; and, by these means, when the winch is turned, the globe or cylinder makes three or four revolutions for one revolution of the wheel. There is an inconvenience generally attending this construction, which is, that the string is sometimes too slack, that the machine cannot work. To remedy this inconvenience, the wheel should be made moveable with respect to the electric, so that by means of a screw it might be fixed at the proper distance; or else the pulley should have several grooves of different radiuses on its circumference.

It has been customary with some, to turn the cylinder simply with a winch, without any accelerated motion; but that seems not sufficient to produce the greatest electric power the glass is capable of giving; for the globe or cylinder should properly make about six revolutions in a second, which is more than can be conveniently done with the winch only. This method, however, on account of its simplicity and easy construction, should not be disregarded, and it may be conveniently used when no very great power is required.

Instead of the pulley and the string as above described, a wheel and pinion, or a wheel and an endless screw, has been also used. This construction may answer tolerably well for small table machines; but it must be constructed with great nicety; otherwise it is apt to make a disagreeable rattling, and, without frequent oiling, soon wears away by the great friction of its parts.

The next thing belonging to the electrical machine necessary to be described, is the rubber which is to excite the electric. The rubber, as it is now made, consists of a cushion of red Basil skin stuffed with hair or flannel, and fastened to a piece of wood well rounded at the edges. To this is glued a flap of Persian black silk, which nearly goes over one half of the cylinder. The method of using the amalgam is by spreading it on a separate piece of leather, and applying it occasionally to the under part of the cylinder while turning. Thus only a very small part of the amalgam is consumed, at the same time that the glass is very strongly excited. The most powerful composition for exciting an electrical cylinder is found to be an amalgam of mercury and zinc, in the proportion of one part of the former to five of the latter. The mercury ought to be previously triturated with some melted grease or bees-wax, by which means the amalgam will be the finer. The composition called Aurum Mucosum, Aurum mysoeum, or Mosaic gold*, will answer very near as well, though somewhat less cleanly and agreeable. The rubber itself should be supported by a spring; by which means it will easily suit any inequalities that may be on the surface of the glass; and by a screw, it support may be made to press more or less as occasion requires. It should likewise be insulated in the most perfect manner; as, when insulation is not required, it may be easily taken off by a chain or wire hung upon it, and thus communicate with the earth or with any electrified body; but where there is no contrivance for insulating the rubber, it is impossible to perform many of the most curious electric experiments. In short, to construct the rubber properly, it must be made in such a manner, that the side it touches in whirling may be as perfect a conductor as it can be made, in order to supply electricity as quick as possible; and the opposite part should be as perfect a non-conductor as possible, in order that none of the fluid accumulated upon the glass may return back to the rubber; which has been found to be the case when the rubber was not made in a proper manner.

Mr William Jones of Holborn, London, instrument-maker, has made a considerable improvement on this part of electrical machines by a very simple contrivance. It consists in a spring placed within the rubber itself; the action of which is found to be better suited for adapting the rubber to the inequalities of the glass, than that placed entirely without the rubber. It consists of a piece of flexible iron or brass, represented edgewise by fig. 1.; and it is evident that it acts in plate a much more parallel and uniform manner than the former,

*See Cleghorn's "History of Electricity," p. 124. former, which is constantly changing the pressure of the line of contact between the rubber and cylinder while it passes from the under to the upper side; and thus rendering the effect inconstant and uncertain.

We come now to consider the prime conductor, or first conductor; which is nothing more than an insulated conducting substance, furnished with one or more points at one end, in order to collect the electricity immediately from the electric. When the conductor is of a moderate size, it is usual to make it of hollow brass; but when it is very large, then, on account of the price of the materials, it is made of pateboard covered with tin-foil or gilt paper. The conductor is generally made cylindrical; but let the form be what it will, it should always be made perfectly free from points or sharp edges: and if holes are to be made in it, which on many accounts are very convenient, they should be well rounded, and made perfectly smooth. Further, that end of the prime conductor which is at the greatest distance from the electric ought to be made larger than the rest, as the strongest exertion of the electric fluid in escaping from the conductor is always at that end.

It has been constantly observed, that the larger the prime conductor is, the longer and denser spark can be drawn from it; and the reason of this is, that the quantity of electricity discharged in a spark, is nearly proportional to the size of the conductor: on this account, the prime conductor is now made much larger than what was formerly used. Its size, however, may be so large, that the diffusion of the electricity from its surface, may be greater than what the electric can supply; in which case, so large a conductor would be nothing more than an unwieldy and disagreeable inconvenience.

Before we quit the electrical machine, it should be observed, that, besides the above-mentioned parts, it is necessary to have a strong frame to support the electric, the rubber, and the wheel. The prime conductor should be supported by stands, with pillars of glass or baked wood, and not by silk strings, which admit of continual motion. In short, the machine, the prime conductor, and any other apparatus actually used, should be made to stand as steady as possible, otherwise many inconveniences will arise.

Besides the electrical machine, the electrician should be provided with glass tubes of different sizes, a pretty large stick of sealing-wax, or a glass tube covered with sealing-wax, for the negative electricity. He should, at least, not be without a glass tube about three feet long and one inch and a half in diameter. This tube should be closed at one end, and at the other end should have fixed a brass cap with a stop-cock; which is useful in case it should be required to condense or rarify the air within the tube.

The best rubber for a tube of smooth glass is the rough side of black oiled silk, especially when it has some amalgam rubbed upon it; but the best rubber for a rough glass tube, a stick of baked wood, sealing-wax, or sulphur, is soft new flannel.

The instruments necessary for the accumulation of electricity are coated electrics; among which, glass coated with conductors obtains the principal place: on account of its strength, it may be formed into any shape, and it will receive a very great charge. The form of the glass is immaterial with respect to the charge Apparatus; it will contain; its thickness only is to be considered; for the thinner it is, the more easily will it receive the utmost charge it can bear; but it is at the same time more subject to be broken: for this reason, therefore, a thin coated jar or plate may be used very well by itself, and it is very convenient for many experiments; but when large batteries are to be constructed, then it is necessary to use glass a little thicker, and care should be taken to have them perfectly well annealed. If a battery is required of no very great power, as containing about eight or nine square feet of coated glass, common pint or half-pint phials may be made use of. They may be easily coated with tin-foil, sheet-lead, or gilt-paper, on the outside, and brass-filings on the inside: they occupy a small space, and, on account of their thinness, hold a very good charge. But when a large battery is required, then these phials cannot be used, for they break very easily; and for that purpose, cylindrical glass jars of about fifteen inches high, and four or five inches in diameter, are the most convenient.

When glass plates or jars, having a sufficiently large opening, are to be coated, the best method is to coat them with tin-foil on both sides, which may be fixed upon the glass with varnish, gum-water, bees-wax, &c., but in case the jars have not an aperture large enough to admit the tin-foil, and an instrument to adapt it to the surface of the glass, then brass-filings, such as are sold by the pin-makers, may be advantageously used; and they may be stuck on with gum-water, bees-wax, &c., but not with varnish, for this is apt to be set on fire by the discharge. Care must be taken that the coatings do not come very near the mouth of the jar, for that will cause the jar to discharge itself. If the coating is about two inches below the top, it will in general do very well: but there are some kinds of glass, especially tinged glass, that when coated and charged, have the property of discharging themselves more easily than others, even when the coating is five or six inches below the edge. There is another sort of glass, like that of which Florence flasks are made, which, on account of some unvitrified particles in its substance, is not capable of holding the least charge. On these accounts, therefore, whenever a great number of jars are to be chosen for a large battery, it is advisable to try some of them first, so that their quality and power may be ascertained.

Electricians have often endeavoured to find some other electric, which might answer better than glass for the purpose, at least be cheaper; but, except Father answering Beccaria's method, which may be used very well, no purpose remarkable discovery has been made relating to this point. He took equal quantities of very pure colophonium, and powder of marble fitted exceedingly fine, and kept them in a hot place a considerable time, where they became perfectly free from moisture: he then mixed them, and melted the composition in a proper vessel over the fire; and, when melted, poured it upon a table, upon which he had previously stuck a piece of tin-foil, reaching within two or three inches of the edge of the table. This done, he endeavoured with a hot iron to spread the mixture all over the table as equally as possible, and to the thickness of one-tenth of an inch: he afterwards coated it with another piece of tin-foil reaching within about two inches of the edge. edge of the mixture: in short, he coated a plate of this mixture like a plate of glass. This coated plate, from what he says, seems to have had a greater power than a glass plate of the same dimensions, even when the weather was not very dry; and if it is not subject to break very easily by a spontaneous discharge, it may be very conveniently used; for it does not very readily attract moisture, and consequently may hold a charge of electricity better, and longer, than glass; besides, if broken, it may be repaired by a hot iron; but glass, when broken, cannot so easily be repaired.

When a jar, a battery, or in general a coated electric, is to be discharged, the operator should be provided with an instrument called the discharging rod, which consists of a metal rod sometimes straight, but more commonly bended in the form of a C: they are made also of two joints, so as to open like a kind of compasses. This rod is furnished with metal knobs at its extremities, and has a non-conducting handle, generally of glass or baked wood, fastened to its middle. When the operator is to use this instrument, he holds it by the handle; and touching one of the coated sides of the charged electric with one knob, and approaching the other knob to the other coated side, or some conducting substance communicating with it, he completes the communication between the two sides, and discharges the electric.

The instruments to measure the quantity, and ascertain the quality, of electricity, are commonly called electrometers, and they are of four sorts: 1. The single thread; 2. The cork or pith balls; 3. The quadrant; and, 4. The discharging electrometer. The second sort of electrometer, i.e., the cork-ball electrometer, was invented by Mr Canton; the discharging electrometer was invented by Mr Lane, and hath been improved by Mr Henley; another on a different principle by Mr Kinnersley; and the quadrant electrometer, which is of latest invention, is a contrivance of Mr Henley.

Besides the apparatus above described, there are several other instruments useful for various experiments; but these will be described occasionally. The electrician, however, ought to have by him, not only a single coated jar, a single discharging rod, or, in short, only what is necessary to perform the common experiments; but he should provide himself with several plates of glass, with jars of different sizes, with a variety of different instruments of every kind, and even tools for constructing them; in order that he may readily make such new experiments as his curiosity may induce him to try, or that may be published by other ingenious persons who are pursuing their researches in this branch of philosophy.

§ 2. Description of the most useful Electrical Machines.

The first which may be mentioned is that described by Dr Priestley in his history of electricity; which, on account of its extensive use, may be deservedly called a universal electrical machine.—The basis consists of two oblong boards a a, which are placed in a situation parallel to one another, about four inches asunder, and kept in that position by two pieces of wood adapted for the purpose. These boards, when set horizontally on a table, and the lowermost of them fixed with iron cramps, form the support of two perpendicular pillars of baked wood, and of the rubber of the machine. One of the pillars, together with the spring supporting the rubber, slides in a groove a, which reaches almost the whole length of the upper board; and, by means of a screw, may be placed at any required distance from the pillar b, which is fixed, being put through a mortice in the upper board, and fastened to the lower. In these two pillars are several holes for the admittance of the spindles of different globes; and as they may be situated at any distance from one another, they may be adapted to receive not only globes, but cylinders and spheroids of different sizes. "In several of these machines (says Dr Priestley), more than one globe or cylinder may be used at once, by fixing one above the other in the different holes of the pillars; and by uniting their adapting to each a proper pulley, they may be whirled all at once, to increase the electricity." But this construction has one capital defect, that rubbers cannot be conveniently applied; so that the power of several globes put together in this manner, though greater than one, is by no means equal to what it would be if the power of them all taken singly were united. Fig. 3 shows a machine of this kind contrived by Dr Watson.

The rubber ought to be made as above directed. It is supported by a socket which receives the cylindrical axis of a round and flat piece of glass or baked wood g, the opposite part of which is inserted into the socket of a bent steel spring b. These parts are easily separated, so that the rubber, or the piece of wood that serves to infuse it, may be changed at pleasure. The spring admits of a twofold alteration of position; being capable of either slipping along the groove, or moving in the contrary direction, the groove being wider than the screw that fastens the spring, so as to give it every desirable position with regard to the globe or cylinder; and it is besides furnished with a screw which makes it press harder or lighter as the operator chooses. The wheel of this machine is fixed to the table at c, and has several grooves for admitting more strings than one, in case that two or three globes or cylinders are used at a time; and as it is disengaged from the frame of the machine, the latter may be screwed at different distances from the former, and so would be suited to the variable length of the string. The chain connected with the rubber at n is for making a communication with the table, when inflation is not wanted. The prime conductor is made of copper, hollow, and in the form of a pear; having its neck placed upwards, and its bottom, or rounded part k, placed on a stand of glass or baked wood. An arched wire l proceeds from its neck, having an open ring at its end, in which some small pointed wires m are hung, that by playing lightly on the globe or cylinder collect the electric fluid from it.

Next to Dr Priestley's machine is one invented by Dr Ingenhousz, and which for its simplicity and convenience makes a fine contrast with the former.—This machine consists of a circular glass-plate about one foot diameter, which is turned vertically by a winch fixed to the iron axis that passes through its middle; and it is rubbed by four cushions, each about two inches long, situated at the opposite ends of the vertical diameter. The frame consists of a bottom board, about a foot square, or a foot long and six inches broad, which, when the machine is to be used, may be fastened by an iron crank to the table. Upon this board two other slender and smaller ones are raised, which lie parallel Apparatus. to one another, and are fastened together at their top by a small piece of wood. These upright boards support in their middle the axis of the plate, and to them the rubbers are fastened. The conductor is of hollow brafs; and from its extremities branches are extended, which, coming very near the extremity of the glass, collect the electricity from it.

The power of this machine is perhaps more than a person would imagine by looking at it. It may be objected, that this construction will not easily admit of the rubbers being insulated, nor consequently be adapted to a great variety of experiments: but at the same time it must be allowed, that it is very portable, that it is not very liable to be out of order, and that it has a power sufficiently strong for physical purposes; on which account it may be conveniently used.

Fig. 4. represents a very portable electrical machine invented by Mr Read, and improved by Mr Lane. A is the glass cylinder, moved vertically by means of the pulley at the lower end of the axis. This pulley is turned by a large wheel B which lies parallel to the table. There are three pulleys of different dimensions marked in the figure; one of which revolves four times for every revolution of the large wheel B. The conductor C is furnished with points to collect the fluid, and is screwed to the wire of a coated jar D, which stands in a socket between the cylinder and the wheel. The figure also represents the manner of applying Mr Lane's electrometer to this machine; of which an account shall be given afterwards.

Electrical machines have of late years undergone some very essential alterations and improvements; both from the suggestions of private electricians and the inventions of Messrs Adams, Nairne, and Jones, instrument makers of London. We shall subjoin a description of the most approved ones.

Fig. 5. represents a most convenient machine for philosophical purposes, and whose power is equal to that of much larger ones of the old construction. The frame of this machine consists of the bottom board ABCD; which, when the machine is to be used, must be fastened to the table by two brafs or iron cramps made for that purpose. Upon the bottom board there are two round pillars EF perpendicularly raised; which will be found after the purpose if made of baked wood. These serve to support the cylinder G by the axles of the brafs or wood caps H. From one of these proceeds the long axle H, going through an hole in the pillar F; having a simple winch I fixed on its square end; or sometimes, as in fig. 6. below a pulley J. On the circumference of this pulley are several grooves in order to suit the variable length of the string a, which goes round one of them, as well as round the large multiplying wheel A. The other cap of the cylinder has a small cavity which fits the conical extremity of a strong screw proceeding from the pillar. The wheel A, which is moved by the handle, turns round a strong axle proceeding from about the middle of the same pillar. In small machines the simple winch may be adopted with great advantage, as is represented fig. 5. as not being liable to disorder; but in large ones the multiplying wheel is indispensably necessary.

In all these machines the rubber is composed of a cushion stuffed with horse-hair or flannel, fastened to a board behind. It is covered with red Basil leather; and from its under edge a piece of black Persian silk is glued, which goes over the cylinder as at a, fig. 5. to near the points of the collector fixed in the conductor. Thus a greater power of electricity is excited than what could have been done by the former machines. In them a piece of leather was fastened to the lower edge of the cushion, bearing against the cushion itself. To this piece of leather another of oiled silk was sewed, covering the cylinder as above described. In this way some of the amalgam above described was to be laid upon the piece of leather, and worked into its substance as much as possible; but in the present method nothing more is necessary than to hold an amalgamated piece of leather once or twice to the cylinder while turning. The rubber is fixed to a glass pillar K (fig. 5.) which is fastened into a wooden base L at the bottom. This turns on an hinge; and by means of a screw at M, going through the base to a fixed block on the frame, the pressure of the cushion may be augmented or diminished at pleasure; at the same time that it is rendered much more steady and uniform than by a flat sliding board and tightening screw as formerly used.

The glass pillar K, as well as all other glass pillars, the glass feet of inflating stools, &c. should be covered with varnish or rather sealing-wax; otherwise they will inflate very imperfectly on account of the moisture they attract from the air in damp weather. It was usual to support the rubber upon two springs screwed to its back, and which proceeded from the wooden cap of the pillar; in order to give way to and avoid the inequalities of the glass; but by this contrivance the line of contact with the cylinder was not always the same, nor its pressure uniform, as already observed: but Mr William Jones has removed this difficulty by the bent spring represented fig. 1. It is fixed by a screw at B, and gives way by sliding notches at a a; its length and breadth are equal to that of the cushion, and its thickness proportional to the diameter and action of the cylinder upon it. In the machine above described, the rubber is well inflated, which is a great advantage when it is necessary to connect with the cushion a conductor, called the negative conductor; and when this happens not to be the case, which usually is in making the common experiments, a chain with a small hook and ring may be hung to one end of the conductor, the other falling upon the table as in fig. 5.

The prime conductor belonging to this machine is represented by N in the same figure. It receives the electric fluid from the cylinder, and is usually made of brass or tin japanned. It is insulated by the glass pillar that supports it, and which is screwed into a wooden base or foot. It is found more convenient to place the conductor parallel to the cylinder than with one of its ends towards it as was formerly done.

The handle of the wheel A, fig. 6. or the simple winch I, fig. 5. should be so turned, that the excited part of the cylinder may revolve from the rubber to the collecting points on the conductor; the prime conductor, standing then as in the figures, will be electrified positively, or overcharged with the electric fluid: for by the action of rubbing, the cylinder pumps, as it were, the fluid from the rubber, and every other body properly connected with it, and gives it to the prime prime conductor. But if negative electricity be required, the chain must be removed from the rubber, and hung to the prime conductor; for in this case, the electricity of the prime conductor will be communicated to the ground, and the rubber remaining insulated will appear strongly negative. If another conductor, equal in size to N, be connected with the rubber, as strong negative electricity may be obtained from the one as positive electricity from the other.

Fig. 6. represents an electrical machine with a conductor in the shape of a T; and an improved medical apparatus, where it is necessary to give the shock in the arms, will be more particularly explained afterwards, under the article Medical Electricity.

Fig. 7. shows Mr Nairne's patent electrical machine for medical purposes. Its principal parts are the glass cylinder, generally about 7 inches in diameter and 12 inches long, with the two conductors parallel to it. It is furnished with wooden caps, and turns in two wooden pieces cemented on the top of two strong glass pillars BB. These pillars are made fast into the bottom board of the machine, which is fastened to the table by means of a crank. There are grooves made in the under part of the bottom of the crank, through which the pieces FF slide. On these pieces the pillars stand by which the two conductors are supported; and in order to place these conductors nearer to the cylinder, or remove them farther from it, the pieces on which they stand are moveable outwards or inwards, and may be fixed by the two screw-nuts LL. The rubber is fastened to the conductor R; and consists of a cushion of leather stuffed, having a piece of silk glued to its under part. This last being turned over the surface of the cushion, and thus interposed between it and the glass, goes over the cylinder, and almost touches the pointed wires which are fixed on the other conductors for the purpose of collecting the electric fluid from the cylinder. The conductors are of tin covered with black lacquer, each of them containing a large coated glass jar, and likewise a smaller one, or a coated tube, which are visible when the caps NN are removed. To each conductor is fixed a knob O, for the occasional suspension of a chain to produce positive or negative electricity. That part of the winch C which acts as a lever in turning the cylinder, is of glass. Thus every part of the machine is insulated, the cylinder itself and its brass caps not excepted; by which means the least quantity possible of electric fluid is dissipated, and hence of course the effects are likely to be the more powerful. And to this the inventor has adapted some flexible conducting joints, a discharging electrometer, and other utensils necessary for the practice of medical electricity.

To these descriptions of electrical machines, we shall add that of a very large and powerful one in Teyler's Museum at Haarlem, and which was constructed by one Mr John Cuthbertson, an English mathematical instrument-maker. It consists of two circular plates of glass, each 65 inches in diameter, and made to turn upon the same horizontal axis, at the distance of 7½ inches from one another. These plates are excited by eight rubbers, each 15½ inches long. Both sides of the plates are covered with a refractory substance to the distance of 16½ inches from the centre, both to render the plates stronger, and likewise to prevent any of the electricity from being carried off by the axis. Apparatus. The prime conductor consists of several pieces, and is supported by three glass pillars 57 inches in length. The plates are made of French glass, as this is found to produce the greatest quantity of the electricity next to English flint, which could not be produced of sufficient size. The conductor is divided into branches which enter between the plates, but collect the fluid by means of points only from one side of the plate. The force of two men is required to work this machine; but when it is required to be put in action for any length of time, four are necessary. At its first construction nine batteries were applied to it, each having 15 jars, every one of which contained about a foot square of coated glass; so that the grand battery formed by the combination of all these contained 135 square feet. The effects of this machine were astonishing, as shall be mentioned in its proper place: but Dr Van Marum, who principally made experiments with it, imagining that it was still capable of charging an additional quantity of coated glass, afterwards added to it 90 jars of the same size with the former; so that it now contains a coated surface of 225 feet, and the effects are found to be proportionable.

We come now to describe some of the other parts of an electrical apparatus, and which, though not essentially necessary for exciting the property called electricity, are absolutely so for communicating it from one body to another, and performing many experiments which the machines themselves, however powerful, could not accomplish. Of these, the first we shall describe is that called the discharger; by which the electricity, whether positive or negative, collected upon one body, may be suddenly transferred from it to another; which is called discharging the electricity of the former, if only one body be perceptibly electrified; or of both, if the one contain positive and the other negative electricity.

Fig. 8. represents Mr Henley's universal discharger; Plate an instrument of very extensive use in forming communications between jars or directing the shock through any particular substance. AB is a flat board 15 inches long, 4 broad, and 1 thick, and forming the basis of the instrument. DC are two glass pillars cemented in two holes upon the board AB, and furnished at their tops with brass caps; each of which has a turning joint, and supports a spring tube, through which the wires EE and TT slide. Each of these caps is composed of three pieces of brass, connected with each other in such a manner, that the wire EE, besides its sliding through the socket, has two other motions, viz. an horizontal one and a vertical one. Each of the wires is furnished with an open ring at one end, and at the other has a brass ball; which, by a short spring socket, is slipped upon its pointed extremity, and may be removed from it at pleasure. HG is a strong circular piece of wood five inches diameter, having a lip of ivory inlaid on its surface, and furnished with a strong cylindric foot, which fits the cavity of the socket I. This socket is fixed in the middle of the bottom board, and has a screw at K; by which the foot of the circular board is made fast at any required height.

Fig. 9. is a small press belonging to this instrument. It consists of two oblong pieces of wood, which which are forced together by the two screws \(aa\). The lower end has a cylindrical foot equal to that of the circular table \(H\). When this pres is to be used, it must be fixed into the socket \(I\), in place of the circular board \(HG\); which in that case is to be removed.

Fig. 10 shows an electrical jar or Leyden vial, for the purpose of giving a shock, or of accumulating a quantity of electricity in such a manner as could not be done in any other way, without using an immense extent of electrified surface. It is coated on the inside with tin-foil to the height of about three inches below the top of the cylindrical part of the glass; and having a wire with a round brass knob at its extremity, which passes through the middle of a piece of wood \(D\), is used as a stopper for the bottle. Its lower end is usually connected with the inside coating by means of a piece of chain or slender wire.

Fig. 11 shows the most approved construction of an electrical battery; a part of the apparatus which takes its name from its construction and formidable effects. It consists of a number of coated jars, placed in such a manner that they may all be charged at the same time, and discharged in an instant; so that the whole power of electricity accumulated in them may be at once exerted upon the substance exposed to the shock.

The battery represented in the figure consists of nine jars connected together by the wires \(a, b, c, d, e, f, g, h, i\); all of which are fastened into the wood-stoppers of the bottles, and meet at top in the brass ball. Thus a communication is made between all the inside coatings of the jars, while their outside coatings are connected by the bottom of the box on which they stand; and which, that it may conduct the better, is covered with tin-foil. In one side of the box near the bottom is a hole through which a brass hook passes, and which communicates with the metallic lining of the box, and consequently with the outside coating of the jars. To this hook a wire or chain is occasionally connected when a discharge is made; and for the more convenient making of this discharge, a ball and wire \(B\) proceed to a convenient length from the centre ball \(A\). When the whole force of the battery is not required, one, two, or three jars may be removed only by pressing down the wires belonging to them, until their extremities can slip out of their respective holes in the brass ball, and then turning them into such a posture that they cannot have any communication with the battery. The number of jars represented in this figure is rather small for some purposes; but it is better to join two or three small batteries together rather than have a single large one, which is inconvenient on account of its weight and unwieldiness.

The construction of jars and batteries is part of the business of an electrician; and he ought to be expert in coating the vials himself, not only because of the expense attending the employment of others, but because he may sometimes be at too great a distance from workmen who are accustomed to operations of this kind. A considerable difficulty arises with respect to the size of the jars and the kind of glass they are to be made of. Fine flint or crystal glass may probably be made use of with greater advantage than any other; but the expense here becomes a very considerable object, especially as the jars of a battery are very apt to break by reason of the inequality of their strength; for it would seem that the force of the fluid in a battery is equally distributed among all the bottles, without any regard to their capacities of receiving a charge singly considered. Thus, if we express the quantity of charge which one jar can easily receive by the number 10, we ought not to combine such a jar in a battery with another whose capacity is only 8; because the whole force of electricity expressed by 10 will be directed also against that whose capacity is only 8; so that the latter will be in danger of being broken. It will be proper, therefore, to compare the bottles with one another in this respect before putting them together in a battery. Besides the consideration of the absolute capacity which each bottle has of receiving a charge, the time which is taken up in charging it must also be attended to; and the jars of a battery ought to be as equal as possible in this respect as well as in the former. The thinner the glass is, the more readily it receives a charge, and vice versa; but it doth not follow from thence, as electricians in general imagine till lately, that, on account of its thinness, it is capable of containing a greater charge than a thicker one. The reverse is actually the case; and though a thick glass cannot be charged in such a short time as a thin one, it is nevertheless capable of containing a greater power of electricity. If the thicknesses of the glass be very great, no charge can indeed be given it; but experiments have not yet determined how great the thickness must be which will prevent any charge. Indeed it is observed, that though a thick glass cannot be charged by a weak electric machine, it may be so by a more powerful one; whence it seems reasonable to suppose that there is no real limit of this kind; but that if machines could be made sufficiently powerful, glasses of any thickness might be charged. Mr Brookes, an ingenious electrician of Norwich, constructed his batteries, which appear to have been very powerful, of green-glass bottles. Some of them, like that represented in the figure, had only nine of these bottles; but when a greater power was wanted, more were added. Jars would have been preferred to bottles on account of their being more easily coated by reason of their wide mouths; but being less easily procured, he was content to put up with this inconvenience. The mean size of these bottles was about eight inches in diameter; they were coated 10 inches high, and made of the thickest and strongest glass that could be procured, weighing from five pounds and a half to seven pounds each. In the construction of a battery of 27 bottles, he disposed of them in three rows; nine of the stoutest and best composing the first row, nine of the next in strength being disposed of in the second, and the third containing the nine weakest. All of these were of green glass, but not of the same kind. Some which stood in the foremost row were composed of a kind very like that of which Frontinian wine-bottles are made; and our author remarks, that this kind of glass seems to be by much the best, as being both harder and stronger, and less liable to break by an high charge. The second and third rows of the battery consisted of bottles whose diameter was from six and a half to ten inches, and which were coated from eight and a half to eleven inches high; none of their mouths being larger than Sect. IV.

Apparatus. an inch and an half, nor less than three quarters of an inch. In case any of the bottles being broken by the discharge of the battery, Mr Brookes found that it could be mended in such a manner as to become serviceable by a cement made according to the following receipt: "Take of Spanish-white eight ounces; heat it very hot in an iron ladle, to evaporate all the moisture; and when cool, sift it through a lawn sieve: add three ounces of pitch, three quarters of an ounce of rosin, and half an ounce of bees-wax: heat them all together over a gentle fire, stirring the whole frequently for near an hour; then take it off the fire, and continue the stirring till it is cold and fit for use." The bottles cemented with this composition, however, were not judged to be sufficiently strong to stand in their original place, but were removed to the second or third row, as it was apprehended they could best sustain the charge. All the bottles of this battery, as well as the single ones he commonly made use of in his experiments, were coated both on the inside and outside with slips of tin-foil from three-eighths to three-fourths of an inch wide, laid on with paste of flour and water, at the distance of about the breadth of a slip between each.

Fig. 12. represents the insulating stool, a very useful part of the apparatus, especially for medical purposes, where it is often necessary to insulate the human body. In these cases it is proper to have it of a magnitude sufficient to hold a chair or other seat, on which the patient may sit during the operation. The stool itself may be conveniently constructed of a mahogany board with glass feet varnished, as already directed. When in use, the insulation will be the more perfect that a piece of dry paper be put upon it.

These are the parts of the electrical apparatus essentially necessary for exhibiting the ordinary experiments; but as many very curious phenomena are to be observed in different substances, without using any part of the apparatus above described, we shall next proceed to give an account of those bodies which naturally exhibit signs of electricity, with the various phenomena attending them.

Sect. IV. A Catalogue of the different Electric Substances, with the general Phenomena attending their Excitation.

The list of substances by which electric phenomena may be produced, is so very extensive, that it may perhaps be doubted whether all terrestrial matters, metals and charcoal only excepted, may not be included in the number. Some, however, have the property much more, or exhibit particular phenomena more obviously, than others; and according to this we may divide them into classes, as shall afterwards be more particularly noticed. The following catalogues enumerate those in which the property in general has been discovered.

| Electric substances | Quality of electricity | Substances with which the electric is rubbed | |---------------------|------------------------|---------------------------------------------| | The back of a cat | Positive | Every substance hitherto tried | | Smooth glass | Positive | Every substance, except the back of a cat |

This table contains most of those substances that exhibit the strongest marks of electricity. The following is composed by Mr Henley, and contains a great number of substances whose electricity is much more equivocal. They were fixed or tied on the end of a stick of sealing-wax, and excited by friction against a woollen garment, or a piece of soft black silk, by which means they became electrified as below. The strongest in power are distinguished by the letter s, and the weakest by the letter w.

**Metals.**

- A new guinea; a smooth sixpence; a brass ferule; tin, and tin-foil; enamelled copper, s; gilding on leather, s; lead ore; copper ore; iron ore; stream tin. - Milled lead; copper, s; a polished steel button, s; a new silver ditto; a metal button gilt, s; tutenague ditto, s; iron. - Lead from a tea-chest, in which there is a mixture of tin, w. - A gilt button, basket-pattern; the juncture at the end of a brass ferule.

**Animal Substances.**

- Tortoise-shell, w; ivory, s; bone, s; horn; lamb's-tooth; horse's-hoof; deer's-hoof; muscle of the leg of a deer, s; cartilage, s; spur of a young cock; bill, claw, and scale from the leg of...

Phenomena.

of a turkey, s; scale of a carp; the chrysalis of a moth, recent from the earth, cleaned; crassifemurum of the human blood exsiccated, w; quills; claw of an unboiled lobster; cowrie and several other smooth shells, s; shell of a hen's egg; tail of a small fish; thigh of the elephant beetle; a small beetle, smooth surface; human hair; red and white horse's and bullock's hair, s; hog's bristles, s; wool; silk from the worm, w; oyster-shell, smooth surface; Mother of pearl, and several other shells.

Muscle and cockle-shells, recent; a recent snail-shell, rough surface; elytra of the flag-beetle; oyster-shell, rough surface.

VEGETABLES.

Rind of chestnut, s; Barcelona nut-shell, s; cashew nut, s; cocoa nut-shell polished; Brazil; lignum vitae; black ebony, s; box, w; cane, s; guaiacum, or Peruvian bark, s; tamarind-stone; coffee-berry roasted, s; nutmeg, s; ginger, s; white pepper, freed from the hulks, s; cinnamon, s; cloves, s; mace, s; all-spice, s; capricium; both sides of the pod, s; hemlock, s; a clove of garlic; ditto of echalot, freed from the hulks, s; a green onion, s; rue, s; cork, s; leaves of laurel, bay, yew, holly, rosemary, with their berries, s; parsley, s; leaf of turnip; ditto of Savoy cabbage, s; celery, s; fago, s; thyme, s; carrot; turnip; potato; an acorn, s; rind of Seville orange, s; a large Windsor bean, s; a white pea; root of the white lily; snow-drop root; seeds of gourd, melon, cucumber, w; a species of long mofs, w; an apple, s; down of the cotton-rush, w; sea-flag; leaf of the American aloe, s; cotton, w.

Hemp; flax; stalk of the tobacco-leaf; spike, from the leaf of the American aloe; palma-chrisi nut; horfe-radish.

A white kidney-bean, smooth surface; black negroe of the same; scarlet of the same.

CORALLINES.

Sea-fan, the horny part, w; rough coral, w.

Spunge, w; coral polished, w.

SALTS.

Alum, w.

Borax,

Nitre purified,

Fossil and Mineral Substances.

Common pebble-stones of all colours, s; marble, s; pit-coal, s; black-

WOOL.

Silk.

lead, w; jet, s; asbestos; mineralized sulphur; thunder-bolt stone; cornuammonis; shark's-tooth; coat of petrifaction.

Several smooth native crystals; brown Iceland ditto; sale, s; Ceylon pebble, smooth and transparent; agate, s; corneian; amethyst, s.

A specimen of gypsum.

ARTIFICIAL SUBSTANCES.

Staffordshire ware, glazed; China ware, s; Wedgwood's ware glazed, s; whale's fin prepared, w; writing-paper; parchment, s; sheep's gut.

Tobacco-pipe, s; Wedgwood's ware unglazed; elastic gum, s; hard under-crust of a leaf; a tallow-candle, w; oiled silk; painted paper, s; silver, Neg.; burnt into glass, unburnished; pearl-barley, w; Indian ink, w; blue vitriol, s.

Dr Lewis's glass porcelain.

Here it must be observed, that a great number of the substances in Mr Henley's table, particularly metals, would have been totally incapable of excitation had they not been insulated; and as they were rubbed against electrics per se, it is by no means fair to conclude that the metal was excited. It seems much more likely that the rubber only was excited, and communicated its electricity to the metal. It must also be observed, that though there is a very remarkable difference between substances with regard to their non-electric or conducting power, yet there seems not to be a perfect electric in nature: for heat will destroy the electric power of glass, and every other substance; and, on the contrary, cold, if not attended with moisture, renders every electric substance more electric than before. The use of warming an electric therefore, before excitation, is only to free it from the moisture which may adhere to it.

From the above catalogues it will readily be apprehended, that the powers of the electric substances not only vary prodigiously from one another, but likewise according to the circumstances in which they are placed. Thus also we find, that, according to the different substances made use of, we may sometimes produce one phenomenon and sometimes another, in a manner exclusive of all the rest. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit, and which we shall do in the following manner.

1. Those which exhibit a strong and permanent attractive and repulsive power; of which the most remarkable is silk.

2. For exhibiting the electric light, attraction and repulsion, and all the other phenomena of electricity in a very vigorous though not durable manner, glass is preferable to all other bodies.

3. Those which exhibit electric appearances for a great length of time, and which communicate to conducting bodies the greatest electric power. Of these the substances called negative electrics are the most remarkable; markable; such as amber, gum-lac, rosin, sulphur, &c., on the properties of which depend the phenomena of the electrophorus, to be afterwards described.

4. Those which readily exhibit electrical phenomena by heating and cooling, of which the principal is the tourmalin.

§ 1. Of the Electrical Phenomena from Silk.

This substance was first discovered to be an electric by Mr Grey, in the manner we have already related*, but as it was by no means remarkable for emitting sparks, which most commonly engages the attention, its electric virtues were almost entirely overlooked till the year 1759. At that time Mr Symmer presented to the royal society some papers containing a number of very curious experiments made with silk stockings, in substance as follows.

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

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

When the stockings were suffered to meet, they stuck together with considerable force. At first Mr Symmer found they required from one to 12 ounces to separate them. Another time they raised 17 ounces, which was 20 times the weight of the stocking that supported them; and this in a direction parallel to its surface. When one of the stockings was turned inside out, and put within the other, it required 20 ounces to separate them; though at that time 10 ounces were sufficient when applied externally. Getting the black stockings new dyed, and the white ones washed, and whitened in the fumes of sulphur, and then putting them one within the other, with the rough sides together, it required three pounds three ounces to separate them. With stockings of a more substantial make, the cohesion was still greater. When the white stocking was put within the black one, so that the outside of the white was contiguous to the inside of the black, they raised nine pounds wanting a few ounces; and when the two rough surfaces were contiguous, they raised 15 pounds one pennyweight and a half. Cutting off the ends of the thread and the tufts of silk which had been left in the inside of the stockings, was found to be very unfavourable to these experiments.

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

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

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

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

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

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

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

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

§ 2. Of the Phenomena produced by excited or electrified Glass.

That glass is an electric substance, was first discovered by Dr Gilbert. It was for a long time, however, thought to possess but a very weak electric virtue; though now it is found to be one of the best, if not the very best, electric as yet known. Notwithstanding the many experiments made upon this substance, it is not yet ascertained what kind of glass is most proper for equal purposes. It has been observed, that the hardest and most completely vitrified glass is often a very bad electric, being sometimes quite a conductor. Glass vessels made for electrical purposes are often rendered fit for them by use and time, though very bad electrics when new. Mr Bergman of Upsal says, that very often, when his glass globes could not be excited to a sufficient degree of strength, he lined them with a thin coating of sulphur, and that then they gave a much stronger positive electricity than before. In Italy, and other places, according to Mr Nollet, it is the custom of electricians to put a coating of pitch or other resinous matter on the inside of their globes, which they say always makes them work well. He gives the preference to the crystal glass of England, Bohemia, &c. It seems doubtful, however, whether the common bottle glass does not answer equally well, or even better.

The most remarkable phenomenon producible by excited glass is that of the Leyden vial. It depends entirely upon the following property of glass, viz. that it is impossible to electrify the outside of a glass positively, at least to any considerable degree, without at the same time electrifying the inside of it negatively: in like manner, it is impossible to electrify the outside negatively; without at the same time electrifying the inside positively. It is also the nature of glass and all other electric substances, when once electrified either by excitation or communication, munication, to part with their electricity very slowly and gradually. Thus, supposing a tube, cylinder, or plate of glass, to be highly electrified; if a finger is brought near any part of it, a spark will be felt to strike the finger with a snapping noise. Part of the electricity will then be discharged from the glass, but not all. If the finger is brought near another part of the glass, a similar spark will be again produced; and so on, by moving the finger to different parts of the glass, till all its electricity is exhausted. It is the nature of conducting substances to discharge all their electricity at once, by a single spark, if another conducting substance is brought near them. This being the case, therefore, it follows, that if every part of one side of a glass plate is covered over with a conducting substance, every point of the glass will give out its electricity to the conductor; and consequently, if another conducting substance is brought near to that by which the glass is covered, the whole electric power in the glass ought to be discharged in one single flash or large spark.

This would no doubt be the case, if it was possible to electrify the glass only on one side. But this is found to be impossible. No method hath yet been found of electrifying one side of a piece of glass positively, without electrifying the other negatively at the same time. There is therefore a necessity for taking off the electricity from both sides of the glass at the same time. This can only be done by covering both sides of the glass with a conducting substance, and presenting other conductors to both sides at the same time; then the electricity of both is discharged in an instant. A strong spark is perceived between both sides of the coated glass and the conducting substances; and if a person holds one in each hand, he will, at the instant of the discharge, feel a very disagreeable sensation, which cannot well be described, in his arms and breast; and this is said to be receiving the electric shock.

If, instead of presenting a conducting substance to both sides of the plate at once, a finger is presented to one side, suppose that which is positively electrified, and another substance very highly electrified positively is presented to the negative side of the glass, a like discharge will ensue, but the shock will be much gentler than in the former case, and probably the electricity of the glass will not be all discharged. If two conducting substances, insulated, suppose two cylinders of metal fixed upon sticks of sealing wax, or suspended by silk threads, are brought to the sides of the coated glass at the same time; each of them will receive a spark of positive or negative electricity, according as the side to which it was applied is positively or negatively electrified. When the metallic cylinders are taken away, they will communicate the electricity they have received to other bodies; and if again applied to the coated glass, they will receive sparks as before; and thus the electricity of both sides will be gradually discharged.

After the discharge has been once made, the glass is found in a short time to recover its electricity, though in a small degree. The side which was originally electrified positively, becomes electrified in the same manner the second time, and so of the negative side. This second electrification is called the residuum of a charge; and, where there is a large surface of coated glass, hath a very considerable degree of power. The same thing, which we have just now observed with regard to a flat surface of glass, takes place with tubes and vials, or glass vessels of any kind; and it is always observed, that the thinnest glass answers best for this purpose. The Leyden vial consists of a glass vial, jar, or bottle, covered on the outside and inside with tin-foil, yet leaving an interval of two or three inches at top without any metallic covering, that the electricity of the one side may not be communicated to the other as fast as it is collected. A more particular description of it will be given when we speak of the electric apparatus. The above will be sufficient to render the following experiments intelligible.

Mr Symmer, when making the experiments we have already related, concerning the strong cohesive power of electrified silk, was induced to try the cohesive power of electrified glass. For this purpose, he got Symmer two panes of common window-glass, the thinnest and smoothest he could meet with. He coated one of the sides with tin-foil, leaving a space uncovered near the edges. The uncovered sides were then put together, and electricity communicated to one of the coatings by means of a machine. In consequence of this, the other side, which was also coated, became electrified with an electricity opposite to the first, and both panes were charged with the electric power, as if they had been but one. After they had received a considerable degree of electric power, they cohered pretty strongly together, but he had no apparatus by which the strength of their cohesion could be measured. He then turned the plates upside down; and discharging from his machine positive electricity upon the negative side of the glass, both panes were immediately discharged, and their cohesion ceased. Placing two panes of glass, each of them coated on both sides, one upon the other, each of them had a positive and negative side, by communicating electricity to one of them, and they did not cohere.

In consequence of these experiments made by Mr Symmer, and another (which we shall presently give an account of) made at Pekin, Mr Beccaria made the following ones. Having charged a coated plate of caria glass, he took off the coating from the negative side, and applied another uncoated and uncharged (or un-electrified) plate of glass close to it. After this, putting a coating upon the uncharged glass (so that the whole resembled one coated plate consisting of two laminae), he made a communication between the two coatings. The consequence of this was an explosion, a discharge of the positive and negative electricity, and a cohesion of the plates. If the plates were separated before the explosion, after they had been in conjunction for some time, the charged plate was positive on both sides, and the uncharged one negative on both sides. If after the explosion he separated and joined them alternately, a small circle of paper, placed under the uncharged plate, adhered to it upon every separation, and was thrown off again upon every conjunction. This could be repeated even 500 times with once charging the plate. This is the experiment made at Pekin as above mentioned.

If, in these experiments, the charged plate was inverted, and the positive side applied to the uncharged plate, all the effects were exactly the reverse of the former. If it was inverted ever so often, after remain- ing some time in contact with the uncharged plate, it would produce a change in the electricity. In the dark, a light was always seen upon the separation of these plates. Laying the two plates together like one, and coating the outsides of them, he discharged them both together; and at the distance of about four feet he distinguished six of the coloured rings mentioned by Sir Isaac Newton, all parallel to one another, and nearly parallel to the edge of the coating. At the angles of the coatings the rings spread to a greater distance. Where the coatings did not quite touch the glass, the rings bent inwards; and where the coatings adhered very close, they retired farther from them. Upon discharging these two plates, the coloured rings vanished, and the electric cohesion ceased with them. On separating the plates before the explosion; that which had received the positive electricity was positive on both sides, and the other negative on both sides. If they were separated after the explosion, each of them was affected in a manner quite the reverse. Upon inverting the plates, that which was the thinner appeared to be possessed of the stronger electricity, and brought the other plate to correspond with it. Charging the two plates separately, and taking off two of the coatings, so that two positive or two negative sides might be placed together, there was no cohesion nor explosion. But joining a positive and a negative side, they immediately cohered; and a communication being formed on the outside, there was an explosion which increased the cohesion.

Mr Henley repeated these experiments with success when he made use of plates of looking-glass, or window and crown glass; but when two plates of Nuremberg glass, commonly called Dutch plates, were used, the result was very different. Each of the plates, when separated after charging, had a positive and a negative surface. When they were replaced, and a discharge made, by forming a communication between the two coatings, the electricity of all the surfaces was changed. It appeared, however, still to be very strong; and the plates continued to give repeated flashes of light when they were alternately closed, touched, and separated, like the other plates above-mentioned. If a clean, dry, uncoated plate of looking-glass was placed between the coated plates, either of looking-glass or crown-glass, before they were charged, that uncoated plate was always found, upon separating them after charging, to be electrified negatively on both sides; but if it was put between the Dutch plates, it acquired, like them, a positive and negative electricity.

The following observation of Mr Epinus is very remarkable. He pressed close together two pieces of looking-glass, each containing some square inches; and found, that when they were separated, and not suffered to communicate with any conductor, they acquired a strong electricity, the one positive and the other negative. When put together again, the electricity of both disappeared; but not if either of them had been deprived of their electricity when they were asunder; for in that case, the two, when united, had the electricity of the other.

These are the most remarkable experiments that have been made with electrified flat plates of glass. Tubes of the same matter, however, afford a variety of curious phenomena of a different nature. One very remarkable one is the conducting power of new flint-glass, which is most easily perceived in tubes, and on which Dr Priestley makes the following observations. He several times got tubes made two or three yards long, terminating in solid rods. These he took almost warm from the furnace, in the finest weather possible; and having immediately inflated them, perceived that the electricity of a charged vial would presently diffuse itself from one end to the other; and this he thought would have been the case at any distance at which the experiment could have been made. When the same tubes were a few months older, the electricity could not be diffused along their surface farther than half a yard.

The diffusive power of glass he thought proper to try in a different manner. A tube was procured of about three feet long, but of very unequal width. About three inches of the middle part of it were coated on both sides. This coated part was afterwards charged, by communicating electricity to the inside of it by means of a wire. The consequence of this was, that not only the part through which the wire was introduced became strongly electrical on the outside, but at the opposite end, where there was neither coating nor wire, the fire crackled under the fingers as the tube was drawn through the hand, and a flame seemed continually to issue out at both ends, while it was at rest and charged. One end of this tube was broken and rough, the other was smooth.

Another tube was procured about three feet and an half in length, and very thin. It was about an inch in diameter, and closed at one end. Three inches of it were coated on both sides, about nine inches below the mouth. This part being charged, the whole tube, to the very extremity of it, was strongly electrical, crackling very loud when the hand was drawn along it, and emitting sparks at about an inch distance all the way. After drawing the whole tube through the hand, all the electricity on the outside was discharged; but, on putting a finger into the mouth, a light stream came from the coating, both towards the finger and towards the opposite end of the tube. After this, all the outside of the tube was become strongly electrical as before; and this electricity might be taken off and recovered many times without charging the tube anew, only it was weaker each time.—Holding this tube by the coated part, and communicating electricity to the uncoated outside, both sides became charged; and, upon introducing a wire, a considerable explosion was made. The discharge made the outside strongly electrical, and by taking off this electricity, the tube became feebly charged. The residuum of these charges was very considerable; and, in one tube, there was a residuum after 20 or 30 discharges.

By being kept for six or seven months, most of the tubes employed in these experiments lost the above-mentioned properties, and the electricity could no longer diffuse itself upon their surfaces. At length they were all broken except one, which had been the most remarkable of the whole. With this old tube, the Doctor tried to repeat the above-mentioned experiments. Phenomena; but to no purpose. He then took it to a glasshouse; and having made it red all over, its diffusive property was restored as before.

He then tried two other tubes which had been made about six weeks, but without being used during all that time, and they answered exactly as if they had been quite new. The charge from a small coated part diffused itself all over the tube; so that, at the distance of a yard from the coating, it gave sparks to the finger of an inch long. On this occasion he observed, that when his finger was brought to the tube about two inches above the coating, a great quantity of the diffusing electricity was discharged; and his whole arm was violently shocked. The old tube, after being heated as above mentioned, showed a prodigious diffusive power. Upon charging a small coated part, the electricity was diffused to the end of the tube; and it gave sparks at the distance of an inch over every part of it. When it was drawn through the hand, in order to take off the diffused electricity, it instantly returned again, and the extremity of the tube would be highly electrified, even while its communication with the coating was cut off by the hand. The middle part of the tube also, which had been oftenest heated, had a much greater diffusive power than any other. It was no sooner taken off, than it appeared again; so that it gave a continual stream of fire. The quantity of residuum after a discharge of this tube was prodigious; so that the outside coating would immediately after give almost a constant stream of fire for some time to any conducting substance placed near it.

The Doctor also observed, that in all the tubes which had the diffusion, there was a considerable noise at the orifice when his hand was drawn from the extremity towards the coating, as if the tube had been gradually discharging itself. In the dark, the electric matter seemed perpetually to pour from the open end, or both ends if they were open; and whenever his hand was drawn over it, the fire streamed from the coating to his hand in a very beautiful manner. The first time he charged any of these tubes after they had stood a while, the diffusion was the most remarkable. It was lessened by every succeeding charge, and at last became exceedingly small; but after the tube had stood a few hours uncharged, it was as vigorous as ever.

Mr Cavallo hath also made some curious discoveries concerning glass-tubes. He took the hint from observing accidentally, that by agitating quicksilver in a glass tube hermetically sealed, and in whose cavity the air was very much rarefied, the outside of the tube was sensibly electrified. The electricity, however, was not constant, nor in proportion to the agitation of the quicksilver. In order to ascertain the properties of these tubes, he constructed several of them, one of which is represented fig. 71. Its length was 31 inches, and its diameter something less than half an inch. The quicksilver contained in it was about three fourths of an ounce; and in order to exhaust it of air, one end of it was closed, while the quicksilver boiled in the other. Before this tube is used, it must be made a little warm and cleaned; then, holding it nearly horizontal, the quicksilver in it is suffered to run from one end to the other, by gently and alternately elevating and depressing its extremities. This operation immediately renders the outside electrical; but with the following remarkable property, viz. that the end of the tube where the quicksilver actually stands is always positive, and all the remaining part of it negative. If elevating this positive end of the tube a little, the quicksilver runs to the opposite end which was negative, then the former instantly becomes negative, and the latter positive. The positive end has always a stronger electricity than the negative. If when one end of the tube, for instance A, is positive, i.e., when the quicksilver is in it, that electricity is not taken off by touching it; then, on elevating this end A, so that the quicksilver may run to the opposite end B, it appears negatively electrified in a very small degree. If by depressing it again it is a second time rendered positive, and that positive electricity is not taken off, then, on elevating the end A again, it appears positive in a small degree. But if, whilst it is positive, its electricity is taken off, then on being elevated, it appears strongly negative. When about two inches of each extremity of this tube is coated with tin-foil, as represented in the figure, that coating renders the electricities at the extremities more perceptible, so that sometimes they will give sparks to a conductor brought near them. Tubes whose glass is about one-twentieth of an inch thick answer better for these experiments than any others.

We shall close this account of the phenomena of excited glass, with some experiments which show the durability of the electric virtue in that substance in certain circumstances. Mr Canton procured some thin glass balls of about an inch and a half in diameter, with stems or tubes of eight or nine inches in length. He electrified them, some positively, and others negatively, on the inside, and then sealed them hermetically. Soon after, he found that they had lost all signs of electricity: but holding them to the fire at the distance of five or six inches, they became strongly electrical in a short time, and more so as they cooled. Heating them frequently he found would diminish their power; but keeping one of them under water a week did not appear in the least to impair it. That which he kept under water was charged on the 22nd of September 1760, was heated several times before it was kept in water, and had been frequently heated afterwards; yet it still retained its virtue to a considerable degree till the 31st of October following. The breaking of two of his balls gave him an opportunity of observing their thickness, which he found to be between seven and eight parts of a thousand of an inch. The balls retained their virtue for five years, but in a less degree. Mr Lullin also found, that a glass tube charged and hermetically sealed, would show signs of electricity when heated.

The most remarkable instances of the continuance of this power in glass, however, are those given by Mr Henly in the 67th volume of the Phil. Trans. One is, of a small bottle, which showed signs of electricity for 70 days after it had been charged, and stood in a cupboard all that time. The other is of a glass cylinder, which after excitation continued to show very strong signs of electricity from the 5th day of February to the 10th of March, though various methods had been used during that time to destroy the electric virtue. These means always proved effectual when they were applied, and the cylinder for some time showed... showed no signs of electricity. They never failed, however, to return without any fresh excitation; and became stronger and weaker, nay, sometimes quite vanished and returned, without any visible cause. In general, the electricity was weak when a fire was kept in the room, or when the door was kept open. When the wind blew from the north, the electricity was vigorous, and likewise after it had been for some time destroyed by flame. The cylinder, however, did not at all times retain its electric virtue for such a length of time without excitation. Very often it would lose all signs of electricity in 12 hours, and at other times in a fortnight, without returning till it was again excited.

§ 3. The Phenomena of excited Sulphur, Gum-lac, Amber, Resin, baked Wood, &c.

The most remarkable property of these, as already mentioned, is the durability of their electric virtue when once excited. They are also capable of being excited by heat without any friction. This last property was discovered by Mr Wilcke, who distinguishes it by the name of spontaneous electricity. He melted sulphur in an earthen vessel, which he placed upon conductors; then, letting them cool, he took out the sulphur, and found it strongly electrical; but it was not so when it stood to cool upon electric substances. He then melted sulphur in glass vessels, whereby they both acquired a strong electricity whether placed upon electrics or not; but a stronger in the former case than in the latter; they acquired a stronger virtue still, if the glass vessel was coated with metal. In these cases, the glass was always positive and the sulphur negative. It was particularly remarkable, that the sulphur acquired no electricity till it began to cool and contract, and was the strongest in the greatest state of contraction; whereas the electricity of the glass was, at the same time, the weakest; and was the strongest of all when the sulphur was shaken out before it began to contract, and acquired any negative electricity.

Performing experiments of this kind, he found, that melted sealing-wax poured into glass acquired a negative electricity, but poured into sulphur a positive one, leaving the sulphur negative. Sealing-wax also, poured into wood, was negative, and the wood positive; but sulphur poured into sulphur, or into rough glass, acquired no electricity at all.

Similar experiments were also made by Mr Æpinus. He poured melted sulphur into metal cups; and observed, that, when the sulphur was cold, the cup and sulphur together showed no signs of electricity, but very strong signs of it the moment they were separated. The electricity always disappeared when the sulphur was replaced in the cup, and revived upon its being taken out. The cup had acquired a negative, and the sulphur a positive electricity; but if the electricity of either of them had been taken off while they were separate, they would both, when united, show signs of that electricity which had not been taken off.

Mr Wilcke also made several curious experiments concerning the effects of different rubbers upon electric substances, the most remarkable of which is the following: viz. that sulphur rubbed against metals was always positive; and this was the only case in which it was so. But, being rubbed against lead, it became negative, and the metal positive.

With regard to the perpetual attractive power of sulphur, &c. which Mr Grey fancied he had discovered *, the most remarkable example he gives is of a large cone of stone sulphur, covered with a drinking glass in which it was made. This he said would never fail to show an attractive power when the glass was taken off. In fair weather, the glass would attract also; but not so strongly as the sulphur, which never failed to attract, let the wind or weather be ever so variable. This experiment has been repeated by Mr Henry; who says he has never known the sulphur to fail of showing signs of electricity on the removal of the glass. Gum-lac, resin, &c. agree in the same general properties with sulphur, but do not become so strongly electrified spontaneously, nor are they so easily excited.

§ 4. Phenomena of the Tourmalin.

These have been accurately observed by Dr Priestley, who gives the following account of the methods he made use of for that purpose.

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

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

3. From Mr Wilcke's experiments on the production of spontaneous electricity, by melting one substance within another, he first conjectured that the tourmalin might collect its electricity from the neighbouring air: To determine which the following experiment was made. Part of a pane of glass was laid on the standard bar of an excellent pyrometer, and upon that glass the tourmalin was placed. This bar was heated heated by a spirit lamp, so that the increase or decrease of heat in the tourmalin could thus be exactly determined. In this situation he observed, that whenever he examined the tourmalin, the glass had acquired an electricity contrary to that side of the stone which lay upon it, and equally strong with it. If, for example, the flat side of the stone had been presented to a feather electrified positively, as the heat was increasing, it would repel it at the distance of about two inches, and the glass would attract it at the same or a greater distance; and when the heat was decreasing, the stone would attract, and the glass repel it at the distance of four or five inches. The case was the same whichever of the sides was presented, as well as when a shilling was fastened with sealing-wax upon the glass; the electricity both of the shilling and glass being always opposite to that of the stone. When it came to the turn, the electricity was very quickly reversed; so that in less than a minute the electricity would be contrary to what it was before. In some cases, however, viz. where the convex surface of the tourmalin was laid upon the glass or shilling, both of these became positive as well as the stone. This he supposed to be owing to the stone touching the surface on which it lay only in a few points, and that its electricity was collected from the air; which supposition was verified: for, getting a mould of Paris plaster made for the tourmalin, and heating it in the mould, fastened to a slip of glass, he always found the mould and glass possessed of an electricity contrary to that of the stone, and equally strong with it. During the time of cooling, the mould seemed to be sometimes more strongly negative than the stone was positive; for once, when the stone repelled the thread at the distance of three inches, the mold attracted it at the distance of near six (a).

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

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

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

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

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

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

10. On heating the convex side by means of a poker, and pressing it against the palm of the hand as soon as it could be borne, it became pretty strongly negative; though it is extremely difficult to procure any appearance

(a) This would probably have been found always the case; for here the stone and mould acted in a manner similar to the electrophorus and its metal plate; the latter of which always discovers a greater electric power than the former. ance of negative electricity from this side; and care must be taken that a slight attraction of the electrified feather, by a body not electrified, be not mistaken for negative electricity.

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

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

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

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

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

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

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

Sect. V. Of the different Theories of Electricity, with the principal Experiments brought in favour of each, and which tend more particularly to show the Nature of the Electric Fluid.

It is not to be supposed, that the phenomena of elec- Electricity would long be observed without attempts to account for them. In fact, this was attempted by Thales, who first observed the attractive power of amber. At this property he was so much surprised, that he reckoned the amber to be animated. With regard to the sentiments of Theophrastus on this subject, we are entirely in the dark; but, among the succeeding electricians, all the phenomena were derived from undulous effluvia emitted by the excited electric. These were supposed to fall upon all bodies in their way, and to carry back with them all that were not too heavy. For, at that time, effluvia of every kind were supposed to return to the bodies from which they were emitted; since nobody could otherwise account for the substance not being sensibly wasted by the constant emission. When these light bodies on which the unctuous effluvia had fallen were arrived at the excited electric, a fresh emission of the effluvia was supposed to carry them back again. But this effect of the effluvia was not thought of till electric repulsion, as well as attraction, had been fully observed.

The discovery of a difference between conducting and non-conducting substances, threw considerable difficulties in the way of those who maintained the hypothesis of unctuous effluvia. When the Newtonian philosophy began to be pretty generally received, the terms attraction and repulsion were quickly introduced into electricity, as well as other branches of philosophy; and the electric effluvia, instead of being of an undulous nature, were said to be of an attractive or repulsive one. At the same time, the apparent stop which is put to the progress of these effluvia by any electric substance, introduced a question not yet well decided, viz. Whether electric bodies are penetrable by the fluid or not?

When Mr Du Fay discovered the two opposite species of electricity, at that time distinguished by the names of vitreous and resinous, and afterwards by those of plus and minus, or positive and negative, he formed the idea of two distinct electric fluids. Both these were supposed to have a repulsive power with respect to themselves, but an attractive one with regard to one another.

As long as electrical attraction and repulsion were the only phenomena to be accounted for, this theory served the purpose well enough. To account for attraction and repulsion by an attractive and repulsive power, was indeed no explication at all; but it afforded a change of terms, which is too frequently mistaken for an explanation both in electricity and other parts of philosophy.—At last, however, Mr Du Fay dropped his opinion concerning the existence of two electric fluids, and thought that all the phenomena might be accounted for from the action of a single one. The vitreous or positive electricity, which was supposed to be the stronger, he thought might attract the negative, or weaker electricity.—It is indeed true, that, in many experiments, the positive electricity doth manifest a superiority in strength over the negative, something like that superior degree of vigour which is observed in one of the poles of a lodestone over the other. According to Mr Du Fay's own principles, however, had this been the case, a body positively electrified ought to have attracted one electrified negatively more weakly than one not electrified at all; which is contrary to experience.

During all this time, it was imagined, that the electric matter, whether it consisted of one or more Electric fluids, was produced from the electric body by friction; but by a discovery of Dr Watson's, it became universally believed, that the glass globes and tubes served only to set the fluid in motion, but by no means to produce it. He was led to this discovery by observing, that, upon rubbing the glass tube, while he was standing upon cakes of wax or rosin (in order, as he expected, to prevent any discharge of the electric matter upon the floor), the power was, contrary to his expectation, so much lessened, that no snapping could be observed upon another person's touching any part of his body; but that, if a person not electrified held his hand near the tube while it was rubbed, the snapping was very sensible. The event was the same when the globe was whirled in similar circumstances. For, if the man who turned the wheel, and who, together with the machine, was suspended upon silk, touched the floor with one foot, the electric fire appeared upon the conductor; but if he kept himself free from any communication with the floor, little or no fire was produced.—He observed, that only a spark or two would appear between his hand and the insulated machine, unless he at the same time formed a communication between the conductor and the floor; but that then there was a constant and copious flux of the electric matter observed between them. From these, and some other experiments of a similar kind, the Doctor discovered what he called the complete circulation of the electric matter. When he found, that, by cutting off the communication of the glass globe with the floor, all electric operations were stopped, he concluded, that the electric fluid was conveyed from the floor to the rubber, and from thence to the globe. For the same reason, feeling the rubber, or the man who had a communication with it, gave no sparks but when the conductor was connected with the floor, he as naturally concluded, that the globe was supplied from the conductor, as he had before concluded that it was supplied from the rubber. From all this he was at last led to Dr Watson form a new theory of electricity, namely, that, in electric operations, there was both an afflux of electric matter to the globe and the conductor, and likewise an efflux of the same electric matter from them. Finding that a piece of leaf-silver was suspended between a plate electrified by the conductor, and another communicating with the floor, he reasons from it in the following manner. "No body can be suspended in equilibrium but by the joint action of two different directions of power: so here the blast of electric ether from the floor setting through it, drives the silver towards the plate electrified. We find from hence, likewise, that the draught of electric ether from the floor is always in proportion to the quantity thrown by the globe over the gun-barrel (the prime conductor at that time made use of), or the equilibrium by which the silver is suspended could not be maintained." Some time after, however, the Doctor retracted this opinion concerning the afflux and efflux, and supposed that all the electric phenomena might be accounted for from the excess or diminution of the quantity of electric matter contained. contained in different bodies. This theory was afterwards adopted by Dr Franklin, and continues to be generally received.

One great difficulty with which the first electricians were embarrassed (and which is yet scarcely removed), was to ascertain the direction of the fluid. At first, all electric powers, as we have already observed, were supposed to reside in the excited globe or glass tube. The electric spark therefore was imagined to proceed from the electrified body towards any conductor that was presented towards it. It was never imagined there could be any difference in this respect, whether it was amber, glass, sealing-wax, or any thing else that was excited. This progress of the electric matter was thought to be quite evident to the senses; and therefore the observation of electric appearances at an insulated rubber occasioned the greatest astonishment.—In this case, the current could not be supposed to flow both from the rubber and the conductor, and yet the first appearances were the same. To provide a supply of the electric matter, therefore, philosophers were obliged to suppose, that notwithstanding appearances were in both cases much the same, the electric fluid was really emitted in one case by the electrified body, and received by it in the other. But now being obliged to give up the evidence from sight for the manner of its progress, they were at a loss, whether, in the usual method of electrifying by excited glass, the fluid proceeded from the rubber to the conductor, or from the conductor to the rubber. It was, however, soon found, that the electricity at the rubber was the reverse of that at the conductor, and in all respects the same with that which had before been produced by the friction of sealing-wax, sulphur, rosin, &c. Seeing, therefore, that both the electricities were produced at the same time, by one and the same electric, and by the same friction, all philosophers were naturally led to conclude, that both were modifications of one fluid; though in what manner that fluid was modified throughout the immense variety of electric phenomena, was a matter not easy to be determined.

On this subject, the Abbé Nollet adopted the doctrine of afflux and efflux already mentioned. He supposed, that, in all electrical operations, the fluid is thrown into two opposite motions; that the afflux of this matter drives all light bodies before it by impulse upon the electrified body, and its efflux carries them back again. He was, however, very much embarrassed in accounting for facts where both these currents must be considered; as in the quick alternate attraction and repulsion of light bodies by an excited glass tube, or other excited electric. To obviate this difficulty, he supposes that every excited electric, and likewise everybody to which electricity is communicated, has two orders of pores, one for the emission of the effluvia, and another for the reception of them. Mr de Tour improved upon Nollet's hypothesis, and supposed that there is a difference between the affluent and effluent current; and that the particles of the fluid are thrown into vibrations of different qualities, which makes one of these currents more copious than the other, according as sulphur or glass is used. It is impossible, however, that suppositions so very arbitrary could be at all satisfactory, or received as proper solutions of the electric phenomena.

No less difficult was it for philosophers to determine the nature of the electric fluid, than its manner of acting. It had been in a manner generally believed, that fire was not a distinct element, but arose from some violent repulsions, rarefactions, &c. among the opinions concerning the electric fluid to elementary fire, however, seemed the nature strongly to militate against this opinion. The hypotrofic fluid, the fire of fire as a distinct principle or element began to revive. Some maintained, that the electric fluid was really this principle; others thought that it was a fluid sui generis, very much resembling that of fire; while others, with Mr Boulanger at their head, imagined that it was nothing more than the finer parts of the atmosphere, which crowded upon the surfaces of electric bodies, when the groser parts had been driven away by the friction of the rubber.

This last opinion, however, soon received a full refutation from the experiments of Dr Watson above-mentioned; by which it was proved, that the electric matter came not from the atmosphere, but from the earth. About the same time the Leyden phial was discovered; and the extraordinary effects of it rendered the inquiries into the nature of the electric fluid much more general than before. But still, the violent prejudice against the existence of fire as a real element or fluid distinct from terrestrial bodies, continued in its full vigour, and the most extravagant theories were acquiesced in, rather than the simple position above mentioned. It would be tedious, and indeed impossible, to give an account of all the theories which were now invented. One of the most remarkable, and most consistent, was that of Mr Wilson. According to this Mr Wilson, gentleman, the chief agent in all the operations of electricity, is Sir Isaac Newton's ether; which is more or less dense in all bodies in proportion to the smallness of their pores, except that it is much denser in sulphureous and nitrous bodies. To this ether are ascribed the principal phenomena of attraction and repulsion: the light, the sulphureous or rather phosphorescent smell with which violent electricity is always attended, and other sensible qualities, are ascribed to the groser particles of bodies driven from them by the forcible action of this ether. He also endeavours to explain many electrical phenomena by means of a subtle medium at the surface of all bodies; which is the cause of the refraction and reflection of the rays of light, and also resists the entrance and exit of this ether. This medium, he says, extends to a small distance from the body, and is of the same nature with what is called the electric fluid. On the surface of conductors this medium is rare, and easily admits the passage of the electric fluid; whereas, on the surface of electrics, it is dense and resists it. The same medium is raised by heat, which thus changes conductors into non-conductors. By far the greater number of philosophers, however, rejected the opinion of Mr Wilson; and as they neither chose to allow the electric fluid to be fire nor ether, they were obliged to own that it was a fluid sui generis, i.e. one of whose nature they were totally ignorant.

But while philosophers were thus embarrassed in their electrical theories, a vast number of interesting phenomena were discovered by the assiduity of a number of different electricians in different countries. Mr Winckler observed, that if glass was rubbed on the inside, Sect. V.

Theory.

fide, it would show strong appearances of electricity on the outside; which seemed to favour the opinion of the permeability of glass to the electric matter. Other German electricians used several globes at a time, and imagined they found effects proportionable; tho' this has since been found a mistake. Such a prodigious force, however, could they excite by means of these globes whirled by a large wheel, and rubbed by the hand or with woollen cloth, that, according to their own accounts, blood could be drawn from a finger by means of the electric spark, the skin would burst, and a wound appear as if made by a caustic. If several globes or tubes were used, they said, that the motion of the heart and arteries would be very perceptibly increased in such as were electrified; and that, if a vein was opened in these circumstances, the blood issuing from it would appear like lucid phosphorus, and run out faster than when the person was not electrified. Mr P. Gordon, a Scots Benedictine monk, and professor of philosophy at Erfurt, increased the electric sparks to such a degree, that they were felt from a man's head to his foot, so that he could hardly take them without falling down with giddiness, and small birds were killed by them. This was effected by conveying the electricity with iron wires to the distance of 200 ells from the place of excitation. He also found that the sparks were stronger when the wires were thick than when they were small.

While the power of electricity was thus tried, another question of great importance was likewise decided, namely, Whether electricity acted according to the largeness of the surface of bodies? This was found to be in proportion to the surface, and not the solid contents. The magnetic effluvia also were found not to interfere in the least with the electrical ones. An electrified loadstone attracted light bodies of all kinds by its electric virtue, at the same time that it attracted iron and steel by its peculiar magnetic virtue. The attractive virtue of electricity was also found to pervade glass so powerfully, that a thread was attracted through five exhausted receivers, and seemingly with more vigour than it would have been by the excited tube alone in the open air.

Such was the state of philosophical opinions concerning electricity, when Dr Franklin first invented his theory concerning positive and negative, or plus and minus, electricity. This had been already suggested by Dr Watson, but was not so fully explained by him as by Dr Franklin; on which account the latter is generally reckoned to be the sole inventor. According to this theory, all the operations in electricity depend upon one fluid sui generis, extremely subtile and elastic. Between the particles of this fluid there subsists a very strong repulsion with regard to each other, and as strong an attraction with regard to other matter. Thus, according to Dr Franklin's hypothesis, one quantity of electric matter will repel another quantity of the same, but will attract and be attracted by any terrestrial matter that happens to be near it. The pores of all bodies are supposed to be full of this subtile fluid; and when its equilibrium is not disturbed, that is, when there is in any body neither more nor less than its natural share, or than that quantity which it is capable of retaining by its own attraction, the fluid does not manifest itself to our senses. The action of the rubber upon an electric disturbs this equilibrium, occasioning a deficiency of the fluid in one place, and a redundancy of it in another. This equilibrium being forcibly disturbed, the mutual repulsion of the particles of the fluid is necessarily exerted to restore it. If two bodies be both of them overcharged, the electric atmospheres repel each other, and both the bodies recede from one another to places where the fluid is less dense. For as there is supposed to be a mutual attraction between all bodies and the electric fluid, such bodies as are electrified must go along with their atmospheres. If both the bodies are exhausted of their natural share of this fluid, they are both attracted by the denser fluid existing either in the atmosphere contiguous to them, or in other neighbouring bodies; which occasions them still to recede from one another as if they were overcharged.

This is the Franklinian doctrine concerning the cause of electric attraction and repulsion; but it is evident, that the reason just now given why bodies negatively attracted ought to repel one another, is by no means satisfactory. Dr Franklin himself had framed his hypothesis before he knew that bodies negatively electrified would repel one another; and when he came afterwards to learn it, he was surprised, and acknowledged that he could not satisfactorily account for it. Other philosophers therefore invented different solutions of this difficulty, of which that above mentioned is one. But by some this was rejected. They said, that as the denser electric fluid, surrounding two bodies negatively electrified, acts equally on all sides of those bodies, it cannot occasion their repulsion. The repulsion, according to them, is owing rather to an accumulation of the electric on the surfaces of the two bodies; which accumulation is produced by the attraction, and the difficulty the fluid finds in entering them. This difficulty is supposed chiefly to be owing to the air on the surface of bodies, which Dr Priestley says is probably a little condensed there. This he deduces from an experiment of Mr Wilson, corrected by Mr Canton. The experiment was made in order to observe the course of the electric light through a Torricellian vacuum. A singular appearance of light was observed upon the surface of the quicksilver, at which the fluid was supposed to enter. Mr Wilson supposed that this was owing to a subtile medium spread over the surface of the quicksilver, and which prevented the easy entrance of the electric fluid. But this was afterwards discovered by Mr Canton to be owing to a small quantity of air which had been left in the tube. It is plain, however, that as the attraction is equal all round, and likewise the difficulty with which the fluid penetrates the air, bodies negatively electrified ought not to repel one another on this supposition more than the former. Nay, they ought to attract each other; because, in the place of contact, the repulsion of the air would be taken off, and the electric fluid could come from all other quarters by the attraction of the bodies.

Mr Cavallo, who seems to have undertaken the defence of this hypothesis in all cases, gives another reason why bodies negatively electrified should repel each other. In a chapter entitled, "A Compendious View of the principal properties of Electricity," among others he gives the following: "No electricity can be ob- observed upon the surface of any electrified body, except that surface is contiguous to an electric, which electric can somehow or other acquire a contrary electricity at a little distance. Otherwise: —No electricity can appear upon the surface of any electrified body, except that surface is opposite to another body which has actually acquired the contrary electricity, and these contrarily electrified bodies are separated by an electric.

On considering this principle (adds he), it may be asked, Why an electricity can be observed upon the surface of an electrified body that is insulated at a considerable distance from other conductors? Or, Which is the electric that is contiguous to the surface of an electrified conductor or excited electric, and which has actually acquired a contrary electricity at a little distance from the said surface? To this question it is answered, that the air is, in general, the electric which is opposite to the surface of any electrified body; which, not being a perfect conductor, does easily acquire a contrary electricity on a stratum of its substance that is at a little distance from the electrified body; and, in consequence of this stratum, it acquires another stratum contrarily electrified, and at a little distance from the former: to this other strata succeed, alternately possessed of positive and negative electricities, and decreasing in power till they vanish. This alteration is easily proved by several experiments, particularly the following. If the end of a pretty long glass tube be presented to a body electrified, for instance, positively, the tube will be found electrified positively also for the space of one or two inches at that end; but beyond that space, will be found two or three inches electrified negatively: after that another positive electricity will appear; and so alternately, a positive and a negative zone will follow one another, always weaker and weaker in power, till at last they quite vanish. This shows, that, in general, when an electric sufficiently dense is presented to an electrified body, it acquires successive zones or strata of positive and negative electricity."

From this fact (which, with the utmost impropriety, he terms a law of electricity, whereas it is most evidently the effect of a law, and not the law itself), Mr Cavallo gives the following reason why bodies negatively electrified repel one another. "As to the repulsion existing between bodies possessed of the same electricity; in order to understand its explanation thoroughly, the reader must be reminded of the principle above-mentioned, which is, that no electricity, i.e., the electric fluid proper to a body, can either be augmented or diminished upon the surface of that body, except the said surface is contiguous to an electric, which can acquire a contrary electricity at a little distance: from whence it follows, that no electricity can be displayed upon the facing surfaces of two bodies that are sufficiently near to one another, and both possessed of the same electricity; for the air that lies between those contiguous surfaces has no liberty of acquiring any contrary electricity. This being premised, the explanation of electric repulsion becomes very easy. Suppose, for instance, that two small bodies are freely suspended by insulated threads; so that, when they are not electrified, they may hang contiguous to one another. Now suppose these bodies to be electrified either positively or negatively, and then they must repel one another: for either the increased or the diminished natural quantity of electric fluid in these bodies will endeavour to diffuse itself equally over every part of the surfaces of these bodies; and this endeavour will cause the said bodies to recede from each other, so that a quantity of air may be interposed between their surfaces, sufficient to acquire a contrary electricity at a little distance from the said surfaces. Otherwise: If the bodies possessed of the same electricity do not repel each other, so that a sufficient quantity of air may be interposed between their surfaces, the increased quantity of electric fluid when the bodies are electrified positively, or the remnant of it when they are electrified negatively, by the above principle cannot be diffused equally throughout or over the surfaces of these bodies; for no electricity can appear upon the surfaces of bodies in contact, or that are very near each other. But the electric fluid, by attracting the particles of matter, endeavours to diffuse itself equally throughout or over the surfaces of these bodies; therefore the said bodies are, by this endeavour, forced to repel one another."

This theory is evidently no solution of the difficulty; seeing it is only explaining one fact by another, which requires explanation at least as much as the first. But though this should be overlooked, it is still insufficient; for, granting that bodies negatively electrified ought to repel one another till the electricity is equally diffused along their surfaces, yet when this is accomplished, the repulsion ought to cease. Now, there is no occasion for supposing the bodies to be electrified while they are in contact, or nearly so. One may be electrified negatively in one corner of a room, and another in the other. The electrification may also be continued for any length of time we please, so that it is not possible to suppose but the electric matter must have diffused itself equally along the surfaces of both: yet, if we attempt to bring these bodies together, we shall find that they will repel each other very violently; which ought not to be the case, according to Mr Cavallo's supposition.

What gave the greatest reputation to Dr Franklin's Dr Frank-theory, however, is the easy solution which it affords in the explanation of all the phenomena of the Leyden phial. The fluid of the Leyden phial is supposed to move with the greatest ease in bodies which are conductors, but with extreme difficulty in Leyden phials, per se, infomuch that glass is absolutely impermeable to it. It is moreover supposed, that all electrics, and particularly glass, on account of the smallness of their pores, do at all times contain an exceeding great, and always an equal quantity of this fluid; so that no more can be thrown into any one part of any electric substance, except the same quantity go out at another, and the gain be exactly equal to the loss. These things being previously supposed, the phenomena of charging and discharging a plate of glass admit of an easy solution. In the usual manner of electrifying by a smooth glass globe, all the electric matter is supplied by the rubber from all the bodies which communicate with it. If it be made to communicate with nothing but one of the coatings of a plate of glass, while the conductor communicates with the other, that side of the glass which communicates with the rubber must necessarily be exhausted in order to supply the conductor, which must convey the whole of it to the side side with which it communicates. By this operation, therefore, the electric fluid becomes almost entirely exhausted on one side of the plate, while it is as much accumulated on the other; and the discharge is made by the electric fluid rushing, as soon as an opportunity is given it by means of proper conductors, from the side which was overloaded to that which is exhausted.

It is not, however, necessary to this theory, that the very same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the space of an equal portion of the natural quantity of fluid belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue till the exhausted side of the glass is supplied by the fluid naturally existing in the conductors contiguous to it. In this case, the motion of the electric fluid, in an explosion, will rather resemble the vibration of the air in sounds, than a current of it in winds.

It will soon be acknowledged (says Dr Priestley), that while the substance of the glass is supposed to contain as much as it can possibly hold of the electric fluid, no part of it can be forced into one of the sides, without obliging an equal quantity to quit the other side: but it may be thought a difficulty upon this hypothesis, that one of the sides of a glass plate cannot be exhausted, without the other receiving more than its natural share; particularly, as the particles of this fluid are supposed to be repulsive of one another. But it must be considered, that the attraction of the glass is sufficient to retain even the large quantity of electric fluid which is natural to it, against all attempts to withdraw it, unless that eager attraction can be satisfied by the admission of an equal quantity from some other quarter. When this opportunity of a supply is given, by connecting one of the coatings with the rubber, and the other with the conductor, the two attempts to introduce more of the fluids into one of the sides are made, in a manner, at the same instant. The action of the rubber tends to disturb the equilibrium of the fluid in the glass; and no sooner has a spark quitted one of the sides, to go to the rubber, than it is supplied by the conductor on the other; and the difficulty with which these additional particles move in the substance of the glass, effectually prevents its reaching the opposite exhausted side. It is not said, however, but that either side of the glass may give or receive a small quantity of the electric fluid, without altering the quantity on the opposite side. It is only a very considerable part of the charge that is meant, when one side is said to be filled while the other is exhausted.

It is a little remarkable, adds Dr Priestley, that the electric fluid, in this and in every other hypothesis, should so much resemble the ether of Sir Isaac Newton in some respects, and yet differ from it so essentially in others. The electric fluid is supposed to be, like ether, extremely subtle and elastic, that is, repulsive of itself; but instead of being, like the ether, repelled by all other matter, it is strongly attracted by it: so that, far from being, like the ether, rarer in the small than in the large pores of bodies, rarer within the bodies than at their surfaces, and rarer at their surfaces than at any distance from them; it must be denser in small than in large pores, denser within the substance of bodies than at their surfaces, and denser at their surfaces than at a distance from them.

To account for the attraction of light bodies, and other electrical appearances, in air of the same density and regularity with the common atmosphere, when glass (which is supposed to be impermeable to electricity) is interposed; it is conceived, that the addition or subtraction of the electric fluid, by the action of the excited electric on one side of the glass, occasions, as in the experiment of the Leyden phial, a subtraction or addition of the fluid on the opposite side. The state of the fluid, therefore, on the opposite side being altered, all light bodies within the sphere of its action must be affected in the very same manner as if the effluvia of the excited electric had actually penetrated the glass, according to the opinions of all electricians before Dr Franklin.

This hypothesis has been in some measure improved by Mr Epinus, in a treatise intitled, "Tentamen Theoriae Electricitatis & Magnetismi." He extends the property of impermeability to air, and all electrics, as well as glass. He supposes impermeability to consist in the great difficulty with which electric substances admit the fluid into their pores, and the slowness with which it moves in them. In consequence of this impermeability of air to the electric fluid, he denies the existence of electric atmospheres, and thinks that Dr Franklin's theory will do much better without them. He also imagines, that all the particles of matter are repulsive of one another: for that otherwise (since all substances have in them a certain quantity of the electric fluid, the particles of which repel one another and are attracted by all other matter), it could not happen, that bodies in their natural state with respect to electricity, should neither attract nor repel one another. He also introduces a number of mathematical calculations; the result of which (says Dr Priestley, with a great deal of probability) cannot be depended upon.

The above is a full explanation of the theory of electricity at present most generally received. It depends on the following principles. 1. All terrestrial substances, as well as the atmosphere which surrounds the earth, are full of electric matter. 2. Glass, and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it. 3. This electric matter violently repels itself, and attracts all other matter. 4. By the excitation of an electric, the equilibrium of the fluid contained in it is broken; and one part of it is overloaded with electricity, while the other contains too little. 5. Conducting substances are permeable to the electric matter through their whole substance, and do not conduct it merely over their surface. 6. Positive electricity is when a body has too much of the electric fluid, and negative electricity when it has too little. Of these positions we shall now adduce those proofs drawn from different sources. different facts, which seem in the strongest manner to confirm them.

I. "All terrestrial substances, as well as the atmosphere which surrounds the earth, are filled with electric fluid."—Of this the proofs are very easy. There is no place of the earth or sea, where the electric fire may not be collected by making a communication between it and the rubber of an electric machine. Therefore, considering that the whole earth is moist, that moisture is a conductor of electricity, and that every part of the earth must thus communicate with another, it is certain that the electric matter must diffuse itself as far as the moisture of the earth reaches; and this we may reasonably suppose to be to the very centre.

With regard to the atmosphere, the case is equally clear. We have formerly mentioned in general, that Dr Franklin, and others, had collected electricity from the atmosphere in great quantity during the time of thunderstorms; but it is now found that it may be collected from the air at any time. The best instrument for this purpose is the electrical kite. Mr Cavallo, who hath made a great many experiments in atmospheric electricity, observes that the whole power of this machine lies in the string. A common school-boy's kite answers the purpose as well as any other. The best method of making the string is by twisting two threads of common twine with one of that copper-thread which is used for trimmings. When a kite constructed in this manner was raised, he says, he always observed the string to give signs of electricity, except once. The weather was warm, and the wind so weak, that the kite was raised with difficulty, and could hardly be kept up for a few minutes. Afterwards, however, when the wind increased, he obtained, as usual, a pretty strong positive electricity. Concerning the management of this kite he gives the following directions.

"In raising the kite, when the weather is very cloudy and rainy, in which time there is danger of meeting with a great quantity of electricity, I generally use to hang upon the string \(AB\) (Plate CLXXVII., fig. 78.) the hook of a chain \(C\), the other extremity of which falls on the ground. Sometimes I use another caution besides, which is to stand upon an insulating stool; in which situation, I think, that if any quantity of electricity, suddenly discharged by the clouds, strikes the kite, it cannot much affect any person. As to insulated reels, and other such like instruments that some gentlemen have used to raise the kite without any danger of receiving a shock; fit for the purpose as they may appear in theory, they are yet very inconvenient to be managed. Except the kite be raised in the time of a thunderstorm, there is no great danger for the operator to receive any shock. Although I have raised my electrical kite hundreds of times without any caution whatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunderstorm, if the kite has not been raised before, I would not advise a person to raise it while the stormy clouds are just overhead; the danger at such a time being very great, even with the precautions above-mentioned: at that time the electricity of the clouds may be observed, without raising the kite, by a cork-ball electrometer held in the hand in an open place, or, if it rains, by the electrometer for rain, to be described hereafter.

"By making use of this instrument, I am obliged to keep the kite up no longer than it is necessary to charge the phial, in order to observe the quality of the electricity in the atmosphere; for after the kite has been drawn in, and brought home, I can then examine the electricity of the inside of the phial, which is the same as that of the kite. When the electricity of the kite is very strong, I fix a chain communicating with the ground, at about six inches distance from the string, which may carry off its electricity in case this should increase so much as to put the bystanders in danger."

With all his caution, however, it seems Mr Cavallo could not always avoid danger, even when there was no thunder; as appears from the following account:

"October 18, 1775. After having rained a great deal down from the morning and night before, the weather became a cloud a little clear in the afternoon, the clouds appearing separated, and pretty well defined. The wind was west, and rather strong, and the atmosphere in a temperate degree of heat. In these circumstances, at three P.M., I raised my electrical kite with 360 feet of string. After the end of the string had been inflated, and a leather ball covered with tin-foil had been hanged to it, I tried the power and quality of the electricity, which appeared to be positive and pretty strong. In a short time, a small cloud passing over, the electricity increased a little; but the cloud being gone, it decreased again to its former degree. The string of the kite was now fastened by the silk lace to a post in the yard of the house, and I was repeatedly charging two coated phials and giving shocks with them. While I was so doing, the electricity, which was still positive, began to decrease, and in two or three minutes it became so weak that it could hardly be perceived with a very sensible cork-ball electrometer. Observing at the same time, that a large and black cloud was approaching the zenith (which, no doubt, caused the decrease of electricity), indicating imminent rain, I introduced the end of the string through a window in a first-floor room, wherein I fastened it by the silk lace to an old chair. The quadrant electrometer was set upon the same window, and was by means of a wire connected with the string of the kite. Being now three quarters after three o'clock, the electricity was absolutely imperceptible; however, in about three minutes time it became again perceptible; but, upon trial, was now found to be negative. It is therefore plain, that its stopping was nothing more than a change from positive to negative; which was evidently occasioned by the approach of the cloud, part of which by this time had reached the zenith of the kite, and the rain also had begun to fall in large drops. The cloud also came farther on; the rain increased; and the electricity keeping pace with it, the electrometer soon arrived at 15°. Seeing now that the electricity was pretty strong, I began again to charge the two coated phials, and to give shocks with them; but the phials had not been charged above three or four times, before I perceived that the index of the electrometer was arrived at 35°, and was keeping still increasing. The shocks being now very smart, I desisted from charging the phials any longer; and, considering the rapid advance of the electricity, thought to take take off the insulation of the string, in case that, if it should increase farther, it might silently be conducted to the earth without causing any bad accident by being accumulated in the insulated string. To effect this, as I had no proper apparatus near me, I thought to remove the silk lace, and fasten the string itself to the chair. Accordingly I disengaged the wire that connected the electrometer with the string; laid hold of the string; untied it from the silk lace, and fastened it to the chair: but while I effected this, which took up less than half a minute of time, I received about 12 or 15 very strong shocks, which I felt all along my arms, in my breast, and legs; shaking me in such a manner, that I had hardly power enough to effect my purpose, and to warn the people in the room to keep their distance. As soon as I took my hands off the string, the electricity (in consequence of the chair being a bad conductor) began to snap between the string and the shutter of the window, which was the nearest body to it. The snappings, which were audible at a good distance out of the room, were at first synchronous with the shocks which I had received; but, in about a minute's time, oftener; so that the people of the house compared their sound to the rattling noise of a jack going when the fly is off. The cloud now was just over the kite; it was black, and well defined, almost of a circular form, its diameter appearing to be about 40°. The rain was copious, but not remarkably heavy. As the cloud was going off, the electrical snapping began to weaken, and in a short time became inaudible. I went then near the string, and finding the electricity weak, but still negative, I insulated it again, thinking to keep up the kite some time longer; but observing that another larger and denser cloud was approaching towards the zenith, and I had then no proper apparatus at hand to prevent every possible bad accident, resolved to pull the kite in: accordingly a gentleman who was by me began pulling it in, while I was winding up the string. The cloud was now very nearly over the kite; and the gentleman told me that he had received one or two slight shocks in his arms; and that, if he was to receive another, he would certainly let the string go: upon which I laid hold of the string, and pulled the kite in as fast as I could without any further observation; being then ten minutes after four o'clock.—N. B. There was neither thunder nor lightning perceived that day, nor indeed for some days before or after."

From his observations on the electricity of the atmosphere, Mr Cavallo deduces the following conclusions:

1. That there is in the atmosphere at all times a quantity of electricity; for whenever I use the abovementioned instrument, it always acquires some electricity.

2. That the electricity of the atmosphere, or fogs, is always of the same kind, namely, positive; for the electrometer is always negative, except when it is evidently influenced by heavy clouds near the zenith.

3. That, in general, the strongest electricity is observable in thick fogs, and also in frothy weather; and the weakest, when it is cloudy, warm, and very near raining; but it does not seem to be less by night than in the day.

4. That in a more elevated place the electricity is stronger than in a lower one; for having tried the atmospheric electrometer both in the stone and iron gallery on the cupola of St Paul's cathedral, I found that the balls diverged much more in the latter than in the former less elevated place. Hence it appears, that if this rule takes place at any distance from the earth, the electricity in the upper regions of the atmosphere must be exceedingly strong."

The conclusions drawn from the experiments with the kites, are as follow:

1. The air appears to be electrified at all times; its electricity is constantly positive, and much stronger in frothy than in warm weather; but it is by no means less in the night than in the daytime.

2. The presence of the clouds generally lessens the electricity of the kite; sometimes it has no effect upon it; and it is very seldom that it increases it a little." To this the above mentioned influence is a most remarkable exception.

3. When it rains, the electricity of the kite is generally negative, and very seldom positive.

4. The aurora borealis seems not to affect the electricity of the kite.

5. The electric spark taken from the string of the kite, or from any insulated conductor connected with it, especially when it does not rain, is very seldom longer than a quarter of an inch; but it is exceedingly pungent. When the index of the electrometer is not higher than 20°, the person that takes the spark will feel the effect of it in his legs; it appearing more like the discharge of an electric jar than the spark taken from the prime conductor of an electrical machine.

6. The electricity of the kite is generally stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it. The electricity, for instance, brought down by a string of 100 yards, may raise the index of the electrometer to 20, when, with double that length of string, the index of the electrometer will not go higher than 25.

7. When the weather is damp, and the electricity is pretty strong, the index of the electrometer, after taking a spark from the string, or presenting the knob of a coated phial to it, rises surprisingly quick to its usual place; but in dry and warm weather it rises exceedingly slow.

II. The second position requisite for establishing Dr Franklin's theory is, "That glass and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it." This assertion evidently has a contradictory appearance. It is very difficult, if not impossible, to conceive, that any substance can be full of a fluid, and yet impermeable by that fluid; especially when we continually talk of putting in an additional quantity into one side, and taking out of the other. Nay, what is still more extraordinary, the thinner the glass is, i.e., the less quantity of electric matter it can contain, the more we are able to put into it; for the thinner a glass is, the more easily does it receive a high charge.

The chief arguments for the impermeability of glass by the electric fluid are drawn from the phenomena of the Leyden phial. It is indeed very plain, that there is in that case an expulsion of fire from the outside at the the same time that it is thrown upon the inside. This appears from numberless experiments, but is most readily observable in the following. Let a coated phial be set upon an inflating stand, and the knob of another phial brought near the coating of the first. As soon then as the electric sparks are discharged from the prime conductor to the knob of the first bottle, an equal number will be observed to proceed from the coating of the first to the knob of the second. This is very remarkable, and an unphilosophical observer will scarce ever fail to conclude, that the fire runs directly through the substance of the glass. Dr Franklin, however, concludes that it does not, because there is found a very great accumulation of electricity on the inside of the glass, which discovers itself by a violent flash and explosion when a communication is made between the outside and inside coatings. But it must be observed, that there is here no other reason for concluding the glass to be impermeable, except that we suppose the electric matter to be accumulated on one side of the glass, and deficient on the other. If this supposition therefore cannot be proved, the evidence of sense, which indeed is very strong in favour of the permeability, must undoubtedly preponderate. It is said, indeed, that if the glass was permeable by the electric matter, a phial would be discharged immediately after being charged, or rather could never be charged at all; because the matter would no sooner be thrown upon one side than it would fly off from the other. This supposition, however, depends entirely upon the above-mentioned one, namely, that in bodies positively electrified there is an accumulation, and in such as are negatively electrified there is a deficiency of fluid; which never can be proved.

Another argument against the permeability of glass and other electrics is, that coated phials, it is said, standing upon electric substances, cannot be charged. This, however, seems to be very much exaggerated. A phial, though ever so perfectly inflated, will always receive a charge from a machine that acts very powerfully. Nay, it is certain, that though a phial is placed in such a manner, that both its knob and outside coating are in contact with the prime conductor, it will still receive a charge; much less indeed in this case than in any other, but still the shock will be perceptible.

In 1759, Mr Wilson read a paper before the Royal Society, in which the permeability of glass by the electric fluid was asserted. The experiments from which he deduced this conclusion were the following. He took a very large pane of glass, a little warmed; and holding it upright by one edge, while the opposite edge rested upon wax, he rubbed the middle part of the surface with his finger, and found both sides electrified plus. He accounted for this from the electrical fluid passing through the glass from his finger to the opposite side. But here Dr Priestley observes, that on Franklin's principles it ought to be so. If one side be rubbed by the finger, it acquires from it some electrical fluid. This being spread on the glass as far as the rubbing extended, repels an equal quantity of that contained in the other side of the glass, and drives it out on that side, where it stands as an atmosphere, so that both sides are found positively electrified. Mr Wilson also tried another experiment, which seemed more decisive than the former: Having by him a pane of glass, one side of which was rough and the other smooth, he rubbed it slightly on one side; upon doing which, both sides were electrified minus. This also Dr Priestley attempts to reconcile with Franklin's hypothesis. "As the electric fluid, contained in the glass (says he), is kept equal in both sides by the common repulsion; if the quantity in one side is diminished, the fluid in the other side, being less repelled, retires inward, and leaves that surface also minus." But here it is impossible to avoid observing, that Dr Priestley's own words, in the strongest manner, militate against the doctrine he means to establish. The quantity of fluid in one side being diminished, that on the other, he says, retires inward. But into what does it retire? if into the substance of the glass, then the glass is undoubtedly permeable by it; and this is the very thing which Dr Priestley argues against.

III. "The electric matter violently repels itself, and attracts all other matter." The proofs of this position are chiefly derived from the following experiment, and others of a similar kind. Let a smooth piece of metal be inflated, and bring an excited glass tube near one end of it. A spark of positive electricity will be obtained from the other end; after which, if the tube is suddenly removed, the metal becomes electrified negatively. Here, then, it is said, is a plain repulsion of one part of the electric fluid by another. That contained in the tube repels the fluid contained in the nearest end of the metal; of consequence it is accumulated in the other end, and when the tube is removed, the metal is found to be deprived of part of its natural quantity of electricity, or is electrified negatively.—On such experiments as this, however, it is obvious to remark, that we ought first to prove that positive electricity consists in an accumulation, and negative electricity in a deficiency, of the electric fluid. But while this is only supposed, it is impossible that any proofs drawn from the supposition can be conclusive.

IV. "By the excitation of an electric, the equilibrium of the fluid contained in it is broken, and one part is overloaded with electricity, while the other contains too little." This position is entirely hypothetical. No electrician hath yet explained, in a satisfactory manner, how the fluid is procured by the excitation of glass or any other electric substance. Dr Priestley, instead of giving an explanation, proposes several queries concerning it. Mr Cavallo tells us, that the act of excitation pumps as it were the electric fluid from the rubber, and consequently from the earth. He adds, "By what mechanism one body extracts the electric fluid from another, is not yet known. The celebrated Father Beccaria supposes, that the action of rubbing increaseth the capacity of the electric, i.e. renders that part of the electric which is actually under the rubber, capable of containing a greater quantity of electric fluid: hence it receives from the rubber an additional share of fluid, which is manifested upon the surface of the electric, when that surface is come out from the rubber; in which state it looses, or, as it were, contracts its capacity. Signior Beccaria's experiment to prove this supposition is the following. He caused a glass plate to be rubbed by a rubber applied on one side of the plate, while it was turning vertically; and holding at the same time a linen thread on the other side of the plate..." Theory. plate just opposite to the rubber, he observed that the thread was not attracted by that part of the glass which corresponded to the rubber, but by that which was opposite to the surface of the glass that had just come out from the rubber; which shows, that the fluid acquired by the glass plate did not manifest its power until the surface of the glass was come out from the rubber." But from this experiment it seems impossible to draw any conclusion concerning the capacity of glass either one way or other. It is evident, therefore, that whatever parts of Dr Franklin's hypothesis rest on this supposition concerning excitation, are entirely void of evidence.

V. "Conducting bodies are permeable by the electric fluid through the whole of their substance, and do not conduct it merely over their surface." The proof of most commonly adduced in favour of this position, is the following experiment. Take a wire of any kind of metal, and cover part of it with some electric substance, as rosin, sealing-wax, &c., then discharge a jar through it, and it will be found that it conducts as well with as without the electric coating. This, says Mr Cavallo, proves that the electric matter passes through the substance of the metal, and not over its surface. A wire, adds he, continued through a vacuum, is also a convincing proof of the truth of this assertion. Even here, however, the proof, if impartially considered, will be found very defective. It is a fact agreed upon by all philosophers, that bodies which to us are apparently in contact, do nevertheless require a very considerable degree of force to make them actually touch one another. Dr Priestley found that a weight of six pounds was necessary to press two shillings into close contact, when lying upon one another on a table. A much greater weight was necessary to bring the links of a chain into contact with each other. It cannot be at all incredible, therefore, that a wire, though covered with sealing-wax or rosin, should still remain at some little distance from the substance which covers it. The following experiments of Dr Priestley also seem to be much in favour of the supposition that the electric fluid passes chiefly over the surface of conducting substances.

"From the very first use of my battery (says he), I had observed a very black smoke or dust to arise on every discharge, even when no wire was melted; and the brass chain I made use of was of a considerable thickness. I observed, that a piece of white paper, on which lay the chain I was using to make the discharge, was marked with a black stain, as if it had been burnt, wherever it had touched it. I neglected the experiment till some time after, observing a very striking appearance of the same kind, I was determined to attend to the circumstances of it a little more particularly. I made my chain very clean, and wrapping it in white paper, I made a discharge of about 40 square feet through it, and found the stain wherever it had touched the paper. Some time after I wrapped the paper, in the same manner, round a piece of brass wire; but, making a discharge through it, saw no stain. To ascertain whether this appearance depended upon the discontinuity of the metallic circuit, I stretched the chain with a considerable weight, and found the paper on which it lay, as the shock passed through it, hardly marked at all. Finding that it depended upon the discontinuity, I laid the chain upon white paper, making each extremity fast with pins stuck through the links; and when I had made the discharge, observed that the black stains were directly opposite to the body of the wire that formed the chain, and not to the intervals, as I had sometimes suspected. A chain five feet four inches long, which weighed one ounce seventeen penny-weights four grains, lost exactly half a grain after each discharge.

"In making the mark above-mentioned, I once happened to lay the chain so as to make it return at a sharp angle, in order to impress the form of a letter upon the paper; and observed, that on the discharge, the part of the chain that had been doubled was displaced, and pulled about two inches towards the rest of the chain. At this I was surprised, as I thought it lay so, that it could not slide by its own weight. Upon this I repeated the experiment with more accuracy. I stretched the whole chain along a table, laying it double all the way, and making it return by a very sharp angle. The consequence always was, that the chain was shortened about two inches, and sometimes more, as if a sudden pull had been given to it by both the ends. Suspecting that the black smoke which rose at every discharge, might come, not from the chain, but from the paper, or the table on which it lay, and which was probably burnt by the contact of it, I let the chain hang freely in the air; but, upon making the discharge, I observed the same great black smoke that had before risen from the paper or the table. Fig. 76 represents the spots made upon the paper by a chain laid over it. The breadth of the spots is about the mean thickness of the wire of the chain, and marks the place to which that part of the chain which returned was thrown back by the discharge.

"Being willing to try what would be the effect of laying the chain in contact with non-conductors, I dipped it in melted rosin till it had got a coating of considerable thickness. When it was quite stiff, I laid it carefully, without bending, upon white paper, and made the discharge through it. The rosin was instantly dispersed from all the outside of the chain, it being left as clean as if none had ever been put on. That with which the holes in the chain had been filled having been impelled in almost all directions, was beaten to powder; which, however, hung together but was perfectly opaque; whereas it had been quite transparent before this stroke. I next laid the chain upon a piece of glass, which was marked in the most beautiful manner wherever the chain had touched it; every spot the width and colour of the link. The metal might be scraped off the glass at the outside of the marks; but in the middle part it was forced within the pores of the glass. On the outside of this metallic tinge was the black dust, which was easily wiped off.

From these experiments it would seem, that the electrical flash had passed over the surface of the chain rather than through its substance; seeing it threw off the rosin with such extreme violence. The same thing appears from the manner in which electricity generally acts, which is not according to the solid contents of any substance, but according to the dimensions of its It is not to be doubted, however, but that, where a great quantity of electric matter is made to pass along a very small wire, it will enter the substance of the metal. This appears from the possibility of melting wires by the force of electric batteries, and even totally dissipating them into small globules. To accomplish this, it is only necessary to connect the hook communicating with the outside coating of a battery, containing at least 30 square feet of coated surface, with a wire that is about one-fiftieth part of an inch thick and about two feet long. The other end of it must be fastened to one end of the discharging rod; this done, charge the battery; and then by bringing the discharging rod near its wires, send the explosion through the small wire, which by this means will be made red hot and melted, so as to fall upon the floor in different glowing pieces. When a wire is melted in this manner, sparks are frequently seen at a considerable distance from it, which are red hot particles of the metal, that, by the violence of the explosion, are scattered in all directions. If the force of the battery is very great, the wire will be entirely dispersed by the explosion, so that none of it can be afterwards found. If it is required to melt such particles as cannot easily be drawn into wires, ores, for instance, or grain-gold, they may be set in a train upon a piece of wax: they are then to be put into the circuit, and an explosion sent through them, which, if sufficiently strong, will melt them as well as the wires. If a wire is stretched by weights, and a shock is sent through it which renders it just red hot, the wire, after the explosion, is found to be considerably lengthened.

VI. The last position on which Dr Franklin's theory depends, and which indeed may be called the foundation of the whole, is, "That positive electricity is an accumulation, or too great a quantity, of electric matter contained in a body; and negative electricity is when there is too little." Of this, however, there is not one solid proof; and all attempts that have hitherto been made to prove it, are only arguing in a circle, or proving the thing by itself. Thus, for instance, a body electrified positively, attracts one that is electrified negatively; because the first has too much, and the other too little, electric matter. But how do we know that one has too much, and the other too little, electricity? Because they attract each other. Again it has been proved, that when a phial is electrified positively, there is as constant a stream of fire from the outside coating, as there is from the conductor to the inside coating. Therefore, it is said, the outside of the glass has too little, and the inside too much, electricity. But how is this known to be the case? Because glass is impermeable by the electric fluid. And how is glass known to be impermeable? Because, in the above experiment, one side has too much, and the other too little, electricity. Thus in every instance, the arguments for Dr Franklin's hypothesis return into themselves, and no conclusion can be drawn from them. In the subsequent section, the nature of the electric fluid is particularly considered, where the improbability of its ever being accumulated in the substance of solid bodies will more plainly appear.

Sect. VI. An Inquiry into the Nature of the Electric Fluid; with an Attempt to explain the principal Phenomena of Electricity, from the known Laws by which other Fluids are observed to act upon one another.

In making this inquiry, or indeed any other, it is proper to take for granted as little as possible. No position should be assumed as the basis of any reasoning whatever, except what has been proved by incontrovertible facts. In the present case, therefore, it is sufficient to assume as a fact what hath been already proved by innumerable experiments, namely, That the air, the earth, and sea, all contain great quantities of electric fluid. The question which most naturally suggests itself when this is once admitted, is, Whence hath the electric fluid come? is it essentially inherent in these bodies, or hath it come from without? This cannot be resolved, without considering the nature of the fluid itself, and whether it is analogous to any other which is more generally known.

§ I. Proofs of the Identity of the Electric Fluid and Elementary Fire or Light of the Sun.

The similarity between the electric matter and fire, naturally suggested to the first observers, that it was nothing other than elementary fire, which pervaded all substances, as we have already mentioned. This, however, was objected to; and the principal objection was, that though the electric matter emitted light, and had the appearance of fire, it nevertheless wanted its most essential quality, namely, burning. In particular, the blast which comes from an electrified point, feels cold instead of being hot; and where great quantities of the fluid are forced with violence through certain substances, and thus set them on fire, it was thought that the fire might be occasioned by the internal commotion excited among their small particles. This objection, however, seems now to be totally removed. The dispute concerning the preferable utility of pointed or knobbed conductors for securing buildings from lightning, occasioned the fitting up of a more magnificent apparatus than had ever appeared before. An immense conductor was constructed at the expense of the board of ordnance, and suspended in the Pantheon. It consisted of a great number of drums covered with tin-foil, which formed a cylinder of above 155 feet in length, and more than 16 inches in diameter; and to this vast conductor were occasionally added 4800 yards of wire. The electric blast from this machine fired gunpowder in the most unfavourable circumstances, by that can be imagined, namely, when it was drawn off the electric by a sharp point, in which case it has generally less force than in any other. The method of doing this was as follows. Upon a staff of baked wood a stem of brass was fixed, which terminated in an iron point at the top. This point was put into the end of a small tube of Indian paper, made somewhat in form of a cartridge, about an inch and a quarter long, and two-tenths of an inch in diameter. When the cartridge was filled with common gun-powder, unbruised, a wire communicating with the earth was then fastened to the bottom of the brass stem. The charge in the great cylinder being continually kept up by the motion of the wheel, the top of the cartridge was brought ery very near the drums, so that it frequently even touched the tin-foil with which they were covered. In this situation a small faint luminous stream was frequently observed between the top of the cartridge and the metal. Sometimes this stream would set fire to the gun-powder the moment it was applied; at others, it would require half a minute or more before it took effect. But this difference in time was supposed to be owing to some small degree of moisture in the powder or the paper, which was always unfavourable to the experiment. Tinder was fired much more readily.

As it therefore appears, that the electric fluid, when it moves through bodies either with great rapidity, or in very great quantity, will set them on fire, it seems scarce disputable, that this fluid is the same with the element of fire. For further proofs of this opinion, which is now adopted by some very eminent philosophers, see the articles Fire and Heat. See also Chemistry-Index. This being once admitted, the source from whence the electric fluid is derived into the earth and atmosphere, must be exceedingly evident, being no other than the sun, or source of light itself. The vast quantity of light which continually comes from him to the earth must of necessity be absorbed by that opaque body, at least in great part. It is impossible it can remain there, because there is a perpetual succession of new quantities coming from the sun. It must be observed, however, that as this fluid receives a great number of different directions after once it enters the earth, it cannot appear in its natural form of fire or light, till it receives a new motion similar to what it had when proceeding from the sun. The solar light only burns, or produces heat, when diverging from a centre, or converging towards one. The heat is always greatest at the central point; and even there, no heat is produced except where the light passes through a resisting medium. In those cases likewise the electric fluid burns. When discharged with violence from an electrified bottle, it flies out on all sides, and then will fire gun-powder, or other combustible substances. The same thing it will do when converging towards a point, if in sufficient quantity, as was observed in the experiment with the large conductor above mentioned. But when the electric fluid neither meets with any considerable resistance, diverges from a centre, nor converges towards one, it is almost always invisible, and without heat. A most remarkable proof of this we have, even when a vast quantity of electric matter is forced to go through a very small wire. Dr Priestley tells us he had once an opportunity of observing what part of the conductors which form an electric circuit are most affected by the explosion. Upon discharging a battery of 51 square feet thro' an iron wire nine inches long, the whole of it was glowing hot, and continued so for some seconds. The middle part grew cool first, while both the extremities were sensibly red. When the wire was afterwards examined, both the extremities were found quite melted; an inch or two of the part next to them was extremely brittle, and crumbled into small pieces on being handled; while the middle part remained pretty firm, but had quite lost its polish, so that it looked dawker than before. This is precisely what would have happened, had both ends been put into a common fire.

We are very sure, that the same quantity of electric matter passed through the middle of the wire, that entered one end of it and went out at the other. Why then did it not produce the same degree of heat in the middle that it did at each end? The reason is plain: At one end it was in a state of convergence from the battery to the point of the wire; at the other, it was in a state of divergence from the point of the wire to the battery. At the points, therefore, an intense heat was produced; but in the middle, where the fluid neither converged nor diverged, but moved forwards in a parallel direction, the heat was much less. Now we know that this is the case with the solar light itself. At the focus of a burning-glass there is an intense heat both where the convergence ends and the divergence begins. But where this divergence considerably ceases, and the motion of the light becomes more parallel, the heat is vastly diminished. The case is the same with a common fire, and with all burning bodies; for heat never acts but from a centre, and is always greatest at the central point. It is true, that we can never produce electric fire without at the same time producing a violent shock exceedingly different from the burning of common fire. But the reason of this is, that we cannot produce a divergence in a stream of electric matter, without at the same time giving it such a motion in some other direction, that its impetus becomes very perceptible. If it was in our power to make the flash produced by an electric bottle keep its place, we cannot suppose that any shock, or other sensation than heat, would be felt. But there is no possibility of hindering it from flying with prodigious celerity from one side of the bottle to the other. Therefore, as it is neither in a state of divergence nor convergence, except where it comes out from and enters into the bottle, no sensation is perceived except what arises from its change of place; and hence it is said, that the electric matter hath no heat.

§ 2. The Identity of Electric Matter and Light farther considered; with some positive Proofs, that Electric Substances are actually penetrated by the Electric Fluid.

The only objection of any strength which can arise to the identity of the electric fluid and light is the supposition that the latter penetrates glass, and the impermeability of glass to the former when a piece of glass or any other electric substance is presented to it. Here, however, it must be observed, that light, as proceeding from a luminous body, must be regulated by very different laws from light which is absorbed by opaque bodies, and consequently subjected to motions quite different from what it originally had. Water, the only fluid with which we are very well acquainted (though all others we know seem to be regulated by the same laws), is capable of two very different motions. The one is a rectilinear one, by which great quantities of it run from one place to another. The other is not so easily explained. It may, however, be very readily observed, by throwing a small stone into a pool of water. A great number of concentric circles will be propagated from the place where the stone fell, as from a centre, which will gradually grow larger and larger. If another stone is thrown in at some distance, similar circles will proceed from the place where it fell. These will meet with the former, and cross them without interfering with each other in the least. It is certain, however, that two streams of water rushing opposite to one another, would shatter and destroy each other. If, therefore, there is a difference in the motion of the electric fluid when it burns, and when it does not (which there certainly is), we may easily suppose it possible, that glass should obstruct one kind of motion and not another: In which case, the glass would seem to be permeable by the fluid when manifesting itself by the first kind of motion, and not so when it manifests itself by the other.

It hath commonly been thought, that the transparency of bodies depends upon the rectilinear direction of their pores, and opacity upon the situation of them in some other direction. Electrical experiments, however, have shown that this is not the case. Sealing-wax and pitch are as opaque bodies as we are acquainted with; yet in Mr Hauksbee's experiments, mentioned no. 4, these substances were both rendered transparent by the action of the electric fluid. These experiments are confirmed by some others still more surprising, mentioned by Dr Priestley. See also below Sect. VIII. One was made by S. Beccaria. He discharged an electric shock through some brass dust sprinkled between two plates of sealing-wax. The whole was perfectly luminous and transparent. The most extraordinary experiment, however, was made by Dr Priestley himself, of which he gives the following account. "I laid a chain in contact with the outside of a jar lightly on my finger, and sometimes kept it at a small distance by means of a thin piece of glass; and, if I made the discharge at the distance of about three inches, the electric fire was visible on the surface of the finger, giving it a sudden concussion, which seemed to make it vibrate to the very bone; and when it happened to pass on that side of the finger which was opposite to the eye, the whole seemed perfectly transparent in the dark."

Experiments of this kind, though they have not hitherto been much pursued by electricians, seem to be more worthy of notice than almost all others. One consequence which may be derived from them is, that there is in bodies, whether electric or non-electric, a certain subtle medium, on the motion of which transparency depends. That is, when the medium is at rest, the body is opaque; but when set in motion, it becomes transparent. This motion, we see, may be given in two different ways. One is by simple electrification in vacuo, according to Mr Hauksbee's experiments. The other is, by sending the spark of an electrified bottle over their surface. In Dr Priestley's experiment, he could determine the motion to be of the vibratory kind; and hence we may easily conclude, that some bodies may be constructed in such a manner, that they are capable of transmitting the vibrations of this fluid, but not any other kind of motion. Such kinds of bodies will be naturally transparent; but others, whose particles are disposed in such a manner that the vibrations cannot be propagated thro' them without considerable violence, are naturally opaque. The question then only is, What is this subtle medium, the vibrations of which occasion transparency? It is scarce possible to answer this question in another manner than by saying, that it is the electric fluid. That it is this fluid which gives the power to electric substances, has never been denied. That the motion of this fluid along the surfaces of bodies throws another fluid within them into vibrations, is also evident from the experiments above mentioned. All bodies are confessed to have much of this fluid in their pores: therefore, if a quantity of the same matter passes over the surface of any body, it must affect what is within its substance with a motion of some kind or other; because it affects that which lies on the outside, and this cannot fail to affect all the rest. This motion Dr Priestley's experiment determines to be of the vibratory or tremulous kind; and, indeed, it is natural to think it should be so. The vibrations of the electrical fluid, therefore, conduct light through opaque bodies. But whatever fluid is conducted by the vibration of another, must itself also vibrate while it is so conducted. Light, therefore, vibrates when emitted from luminous bodies. In the present case, these vibrations are originally occasioned by the electric spark. They are conducted thro' opaque bodies by the vibrations of the electric fluid. The air is also full of the same fluid. The air is naturally transparent; but we have seen that transparency confines only in the easy transmission of a vibratory motion of the electric fluid. The light, therefore, is perpetually conducted by means of the vibrations of this fluid: therefore, the vibrations of the electric fluid and vibration of light are the same; for no two fluids are always capable of setting one another in motion precisely in the same manner, unless their nature is in all respects exactly the same.

These experiments seem in the strongest manner to prove the identity of the electric fluid and light, and that both are transmitted through electric as well as other substances. The reason, therefore, of the seeming stop, which is observed in our electrical operations by the intervention of glass, is, that in all artificial electricity, the fluid has a very considerable progressive motion, which cannot be easily propagated through the solid substance of any body, especially where there is a pretty strong resistance on the other side; which shall afterwards be shown to be the case with this fluid when passing through electric substances.

§ 3. Of the Passage of the Electric Fluid over the Surface, and through the Substance, of different Bodies.

Dr Priestley hath made many very curious experiments concerning the discharging of electric shocks ley's experiments over the surface of different bodies; and finds, that by this means a battery may be made to discharge itself at a much greater distance than it would do if sent directly through the air. The experiments were begun with ice; and he first accidentally discovered, that, when the shock of a common jar was discharged on a plate of ice, it would sometimes run over the surface and strike the chain directly on the other side. With a single jar, however, the distance was not much greater than what it would have passed over in the usual way; but, with a battery, it exceeded the usual distance in a very great degree. Endeavouring to make a circular spot, such as he had formerly made on metals, upon a fleish; piece of raw flesh, he took a leg of mutton, and laying the chain that communicated with the outside of the battery over the shank, he took the explosion on the outward membrane, about seven inches from the chain; but was greatly surprized to observe the electric fire not. Theory. to enter the flesh, but to pass in a body along the surface of it to come to the chain. Thinking that this might be occasioned by the fatty membrane on which the explosion was made, he again laid the chain in the same manner over the shank, and took the explosion upon the muscular fibres, where they had been cut off from the rest of the body; but still the fire avoided entering the flesh, made a circuit of near an inch round the edge of the joint, and passed along the surface to come to the chain as before, though the distance was near 11 inches. Imagining that this effect was promoted by the chain lying lightly on the surface of the flesh, and therefore not actually in contact with it, he took another explosion upon the hook of the chain, which was thrust into the flesh. On this the fire entered the mutton; and as he held it in his hands, both his arms were violently shocked up to his shoulders.

The Doctor next determined to try the effect of different conducting substances in the same manner; and of these water was the most obvious. "Next day, (says he) I laid a brass rod communicating with the outside of the battery, very near the surface of a quantity of water (to resemble the chain lying upon the surface of the flesh, without being in contact with it), and, by means of another rod furnished with knobs, made a discharge on the surface of the water, at the distance of several inches from any part of the rod; when the electric fire struck down to the water, and, without entering it, passed visibly over its surface till it arrived at that part of the rod which was nearest the water, and the explosion was exceedingly loud. If the distance at which I made the discharge exceeded seven or eight inches, the electric fire entered the water, making a beautiful star upon its surface, and yielding a very dull sound. When I first made this experiment of the electric flash passing over the surface of water, I thought it necessary, that neither the piece of metal communicating with the outside, nor that communicating with the inside, of the jars, should touch the water immediately before the discharge. But I afterwards found, that the experiment would answer, tho' either, or even both of them, were dipped in the water: for, in this case, the explosion would still prefer the surface to the water itself, if the distance was not very great; and would even pass at a greater distance along the surface, when there was a nearer passage from one rod to the other in the water."

He afterwards tried to pass the electric spark over the surfaces of a great number of different bodies, but found it impossible with many of them. He therefore imagined that this property of conducting a shock over its surface was peculiar to water and raw flesh. It was found, however, that the spark passed over the surface of a touch-stone, and likewise over a piece of the best kind of iron ore, exceedingly smooth on some of its sides. The piece was about an inch thick, and three inches in its other dimensions. The full charge of a jar of three square feet would not enter it. The explosion passed over the surface of oil of vitriol, with a dull sound and a red colour; but in all other cases, if it passed at all, it was in a bright flame, and with a report peculiarly loud. It passed over the surface of the most highly rectified spirit of wine without firing it; but when too great a distance was taken, the electric fire entered the spirit, and the whole was in a blaze in a moment.

This was the case when such substances were employed as are but indifferent conductors of electricity; raw flesh, for instance, water, &c. When good conductors were used, such as charcoal of different kinds, no remarkable appearances were produced. So far was the shock from passing visibly over the surface of any metal, that, if the distance through the air, in order to a passage through the metal, was ever so little nearer than the distance between the two surfaces, it never failed to enter the metal; so that its entering the surface of the metal, and its coming out again, seemed to be made without obstruction. If as much water was laid on a smooth piece of brass as could lie upon it, it would not go over the surface of the water, but always struck thro' the water into the metal. But if the metal lay at any considerable depth under the water, it would prefer the surface. It even passed over three or four inches of the surface of water as it was boiling in a brass pot, amidst the steam and bubbles, which seemed to be no hindrance to it. Animal fluids, however, of all kinds, seemed peculiarly to favour this passage of the electric matter over their surface; and the report of these explosions was manifestly louder than when water was used. In all cases of this kind, the report was considerably louder than when the discharge was made in the common way. The explosions were observed by persons out of the house, and in a neighbouring house, very much to resemble the smart cracking of a whip. "But (says Dr Priestley) the sound made by these explosions, though by far the loudest that ever I heard of the kind, fell much short of the report made by a single jar, of no very great size, of Mr Rackrow's; who says, that it was as loud as that of a pistol." He also observes, that when the electrical explosion does not pass over the surface of the water, but enters it, a regular star is made upon the surface, consisting of ten or a dozen rays; and what is very remarkable, those rays which stretch towards the brass rod that communicates with the outside of the battery are always longer than the rest; and if the explosion is made at such a distance as to be very near taking the surface, those rays will be four or five times longer than the rest, and a line bounding the whole appearance will be an ellipse, one of whose foci is perpendicularly under the brass knob with which the discharge is made.

When an electric battery is discharged upon smooth circular pieces of metal, the effects are very different from any spots produced of those we have yet mentioned. Dr Priestley having constructed some large batteries, determined to try what would be the effects of a very great electric power discharged upon metals and other substances; and, in the course of his random experiments, he made the following discoveries. "June 13, 1766 (says he), after having discharged a battery of about 40 square feet with a smooth brass knob, I accidentally observed upon it a pretty large circular spot, the centre of which seemed to be superficially melted, in a great number of dots; larger near the centre, and smaller at a distance from it. Beyond this spot was a circle of black dust which was easily wiped off: but what I was most struck with was, that after an interruption of melted places, there was an entire and exact circle of shining dots, consisting of places superficially melted like those at the The appearance of the whole, exclusive of the black dust, is represented Plate CLXXVII. fig. 75. n° 1.

"June 14th, I took the spot upon smooth pieces of lead and silver. It was in both cases like that on the brass knob; only the central spot on the silver consisted of dots disposed with the utmost exactness, like radii from the centre of a circle, each of which terminated a little short of the external circle. I took the circular spot upon polished pieces of several metals with the charge of the same battery, and observed that the cavities in some of them were deeper than in others; as I thought in the following order, beginning with the deepest, tin, lead, brass, gold, steel, iron, copper, silver. I will not be positive as to the order of some of the metals; but silver was evidently not affected a fourth part so much as gold, and much less than any of the others. The circles were marked as plain, but the impression was more superficial.

"I also made the explosion between a piece of lead just solid after melting, and another smooth piece that I had kept a considerable time. The piece of fresh lead was melted more than the other, but there was no other difference between them. The semimetals, as bismuth and zinc, received the same impression as the proper metals; being melted nearly as much as iron. I made three discharges between a piece of highly polished steel and a piece of very smooth iron, and in all cases thought the steel was more deeply melted than the iron.

"Presently after I had observed the single circle, I imagined, that, whatever was the cause of the appearance, it was not improbable but that two or more concentric circles might be procured, if a greater quantity of coated glass was used, or perhaps if the explosion was received upon metals that were more easily fused than brass. Accordingly, June 27, taking the moderate charge of a battery, consisting of about 38 square feet, upon a piece of tin, I first observed a second outer circle, at the same distance from the first, as the first was from the central spot. It consisted of very fine points hardly visible, except when held in an advantageous light; but the appearance of the whole was very beautiful, and was such as is represented Plate CLXXVII. fig. 75. n° 2.

"Having hitherto found the circles the most distinct on metals that melt with the least degree of heat, I soon after procured a piece of that composition which melts in boiling water; and having charged 60 square feet of coated glass, I received the explosion with it, and found three concentric circles; the outermost of which was not quite so far from the next to it, as that was from the innermost. All the space within the first circle was melted; but the space was very well defined, and by no means like a central spot, which in this case was quite obliterated. The appearance of these three concentric circles is represented Plate CLXXVII. fig. 75. n° 3. The distance at which the discharge was made occasioned no difference in the diameter of these circular spots. When, by putting a drop of water upon the brass rod communicating with the inside of the battery, I made the discharge at the distance of two inches; the spot was just the same as if it had been received at the distance of half an inch. i.e., about a quarter of an inch in diameter. Attempting to send an electric shock over the surface of quicksilver or melted lead, I found that it would not pass; though neither of the rods with which the discharge was made touched the metals. A dark impression was made on the surfaces of both the quicksilver and the lead of the usual size of the circular spot; and remained very visible notwithstanding the state of fusion in which the metals were."

§ 4. The Electric Fluid moves through the Substance of Electrics, though with difficulty. In most Cases, it passes over the Surface of good Conductors.

This will appear from a consideration of the phenomena above mentioned, and some others. The electric fluid universally present is air. That the fluid pervades its substance is evident to our eye-sight; for if a pointed body is placed on the prime conductor, and at the same time the cylinder is briskly turned, a continual stream of blue fire will be observed to issue from the point. This is undoubtedly the fluid itself made visible by the resistance it meets with from the air. That the electric fluid in this case pervades the air to a considerable distance, is also evident from the different methods by which the air of a room may be electrified. One method is that above mentioned: One or more needles are fixed on the prime conductor, which is kept strongly electrified for about 10 minutes. If, afterwards, an electrometer is brought into the room, the air will show that it has received a considerable quantity of electricity; for the balls will separate, and continue to do so even after the apparatus has been quite removed out of the room. Another method of electrifying the air is to charge a large jar and insulate it; then connect a sharp-pointed wire, or a number of them, with the knob of the jar; and make a communication from the outside coating to the table. If the jar is charged positively, the air of the room will likewise soon become electrified positively; but if the jar is charged negatively, the air will also become negative. To this it may be replied, that the air is always full of conducting substances, and that by means of them the electricity is propagated from one part of the air to another. But whether this is the case or not, it is certain that the air, notwithstanding all the conducting substances it may contain, is in fact an electric, and capable of receiving a charge like glass or any other electric substance. To this purpose there is a very curious experiment made in the following manner. Take two plates of smooth boards, of a circular form, and each about three or four feet in diameter. Coat one side of each with tin-foil, which should be patted down and burnished, and turned over the edge of the board. These boards must be both insulated, parallel to one another, in a horizontal position. They must be turned with their coated sides towards each other; and should be placed in such a manner as to be easily moved to or from each other; to do which, it will be proper to fix to one of the boards a strong supporter of glass or baked wood, and to suspend the other by silk strings from the ceiling of the room; from which it may be lowered at pleasure by means of a pulley. When these boards are placed in the manner above described, and about an inch distant from one another, they may be used exactly as the coatings of a pane of glass. If a spark is given from the conductor to the upper board, a spark will... instantly be discharged from the lower one, if any conducting substance is presented to it. By continuing to give sparks to the upper board, and to take them from the lower one, the air between them will at last become charged like a piece of glass; and if a communication is made between them, they will explode, give the shock, &c. like glass.

In this experiment it seems impossible to deny that the air is penetrated by electric fluid. The distance of an inch is so small, that it must appear ridiculous to say that this space is penetrated only by a repulsive power, when in other cases we plainly see the fluid penetrating it to three or four times that distance. The flat surface of the boards indeed makes the motion of the electric fluid through the plate of air gradual and equal, so that it is not seen to pass in sparks or otherwise; but this is necessary to its receiving a charge, as will be afterwards explained.

If one electric substance is penetrable by the electric fluid, we must be led strongly to suspect at least, that all the rest are so too. That rosin, pitch, sealing-wax, &c. are so, hath been already proved; and from thence, if we reason analogically, we must conclude, that glass is likewise penetrable by it. A very strong additional proof of this is, that the electric shock cannot be felt over the surface of glass. If this substance was altogether impenetrable to the fluid, it is natural to think, that it would run over the surface of glass very easily. But instead of this, so great is its propensity to enter, that a shock sent through between two glass plates, if they are pressed pretty close together, always breaks them to pieces, and even reduces part of them to a powder like sand. This last effect cannot be attributed to any other cause than the electric fluid entering the pores of the glass; and, meeting with resistance, the impetus of its progressive motion violently forces the vitreous particles asunder in all directions.

To this violent impetus of the electric fluid, when once it is set in motion, we may also with some probability ascribe the bursting of electric globes, both such as are made of glass, and other materials, in the act of excitation. Dr Priestley hath given several instances of this accident. "The fragments (says he) have been thrown with great violence in every direction, so as to be very dangerous to the bystanders. This accident happened to Mr Sabbatelli in Italy, Mr Nollet in France, Mr Berand at Lyons, Mr Boze at Wittenberg, Mr Le Cat at Rouen, and Mr Robein at Rennes. The air in the inside of Mr Sabbatelli's globe had no communication with the external air, but that of the Abbe Nollet had. This last, which was of English flint glass, had been used for more than two years, and was above a line thick. It burst like a bomb in the hands of a servant who was rubbing it, and the fragments, none of which were above an inch in diameter, were thrown to a considerable distance. The Abbe says, that all the globes which were burst in that manner, exploded after five or six turns of the wheel; and he attributes this effect to the action of the electric matter making the particles of glass vibrate in a manner he could not conceive.

"When Mr Berand's globe burst (and he was the first to whom this accident was ever known to happen), he was making some experiments in the dark on the 8th of February 1750. A noise was first heard as of something rending to pieces; then followed the explosion; and when the lights were brought in, it was observed that those places of the floor which were opposite to the equatorial diameter of the globe were strewed with smaller pieces, and in greater numbers, than those which were opposite to other parts of it. This globe had been cracked, but it had been in constant use in that state above a year; and the crack had extended itself from the pole quite to the equator. The proprietor ascribed the accident to the vibrations of the glass, and thought the crack had some way impeded these vibrations. When Mr Boze's globe broke, he says that the whole of it appeared, in the act of breaking, like a flaming coal. Mr Boulanger says, that glass globes have sometimes burst like bombs, and have wounded many persons, and that their fragments have even penetrated several inches into a wall. He also says, that if globes burst in whirling by the gun-barrel's touching them, they burst with the same violence, the splinters often entering into the wall. The Abbe Nollet had a globe of sulphur which burst as he was rubbing it with his naked hands, after two or three turns of the wheel, having first cracked inwardly. It broke into very small pieces, which flew to a great distance, and into a fine dust; of which part flew against his naked breast, where it entered the skin so deep, that it could not be got off without the edge of a knife."

From these appearances we must necessarily conclude, not only that the electric fluid moves within the electric substance of electric bodies, but that it sometimes moves with extreme violence; so that its repulsive force separates even the minutest particles from each other; of conductors and this could not happen without a thorough penetration of the electric body. It seems more difficult to prove, that the electric matter does not generally pass directly through the substance of metals, but over their surface. A little consideration, however, will show, that this must very probably be the case. If we compare Dr Priestley's experiments on metals related in § 3, with the effects of the solar light collected in the focus of a burning-glass upon the same metals, we shall find a considerable degree of resemblance. Under the article Burning-glass, it is observed, that notwithstanding the prodigious power of that concave mirror with which Mr Macquer melted platinum, all bodies did not melt equally soon in the focus. In particular, polished silver, though a very fusible metal, did not melt at all. It is not to be doubted, that this was owing to the complete reflection of the light by the silver; and had polished pieces of all the metals been tried, it is equally certain, that the difficulty of melting them would have been found exactly proportioned to their reflective power. Something like this happened with Dr Priestley; for silver was less touched by the electric explosion than any other metal. The violent progressive motion of the fluid indeed forced it into the metal, but at the same time the reflective power of the silver hindered it from going so deep as it had done in the others. The case was still more evident when melted lead and quicksilver were used. These have a very great reflective power; and though by reason of the extreme violence wherewith the fluid struck them, part of their substance might naturally have... have been supposed to be dissipated in the hard metals, yet we find this was not the case. Only a black spot was made on the surface, and the fluid was immediately dispersed, most probably over the surface of the metal.

It is not indeed easy to bring a decisive proof in favour of this hypothesis. The extreme subtilty, and, in most cases, invisibility, of the electric fluid, render all reasoning about its motions precarious. It is incredible, however, that this fluid should pass through the very substance of metallic bodies, and not be in the least retarded by their solid particles. In those cases, where the solid parts of metals are evidently penetrated, i.e., when wires are exploded, there is a very manifest resistance; for the parts of the wire are scattered about with violence in all directions. The like happened in Dr Priestley's circles made on smooth pieces of metal. Part of the metal was also dispersed and thrown off, for the circular spots were composed of little cavities. If therefore the fluid was dispersed throughout the substance; and not over the surface of the metal, it is plain, that a wire whose diameter was equal to one of those circular spots, ought also to have been destroyed by an explosion of equal strength sent through it. But this would not have been the case. A wire whose diameter is equal to one of those circular spots represented in figs. 1, 2, 3, fig. 75, Plate CLXXVII, would without injury conduct a shock much greater than any battery hitherto constructed could give. It is most probable therefore, that though violent flashes of electricity, which act also as fire, will enter into the substance of metals and consume them; yet it immediately disperses itself over their surface, without entering the substance any more, till being forced to collect itself into a narrow compass it again acts as fire.

In many cases, the electric fluid will be conducted very well by metals reduced to a mere surface, so that we can scarcely say they have any thickness at all. A piece of white paper will not conduct a shock without being torn in pieces, as it is an electric substance. But a line drawn upon it with a black-lead pencil will safely convey the charge of several jars. It is impossible we can think that the fire here passes through the substance of the black-lead stroke. It must run over its surface; and if we consider some of the properties of metals, we shall find, that there is very great reason for believing that their conducting power lies at their surface.

The metals are, of all terrestrial substances, those which reflect the light most powerfully. Sir Isaac Newton hath shown that this reflective power they have not from their substance as metals, but from what he calls a repulsive power, spread equally over their surface. The existence of this repulsive power hath already been taken notice of in several instances, particularly in that of a chain, whose links cannot be brought into contact with each other without a considerable degree of force. It is exceedingly probable, that the repulsive power by which the links of the chain are kept asunder, and that by which the rays of light are reflected, are one and the same. As the electric fluid is known to pervade all substances, and metals as well as others, it seems also probable, that the repulsive and reflective power on the substance of metals is no other than the electric fluid itself in a quiescent state. Perhaps it may be thought absurd to ascribe the reflection of light to a substance of such extreme fluidity and tenacity as the electric fluid is; but we find that the vacuum of an air-pump, a medium of nearly equal tenacity with the electric fluid (as will elsewhere be proved), is in some cases capable of reflecting light very powerfully. Now it is certain, that nothing can be supposed to give such an easy passage to the electric fluid as itself; because it is the thinnest and most subtle of all the substances we know, and therefore must make the least resistance. Hence the fluid slides over the surface of a piece of metal with surprising ease; and when a large surface of metal is electrified, the effect is proportionable to the extent of it, because all that quantity of electric fluid which is spread over the surface, easily receives the motion communicated by the electrical machine.

The vacuum of an air-pump is found to be a very good conductor, and by means of it the motion of the fluid is rendered visible. Hence this is brought as an argument that the electric fluid always passes through the substance of conductors. That it doth so in some cases is indeed very evident, but it then meets with considerable resistance; and, even in the present instance, the passing through the vacuum of an air-pump, where it is opposed by a considerable quantity of the same kind of fluid, gives such a considerable resistance, that it will prefer a passage along a metallic rod to one through a vacuum. With regard to charcoal, and other conductors of that kind, as they are very porous, and likewise composed of fine spicules, it is probable the fluid may run along the surface of the spicules, and at the same time through the substance of the coal. Even in passing over the best conductors, however, this fluid meets with some resistance, so it will prefer a short passage through the air to a long one through the best conductors.

§ 5. The exceeding great Velocity and Strength of the Electric Fluid are not owing to a repulsive Power among its Particles, but to the mutual Action of the Air and Electric Fluid upon themselves and one another.

The arguments for a repulsive power existing between the particles of the electric fluid are very inconclusive. Some of them have been already taken notice of. The strongest is that drawn from the appearance of the electric fire issuing from a point, or from any fluid shown body highly electrified. In the open air this diverges excessively; and very often divides into several distinct rays, which by avoiding each other seem to be violently repulsive. That they are not so in reality, however, is plain from the appearance they have in vacuo; when, the resistance of the atmosphere being taken off, the electric light would have room to spread more widely. Fig. 27, Plate CLXXIV, represents an exhausted receiver with an electrified wire discharging a stream of this fluid from itself, by means of its communication with a machine. If the electric matter then was really elastic, or endowed with a power repulsive of itself, it is impossible it could pass in an uninterrupted column through an exhausted receiver as in the figure. A column of air, if blown swiftly through the orifice of a small pipe, will go forward a considerable way, if it is counterbalanced by air like itself on every side. But if such a column enters a vacuum, Theory. what we call its elasticity, occasions it to be dissipated in a moment, and equally diffused through the whole exhausted receiver. But this by no means happens to the electric fluid; for even the small divergency represented in the figure seems entirely owing to some quantity of air left in the air-pump. Dr Watson, by means of a long bent tube of glass filled with mercury, and inverted, made all the bended part which was above the mercury the most perfect vacuum that could be made. This vacuum he insulated; and one of the basins of mercury being made to communicate with the prime conductor, when some non-electric substance touched the other, the electric matter pervaded the vacuum in a continued arch of lambent flame, and, as far as the eye could follow it, without the least divergence. From these experiments it appears, that there is in the vacuum of an air-pump, as well as in the Torricellian vacuum, a fluid of nearly the same density with the electric one; that the electric fluid is not repulsive of itself, but is resisted by the atmosphere; and therefore all appearances of electrical light are less bright in vacuo than in the open air; because, the more resistance the matter meets with, the brighter is the flash.

Thus, as long as a stream of electric fluid is moved through a medium of an equal density with itself, the equable pressure of the fluid all round will keep the luminous streams from diverging; but if the pressure is taken off from any part of the receiver, the pressure of the rest will immediately force the stream to that place, as represented fig. 28. That it is by a pressure of this kind, and not by any obscure attractive power, that this is occasioned, will be rendered very probable from the following example. Suppose a pot or kettle is boiling violently over a fire, and in such a situation that there is very little agitation in the surrounding air. The equal pressure of the atmosphere will then force the steam straight upwards in a cylindrical column; but if any object is brought near the edge of the pot, so that the pressure of the atmosphere is taken off on one side, the steam will be directly forced upon that body, or seemingly attracted by it. The electric matter therefore, being capable of having its motions resisted by the air, must immediately fly to that place where the resistance is least; but in the case above mentioned, this is best done by applying a conducting substance to the side of the receiver, or one along which the fluid can run downward to the earth. This, however, will be more fully explained when we speak of the phenomena of the Leyden phial.

From this simple principle, viz. that fluids impelled by any force will always tend towards that place where there is the least resistance, most of the phenomena of electricity may be explained. The first thing to be considered is, From what source it originally derives the astonishing agility and strength displayed in its motions. If it is granted that the electric fluid is the same with the solar light, the ultimate cause of its momentum must be the power by which the light of the sun is emitted. As this power extends through regions of space which to our conceptions are truly infinite, so must the power itself be; and it is plain, that by its equable action all round, throughout the whole space thro' which the sun's light is propagated, the pressure of it upon all bodies must be equal all round, and consequently it can neither move them one way nor another. But if, by the intervention of some other power, the pressure is lessened upon any particular part, a current of electric matter will set towards that part, with a force exactly proportioned to the diminution of the pressure. Thus, in the common experiments of the air-pump, when the air is exhausted from a glass vessel, the pressure of the superincumbent atmosphere is directed towards every part of the glass; so that if it is of a flat square shape, and not very strong, it will certainly be broken. But after the air is exhausted, the vessel is discovered to be full of another subtle fluid of the same nature with the electric one *. If this could also be extracted from the vessel, the pressure on its sides would necessarily be much greater, because not only the atmosphere, but the whole surrounding ether or electric matter, would urge towards the place; and it is not probable, that this pressure could be resisted by any terrestrial power whatever. The momentum of the electric matter therefore, in our experiments, depends on two causes, viz. the pressure of the atmosphere upon the electric matter, and the pressure of one part of this matter upon another. The celerity with which it moves may be explained from its parts lying in contact with each other throughout the wide immensity of space. Hence the great tendency of the fluid to circulate; because, from whatever point a stream of it is sent off, there the pressure is lessened, and the stream, finding no place empty for its reception, must necessarily have a tendency to return to the place from whence it came, as there it meets with the least resistance; and hence, when a passage is opened for it, by which it can return to this point, it is urged thither with great violence, the equable pressure is restored, and the artificial motion ceases.

§ 6. The Manner in which an Electric Substance becomes excited, or diffuses its Electric Virtue.

This will easily appear from considering the means taken for the excitation of a common cylinder for electric experiments. The glass is a substance, as we have already seen, into which the electric matter is very apt to enter. To the surface of the glass is applied some amalgam spread on leather. This is a metallic substance which has an exceeding great reflective power, being that which is employed for silvering looking-glasses. The electric fluid therefore runs over its surface with great ease, and there is always a certain quantity of this fluid in a state of stagnation on its surface. At the place where the cylinder touches the amalgam, the air is excluded, and consequently the electric fluid hath there a tendency to rise more than at any other part of the surface where the atmosphere presses with its full force. When the cylinder begins to turn, it necessarily forces before it a small quantity of that electric matter which lay upon the surface of the amalgam. To understand this the more easily, we must consider that property which glass has of transmitting the electric fluid through it, and refusing it a passage along its surface. Thus we may conceive it to be formed of a vast number of exceedingly small tubes placed close to each other. If we suppose any substance made by art of such a texture, we would find it impossible to pour water along its surface, though it would very easily run through it. If such a substance was made in the shape of a cylinder, and turned briskly round, with its surface just touching a quantity of water contained in a vessel, the consequence would be, that the water would be scattered around in all directions. The case seems to be the same with the more subtle electric fluid. The glass cylinder throws out part of the electric fluid lying on the surface of the amalgam. This quantity is perpetually renewed from the conducting side of the rubber. The quantity which is thrown out cannot be conducted over the surface of the glass, nor can it pass through it; because it is resisted by the air in the inside, and, in some measure, by the glass itself. It is also resisted by the air on the outside; but as that resistance is less than what is made by the air and glass both put together, the fluid naturally forces itself into the open air. Still, however, there neither is, nor can be, any accumulation of the matter itself. It cannot enter the air without displacing the electric matter which was there before. This will displace more of the same kind, and so on, till at last the motion is communicated to the electric matter lodged in some part of the earth. From thence it is propagated to the rubber of the electric machine, and thus a kind of circulatory motion is carried on. By the excitation of an electric substance, therefore, the fluid is not accumulated, but only set in motion. The reason of that seeming accumulation observable about the excited cylinder is, the resistance which the fluid meets with from the air. This instantly produces a divergency in the stream of electric matter, and a vibratory struggle between it and the air; which again produces the appearances of fire and light, for the reasons already given.

That this kind of vibratory motion or struggle between the electric fluid and air always takes place when the latter is set in motion, seems evident from the sensation which is felt when a strongly excited electric is brought near any part of the human body. This is such as would be occasioned by a spider's web drawn lightly along the skin, or rather by a multitude of small insects crawling upon the body. It is, however, more clearly proved by an experiment made by Dr Priestley. He was desirous to know whether the electric fluid was concerned in the freezing of water or not. For this purpose, he exposed two dishes of water to the open air in the time of a severe frost. One of them he kept pretty strongly electrified; but could observe no difference in the time either when it began to freeze, which was in about three minutes, or in the thickness of the ice, when both had been frozen for some time. Happening to look out at the window through which he had put the dishes, he observed on each side of the electrified wire the same dancing vapour which is seen near the surface of the earth in a hot day, or at any time near a body strongly heated.

If the glass cylinder which we want to excite is exhausted of air, the electric matter, instead of flying off into the air, runs directly through the glass; and, meeting with some resistance from the vacuum as it is called, a weak light is produced in the inside, but no signs of electricity are perceived on the outside of the glass. The same thing happens by giving the cylinder or tube a metallic coating. The fluid collected from the rubber runs directly through the glass, and along the surface of the metallic coating, which keeps off the pressure of the air contained in the glass. If an electric lining is used, and the glass is exhausted of air, the motion of the fluid becomes visible through both, and the whole is transparent, as already observed. If the cylinder is lined with an electric substance, and the air is not exhausted, the electricity on the outside is often considerably increased; but the reason of this is not evident. Most probably it is owing to the different kind of electricity acquired by the inside lining; for electricity of any kind always produces its opposite at a small distance, the reason of which shall be afterwards given.

If the air within the cylinder is condensed, the electrical appearances on the outside are lessened in proportion. The reason of this seems to be, that though it is necessary that the fluid should not go through the substance of the glass very easily, yet it is requisite that its passage should not be totally obstructed; and therefore the electric experiments succeed best when the air within the glass is a little rarefied. We must also consider, that when an additional quantity of air is forced into the cylinder, an equal bulk of electric matter is forced out. The rest of the matter, therefore, which is contained all round the glass, presses violently into its pores; but this pressure, being directly opposite to what happens when the glass is excited, must of consequence hinder the excitation. If the glass is now made very hot, the pressure of the atmosphere is kept off, and the passage of the electric fluid through the glass and condensed air is rendered easier, and therefore the electric appearances on the outside return.

On the same principles may we explain the excitation of a solid stick of glass, sealing-wax, or sulphur. Though these have no air within them, yet they have a very considerable quantity of electric matter, which resists an expulsion from its place; and therefore, though it may yield a little when the rubber is applied to the outside, yet it will instantly throw off into the atmosphere what the rubber has left on the surface; because the resistance is least towards that place, as soon as the electric has come out from under the rubber. Hence also we see the reason why no signs of electricity are observed on glass to which the rubber is immediately applied; namely, because the pressure being equally great all round, no part of the electric fluid can be thrown off into the atmosphere, in order to let the rest in motion.

The only thing necessary to be added in confirmation of this theory of excitation is, that electric substances of the same kind cannot be excited by rubbing them against one another. Thus glass cannot be excited by rubbing it against glass, &c. Mr Willeke observed, that when two pieces of glass were rubbed upon each other in the dark, a very vivid light appeared upon them; which, however, threw out no rays, but adhered to the place where it was excited. It was attended with a strong phosphorescent smell, but no attraction or repulsion. From this experiment he inferred, that friction alone would not excite electricity; but that to produce this effect, the bodies rubbed together must be of different natures with respect to their attracting the electric fluid. § 7. Of Positive and Negative Electricity.

From what hath been already advanced, it will pretty plainly appear, that to increase the quantity of electric fluid in any body is a thing impossible, unless we also augment the size of the body. All the fine pores of every terrestrial fluid are exceedingly full, and unless we separate the minute particles of the body farther from one another than they are naturally, we cannot introduce more of the electric fluid into it than there was before. This fluid, we have already seen, is not, like the air, endued with a repulsive force between its particles; and therefore it must be incompressible. If it is incompressible, all the phenomena attending it must be owing to its various motions, and the seeming accumulations of it must be owing only to its more brisk action in some places than in others. But before a complete solution of the phenomena of positive and negative electricity can be given, it is necessary to show that these are not so essentially distinct and opposite as they have been thought to be, but may be converted into each other in such cases as we cannot possibly suppose either an addition or subtraction of the electric fluid.

This position, however opposite to the common opinions on the subject, may be proved by the following experiments. 1. Let a coated phial be set upon an insulating stand, and let its knob be touched by the knob of another phial negatively electrified. A small spark will be observed between them, and both sides of the insulated phial will instantly be electrified negatively. Now, though we may suppose the one side of the phial which is touched by the negatively electrified one to lose part of its fire, yet this cannot be the case with the other, because there is nothing to take it away, and therefore it ought to appear in its natural state. 2. Let a phial, having a pith-ball electrometer fastened to its outside coating, be slightly charged positively, and then set upon an insulating stand. The outside is then negatively electrified, or, according to Dr Franklin's theory, has too little electric matter in it. The pith-balls, however, will touch each other, or separate but in a very small degree: but let the knob of another bottle, which hath received a strong charge of positive electricity, be brought near to the knob of the first, and the pith-balls on the outside will diverge with positive electricity. Now, it is impossible that any substance can have both too much and too little electric matter at the same instant: yet we see that negative electricity may thus instantaneously be converted into the positive kind, in circumstances where no addition of fire to the outside can be supposed. 3. Let the same phial, with the pith-balls affixed to its outside coating, be slightly charged negatively, and then insulated. The outside is now electrified positively, or, according to Dr Franklin's hypothesis, has too great a quantity of electric fluid. Nevertheless, upon bringing the knob of a phial strongly electrified negatively to that of the insulated one, the pith-balls will instantly diverge with negative electricity. 4. Let a phial receive as full a charge of positive electricity as it can contain, and then insulate it. Charge another very highly with negative electricity. Bring the knob of the negative bottle near that of the positive one, and a thread will play briskly between them. But when the knobs touch each other, the thread after being attracted will be repelled by both. The negative electricity is somehow or other superinduced upon the positive; and, for a few moments after the bottles are separated, both will seem to be electrified negatively. But if the finger is brought near the knob of that bottle on which the negative electricity was superinduced, it will instantly be dissipated, a small spark strikes the finger, and the bottle appears positively charged as before.

From these metamorphoses of positive into negative, or negative into positive, electricity, it seems proven in the most decisive manner, that positive electricity doth not consist in an accumulation, nor the negative kind in a deficiency, of the electric fluid. We are obliged, therefore, to adopt the only probable supposition, namely, that both of them arise entirely from the different directions into which the fluid is thrown in different circumstances; and of consequence, the only method of giving an intelligible explanation of positive and negative electricity is by considering the different direction of the fluid in each.

A great variety of methods have been contrived to ascertain the direction of the electric fluid, but all of them seem uncertain except that which is drawn from the appearance of electric light. The luminous matter appearing on a point negatively electrified is very small, resembling a globule; it makes little noise, and has a kind of hissing sound. The positive electricity, on the other hand, appears in a diverging luminous stream, which darts a considerable way into the air, with a crackling noise. Now, it is certain, that in whatever case the electric fluid darts from the point into the air, in that case it must be the most resisted by it; and this is evident in the positive electricity. In this, the rays evidently diverge from the points. We may, indeed, suppose them to be converging from many points in the surrounding air towards the metallic point. But why should we imagine that a visible ray would break out from one place of the atmosphere more than another? The air, we know, resists the motion of the electric fluid, and it certainly must resist it equally. Of consequence, when this fluid is coming from the air towards a pointed conductor, it must percolate slowly and invisibly through the air on all sides equally, till it comes so near that it is able to break through the intermediate space; and as this will likewise be equal, or nearly so, all round, the negative electricity must appear like a steady luminous globule on the point, not lengthening or shortening by flashes as the positive kind does. Electricians have therefore determined with a great deal of reason, that when a point is electrified positively the matter flows out from it.

It is to be remarked, however, that in most cases, if not in all, a body cannot be electrified negatively till it has first become positively electrified; and it is in the act of discharging its positive electricity that it becomes negative. Thus, suppose a coated phial to be set upon an insulated stand, and its knob is approached by that of another bottle charged positively; a small spark is observed between them, and both sides of the insulated bottle are electrified positively; but as soon as the finger is brought near to the outside, the positive electricity is discharged by a spark, and a negative one appears. But from what hath been already advanced, it is evident, that positive electricity is when the fluid hath a tendency to leave any body, and the negative electricity when it hath the same tendency to enter it. Therefore, as the electric fluid is subject to mechanical laws as well as other fluids, it must follow, that these tendencies are produced and kept up by the motions excited originally in the air, and electric fluid in the air, surrounding these bodies. If this principle is kept in view, it will lead us to an easy explanation of many electrical phenomena, for which no satisfactory reason hath hitherto been given.

§ 8. Of Electric Attraction and Repulsion.

It hath now been shown, that, in bodies electrified positively, there is a flux of electric matter from their surface all round; that is, the fluid contained in their pores pushes out on every side, and communicates a similar motion to the electric fluid contained in the adjacent atmosphere. This must of necessity very soon exhaust the body of its electric matter altogether, if it was not instantaneously supplied after every emission. But this supply is immediately procured from the surrounding atmosphere. The quantity sent off is instantly returned from the air, and the vibratory motion or struggle between the air and electric fluid, which hath been often mentioned, immediately takes place. The positive electricity therefore consists in a vibratory motion in the air and electric fluid; and the force of this vibration is directed outwards from the electrified body. In bodies negatively electrified, the fluid contained in the neighbouring atmosphere is directed towards the body so electrified. But it is certain, that this motion inwards cannot be continued unless there is also a motion of the fluid outwards from the body. In this case also there is a vibratory motion, but the force of it is directed inwards, and as the source of it lies not in the body, but in the surrounding atmosphere, it manifests itself somewhat less vigorously.

The reason why these motions are continued for such a length of time as we see they are, is, the extreme mobility of the electric fluid. It doth not indeed appear from any experiments, that this fluid hath the least friction among its parts. A motion once induced into it must therefore continue forever, until it is counteracted by some other motion of the same fluid. Hence, when a vibratory motion is once introduced among the particles of the electric fluid contained in any substance, that motion will be kept up by the surrounding fluid, let the body be removed to what place we please. There is no occasion indeed for supposing any thing like an electric atmosphere round the electrified body. The case is exactly the same as with a burning body. Let a candle be carried to what place we will, it will still burn; but it would be absurd to say, that the fire surrounded it like an atmosphere, as we know the fire is kept up by the air only, which is changed every moment. In like manner, the positive and negative electricities, which are two different motions of the electric fluid, are kept up by the air and electric matter contained in it; and, wherever the electrified body is carried, these fluids are equally capable of continuing them.

The phenomena of attraction and repulsion are now easily explained. Let us suppose a body positively electrified suspended by a small thread, at a distance from any other. The vibration above-mentioned, in which positive electricity consists, being kept up by the equable pressure on all sides, the body is neither moved to one side nor another. But when a negatively electrified body is brought near, the force of the vibration being directed outwards in the one, and inwards in the other, the pressure of the fluid in the intermediate space between them is greatly lessened; and of consequence the pressure on the other sides drives them together, and they are said to attract each other. If another body, electrified also positively, is brought near to the first, the force of the vibrations are directly opposed to one another, and therefore the bodies recede from each other, and are said to repel one another. The case is the same with two bodies negatively electrified: for there the electricity, as far as it extends round the bodies, consists of a vibratory motion of the electric fluid; and the vibrations being directed towards both the bodies, as towards two different centres, must necessarily cause them recede from each other; because, if they remained in contact, the vibratory motions would interfere with and destroy one another.

When a small body is brought within the sphere of another's electricity, the equable pressure of that vibratory or electrical sphere is somewhat lessened upon the side near which the second body is brought; and therefore it is immediately impelled towards the first by the action of the surrounding fluid, in order to keep up the equilibrium. As soon as it arrives there, the vibrations of the fluid around the first body being communicated to that within the pores of the second, it immediately acquires a sphere of electricity as well as the first, and is consequently repelled. The repulsion continues till the vibration ceases either by the action of the air, or by the body coming in contact with another much larger than itself; in which case the electricity is said to be discharged. If, after this discharge of electricity, the second body is still within the electric sphere of the first, it will immediately be attracted, and very soon after repelled, and so on alternately till the electricity of the former totally ceases.

§ 9. Of the Discharge of Electricity by Sparks upon blunt Conductors, and Silently by pointed Ones.

The manner in which this is accomplished will best appear from considering the nature of what is commonly called electricity. This cannot appear but in an electric substance; and the substance in which it does only show appear is the air. The prime conductor of an electric machine discovers no other properties in itself, when electrified, than it had before. The metal is equally hard, shining, and impenetrable. The electricity, or properties of attracting, repelling, &c., are all lodged in the air; and if the conductor is placed in vacuo, they instantly cease. It hath already been shown, that the electric matter runs over the surface of conducting substances in great quantities, like a stream of water running from one place to another. In this manner it will not pass over the surface of electrics. It enters their substance, and passes through it with a vibratory motion. This vibratory motion always shows a resistance; nor is it in any case possible to induce a vibration without first impressing a motion in one direction, and then resisting it by a contrary motion. Theory. Round the surface of an electrified body suspended in the air, therefore, there is always an equable pressure, by which the emission of the electric fluid is every moment checked, and by which its vibrations are occasioned. When a metallic substance is brought near the electrified body, the fluid has an opportunity of making its escape, provided it could get at the metal, because it could run along its surface. The pressure of the air is also lessened on that side which the conducting substance approaches. The whole effort of the electric matter contained in the vibratory sphere is exerted against that single place, because the resistance is least. If the body has a broad surface, however, the disproportion between these resistances is not so great as when its surface is less. Let us suppose, for instance, that the surface of the conducting substance contains an inch square, and that the whole surface of the electrified sphere contains only six square inches. When the conducting substance approaches, all the pressure is directed towards that place; and the effort made by the electric matter to escape there, is five times as great as what it is anywhere else. Nevertheless, though it has a vibratory motion in the substance of the air, it cannot have a progressive motion through it without violently displacing its parts; and an inch square of air makes a considerable resistance. At last, however, if this resistance is every moment made less by approaching the conducting substance nearer to the electrified body, the electric matter breaks through the thin plate of air, strikes the conductor, and runs along it. The spark is produced by the resistance it meets with from the air. But if, instead of a body with a broad surface, we present the point of a needle, whose surface is perhaps not above the ten-thousandth part of a square inch, the effort of the electric matter to discharge itself there will be 60,000 times greater than at any other place, because the whole effort of the six square inches, of which we suppose the surface of the electric sphere to consist, is exerted against that single point. The air also resists, as in the former case; but it can resist only in proportion to the extent of its surface which covers the conducting body; and this, being only the ten-thousandth part of a square inch, must be exceedingly little. As soon therefore as a needle, or any other fine pointed body, is presented to an electrified substance, the electric matter is urged thither with great velocity; and as it hath an opportunity of running along the needle, its vibrations quickly cease, and the electricity is said to be drawn off.—This drawing off, however, does not extend all round the electrified body, if means are used to keep up the electricity perpetually. Thus, if, on the end of the prime conductor, there are fastened a number of fine threads, hairs, &c., when the cylinder is turned, the threads on the end will diverge, and spread out like as many rays proceeding from a centre. If a point is presented on one side of the conductor, though at a considerable distance, the threads on one side will lose their divergence and hang down, but those on the other side will continue to diverge. The reason of this is, the difficulty with which the electric fluid gets through the atmosphere, even where the resistance of it is made as little as possible; and hence also we may see why more conductors than one may be necessary for the safety of large buildings. See Lightning.

§ 10. Why Positive Electricity hath a Tendency to induce the Negative Kind on any Body kept within its Sphere of Action, and why Negative Electricity produces the Positive Kind in similar Circumstances.

This is one of the electrical phenomena most difficult to be solved; and indeed seems totally insoluble, unless we give up the idea of accumulation and deficiency of the electric fluid in different bodies. On Dr Franklin's principles, no solution hath been attempted. Mr Cavallo places this among the properties of electricity for which he doth not pretend to account, but gives as the causes of other phenomena. It is indeed certain, that if a body hath already too much electricity or anything else, it cannot be continually taking from those around it; and if it hath too little, it cannot be continually giving them. By attending to the principles above laid down, however, this phenomenon admits of an easy solution. As positive electricity consists in a vibratory motion of the electric matter in the pores of any body, and to some distance through the air, while at the same time the force is directed outwards from the body, it is plain, that if any other body is brought within this sphere, the direction of the vibration is changed; for what is outwards from the one is inwards to the other. But a vibratory motion, the force of which is directed inwards, is what constitutes negative electricity; and, therefore, no sooner is any body placed at some distance from one positively electrified, than it immediately becomes negatively so. The same reason may be given why negative electricity produces the positive kind on a body placed near it. In the negative kind, the force of the vibration is directed inwards. If another body is brought near, the vibration which is inwards to the first must be outwards from the second, which thus electric becomes positively electrified. The only difficulty here, is to account for this motion, which is only inward or outward to one side of the body brought near the electrified one, being so suddenly propagated all round. This, however, must easily be seen to arise from the extreme subtlety of the electric fluid, and its effort to keep up an equilibrium in all parts, which it will never suffer to be broken. When this fluid pushes inward to one side of a body, the fluid contained in that body would immediately yield, and allow a free passage to what came after, if its yielding was not obstructed by something on the other side. This obstruction arises from the air, which cannot admit a progressive motion of electric matter through it. No sooner, therefore, is a push made against one side than a contrary one is made against the other; and thus the body instantly becomes electrified all round.

On these principles, also, may we account for the zones of positive and negative electricity which are to positive and be found on the surface of glass tubes*; and especially negative in electrified air. When the prime conductor of a machine is strongly electrified positively, it is throwing forth the fluid from it in all directions. The air cannot receive this fluid without throwing out that which it also contains; and this shows, that simple electrification can neither increase nor diminish the density of the air, which is also vouched by numberless experiments. But if the air throws out its electric fluid in all directions, it must throw part of it back upon the conduc- tor, and consequently obstruct its operations. This likewise is found to be the case; for it is impossible to make an electric machine act long with the same degree of strength, owing to the electricity communicated from it to the air. But if the conductor and air are thus reciprocally throwing the electric matter back upon one another, it is impossible but another zone of air which lies at a greater distance must be continually receiving it, or be electrified negatively. But this cannot receive, without also emitting the fluid it contains; which, therefore, will be thrown upon another zone behind it, and partly back upon the first. The original force of the fluid being now spread over a large space, will consequently be diminished; and the succeeding zone will be electrified weakly, though positively. In like manner, a succeeding zone must yield, and receive the fluid from this; which will consequently be electrified negatively, though weaker than the former; and thus zones of positive and negative electricity will gradually succeed each other in the air, till no traces of either are to be found.—In these zones, it must be remembered, that there is a centre peculiar to each, and from this centre the vibrations proceed either inward or outward. Thus, when the machine is first set in motion, a vibration is propagated from it as from a centre to some distance in the air, and the air is at first negatively electrified. But as this vibratory motion cannot be extended far in one direction, vibrations begin to be propagated in all directions from another centre at some distance. The conductor becomes then less positively electrified than before; however, by means of the machine, its electricity is still kept up, though weaker; but a zone of air beyond the first, where the repulsion is much less, becomes negatively electrified. This again cannot continue long till vibrations outwards arise from another centre, and so on. It is scarce needful to add here, that the longer the electrification is continued, and the stronger it is, the broader these zones must be.

§ 11. Of the Leyden Phial.

The phenomena of the Leyden phial are easily explained from what hath been already advanced. Glass and other electric substances are so constituted, that they can transmit the vibratory motions of the electric matter, though they cannot admit of any considerable progressive one. Conducting substances, on the other hand, admit of a progressive motion, but not so easily of a vibratory one. When the electric fluid is procured from the earth by an electric machine, if the conductor had a communication with the earth, all the matter collected by the cylinder would run along the conductor into the earth, and not a spark or other appearance of electricity would be procured in the air. But when the conductor is insulated, the matter is forced to go off into the air, and there produces the vibratory motions already mentioned. If a pane of glass which has no metallic coating touches the conductor, though it is permeable by the vibratory motion of the fluid, yet a considerable resistance is made, and the fluid cannot easily diffuse itself over its surface. Nevertheless, it will soon show signs of having received electricity, that is, of having the fluid within its pores thrown into a vibratory motion. This motion is directed outwards, from the middle of the substance of the glass, to the surface, and a considerable way beyond it on both sides. Both sides of the glass are then positively electrified. If a conducting substance touches one of the sides of the glass, the vibrations on that side are destroyed; because the fluid which occasioned them yields to the repulsion it met comes charged, and runs along the conductor into the earth. But if no sooner is this done, than the power which refilled the vibration outward from the glass having got the better in the manner just now explained, a new vibration is produced by that refilling power; and the force of this vibration is directed towards the side from whence the electricity was drawn off, which therefore becomes electrified negatively. Thus may we understand how a pane of glass, or any other electric, may receive positive electricity on the one side and negative on the other, to as high a degree as we please. But there is found to be a limit to every charge of electricity we can give; and this limit is the repulsion of the air. A phial will contain double the charge in air doubly condensed that it does in the common atmosphere; and when once the vibration becomes too great to be borne, the positive side of the glass throws out pencils of light, and will receive no more electricity in that state of the atmosphere.

Thus, in every charged phial, there is a violent impulse or vibration of the fluid, outward from the positive, and inward to the negative, side. As long as these continue, the phial continues charged. As the electric fluid seems to be subject to no other natural power, but controls all its own actions only by moving in opposite directions, it is plain, that if a charged phial is carefully kept from any of those means by which it is known to be discharged, it must keep its charge for a long time; and thus, by keeping phials within glass cases, their charge will be retained for six or eight weeks, or perhaps a great deal longer. The only method of discharging a phial, is by making a communication between its coatings. The fluid pressing out of the positive side, now yields to the pressure of that from the negative side, and runs along the conductor. But no sooner does it come near the negative charge, side of the phial, than, meeting with more of the same kind, the current of which is directed the same way, both together break through the air with a violent flash and crack, and all appearances of electricity cease.—In this, as in all other electrical experiments, it is easy to see, that the force, velocity, &c. of the fluid depends entirely on the pressure of that which surrounds us. Nature hath appointed a certain constitution or modification of the electric fluid in all terrestrial bodies, and likewise all round the earth. In our electrical experiments, we violate this constitution in some degree. When this violation is but small, the powers of nature operate gently in repairing the disorder we have introduced; but when any considerable deviation is occasioned, the natural powers restore the original constitution with extreme violence.

§ 12. The Phenomena of the Electrophorus accounted for.

The electrophorus is a machine represented Plate CLXXVII. fig. 74. It consists of two plates, A and B, usually of a circular form; though they may be made square, or of the figure of a parallelogram, with more ease, and with equal advantage. At first the under plate was Theory. of glass, covered over with sealing-wax; but there is little occasion for being particular either with regard to the substance of the lower plate, or the electric which is put upon it. A metallic plate, however, is perhaps preferable to a wooden one, though the latter will answer the purpose very well. This plate is to be covered with some electric substance. Pure sulphur answers very neat as well as the dearer electrics, sealing-wax, gum-lac, &c.; but it hath this bad quality, that, by rubbing it, some exceeding subtle streams are produced, which infect the person's clothes, and even his whole body, with a very disagreeable smell, and will change silver in his pocket to a blackish colour.

The upper plate of the electrophorus is a brass plate, or a board or piece of palteboard covered with tin-foil or gilt paper, nearly of the same size with the electric plate, though it will not be the worse that it is somewhat larger. It is furnished with a glass handle (T), which ought to be screwed into the centre. The manner of using this machine is as follows. First, the plate B is excited by rubbing its coated side with a piece of new white flannel, or a piece of hare's skin. Even a common hard shoe-brush, having the hair a little greased, will excite sulphur extremely well. When this plate is excited as much as possible, it is set upon the table with the electric side uppermost. Secondly, the metal plate is laid upon the excited electric, as represented in the figure. Thirdly, the metal plate is touched with the finger or any other conductor, which, on touching the plate, receives a spark from it. Lastly, the metal plate A, being held by the extremity of its glass handle (T), is separated from the electric plate; and, after it is elevated above that plate, it will be found strongly electrified with an electricity contrary to that of the electric plate; in which case, it will give a very strong spark to any conductor brought near it. By setting the metal upon the electric plate, touching it with the finger, and separating it successively, a great number of sparks may be obtained apparently of the same strength, and that without exciting again the electric plate. If these sparks are repeatedly given to the knob of a coated phial, it will presently become charged.

"As to the continuance of the virtue of this electric plate (says Mr Cavallo), when once excited, without repeating the excitation, I think there is not the least foundation for believing it perpetual, as some gentlemen have supposed; it being nothing more than an excited electric, it must gradually lose its power by imparting continually some of its electricity to the air, or other substances contiguous to it. Indeed its electricity, although it could never be proved to be perpetual by experiments, lasts a very long time, it having been observed to be pretty strong several days, and even weeks, after excitation. The great duration of the electricity of this plate, I think, depends upon two causes: first, because it does not lose any electricity by the operation of putting the metal plate upon it, &c., and, secondly, because of its flat figure, which exposes it to a less quantity of air, in comparison with a stick of sealing-wax, or the like, which, being cylindrical, exposes its surface to a greater quantity of air, which is continually robbing the excited electrics of their virtue.

"The first experiments that I made, relative to this machine, were with a view to discover which substance would answer best for coating the glass plate, in order to produce the greatest effect. I tried several substances either simple or mixed; and at last I observed, that the strongest in power, as well as the easiest, I could construct, were those made with the second sort of sealing-wax, spread upon a thick plate of glass. A plate that I made after this manner, and no more than five inches in diameter, when once excited, could charge a coated phial several times successively, so strongly as to pierce a hole through a card with the discharge. Sometimes the metal plate, when separated from it, was so strongly electrified, that it darted strong flashes to the table upon which the electric plate was laid, and even into the air, besides causing the sensation of the spider's web upon the face brought near it, like an electric strongly excited. The power of some of my plates is so strong, that sometimes the electric plate adheres to the metal when this is lifted up, nor will they separate even if the metal plate is touched with the finger or other conductor. It is remarkable, that sometimes they will not act well at first, but they may be rendered very good by scraping with the edge of a knife the shining or glossy surface of the wax. This seems analogous to the well-known property of glass, which is, that new cylinders or globes, made for electrical purposes, are often very bad electrics at first; but that they improve by being worked, i.e., by having their surface a little worn. Paper also has this property.

"If, after having excited the sealing-wax, I lay the experiment plate with the wax upon the table, and the glass upwards with pernol, i.e., contrary to the common method; then, the electrostatics making the usual experiments of putting the metal plate on it, and taking the spark, &c., I observe it to be attended with the contrary electricity: that is, if I lay the metal plate upon the electric one, and, while in that situation, touch it with an insulated body, that body acquires the positive electricity; and the metallic, removed from the electric plate, appears to be negative; whereas it would become positive, if laid upon the excited wax. This experiment, I find, answers in the same manner if an electric plate is used which has the sealing-wax coating on both sides, or one which has no glass plate.

"If the brass plate, after being separated from, be presented with the edge toward the wax, lightly touching it, and thus be drawn over its surface, I find that the electricity of the metal is absorbed by the sealing-wax, and thus the electric plate loses part of its power; and if this operation is repeated five or six times, the electric plate loses its power entirely, so that a new excitation is necessary in order to revive it.

"If, instead of laying the electric plate upon the table, it is placed upon an electric stand, so as to be accurately insulated, then the metal plate set on it acquires so little electricity, that it can only be discovered with an electrometer; which shows, that the electricity of this plate will not be conspicuous on one side of it, if the opposite side is not at liberty either to part with or acquire more of the electric fluid. In consequence of this experiment, and in order to ascertain how the opposite sides of the electric plate would be affected in different circumstances, I made the following experiments.

"Upon an electric stand E, (Plate CI.XXVII, fig. 74.) I placed a circular tin-plate, nearly six inches in diameter, which by a slender wire H communicated with an electrometer of pith-balls G, which was also insulated upon the electric stand F. I then placed the excited electric plate D of six inches and a quarter in diameter, upon the tin-plate, with the wax uppermost; and on removing my hand from it, the electrometer G, which communicated with the tin-plate, i.e., with the under side of the electric plate, immediately opened with negative electricity. If, by touching the electrometer, I took that electricity off, the electrometer did not afterwards diverge. But if now, or when the electrometer diverged, I presented my hand open, or any other uninsulated conductor, at the distance of about one or two inches, over the electric plate, without touching it, then the pith-balls diverged; or, if they diverged before, came together, and immediately diverged again with positive electricity;—I removed the hand, and the balls came together;—approached the hand, and they diverged; and so on.

"If, while the pith-balls diverged with negative electricity, I laid the metal plate, holding it by the extremity K of its glass handle, upon the wax, the balls came, for a little time, towards one another, but soon opened again with the same, i.e., negative electricity.

"If, whilst the metallic rested upon the electric plate, I touched the former, the electrometer immediately diverged with positive electricity; which if, by touching the electrometer, I took off, the electrometer continued without divergence.—I touched the metal plate again, and the electrometer opened again; and so on for a considerable number of times, until the metal plate had acquired its full charge. On taking now the metal plate up, the electrometer G instantly diverged with strong negative electricity.

"I repeated the above-described experiments, with this only difference in the disposition of the apparatus, i.e., I laid the electric plate D with the excited sealing-wax upon the circular tin-plate, and the glass uppermost; and the difference in their result was, that where the electricity had been positive in the former disposition of the apparatus, it now became negative, and vice versa; except that, when I first laid the electric plate upon the tin, the electrometer G diverged with negative electricity, as well in this as in the other disposition of the apparatus.

"I repeated all the above experiments with an electric plate, which, besides the sealing-wax coating on one side, had a strong coat of varnish on the other side, and their result was similar to that of those made with the above-described plate."

This is Mr Cavallo's account of the electrophorus; but there is one part of it in which he must certainly be mistaken. He tells us, that "if instead of laying the electric plate upon the table, it is set upon an electric stand, so as to be accurately insulated, then the metal plate set on it acquires so little electricity, that it can only be discovered by an electrometer." In what manner this gentleman came to mistake a plain fact so egregiously, is not easy to determine; but it is certain, than an electrophorus, instead of having its virtue impaired by being insulated, has it greatly increased, at least the sphere of its activity is greatly enlarged. When lying on the table, if the upper plate is put upon it without being touched with the finger, it will not show much sign of electricity. But as soon as it is put on the electric stand, both the upper and under side appear strongly negative. A thread will be attracted at the distance of eight or ten inches. If both the upper and under side are touched at the same time, a strong spark will be obtained from both, but always of the same kind of electricity, namely, the negative kind. If the upper plate is now lifted up, a strong spark of positive electricity will be obtained from it; and on putting it down again, two sparks of negative electricity will be produced.

The singularity of this experiment is, that it produces always double the quantity of negative electricity appearance that it doth of the positive kind; which cannot be done by any other method yet known. Another very surprising circumstance is, that when the electrophorus remains in its insulated situation, you need not always touch the upper and under side of the plates at once, in order to procure positive electricity from the upper plate: It is sufficient to touch both sides only once. On lifting up the upper plate, a spark of positive electricity is obtained as already mentioned. On putting it down again, a spark of the negative kind is obtained from the upper plate, even though you do not touch the lower one. On lifting up the upper plate, a spark of positive electricity is obtained, but weaker than it would have been had both sides been touched at once. Putting down the upper plate again without touching both, a still weaker spark first of negative and then of positive electricity will be obtained from the upper one. Thus the sparks will go on continually diminishing, to the number perhaps of two or three hundred. But at last, when the electricity of the whole machine seems to be totally lost, if both sides are touched at once, it will instantly be restored to its full strength, and the double spark of negative, with the single one of positive electricity, will be obtained without intermission as before.

To account for all these phenomena very particularly, is perhaps impossible, without a greater degree of knowledge concerning the internal fabric of bodies than we have access to attain. In general, however, it is evident, that the phenomena of the electrophorus arise from the disposition that the electric matter hath to keep up an equilibrium within itself throughout every part of the universe. In consequence of this, no motion of the electric matter can be produced upon the one side of a body, but it must immediately be balanced by a corresponding one on the opposite side; and in proportion to the strength of the one, so will the strength of the other be. When the under plate of the electrophorus is excited, the negative electricity or vibratory action of the electric matter towards the excited side, is produced; and the moment that such an action is produced on one side, it is reflected by a similar one on the opposite side, and thus the electrophorus becomes negatively electrified on both sides. As long as the under part of the machine communicates with the earth, the vibratory motion is impeded by the progressive one towards the earth. This makes the reflexion on the under side less, and therefore the vibratory motion on the upper part extends but a small way. When the plate is insulated, the electric matter has not an opportunity of escaping to the earth as before, because it is strongly resisted by the air; a vibration bration therefore takes place on both sides, and extends to a great distance from the plate. When the upper plate is set upon the electrophorus, the same kind of electricity, viz. the negative kind, is communicated to it. When both sides are touched with the finger, or with any other conducting substance, both electricities are suddenly taken off, because the electric matter running along the conducting substance on both sides, puts an end to the vibratory motion in the air, which constitutes the very essence of what we call electricity. There is now a quiet and equal balance of the electric matter on both sides, and therefore no signs of electricity are shown. But as soon as the upper plate is taken off, this balance is destroyed. The fluid in the metal plate had not been able to penetrate the electric substance in such a manner as to put a stop to the vibrations of what was within it. As soon then as the plate is taken off, the electricity or vibratory motion towards the electric breaks out at that side. But this motion inwards to the electric, which constitutes negative electricity, necessarily becomes outward from the plate; and as no motion of the fluid can be produced on one side of a body, but what is immediately communicated to the other, the upper plate becomes electrified positively, and the under one negatively on both sides.

Sect. VII. Of the Method of using the Electrical Apparatus already described, with some practical Rules for performing Experiments with it to the best Advantage.

The machines already described are calculated for exhibiting the phenomena of electricity in a very high degree; and in general the following effects may be expected from them.

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

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

Vol. VI. Part II.

(b) This is a kind of sensation always produced by strong electricity, something resembling the creeping of insects or the motion of a light body, such as a spider's web, over the skin, as already mentioned. It seems to proceed from the attraction and electrification of the small hairs with which the body is covered. Method of using the Electrical Apparatus, &c.

Method of using the prime conductor of the machine, a stream of fire was perceived between them. This was crooked, and darting out many lateral brushes of a very large size, in the manner already mentioned. A Leyden phial, containing about one square foot of coated surface, was fully charged by about half a turn of the winch so as to discharge itself; and by repeated trials it was found, that in one minute it discharged itself 76, 78, and frequently 80 times. Lastly, it was found, that though the conductor, which received the sparks from the prime one of the machine, communicated with the earth by a wire 1/3 this of an inch in diameter, this wire would give small sparks to any conducting body brought near it, as if even this wire had not been sufficient to conduct the quantity of electricity it received from the machine very readily to the earth.

Though these effects are not to be expected from our ordinary electrical machines, yet it is certain, that by taking proper care of them they will be found to act much more powerfully than if neglected. The following directions therefore will be found useful for such as wish to make electrical experiments.

1. The first thing to be observed is, the preservation and care of the instruments. The electrical machine, the coated jars, and in short every part of the electrical apparatus, should be kept clean, and as free as possible from dust and moisture.

2. When the weather is clear, and the air dry, especially in clear and frosty weather, the electrical machine will always work well. But when the weather is very hot, the electrical machine is not so powerful; nor in damp weather, except it be brought into a warm room, and the cylinder, the stands, the jars, &c. be made thoroughly dry.

3. Before the machine be used, the cylinder should be first wiped, very clean with a soft linen cloth that is dry, clean, and warm; and afterwards with a clean hot flannel, or an old silk handkerchief: this done, if the winch be turned when the prime conductor and other instruments are removed from the electrical machine, and the knuckle be held at a little distance from the surface of the cylinder, it will be soon perceived, that the electric fluid comes like a wind from the cylinder to the knuckle; and, if the motion be a little continued, sparks and crackling will soon follow. This indicates that the machine is in good order, and the electrician may proceed to perform his experiments. But if, when the winch is turned for some time, no wind is felt upon the knuckle, then the fault is, very likely, in the rubber: and to remedy that, use the following directions: By loosening the screws on the back of the rubber, remove it from its glass pillar, and keep it a little near the fire, so that its silk part may be dried; take now a dry piece of mutton fat, or a little tallow from a candle, and just pass it over the leather of the rubber; then spread a small quantity of the above described amalgam over it, and force it as much as possible into the leather. This done, replace the rubber upon the glass pillar; let the glass cylinder be wiped once more, and then the machine is fit for use.

4. Sometimes the machine will not work well because the rubber is not sufficiently supplied with electric fluid; which happens when the table, upon which the machine stands, and to which the chain of the rubber is connected, is very dry, and consequently in a bad condition of ducting slate. Even the floor and the walls of the room are, in very dry weather, bad conductors, and they cannot supply the rubber sufficiently. In this case &c., the best expedient is, to connect the chain of the rubber, by means of a long wire, with some moist ground, a piece of water, or with the iron work of a water-pump; by which means the rubber will be supplied with as much electric fluid as is required.

5. When a sufficient quantity of amalgam has been accumulated upon the leather of the rubber, and the machine does not work very well, then, instead of putting on more amalgam, it will be sufficient to take the rubber off, and to scrape a little that which is already upon the leather.

6. It will be often observed, that the cylinder, after being used some time, contracts some black spots, occasioned by the amalgam, or some foulness of the rubber, which grow continually larger, and greatly obstruct its electric power. These spots must be carefully taken off, and the cylinder must be frequently wiped in order to prevent its contracting them.

7. In charging electric jars in general, it must be observed, that not every machine will charge them equally high. That machine whose electric power is the strongest, will always charge the jars highest. If the coated jars, before they are used, be made a little warm, they will receive and hold the charge the better.

8. If several jars are connected together, among which there is one that is apt to discharge itself very soon, then the other jars will soon be discharged with that; although they may be capable of holding a very great charge by themselves. When electric jars are to be discharged, the electrician must be cautious, lest by some circumstance not adverted to, the shock should pass through any part of his body; for an unexpected shock, though not very strong, may occasion several disagreeable accidents. In making the discharge, care must be taken that the discharging rod be not placed on the thinnest part of the glass, for that may cause the breaking of the jar.

9. When large batteries are discharged, jars will be often found broken in it, which burst at the time of the discharge. To remedy this inconvenience, Mr Nairne says, he has found a very effectual method, which is, never to discharge the battery through a good conductor, except the circuit be at least five feet long. Mr Nairne says, that ever since he made use of this precaution, he has discharged a large battery near a hundred times without ever breaking a single jar, whereas before he was continually breaking them. But here it must be considered, that the length of the circuit weakens the force of the shock proportionally; the highest degree of which is in many experiments required.

10. It is advisable, when a jar, and especially a battery, has been discharged, not to touch its wires with the hand, before the discharging rod be applied to its sides a second and even a third time; as there generally remains a residuum of the charge, which is sometimes very considerable.

11. When any experiment is to be performed which requires but a small part of the apparatus, the remaining part of it should be placed at a distance from the machine. machine, the prime conductor, and even from the table, if that is not very large. Candles, particularly, should be placed at a considerable distance from the prime conductor, for the effluvia of their flames carry off much of the electric fluid.

Sect. VIII. Entertaining Experiments.

I. The Electrified Cork-ball Electrometer.

Fix at the end of the prime conductor a knobbed rod, and hang on it two small cork-balls suspended by threads of equal length. The balls will now touch one another, the threads hanging perpendicularly, and parallel to each other. But if the cylinder of the machine be whirled by turning the winch, then the cork-balls will repel one another; and more or less according as the electricity is more or less powerful.—If the electrometer be hung to a prime conductor negatively electrified, i.e., connected with the insulated rubber of the machine, the cork-balls will also repel each other. If, in this state of repulsion, the prime conductor is touched with some conducting substance not insulated, the cork-balls will immediately come together. But if, instead of the conducting substance, the prime conductor is touched with an electric, as for instance a stick of sealing-wax, a piece of glass, &c., then the cork-balls will continue to repel each other; because the electric fluid cannot be conducted through that electric: hence we have an easy method of determining what bodies are conductors and what electrics. This electrical repulsion is also shown by a large downy feather, or still more agreeably by the representation of a human head with hair, as shown fig. 47; for there the electric repulsion will make the hair erect itself in a strange manner. If the feather is used, it will appear beautifully swelled by the divergency of its down.

II. Attraction and Repulsion of light Bodies.

Connect with the prime conductor, by means of the hook H, the two parallel brass plates F, G, as represented in fig. 38, at about three inches distance from one another; and upon the lower plate put any kind of light bodies, as bran, bits of paper, bits of leaf-gold, &c.; then work the machine, and the light bodies will soon move between the two plates, leaping alternately from one to the other with great velocity. If, instead of bran or irregular pieces of other matter, small figures of men or other things cut in paper and painted, or rather made of the pith of alder, be put upon the plate, they will generally move in an erect position, but will sometimes leap one upon another, or exhibit different postures, so as to afford a pleasing spectacle to an observing company. When bran or other substances of that kind are made use of, it will be proper to inclose both plates in a glass cylinder, by which the bran will be kept from dispersing and flying about the room.

The phenomena of electric attraction and repulsion may be represented also with a glass tube, or a charged bottle, and some of them in a manner more satisfactory than with the machine.

III. The Flying-feather, or Shuttle-cock.

Take a glass tube (whether smooth or rough is not material); and after having rubbed it, let a small light feather be let out of your fingers at the distance of about eight or nine inches from it. This feather will be immediately attracted by the tube, and will stick very close to its surface for about two or three seconds, and sometimes longer; after which time it will be repelled; and if the tube be kept under it, the feather will continue floating in the air at a considerable distance from the tube, without coming near it again, except it first touches some conducting substance; and if you manage the tube dexterously, you may drive the feather through the air of a room at pleasure.

There is a remarkable circumstance attending this experiment; which is, that if the feather be kept at a distance from the tube by the force of electric repulsion, it always presents the same part towards the tube:—You may move the excited tube about the feather very swiftly, and yet the same side of the feather will always be presented to the tube.

This experiment may be agreeably varied in the following manner: A person may hold in his hand an excited tube of smooth glass, and another person may hold an excited rough glass tube, a stick of sealing-wax, or in short another electric negatively electrified, at about one foot and a half distance from the smooth glass tube: a feather now may be let go between these two differently excited-electrics, and it will leap alternately from one electric to the other; and the two persons will seem to drive a shuttle-cock from one to the other by the force of electricity.

IV. The Electric Well.

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

The following experiments require to be made in the dark: for although the electric light in several circumstances may be seen in the day-light, yet its appearance in this manner is very confused; and that the electrician may form a better idea of its different appearances, it is absolutely necessary to perform such experiments in a darkened room.

V. The Star and Pencil of Electric Light.

When the electrical machine is in good order, and the prime conductor is situated with the collector sufficiently near the glass cylinder, turn the winch, and you will see a lucid star at each of the points of the collector. This star is the constant appearance of the electric fluid that is entering a point. At the same time you will see a strong light proceeding from the rubber. rubber, and spreading itself over the surface of the cylinder; and if the excitation of the cylinder is very powerful, dense streams of fire will proceed from the rubber, and, darting round almost half the circumference of the cylinder, will reach the points of the collector. If the prime conductor is removed, the dense streams of fire will go quite round the cylinder; reaching from one side of the rubber to the other. If the chain of the rubber is taken off, and a pointed body, as for instance the point of a needle or a pin, is presented to the back of the rubber, at the distance of about two inches, a lucid pencil of rays will appear to proceed from the point presented, and diverge towards the rubber. If another pointed body be presented to the prime conductor, it will appear illuminated with a star; but if a pointed wire or other pointed conducting body be connected with the prime conductor, it will throw out a pencil of rays.

VI. Drawing Sparks.

Let the prime conductor be situated in its proper place, and electrify it by working the machine; then bring a metallic rod with a round knob at each end, or the knuckle of a finger, within a proper distance of the prime conductor, and a spark will be seen between that and the knuckle or knobbed wire. The longer and stronger spark is drawn from that end of the prime conductor which is farthest from the cylinder, or rather from the extremity of the knobbed rod fixed at its end; for the electric fluid seems to acquire an impetus by going through a long conductor, when electrified by a powerful machine. This spark appears like a long line of fire, reaching from the conductor to the opposed body, and often (particularly when the spark is long, and different conducting substances are near the line of its direction) it will have the appearance of being bended to sharp angles in different places, exactly resembling a flash of lightning. It often darts flashes of light likewise in every direction.

VII. The Electric Light flashing between two Metallic Plates.

Let two persons, one standing upon an insulated stool, and communicating with the prime conductor, and another standing upon the floor, each hold in one of his hands a metal plate, in such a manner that the plates may stand back to back in a parallel situation, and about two inches asunder. Let the winch of the machine be turned, and you will see the flashes of light between the two plates so dense and frequent, that you may easily distinguish any thing in the room. By this experiment the electric light is exhibited in a very copious and beautiful manner, and it bears a striking resemblance to lightning.

VIII. To fire Inflammable Spirits.

The power of the electric spark to set fire to inflammable spirits, may be exhibited by several different methods, but more easily thus: Hang to the prime conductor a short rod having a small knob at its end; then pour some spirits of wine, a little warmed, into a spoon of metal; hold the spoon by the handle, and place it in such a manner, that the small knob on the rod may be about one inch above the surface of the spirits. In this situation, if, by turning the winch, a spark be made to come from the knob, it will set the spirits on fire. It will generally be found more advantageous to fix the dish containing the spirits upon the prime conductor, as represented fig. 48.

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

IX. The artificial Bolognian Stone illuminated by the Electric Light.

The most curious experiment to show the penetrability of the electric light, is made with the real, or more easily with the artificial, Bolognian stone, invented by the late Mr J. Canton. This phosphorus is a calcareous substance, generally used in the form of a powder, which has the property of absorbing light when exposed to it, and afterwards appearing lucid when brought into the dark*. Take some of this powder,* See Ch. and, by means of spirits of wine or ether, stick it all miftry, over the inside of a clear glass phial, and stop it with n°144, a glass stopper, or a cork and sealing-wax. If this phial be kept in a darkened room (which for this experiment must be very dark), it will give no light; but let two or three strong sparks be drawn from the prime conductor, when the phial is kept at about two inches distance from the sparks, so that it may be exposed to that light, and this phial will receive that light, and afterwards will appear illuminated for a considerable time. The powder may be stuck upon a board by means of the white of an egg, so as to represent figures of planets, letters, or any thing else at the pleasure of the operator; and the figures may be illuminated in the dark, in the same manner as the above-described phial.

A beautiful method to express geometrical figures with the above phosphorus, is to bend small glass tubes of about the tenth part of an inch diameter, in the shape and figure desired, and then fill them with the phosphorus powder. These may be illuminated in the manner described, and they are not subject to be spoiled as the figures represented upon the board frequently are. The best method of illuminating this phosphorus, and which Mr W. Canton generally used, is to discharge a small electric jar near it.

X. The Luminous Conductor.

Fig. 24. represents a prime conductor invented Plate by Mr Henley, which shows clearly the direction of the electric fluid passing through it, from whence it is called the luminous conductor. The middle part E F of this conductor is a glass tube about 18 inches long and three or four inches in diameter. To both ends of this tube the hollow brass pieces F D, B E, are cemented air-tight, one of which has a point C, by which which it receives the electric fluid, when set near the excited cylinder of the electrical machine, and the other has a knopped wire G, from which a strong spark may be drawn; and from each of the pieces F D, B E, a knopped wire proceeds within the cavity of the glass tube. The brass piece F D, or B E, is composed of two parts; i.e., a cap F cemented to the glass tube, and having a hole with a valve, by which the cavity of the glass tube is exhausted of air; and the ball D, which is screwed upon the cap F. The supports of this instrument are two glass pillars fastened in the bottom-board H, like the supports of the prime conductor. When the glass tube of this conductor is exhausted of air by means of an air-pump, and the brass piece is screwed on, as represented in the figure, then it is fit for use, and may serve for a prime conductor to an electrical machine. If the point G of this conductor is set near the excited cylinder of the machine, it will appear illuminated with a star; at the same time the glass tube will appear all illuminated with a weak light; but from the knopped wire that proceeds within the glass from the piece F D, a lucid pencil will issue out, and the opposite knob will appear illuminated with a star, which, as well as the pencil of rays, is very clear, and discernible among the other light that occupies the greatest part of the cavity of the tube. If the point G, instead of being presented to the cylinder, be connected with the rubber of the machine, the appearance of light within the tube will be reversed; the knob which communicates with the piece F D appearing illuminated with a star, and the opposite with a pencil of rays; because in this case the direction of the electric fluid is just the contrary of what it was before; it then going from D to B, and now coming from B and going to D. If the wires within the tube E F, instead of being furnished with knobs, be pointed, the appearance of light is the same; but it seems not so strong in this as in the other case.

XI. The Conducting Glass Tube.

Take a glass tube of about two inches diameter, and about two feet long; fix to one of its ends a brass cap, and to the other a stop-cock or a valve; then, by means of an air-pump, exhaust it of air. If this tube be held by one end, and its other end be brought near the electrified prime conductor, it will appear to be full of light whenever a spark is taken by it from the prime conductor, and much more so if an electric jar be discharged through it. This experiment may also be made with the receiver of an air-pump: take, for instance, a tall receiver, clean and dry; and through a hole at its top insert a wire, which must be cemented air-tight. The end of the wire that is within the tube must be pointed, but not very sharp; and the other end must be furnished with a knob. Put this receiver upon the plate of the air-pump, and exhaust it. If now the knob of the wire at the top of the receiver be touched with the prime conductor, every spark will pass through the receiver in a dense and large body of light, from the wire to the plate of the air-pump. When anything is to be touched with the prime conductor that is not very portable, as the air-pump above mentioned, the communication between the former and the latter may be made by means of a rod furnished with an electric handle, or the like.

XII. The Aurora Borealis.

Take a phial nearly of the shape and size of a Florence flask; fix a stop-cock or a valve to its neck, and exhaust it of air as much as possible with a good air-pump. If this flask is rubbed in the common manner fig. 21, used to excite electrics, it will appear luminous within, being full of a flashing light, which plainly resembles the aurora borealis or northern light. This phial may also be made luminous, by holding it by either end, and bringing the other end to the prime conductor; in this case, all the cavity of the flask will instantly appear full of flashing light, which remains in it for a considerable time after it has been removed from the prime conductor. Instead of the above-described flask vessel, a glass tube exhausted of air and hermetically sealed may be used, and perhaps with better advantage. The most remarkable circumstance of this experiment is, that if the phial, or tube, after it has been removed from the prime conductor (and even several hours after its flashing light hath ceased to appear), be grasped with the hand, strong flashes of light will immediately appear within the glass, which often reach from one of its ends to the other.

XIII. The Visible Electric Atmosphere.

G I, fig. 26, represents the receiver with the plate plate of an air-pump. In the middle of the plate I P, a short rod is fixed, having at its top a metal ball B nicely polished, whose diameter is nearly two inches. From the top of the receiver, another rod A D, with a like ball A, proceeds, and is cemented air-tight in the neck C; the distance of the balls from one another being about four inches, or rather more. If, when the receiver is exhausted of air, the ball A be electrified positively, by touching the top D of the rod A D with the prime conductor, or an excited glass tube, a lucid atmosphere appears about it, which although it consists of a feeble light, is yet very conspicuous, and very well defined; at the same time, the ball B has not the least light. This atmosphere does not exist all round the ball A; but reaches from about the middle of it, to a small distance beyond that side of its surface which is towards the opposite ball B. If the rod with the ball A be electrified negatively, then a lucid atmosphere, like the above described, will appear upon the ball B, reaching from its middle to a small distance beyond that side of it that is towards the ball A; at the same time, the negatively electrified ball A remains without any light. The operator in this experiment must be careful not to electrify the ball A too much; for then the electric fluid will pass in a spark from one ball to the other, and the experiment will not have the desired effect. A little practice, however, will render the operation very easy and familiar.

XIV. Of charging and discharging a Phial in general.

Take a coated jar, and place it upon the table near the prime conductor, so that the knob of its wire, and that only, may be in contact with it; fix the quadrant electrometer fig. 15, upon the prime conductor, and then turn the winch of the machine. You will observe, that as the jar is charging, the index of the e- Electrometer will rise gradually as far as 90° or thereabouts, and then rest; when this happens, you may conclude that the jar has received its full charge. If now you take a discharging rod, and holding it by the glass handle, apply first one of its knobs to the outside coating of the jar, and then bring the other knob near the knob of the wire of the jar, or near the prime conductor that communicates with it, you will hear a report, and see very vivid sparks between the discharging rod and the conducting substances, communicating with the sides of the jar. This operation discharges the jar. If, instead of using the discharging rod, you touch the outside of the jar with one hand, and bring the other hand near the wire of the jar, the same spark and report will follow; but now you will feel a shock which affects your wrists, elbows, and if strong, your breast also. If a number of persons join hands, and the first of them touches the outside of the jar, and the last touches the wire communicating with the inside, they will all feel the shock, and precisely at the same perceivable time. This shock, bearing no resemblance to any sensation otherwise felt, cannot consequently be described; and in order that a person may form a just idea of it, he must absolutely feel it. A shock may be given to any single part of the body, if that part only be brought into the circuit.

XV. The Leyden Vacuum.

Fig. 22. 23. represent a small phial coated on the outside, about three inches up the sides, with tin-foil; at the top of the neck of this phial, a brass cap is cemented, having a hole with a valve, and from the cap a wire proceeds a few inches within the phial, terminating in a blunt point. When this phial is exhausted of air, a brass ball is screwed upon the brass cap, which is cemented into its neck, so as to defend the valve, and prevent any air from getting into the exhausted glass. This phial exhibits clearly the direction of the electric fluid, both in charging and discharging; for if it be held by its bottom, and its brass knob be presented to the prime conductor positively electrified, you will see that the electric fluid causeth the pencil of rays to proceed from the wire within the phial, as represented fig. 22.; and if it is discharged, a star will appear in the place of the pencil, as represented in fig. 23. But if the phial is held by the brass cap, and its bottom be touched with the prime conductor, then the point of the wire on its inside will appear illuminated with a star when charging, and with a pencil when discharging. If it be presented to a prime conductor electrified negatively, all these appearances, both in charging and discharging, will be reversed.

The apparatus represented fig. 25. will be found very convenient for the various experiments upon the luminous conductor, Leyden vacuum, jars charged positively or negatively, with their different states of insulation. A is an insulating pillar of glass, which is screwed to the wooden foot B; and on this pillar all the apparatus may be screwed alternately. CD is an exhausted tube of glass, furnished at each end with brass caps; at the end D is a valve properly secured under the brass plate; a brass wire with a ball projects from the upper cap; a pointed wire proceeds from the bottom plate; and this tube is called the luminous conductor. The flask represented at E is called the Leyden vacuum. It is furnished with a valve under the ball E; to come at which the more readily, the ball may be unfrosted: a wire, with a blunt end, projects to within a little of the bottom of the flask, the latter being coated with tin-foil; and a female screw is cemented to the bottom, in order to screw it on the pillar A. F is a syringe to exhaust the air occasionally, either from the luminous conductor or the Leyden vacuum. To do this, unfrost the ball of the Leyden vacuum, or the plate of the luminous conductor, and then screw the syringe in the place of either of these pieces, being careful that the bottom of the female screw G bears close against the leather which covers the shoulders a b c d; then work the syringe, and in a few minutes the glasses will be sufficiently exhausted. H and I are two Leyden bottles; each of which has a female screw fitted to the bottom, in order that they may be conveniently screwed on the pillar A; and the bottle H is furnished with a belt by which it may be screwed likewise to the same. K and L are two small wires, to be screwed occasionally either into the ball E, the knobs e or f, the cap e, or the socket g on the top of the pillar: the balls may be unfrosted from these wires, which will then exhibit a blunt point. M is a wooden table to be screwed occasionally on the glass pillar.

XVI. To pierce a Card and other Substances with the Electric Explosion.

Take a card, a quire of paper, or the cover of a book, and keep it close to the outside coating of a charged jar; put one knob of the discharging rod upon the card, quire of paper, &c. so that between the knob and coating of the jar the thicknesses of that card, or quire of paper, only is interposed; lastly, by bringing the other knob of the discharging rod near the knob of the jar, make the discharge, and the electric matter will pierce a hole (or perhaps several) quite through the card or quire of paper. This hole has a bur raised on each side, except the card, &c. be pressed hard between the discharging rod and the jar; which shows that the hole is not made in the direction of the passage of the fluid, but in every direction from the centre of the resisting body. If this experiment be made with two cards instead of one, which however must be kept very little distant from one another, each of the cards, after the explosion, will be found pierced with one or more holes, and each hole will have burns on both surfaces of each card. The hole, or holes, are larger or smaller, according as the card, &c. is more damp or more dry. It is remarkable, that if the nostrils are presented to it, they will be affected with a sulphureous, or rather a phosphoreal, smell, just like that produced by an excited electric.

If, instead of paper, a very thin plate of glass, rosin, sealing-wax, or the like, be interposed between the knob of the discharging rod and the outside coating of the jar, on making the discharge, this will be broken in several pieces. Small insects may also be killed in this manner. They may be held between the outside coating of the jar and the knob of the discharging rod, like the above card; and a shock of a common phial sent through them, will instantly deprive them of of life, if they are pretty small; but if larger, they will be affected in such a manner, as to appear quite dead on first receiving the stroke; but will, after some time, recover; this, however, depends on the quantity of the charge sent through them.

XVII. To show the Effect of the Shock sent over the Surface of a Card or other Substances.

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

If, instead of the card, the explosion is sent over the surface of a piece of glass, this will be marked with an indelible track, which generally reaches from the extremity of one of the wires to the extremity of the other. In this manner, the piece of glass is very seldom broken by the explosion. But Mr Henley has discovered a very remarkable method to increase the effect of the explosion upon the glass; which is by pressing with weights that part of the glass which lies between the two wires (i.e., that part over which the shock is to pass). He puts first a thick piece of ivory upon the glass, and places upon that ivory a weight at pleasure, from one quarter of an ounce to five pounds; The glass in this manner is generally broken by the explosion into innumerable fragments, and some of it is absolutely reduced into an impalpable powder. If the glass is very thick, and resists the force of the explosion, so as not to be broken by it, it will be found marked with the most lively prismatic colours, which are thought to be occasioned by very thin laminae of the glass, in part separated from it by the shock. The weight laid upon the glass is always shook by the explosion, and sometimes it is thrown quite off from the ivory. This experiment may be most conveniently made with the universal discharger, fig. 8.

XVIII. To swell Clay, and break Small Tubes, by the Electric Explosion.

Roll up a piece of soft tobacco-pipe clay in a small cylinder, and insert in it two wires, so that their ends without the clay may be about a fifth part of an inch from one another. If a shock be sent through this clay, by connecting one of the wires with the outside of a charged jar, and the other with the inside, it will be inflated by the shock, i.e., by the spark, that passes between the two wires, and, after the explosion, will appear swelled in the middle. If the shock sent through it is too strong, and the clay not very moist, it will be broken by the explosion, and its fragments scattered in every direction. To make this experiment with a little variation, take a piece of the tube of a tobacco-pipe, about one inch long, and fill its bore with moist clay; then insert in it two wires, as in the above rolled clay; and send a shock through it. This tube will not fail to burst by the force of the explosion, and its fragments will be scattered about to a great distance.

If, instead of clay, the above-mentioned tube of the tobacco-pipe, or a glass tube (which will answer as well), be filled with any other substance, either electric or non-electric, inferior to metal, on making the discharge, it will be broken in pieces with nearly the same force. This experiment is the invention of Mr Lane, F.R.S.

XIX. To make the Electric Spark visible in Water.

Fill a glass tube of about half an inch diameter, and six inches long, with water; and to each extremity of the tube adapt a cork, which may confine the water; through each cork insert a blunt wire, so that the extremities of the wires within the tube may be very near one another; lastly, connect one of these wires with the coating of a small charged phial, and touch the other wire with the knob of it; by which means the shock will pass through the wires, and cause a vivid spark to appear between their extremities within the tube. In performing this experiment, care must be taken that the charge be exceedingly weak, otherwise the tube will burst. If we place in a common drinking glass, almost full of water, two knotted wires, so bent, that their knobs may be within a little distance of one another in the water, and if one of these wires be connected with the outside coating of a pretty large jar, and the other wire be touched with the knob of it; the explosion which must pass through the water from the knob of one of the wires to that of the other, will disperse the water, and break the glass with a surprising violence. This experiment is very dangerous if not conducted with great caution.

XX. To fire Gun-powder.

Make a small cartridge of paper, and fill it with gun-powder, or else fill the tube of a quill with it; insert two wires, one at each extremity, so that their ends within the quill, or cartridge, may be about one fifth of an inch from one another; this done, send the charge of a phial through the wires; and the spark between their extremities, that are within the cartridge, or quill, will set fire to the gun-powder. If the gunpowder be mixed with steel-fillings, it will take fire more readily, and with a very small shock.

XXI. To strike Metals into Glass.

Take two slips of common window-glass about three inches long, and half an inch wide; put a small slip of gold, silver, or brass leaf, between them, and tie them together, or else press them together between the boards of the press H, belonging to the universal discharger fig. 9, Plate CLXXXIV. Leaving a little of the metallic leaf out between the glasses at each end; then send a shock through this metallic leaf, and the force of the explosion will drive part of the metal into so close a contact with the glass, that it cannot be wiped off, or even be affected by the common menstrua which otherwise would dissolve it. In this experiment the glasses are often shattered to pieces; but whether they are broken or not, the indelible metallic tinge will always be found in several places, and sometimes thro' the whole length of both glasses. XXII. To stain Paper or Glass.

Lay a chain, which forms a part of the circuit between the two sides of a charged jar, upon a sheet of white paper; and if a shock be sent through it, the paper will be found stained with a blackish tinge at the very juncture of the links. If the charge be very large, the paper, instead of being stained with spots, is burnt through. If the chain be laid upon a pane of glass instead of paper, the glass will often be found stained with spots in several places, but (as might be expected) not so deep as the paper. If this experiment be made in the dark, a spark will be seen at every juncture of the links; and if the links are small, and the shock pretty strong, the chain will appear illuminated like a line of fire.

XXIII. The Lateral Explosion.

If a jar be discharged with a discharging rod that has no electric handle, the hand that holds it, in making the discharge, feels some kind of shock, especially when the charge is considerable. In other words: A person, or any conducting substance, that is connected with one side of a jar, but forms no part of the circuit, will feel a kind of shock, i.e., some effect of the discharge. This may be rendered visible in the following manner. Connect with the outside of a charged jar a piece of chain; then discharge the jar thro' another circuit, as for instance with a discharging rod in the common way, and the chain that communicates with the outside of the jar, and which makes no part of the circuit, will appear lucid in the dark, i.e., sparks will appear between the links; which shows, that the electric fluid, natural to that chain, must by some means have been disturbed. This chain will also appear luminous, if it is not in contact with the outside of the jar, but only very near it; and on making the discharge, a spark will be seen between the jar and the end of the chain near it. This electrical appearance out of the circuit of a discharging jar, is that which we call the lateral explosion; and to make it appear in the most conspicuous manner, observe the following method, which is that of Dr Priestley.

When a jar is charged, and stands upon the table as usual, infuse a thick metallic rod, and place it so that one of its ends may be contiguous to the outside coating of the jar; and within about half an inch of its other end place a body of about six or seven feet in length, and a few inches in breadth; then put a chain upon the table, so that one of its ends may be about an inch and a half distant from the coating of the jar: at the other end of the chain apply one knob of the discharging rod, and bring the other knob to the wire of the jar, in order to make the explosion. On making the discharge in this manner, a strong spark will be seen between the insulated rod, which communicates with the coating of the jar and the body near its extremity, which spark does not alter the state of that body in respect to electricity. Whether this lateral explosion is received on flat and smooth surfaces, or upon sharp points, the spark is always equally long and vivid.

XXIV. To discharge a Jar silently.

When a large jar is fully charged, which would give

XXV. Drawing the Electricity from the Prime Conductor by a Point.

Let a person hold the knob of a brass rod at such a distance from the prime conductor, that sparks may easily fly from the latter to the former, when the machine is in motion. Then let the winch be turned; and while the sparks are following one another, present the sharp point of a needle at nearly twice the distance from the prime conductor, that the knobbed rod is held; and you will observe that no more sparks will go to the rod—remove the needle entirely, and the sparks will be seen again;—present the needle, and the sparks disappear: which evidently shows, that the point of the needle draws off silently almost all the fluid that the cylinder throws upon the prime conductor.

If the needle be fixed upon the prime conductor with the point outward, and the knob of a discharging rod, or the knuckle of a finger, be brought very near the prime conductor, though the excitation of the cylinder may be very strong, yet you will perceive that no spark, or an exceeding small one, can be obtained from the prime conductor.

XXVI. The Electrified Cotton.

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

XXVII. The Electrified Bladder.

Take a large bladder well blown, and cover it with gold, silver, or brass leaf, sticking it with gum-water; suspend this bladder at the end of a silk thread, at least six or seven feet long, hanging from the ceiling of the room; and electrify the bladder, by giving it a strong spark with the knob of a charged bottle: this done, take a knobed wire, and present it to the bladder when motionless; and you will perceive, that as the knob approaches the bladder, the bladder also moves towards the knob, and, when nearly touching it, gives it the spark which it received from the charged phial, and thus it becomes unelectrified. Give it another spark, and, instead of the knotted wire, present the point of a needle towards it, and you will perceive that the bladder will not be attracted by, but rather recede from, the point, especially if the needle be very suddenly presented towards it.

XXVIII. The Spider seemingly animated by Electricity.

Fig. 51. represents an electric jar, having a wire CDE fastened on its outside, which is bended so as to have its knob E as high as the knob A.—B is a spider made of cork, with a few short threads run through it to represent its legs. This spider is fastened at the end of a silk thread, proceeding from the ceiling of the room, or from any other support, so that the spider may hang mid-way between the two knobs A, E, when the jar is not charged. Let the place of the jar upon the table be marked; then charge the jar, by bringing its knob A in contact with the prime conductor, and replace it in its marked place. The spider will now begin to move from knob to knob, and continue this motion for a considerable time, sometimes for several hours.

The inside of the jar being charged positively, the spider is attracted by the knob A, which communicates to it a small quantity of electricity; the spider then becoming possessed of the same electricity with the knob A, is repelled by it, and runs to the knob E, where it discharges its electricity, and is then attracted by the knob A, and so on. In this manner the jar is gradually discharged; and when the discharge is nearly completed, the spider finishes its motion.

XXIX. The Dancing Balls.

Fix a pointed wire upon the prime conductor, with the point outward; then take a glass tumbler, grasp it with your hands, and present its inside surface to the point of the wire upon the prime conductor while the machine is in motion; the glass in this manner will soon become charged; for its inside surface acquires the electricity from the point, and the hands serve as a coating for the outside. This done, put a few pith balls upon the table, and cover them with this charged glass tumbler. The balls will immediately begin to leap up along the sides of the glass as represented fig. 39, and will continue their motion for a considerable time.

XXX. The Electrical Jack.

This is an invention of Dr Franklin's, and turns with considerable force, so that it may sometimes be used for the purpose of a common jack. A small upright shaft of wood passes at right angles through a thin round board of about 12 inches diameter, and turns on a sharp point of iron fixed in the lower end, while a strong wire in the upper end, passing through a small hole in a thin brass plate, keeps the shaft truly vertical. About 30 radii, of equal length, made of fish-glass cut into narrow slips, issue horizontally from the circumference of the board, the ends most distant from the centre being about four inches apart. On the end of every one a brass thimble is fixed. If now the wire of a bottle electrified in the common way be brought near the circumference of this wheel, it will attract the nearest thimble, and so put the wheel in motion. That thimble, in passing by, receives a spark; and thereby being electrified, is repelled, and so driven forwards; while a second, being attracted, approaches the wire, receives a spark, and is driven after the first; and so on, till the wheel has gone once round; when the thimbles before electrified approaching the wire, instead of being attracted, as they were at first, are repelled, and the motion presently ceases. But if another bottle which had been charged through the coating, or otherwise negatively electrified, is placed near the same wheel, its wire will attract the thimble repelled by the first, and thereby double the force that carries the wheel round. The wheel therefore moves very swiftly, turning round 12 or 15 times in a minute, and with such force, that a large fowl spitted on the upper shaft may be roasted by means of it.

XXXI. The Self-moving Wheel.

This appears more surprising than the former, tho' constructed upon the same principles. It is made of a thin round plate of window-glass 17 inches in diameter, well gilt on both sides, all but two inches next the edge. Two small hemispheres of wood are then fixed with cement to the middle of the upper and under sides, centrally opposite; and in each of them a strong thick wire eight or ten inches long, which together make the axis of the wheel. It turns horizontally on a point at the lower end of its axis, which rests on a bit of brass cemented within a glass salt-cellar. The upper end of its axis passes through a hole in a thin brass plate, cemented to a long and strong piece of glass; which keeps it six or eight inches distant from any non-electric, and has a small ball of wax or metal on its top to keep in the fire.

In a circle on the table which supports the wheel, are fixed 12 small pillars of glass, at about 11 inches distance, with a thimble on the top of each. On the edge of the wheel is a small leaden bullet, communicating by a wire with the gilding of the upper surface of the wheel; and about six inches from it is another bullet communicating in like manner with the under surface. When the wheel is to be charged by the upper surface, a communication must be made from the under surface to the table. As soon as it is well charged, it begins to move. The bullet nearest to a pillar moves towards the thimble on that pillar; and, passing by, electrifies it, and is then repelled from it. The succeeding bullet, which communicates with the other surface of the glass, more strongly attracts that thimble on account of its being electrified before by the other bullet; and thus the wheel increases its motion, till the resistance of the air regulates it. It will go half an hour; and make, one minute with another, 20 turns in a minute, which is 600 turns in the whole; the bullet in the upper surface giving in each turn 12 sparks to the thimbles, making in all 2500 sparks; while the same quantity of fire is thought to be received by the under bullet. The whole space moved over by these bullets in the mean time is 2500 feet. If, instead of two bullets, you put eight, four communicating with the upper and four with the under surface, the force and swiftness will be greatly increased, and the wheel will make about 50 turns in a minute; but then it will not continue moving for such a long time. These wheels may be applied to the ringing of chimes, and the moving of small orreries, &c.

XXXII. The Magic Picture.

This is a contrivance of Mr Kinnerley; and is perhaps more calculated to give surprise than any other experiment in electricity. It is made in the following manner: Having a large mezzotinto, with a frame and glass (suppose of the king), take out the print, and cut a panel out of it near two inches distant from the frame all round. If the cut be through the picture, it is nothing the worse. With thin paste, or gum-water, fix the board that is cut off on the inside of the glass, pressing it smooth and close; then fill up the vacancy, by gilding the glass well with leaf-gold or brass. Gild likewise the inner edge of the back of the frame all round, except the top part, and form a communication between that gilding and the gilding behind the glass; then put in the board, and that side is finished. Turn up the glass, and gild the forehead exactly over the back gilding; and when it is dry, cover it, by pasting on the panel of the picture that has been cut out; observing to bring the corresponding parts of the board and picture together, by which the picture will appear of a piece as at first; only part is behind the glass and part before. Lastly, hold the picture horizontally by the top, and place a little moveable gilt crown on the king's head. If now the picture is moderately electrified, and another person take hold of the frame with one hand, so that his fingers touch its inside gilding, and with the other endeavour to take off the crown, he will receive a terrible blow, and fail in the attempt. The operator, who holds the picture by the upper end, where the inside of the frame is not gilt, to prevent its falling, feels nothing of the shock; and may touch the face of the picture without danger, which he pretends to be a test of his loyalty.

XXXIII. The Thunder-house.

Fig. 52. is an instrument representing the side of a house, either furnished with a metallic conductor, or not; by which both the bad effects of lightning striking upon a house not properly secured, and the usefulness of metallic conductors, may be clearly represented. A is a board about three quarters of an inch thick, and shaped like the gable-end of a house. This board is fixed perpendicularly upon the bottom-board B, upon which the perpendicular glass pillar C D is also fixed in a hole about eight inches distant from the base of the board A. A square hole I L M K, about a quarter of an inch deep, and nearly one inch wide, is made in the board A, and is filled with a square piece of wood nearly of the same dimensions. It is mentioned nearly of the same dimensions, because it must go easily into the hole, that it may drop off by the least shaking of the instrument. A wire L K is fastened diagonally to this square piece of wood. Another wire I H of the same thickness, having a brass ball H, screwed on its pointed extremity, is fastened upon the board A; so also is the wire M N, which is shaped in a ring at O. From the upper extremity of the glass pillar C D, a crooked wire proceeds, having a spring locket F, through which a double knotted wire slips perpendicularly, the lower knob G of which falls just above the knob H. The glass pillar D G must not be made very fast into the bottom board; but it must be fixed so as it may be pretty easily moved round its own axis; by which means the brass ball G may be brought nearer or farther from the ball H, without touching the part E F G. Now when the square piece of wood L M I K (which may represent the shutter of a window or the like) is fixed into the hole so, that the wire L K stands in the dotted representation I M, then the metallic communication from H to O is complete, and the instrument represents a house furnished with a proper metallic conductor; but if the square piece of wood L M I K is fixed so, that the wire L K stands in the direction L K, as represented in the figure, then the metallic conductor H O, from the top of the house to its bottom, is interrupted at I M, in which case the house is not properly secured.

Fix the piece of wood L M I K so, that its wire may be as represented in the figure, in which case the metallic conductor H O is discontinued. Let the ball G be fixed at about half an inch perpendicular distance from the ball H; then, by turning the glass pillar D G, remove the former ball from the latter; by a wire or chain connect the wire E F with the wire Q of the jar P, and let another wire or chain, fastened to the hook O, touch the outside coating of the jar. Connect the wire Q with the prime conductor, and charge the jar; then, by turning the glass pillar D G, let the ball G come gradually near the ball H; and when they are arrived sufficiently near one another, you will observe that the jar explodes, and the piece of wood L M I K is pushed out of the hole to a considerable distance from the thunder-house. Now the ball G, in this experiment, represents an electrified cloud, which, when it is arrived sufficiently near the top of the house A, the electricity strikes it; and as this house is not secured with a proper conductor, the explosion breaks off a part, i.e., knocks off the piece of wood I M.

Repeat the experiment with only this variation, viz., that this piece of wood I M is situated so, that the wire L K may stand in the situation I M, in which case the conductor H O is not discontinued; and you will observe, that the explosion will have no effect upon the piece of wood L M, this remaining in the hole unmoved; which shows the usefulness of the metallic conductor.

Further. Unscrew the brass ball H from the wire H I, so that this may remain pointed. With this difference only in the apparatus, repeat both the above experiments; and you will find that the piece of wood I M is in neither case moved from its place, nor any explosion will be heard; which not only demonstrates the preference of the conductors with pointed termination to those with blunted ones; but also shows that a house furnished with sharp terminations, although not furnished with a regular conductor, is almost sufficiently guarded against the effects of lightning.

This apparatus is sometimes made in the shape of a house, as represented fig. 53, where, for the sake of XXXIV. The Electric Fly.

This fly is composed of small brass wires, fig. 49, fixed into a cap of brass also, easily moveable upon an axis of the same metal, and exactly balanced, so that they may turn with the smallest force. The ends, which ought to be very sharp, are all bent one way, with regard to one another, as those belonging to a, b, in the figure; though the two sets of points constituting the two flies there represented, are contrary to each other; so that the whole flies must have a contrary motion. Fixing the axle with the two flies upon the prime conductor, and working the machine, both will begin to turn very swiftly, each in a direction contrary to that of the points. In this manner, with a powerful machine, a great many flies may be made to turn either in the same or in contrary directions; and by their gradual increase or decrease in size may represent a cone or other figure; for the course of each will be marked by a line of fire, and thus the whole will exhibit a beautiful appearance in the dark. The light is said to be more brilliant when the ends are slightly covered with sealing-wax, grease, or other electric matter.

In this experiment the fly will turn the same way whether the electricity be positive or negative; the reason of which will easily be conceived from the theory already laid down, viz. that in positive electricity the fluid issues from the body electrified, and that in negative electricity it enters into it. In the former case, the recoil of the fluid, which acts equally on the air and on the point from whence it issues, must continually put the point the contrary way; and in negative electricity, when the point solicits a continual draught of electric matter from the air, the direct impulse of the former must also produce a motion in the point in the course in which the fluid itself moves. In vacuo no motion is produced; because there is no air on which the fluid may act when it issues from the point. In like manner, when air is inclosed in a glass vessel, the motion of the electric fly soon stops; because the fluid cannot easily get through the air and the glass, and therefore its motions are impeded so that it cannot press with force sufficient to produce motion. On applying a conductor to the outside of the glass, the fly renews its motion; because an opportunity is now given to the fluid to escape, by running through the glass. But this, for the reasons already given, must soon cease, because a contrary action of the fluid instantly begins to take place; and in a short time becomes equal to that which urges it forward from the machine. The motion of the fly, therefore, stops for the same reason that a Leyden phial becomes at last saturated and cannot receive a greater charge; and which has been already so fully discussed, that it would be superfluous to say more on the subject. Fig. 50 shows another fly which turns perpendicularly, and which will be readily understood from what has been already said.

XXXV. The Electrified Bells.

Fig. 35 represents an instrument having three bells, which are made to ring by electric attraction and repulsion. B is a brass rod, furnished with a ring A of the same metal, by which it is suspended from another rod fixed in the prime conductor. The outer bells C and E are suspended by brass chains; but the middle bell D and the two small brass clappers between C D and D E are suspended by silk threads. From the concave under part of the bell D a chain proceeds, which falls upon the table, and has a silk thread E at its extremity. When this apparatus is hung to the conductor by the ring A, and the cylinder of the machine gently turned, the clappers will fly from bell to bell with a rapid motion, and the bells will ring as long as they are kept electrified. The two bells C and E being suspended by brass chains, are first electrified; hence they attract the clappers, communicate to them a little electricity, and repel them to the unelectrified bell D; upon which the clappers deposit their electricity, and move again to the bells C E, from which they acquire more, and so on. If, by holding the silk thread F, the chain of the middle bell be raised from the table, the bells after ringing a little while will stop; because the bell D will have no opportunity of conveying the electricity it receives from the clappers to the ground, being inflated by the silk thread. In the dark, sparks will be seen between the clappers and bells.

Fig. 36 represents a set of bells more elegantly mounted, and which produce a better sound. In these the knob a must communicate with the conductor when the apparatus is made use of. Fig. 37 represents a set of eight bells otherwife constructed. The clapper \( h \) is here suspended by a filken thread from the fly \( a b c d \); the axis of the fly rests in a small hole on the top of a glass pillar; and its upper part moves freely in, and is confined by a hole in the brass arm \( g \). To make use of these bells they must be applied to the cylinder of the machine, or at least brought very near it when the conductor is removed; so that the fly \( a b c d \) may be about the height of the centre of the cylinder. The latter being then put in motion, the electricity from it proceeding to the fly, will cause it to turn round in the manner described in the foregoing experiment, and the clapper attracted by each of the bells alternately in its rotation; which, if they are properly turned, will produce a pleasing and harmonious sound.

XXXVI. To fire a Pistol or Cannon by Inflammable Air.

Fig. 40 represents a brass pistol for inflammable air. It consists principally of a chamber, to the mouth \( D \) of which a cork is fitted; a glass tube \( F \) is cemented into the top of the chamber, through which a brass wire passes, and is bent within side so as to approach within an eighth part of an inch of the side. On the outside end of this wire is screwed a brass ball \( A \), which serves to receive a spark from the conductor of the machine, and conduct it in that form to the inside of the pistol. The inflammable air with which the pistol is to be charged may be made in a common stone-ware or glass bottle, by mixing a handful of iron-flings with about two wine-glassesfuls of water and near one of oil of vitriol. The air, when thus made, should be kept in a bottle corked up. To make use of the pistol, take out the cork from the bottle, and instantly apply the mouth of the pistol to the mouth of the bottle; and in about ten seconds it will be sufficiently charged; then remove it, and cork both the pistol and bottle with the utmost expedition; then bring the ball \( A \) near the prime conductor or the knob of a charged jar; and the spark that passes through the ball, and between the end of the wire within side and the side of the chamber, will fire the inflammable air with a loud report, and drive the cork to a considerable distance. Instruments to fire inflammable air are often made in the form of a cannon with its carriage, as in fig. 41.

XXXVII. The Spiral Tube.

Fig. 42 represents an instrument composed of two glass tubes \( C D \), one within another, and closed with two knopped brass caps \( A \) and \( B \). The innermost of these has a spiral row of small round pieces of tin-foil stuck upon its outside surface, and lying at about one-thirtieth of an inch from each other. If this instrument be held by one of its extremities, and its other extremity be presented to the prime conductor, every spark that it receives from the prime conductor will cause small sparks to appear between all the round pieces of tin-foil stuck upon the innermost tube; which in the dark affords a pleasing spectacle, the instrument appearing encompassed by a spiral line of fire.

Fig. 43 represents several spiral tubes placed round a board, in the middle of which is screwed a glass pillar, and on the top of this pillar is cemented a brass cap with a fine steel point. In this a brass wire turns, having a brass ball at each end, nicely balanced on the wire. To make use of this apparatus, place the middle of the turning wire under a ball proceeding from the conductor, so that it may receive a succession of sparks from the ball; then push the wire gently round; and the balls in their relative motions will give a spark to each tube, and thereby illuminate them down to the board, which from its brilliancy and rapid motion afford a most beautiful and pleasing sight.

The small pieces of tin-foil are sometimes stuck on a flat piece of glass \( A B C D \), fig. 44. So as to represent various fanciful figures. Upon the same principle is the luminous word light produced. It is formed by the small separations of the tin-foil pasted on a piece of glass fixed in a frame of baked wood, as represented fig. 45. To use this, the frame must be held in the hand, and the ball \( G \) presented to the conductor. The spark then will be exhibited in the intervals composing the word; from whence it passes to the hook at \( h \), and thence to the ground by a chain. The brilliancy of this is equal to that of the spirals.

XXXVIII. To fire a Piece of Iron-wire in Dephlogisticated Air.

The apparatus for this is represented fig. 28, no. 2. Where the wire is twisted into a spiral figure. When this is done, it may easily be inserted in the brass knob \( D \). The jar comes out of the bottom \( C \), and is filled with the dephlogisticated air, as directed under the article Aerology. The electricity of a common jar being then instantly sent down through the ball and wire at \( A \), an explosion takes place betwixt the end of the small wire and the lower ball \( B \), which sets the end of the former on fire. It burns with remarkable brightness; and by reason of the spiral shape into which it is twisted, shows the appearance of a small sun moving from the top to the bottom of the jar, and slowly moving round as the wire, which is of a spiral shape, gradually burns away.

XXXIX. The Electrified Capillary Syphon.

Let a small bucket of metal filled with water be suspended from the prime conductor, and put in a glass syphon so narrow in the extremity that the water may just drip from it. If in this disposition of the apparatus the winch of the machine be turned, the water, which when not electrified run out only by drops, will now run in a full stream, or even be subdivided into smaller streams; and if the experiment be made in the dark, the appearance will be very beautiful. The same phenomenon will be exhibited by a small bucket with a jet, as represented fig. 46, or the experiment may be agreeably varied, by hanging one bucket from a positive conductor and another from a negative one; so that the ends of the tubes or jets may be about three or four inches from each other. The stream issuing from the one will be attracted by that issuing from the other, and both will unite into one; but though both are luminous in the dark before meeting, the united stream will not be so unless the one electricity has been stronger than the other.

XL. To illuminate Eggs.

Fig. 55 represents a mahogany stand so constructed as to hold three eggs at a greater or smaller distance, according to the position of the sliding pieces. A chain \( C \) is placed at the bottom in such a manner as to touch the bottom of the egg at B with one end, and with its other the outside coating of a charged jar. The sliding wire A at top is made to touch the upper egg; and the distance of the eggs' surfaces should not exceed the quarter or eighth part of an inch. The electricity being by means of the discharging rod sent down the ball and wire at A, will in a darkened room render the eggs very luminous and transparent, as has already been mentioned.

XLII. To render Ivory or Boxwood luminous.

Place an ivory ball on the prime conductor of the machine, and take a strong spark, or send the charge of a Leyden bottle through its centre, the ball will appear perfectly luminous; but if the charge be not taken through the centre, it will pass over the surface of the ball and corrode it. A spark taken through a ball of boxwood not only illuminates the whole, but makes it appear of a beautiful crimson or rather fine fætus colour.

XLIII. To illuminate Water.

Connect one end of a chain with the outside of a charged jar, and let the other lie upon the table. Place the end of another piece of chain at about one quarter of an inch from the former; then set a decanter of water on these separated ends; and on making a discharge, the water will appear perfectly and beautifully luminous.

XLIV. To make a beautiful Appearance in vacuo.

Fig. 58. represents a glass barometer tube, having on the end B a steel cap fastened to the glass with cement. From this proceed a wire and ball c d. Fill this tube with quicksilver; and then by sending up a large bubble of air, and repeatedly inverting the tube, free the quicksilver and iron ball from air: then put a small drop of ether on the quicksilver, and put the finger on the end of the glass tube; and then invert the end f in a basin of quicksilver, taking care not to remove the finger from the end of the tube till the latter be immersed under the surface of the quicksilver. When the finger is removed, the mercury will descend, and the ether expand itself; present the metallic top of the tube to a large charged conductor, and a beautiful green spark will pass through the vapour of the ether from the ball d to the quicksilver. By admitting a small quantity of air into the tube, an appearance something like a falling star is produced.

XLV. To render Gold-leaf or Dutch-metal luminous.

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

XLVI. The Inflammable Air-lamp.

Fig. 60. represents this machine, which is an invention of M. Volta. A is a glass globe to contain the inflammable air; B, a glass basin or reservoir to hold water; D, a cock to form occasionally a communication between the reservoir of water and that of air. The water passes into the latter through the metal pipe gg, which is fixed to the upper part of the reservoir A; as s is a small cock to cut off or open a communication with the air in the ball and the jet K.—N is a small pipe to hold a piece of wax taper; L, a brass pillar, on the top of which is a ball of the same metal; a is a pillar of glass with a socket at top, in which the wire b slides, having a ball screwed on the end of it. F is a cock by which the ball A is filled with inflammable air, and which afterwards serves to confine the air, and what water falls from the basin B into the ball A.

To use this instrument, after having filled the reservoir A with pure inflammable air and the basin with water, turn the cocks D and s, and the water which falls from the basin B will force out some of the inflammable air, and cause it to pass through the jet K into the air. If an electric spark be made to pass from the brass ball m to that marked n, the inflammable jet which passes through the pipe K will be fired. To extinguish the lamp, first shut the cock s, and then the cock D. The inflammable air is made of the usual ingredients, viz., iron-filings and vitriolic acid; and the reservoir is filled in the following manner: Having previously filled A with water, place the foot R in a tub of that fluid which may cover it, so that the bent glass tube through which the air passes may pass commodiously below the foot of the lamp. When the air has nearly driven out all the water, turn the cock F, and the apparatus is ready for use. This instrument is convenient for preserving a quantity of inflammable air ready for any occasional experiment, as charging the inflammable air-pistol, &c. It is also convenient for lighting a candle for economical purposes, as the least spark from an electrophorus or a small bottle is sufficient to fire the air.

XLVII. Imitations of the Planetary Motions.

See below, Uses of the Electric Fluid in the System of Nature.

XLVIII. Beautiful Figures produced in Powdered Rosin, &c., screwed over an Electric Substance. Ibid. Sect. IX. Experiments of a Miscellaneous Nature, viz. those relating to the Effects of the Electric Fluid on Colours; on its Velocity; the Changes of Electric into Conducting Substances; the impossibility of forcing the Fluid through a perfect Vacuum; the Power of Batteries; its Direction in various Cases; Improvements in the Method of Excitation, &c.

These experiments, though far from being uninteresting, we have thought proper to clas under a different title, as many might wish to amuse themselves with producing an agreeable and beautiful phenomenon who would not choose to make experiments for the sake of investigating unknown subjects, where perhaps little else than the labour of making the experiment might be the reward of the operator. These experiments also may be truly said to be of an anomalous nature; as not being founded upon any known laws of electricity, but rather a collection of facts; from some of which we may afterwards deduce the laws by which this subtle fluid is occasionally governed. We shall begin with experiments made by Mr Cavallo upon substances painted over with colours of different kinds. They were occasioned by his having observed that an electric shock, sent over the surface of a card, made a black stroke upon a red spot, from which he was induced to try the effect of sending shocks over cards painted with different water-colours. The force employed was generally about one foot and an half of charged glass; and the shocks were sent over the cards while the latter were in a very dry state.

Vermilion was marked with a strong black track, about one-tenth of an inch wide. This stroke is generally single, as represented by AB, fig. 74, n° 2. of Plate CLXXVII. Sometimes it is divided in two towards the middle, like EF; and sometimes, particularly when the wires are set very distant from one another, the stroke is not continued, but interrupted in the middle, like GH. It often, although not always, happens, that the impression is marked stronger at the extremity of that wire from which the electric fluid issues, as it appears at E, supposing that the wire C communicates with the positive side of the jar; whereas the extremity of the stroke, contiguous to the point of the wire D, is neither so strongly marked, nor surrounds the wire so much, as the other extremity E.

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

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

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

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

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

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

Defirous of carrying this investigation on colours a little further, with a particular view to determine something relative to the properties of lamp-black and oil (c), I procured some pieces of paper painted on both sides with oil colours; and pending the charge of two feet of coated glass over each of them, by making the interruption of the circuit upon their surfaces, I observed that the pieces of paper painted with lamp-black, Prussian blue, vermilion, and purple brown, were torn by the explosion; but white lead, Naples yellow, English ochre, and verdigris, remained unhurt.

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

I. A coat of oil-paint over any substance, defends it from the effects of such an electric shock as would otherwise injure it; but by no means defends it from any electric shock whatever. II. No one colour seems preferable to the others, if they are equal in substance, and equally well mixed with oil; but a thick coating does certainly afford a better defence than a thinner one.

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

It is remarkable, that vermilion receives the black impression, when painted with linseed oil, nearly as well as when painted with water. The paper painted with white lead and oil receives also a black mark; but its nature is very singular. The track, when first made, is almost as dark as that marked on white-lead painted

(c) "It has often been observed, that when lightning has struck the masts of ships, it has passed over such parts of the masts as were covered with lamp-black and tar, or painted with lamp-black and oil, without the least injury, at the same time that it has shivered the uncoated parts in such a manner as to render the masts useless." painted with water; but it gradually loses its blackness, and in about an hour's time (or longer, if the paint is not fresh) it appears without any darkness; and when the painted paper is laid in a proper light, appears only marked with a colourless track, as if made by a finger-nail. I sent the shock also over a piece of board which had been painted with white-lead and oil about four years before, and the explosion marked the black track upon this also: this track, however, was not so strong, nor vanished so soon, as that marked upon the painted paper; but in about two days' time it also vanished entirely."

Another very remarkable property of the electric fluid is, that it both calcines, vitrifies, and revivifies metals. The calcination of them appears from Dr Priestley's experiments with the bras chain, formerly mentioned, where the black dust was plainly a calx of the metal. The vitrification is performed by exploding small wires of any kind with the shock of a battery. In this case, the small globules of metal, even though gold, silver, or platinum, are found to be completely vitrified. The revivification is an experiment of Mr Beccaria. This he did by making the explosion between two pieces of the calces; and thus he revivified several metallic substances, particularly zinc, and even produced real quicksilver from cinnabar. In this case, he always observed streaks of black beyond the coloured metallic stains; owing, as he supposed, to the phlogiston driven from the parts that were vitrified, when the other part revivified the calx.

Mr Beccaria also discovered another very remarkable property of the electric matter; namely, that when it is obliged to pass through air, or any other substance through which it makes its way with difficulty, it throws before it all light conducting substances it can find, in order to facilitate its own passage; and thus it will pass through a greater quantity of resisting medium than it would otherwise be able to do. The experiments from which Mr Beccaria drew this conclusion, were the following. He put a narrow piece of leaf-fiber between two plates of wax, laying it across them, but so that it did not quite reach one of the sides. The discharge being made through this strip of metal, by bringing a wire opposite to the silver, at the place where it was discontinued; the silver was found melted, and part of it dispersed all along the track that the electric matter took between the plates of wax, from the silver to the wire. Happening once to receive, inadvertently, the charge of a small jar through some smoke of spirit of nitre, a hole was made in his thumb, where the fire entered, and which he thought could only have been made by the acid carried along by the electric fluid. Dr Priestley hath made several more experiments, in order to ascertain this remarkable property; and of which he gives the following account.

"I discharged frequent shocks, both of a common jar, and another of three square feet, through trains of bras dust, laid on a stool of baked wood, making interruptions in various parts of the train; and always found the bras dust scattered in the intervals, so as to connect the two disjointed ends of the train; but then it was likewise scattered nearly as much from almost all other parts of the train, and in all directions. The scattering from the train itself was probably occasioned by small electric sparks between the particles of the dust; which, causing a vacuum in the air, drove all that light matter to a considerable distance. But the particles of the dust, which were thrown in the intervals of the train, some of which were at least three inches, could hardly be conveyed in that manner.

"When small trains were laid, the dispersion was the most considerable, and a light was very visible in the dark, illuminating the whole circuit. It made no difference, in any of these experiments, which way the shock was discharged.

"When I laid a considerable quantity of the dust at the ends of two pieces of chain, through which the shock passed, at the distance of about three inches from one another, the dust was always dispersed over the whole interval, but chiefly laterally; so that the greatest quantity of it lay in arches, extending both ways, and leaving very little of it in the middle of the path. It is probable, that the electric power would have spread it equably, but that the vacuum made in the air, by the passage of the fluid from one heap of dust to the other, dispersed it from the middle part.

"I then inflated a jar of three square feet, and upon an adjoining glass-stand laid a heap of bras dust; and at the distance of seven or eight inches a bras rod communicating with the outside of the jar. Upon bringing another rod, communicating with the inside, upon the heap of dust, it was dispersed in a beautiful manner, but not one way more than another. However, it presently reached the rod communicating with the outside.

"Making two heaps, about eight inches asunder, I brought one rod communicating with the inside upon one of them, and another rod communicating with the outside upon the other. Both the heaps were dispersed in all directions, and soon met; presently after which the jar was discharged, by means of this dispersed dust, in one full explosion. When the two heaps were too far asunder to promote a full discharge at once, a gradual discharge was made through the scattered particles of the dust.

"When one heap of dust was laid in the centre of the stand, and the two rods were made to approach on each side of it, they each attracted the dust from the side of the heap next to them, and repelled it again in all directions. When they came very near the heap, the discharge was made through it, without giving it any particular motion.

"All these experiments show, that light bodies possessed of a considerable share of electricity, disperse in all directions, carrying the electric matter to places not abounding with it; and that they sometimes promote a sudden discharge of great quantities of that matter from places where it was lodged, to places where there was a defect of it. But an accident led me to a much more beautiful, and perhaps a more satisfactory, manner of demonstrating the last part of this proposition, than any that I hit upon while I was pursuing my experiments with that design.

"Hanging a drop of water upon the knob of a bras rod communicating with the inside of my battery, in order to observe what variety it might occasion in the circular spots above mentioned, I was greatly surprised to find the explosion made all at once, at the distance of two inches." Miscellaneous. "I afterwards put some brass dust upon a plate of our expert metal communicating with the inside of the battery; and making the discharge through the dust, it exploded at the distance of an inch and a half. The dust rose towards the discharged rod, and from thence was dispersed in all directions.

"These experiments are the more remarkable, as they demonstrate so great a difference between the distance at which the battery may be made to discharge at once, by the help of these light bodies, and without them. When the discharge of a battery by the knobs of brass rods, in the open air, is at the distance of about half an inch; it will, by this means, be made at about two inches."

The motions of the electric fluid, though prodigiously quick, are not instantaneous. The shock of the Leyden phial, indeed, hath been transmitted through wires of several miles in length, without taking up any sensible space of time. That is, supposing two persons to hold the ends of the wire, one communicating with the knob, and the other with the outside coating of the phial, both would feel the shock at the same instant; nor would it make any alteration though a considerable part of the surface of the ground was made part of the conductor. Dr Priestley relates several very curious experiments made with a view to ascertain this point soon after the Leyden phial was discovered. These experiments were planned and directed by Dr Watson, who was present at every one of them. His chief assistants were Martin Folkes, Esq.; president of the royal society, Lord Charles Cavendish, Dr Bevis, Mr Graham, Dr Birch, Mr Peter Daval, Mr Trembley, Mr Ellicott, Mr Robins, and Mr Short. Many other persons, and some of distinction, gave their attendance occasionally.

Dr Watson, who wrote the history of their proceedings, in order to lay them before the royal society, begins with observing (what was verified in all their experiments), that the electric shock is not, strictly speaking, conducted in the shortest manner possible, unless the bodies through which it passes conduct equally well; for that, if they conduct unequally, the circuit is always formed through the best conductor, though the length of it be ever so great.

The first attempt these gentlemen made, was to convey the electric shock across the river Thames, making use of the water of the river for one part of the chain of communication. This they accomplished on the 14th and 18th of July 1747, by fastening a wire all along Westminster bridge, at a considerable height above the water. One end of this wire communicated with the coating of a charged phial, the other being held by an observer, who, in his other hand, held an iron rod, which he dipped into the river. On the opposite side of the river stood a gentleman, who likewise dipped an iron rod in the river with one hand; and in the other held a wire, the extremity of which might be brought into contact with the wire of the phial.

Upon making the discharge, the shock was felt by the observers on both sides the river, but more sensibly by those who were stationed on the same side with the machine; part of the electric fire having gone from the wire down the moist stones of the bridge, thereby making several shorter circuits to the phial, but still all passing through the gentlemen who were stationed on the same side with the machine. This was, in a manner, demonstrated by some persons feeling a sensible shock in their arms and feet, who only happened to touch the wire at the time of one of the discharges, when they were standing upon the wet steps which led to the river. In one of the discharges made upon this occasion, spirits were kindled by the fire which had gone through the river.

Upon this, and the subsequent occasions, the gentlemen made use of wires in preference to chains; for this, among other reasons, that the electricity which was conducted by chains was not so strong as that which was conducted by wires. This, as they well observed, was occasioned by the junctures of the links not being sufficiently close, as appeared by the snapping and flashing at every juncture where there was the least separation. These lesser snappings, being numerous in the whole length of a chain, very sensibly lessened the great discharge at the gun-barrels.

Their next attempt was to force the electrical shock to make a circuit of two miles, at the New River at Stoke Newington. This they performed on the 24th of July 1747, at two places; at one of which the distance by land was 800 feet, and by water 2000; in the other, the distance by land was 2800 feet, and by water 8000. The disposition of the apparatus was similar to what they before used at Westminster bridge, and the effect answered their utmost expectations. But as, in both cases, the observers at both extremities of the chain, which terminated in the water, felt the shock as well when they stood with their rods fixed into the earth 20 feet from the water, as when they were put into the river; it occasioned a doubt, whether the electric circuit was formed through the windings of the river, or a much shorter way, by the ground of the meadow; for the experiment plainly showed, that the meadow-ground, with the grass on it, conducted the electricity very well.

By subsequent experiments they were fully convinced, that the electricity had not in this case been conveyed by the water of the river, which was two miles in length, but by land, where the distance was only one mile; in which space, however, the electric matter must necessarily have passed over the New River twice, have gone through several gravel pits, and a large stubble field.

July 28th, they repeated the experiment at the same place, with the following variation of circumstances. The iron wire was, in its whole length, supported by dry sticks, and the observers stood upon original electrics; the effect of which was, that they felt the shock much more sensibly than when the conducting wire had lain upon the ground, and when the observers had likewise stood upon the ground, as in the former experiment.

Afterwards, every thing else remaining as before, the observers were directed, instead of dipping their rods into the water, to put them into the ground, each 150 feet from the water. They were both finitely struck, though they were distant from each other above 500 feet.

The same gentlemen, pleased with the success of their former experiments, undertook another, the object of which was, to determine whether the electric virtue virtue could be conveyed through dry ground; and, at the same time, to carry it through water to a greater distance than they had done before. For this purpose they pitched upon Highbury-barn beyond Ilfordton, where they carried it into execution on the 5th of August 1747. They chose a station for their machine almost equally distant from two other stations for observers upon the New River; which were somewhat more than a mile asunder by land and two miles by water. They had found the streets of London, when dry, to conduct very strongly for about 40 yards; and the dry road at Newington about the same distance.

The event of this trial answered their expectations. The electric fire made the circuit of the water, when both the wires and the observers were supported upon original electrics, and the rods dipped into the river. They also both felt the shock, when one of the observers was placed in a dry gravelly pit, about 300 yards nearer the machine than the former station, and 100 yards distant from the river: from which the gentlemen were satisfied, that the dry gravelly ground had conducted the electricity as strongly as water.

From the shocks which the observers received in their bodies, when the electric power was conducted upon dry sticks, they were of opinion, that, from the difference of distance simply considered, the force of the shock, as far as they had yet experienced, was very little if at all impaired. When the observers stood upon electrics, and touched the water or the ground with the iron rods, the shock was always felt in their arms or wrists; when they stood upon the ground with their iron rods, they felt the shock in their elbows, wrists, and ankles; and when they stood upon the ground without rods, the shock was always felt in the elbow and wrist of that hand which held the conducting wire, and in both ankles.

The last attempt of this kind which these gentlemen made, and which required all their sagacity and address in the conduct of it, was to try whether the electric shock was perceptible at twice the distance to which they had before carried it, in ground perfectly dry, and where no water was near; and also to distinguish, if possible, the respective velocity of electricity and sound.

For this purpose they fixed upon Shooter's-hill, and made their first experiments on the 14th of August 1747; a time when, as it happened, but one shower of rain had fallen during five preceding weeks. The wire communicating with the iron rod which made the discharge, was 6732 feet in length, and was supported all the way upon baked sticks; as was also the wire which communicated with the coating of the phial, which was 3868 feet long, and the observers were distant from each other two miles. The result of the explosion demonstrated, to the satisfaction of the gentlemen present, that the circuit performed by the electric matter was four miles, viz., two miles of wire and two of dry ground, the space between the extremities of the wires; a distance which, without trial, as they justly observed, was too great to be credited. A gun was discharged at the instant of the explosion, and the observers had stop-watches in their hands, to note the moment when they felt the shock; but, as far as they could distinguish, the time in which the electric matter performed that vast circuit might have been instantaneous.

In all the explosions where the circuit was made of considerable length, it was observed, that though the phial was very well charged, yet that the spark at the gun-barrel, made by the explosion, was not near so loud as when the circuit was formed in a room; so that a bystander, says Dr Watson, though versed in these operations, would not imagine, from seeing the flash, and hearing the report, that the stroke at the extremity of the conducting wire could have been considerable; the contrary whereof, when the wires were properly managed, he says, always happened.

Still the gentlemen, unwearyed in these pursuits, were desirous, if possible, to ascertain the absolute velocity of electricity at a certain distance; because, though in the last experiment, the time of its progress was certainly very small, if any, they were desirous of knowing, small as that time might be, whether it was measurable; and Dr Watson had contrived an excellent method for that purpose.

Accordingly, on the 5th of August 1748, the gentlemen met once more, and the last time, at Shooter's-hill; when it was agreed to make an electric circuit of two miles, by several turnings of the wire in the same field. The middle of this circuit they contrived to be in the same room with the machine, where an observer took in each hand one of the extremities of the wires, each of which was a mile in length. In this excellent disposition of the apparatus, in which the time between the explosion and the shock might have been observed to the greatest exactness, the phial was discharged several times; but the observer always felt himself shocked at the very instant of making the explosion. Upon this the gentlemen were fully satisfied, that through the whole length of this wire, which was 12,276 feet, the velocity of the electric matter was instantaneous.

With all this surprising velocity, however, it is certain, that both sides of a charged phial may be touched so quickly, even by the best conductors, that all the electric matter hath not time to make the circuit, and slowly, the phial will remain but half discharged. If the upper plate of an electrophorus also is very suddenly touched with the finger, or any other conductor, a very small spark will be obtained on lifting it up; though a very strong one would be got if the finger was kept longer upon it. But how this seeming flowery can be reconciled with the immeasurable velocity above-mentioned, doth not appear. It is certain, indeed, that this fluid is considerably resisted in its passage through or over every substance. It will even prefer a short passage in the air where it is violently resisted to one along a wire of very great length; but here, as in every other case, it seems to divide its force, and to break out through several different passages at once.

A method of ascertaining this hath been contrived by Dr Priestley, thus. Bend a wire, about five feet long, so that one part may come within half an inch of the other; then connect the extremities of the wire with the hook of the battery, and send a shock through it. On making the explosion, a spark will be seen between the two parts which approach nearest to each other; which shows that the fluid chooses a short passage through the air, rather than the long one through the wire. The charge, however, does not pass entirely between these two parts, but some of it goes also thro' the wire. This may be proved by putting a slender wire between the two approaching parts: for, on making the discharge with only this addition in the apparatus, the small wire will hardly be made red hot; whereas, if the large wire be cut so as to discontinue the circuit, the small wire will be melted, and even exploded, by the same shock that before made it scarcely red hot. But though we can easily show that the electric fluid always meets with resistance, it is by no means easy to show why the same resistance which puts a temporary stop to its motions in some cases, doth not so in all.

Another curious experiment in electricity is the converting of conducting substances into electrics by cold, and of changing electrics into conductors by heat. The first hath yet been done only in the instance of water. This is a discovery of Mr Achard's at Berlin, who, in the month of January 1776, observed, that water frozen to the 20th degree below the freezing point of Reaumur's thermometer, answering to the 13th below 0 of Fahrenheit's, is an electric. He tried his experiments in the open air, where he found, that a rod of ice two feet long and two inches thick, was a very imperfect conductor when Reaumur's thermometer was at six degrees below 0; and that it would not in the least conduct when the thermometer was sunk to 20°. By whirling a spheroid of ice in a proper machine, he even electrified the prime conductor so as to attract, repel, give sparks, &c. The ice made use of was free from air bubbles, and quite transparent; to produce which, he used to set a vessel containing distilled water to be frozen, upon the window of a room which was rather warm with respect to the ambient air; so that the water began to freeze on the one side of the vessel, while on the other it was still liquid.

To prove that glas and other electrics become conductors when very hot: Take a small glass tube of about one-twentieth of an inch in diameter, and above a foot long; close it at one end, and introduce a wire into it, so that it may be extended through its whole length; let two or three inches of this wire project above the open end of the tube, and there fasten it with a bit of cork; tie round the closed end of the tube another wire, which will be separated from the wire within the tube only by the glass interposed between them. In these circumstances, endeavour to send a shock through the two wires, i.e., the wire inserted in the glass tube and that tied on its outside, by connecting one of them with the outside, and touching the other with the knob of a charged jar; and you will find that the discharge cannot be made unless the tube be broken; because the circuit is interrupted by the glass at the end of the tube, which is interposed between the two wires. But put that end of the tube to which the wire is tied into the fire, so that it may become just red-hot, then endeavour to discharge the jar again through the wires, and you will find that the explosion will be easily transmitted from wire to wire through the substance of the glass, which, by being made red-hot, is become a conductor.

In order to ascertain the conducting quality of hot resinous substances, oils, &c. bend a glass tube in the form of an arch CEF, fig. 77. Plate CLXXVII; and tie a silk string GCD to it, which serves to hold it by when it is to be set near the fire; fill the middle part of this tube with rosin, sealing-wax, &c. then introduce two wires AE, BF, through its ends, so that they may touch the rosin, or penetrate a little way in it. This done, let a person hold the tube over a clear fire, so as to melt the rosin within it; at the same time, by connecting one of the wires A or B with the outside of a charged jar, and touching the other with the knob of the jar, endeavour to make the discharge through the rosin, and you will observe, that while the rosin is cold, no shocks can be transmitted through it; but it becomes a conductor according as it melts; and when totally melted, then the shocks will pass through it very freely.

To show that hot air is a conductor, electrify one of the cork-ball electrometers suspended upon the stand fig. 13. of Plate CLXXIV. or electrify the prime conductor with the quadrant electrometer; then bring a red-hot iron within a sufficient distance of the electrometer or prime conductor, and you will find that they soon lose their electricity, which is certainly conducted by the hot air contiguous to the iron; for if the experiment be repeated with the same iron when cold, i.e., by bringing it within the same distance of the electrified electrometer or prime conductor, their electricity will not be conducted away as before. It has been observed, that a battery may be discharged by introducing a red-hot iron between two knobs interposed, and standing at some distance from each other in the circuit: but if, instead of iron, there be introduced a piece of red-hot glass between the knobs (the distance between them remaining as at first), the battery cannot be discharged: whence we may infer, that either hot air is not so good a conductor as has been imagined; or else, that air heated by iron is stronger with respect to its conducting power, than when heated by the red-hot glass.

The impossibility of forcing the electric fluid through a perfect vacuum, is a doctrine which militates so directly against the theory laid down in Sect. VI. that power of a cannot by any means omit a very full consideration of perfect vacuum. As this, however, would lead us here into a theoretical disquisition, we shall not enter into any explanation in this place, but defer what is to be said on that subject to the last section, where the uses of the electric fluid in the system of nature are considered. The experiment on which this supposition is founded, was originally made by Mr Walsh; who found that it was possible to cleanse a barometrical tube so perfectly of air, that no electric light would be visible in it upon agitating the mercury, as is the case with the common barometers. It has since been repeated to more advantage by Mr William Morgan, who from his observations has deduced some conclusions concerning the action of the electric fluid very inconsistent with that extensive operation which many philosophers have ascribed to it, and which is ascribed to it in various articles of this work. His experiment is published in the Phil. Transf. for 1785, which we shall here extract.

"The non-conducting power of a perfect vacuum, Mr Morgan is a fact in electricity which has been much controverted among philosophers. The experiments made by Mr Walsh, F. R. S. in the double barometer tube clear, this subject demonstrated the impermeability of the electric light through a vacuum; nor was it, I think, precipitate." to conclude from them the impermeability of the electric fluid itself. But this conclusion has not been universally admitted; and the following experiments were made with the view of determining its truth or fallacy. When I first attended to the subject, I was not aware that any other attempts had been made before those of Mr Walsh; and though I have since found myself to have been in part anticipated in one of my experiments, it may not perhaps be improper to give some account of them, not only as they are an additional testimony in support of this fact, but as they led to the observation of some phenomena which appear to be new and interesting.

"A mercurial gage B, about 15 inches long, carefully and accurately boiled till every particle of air was expelled from the inside, was coated with tin-foil five inches down from its sealed end A, and being inverted into mercury thro' a perforation D, in the brass cap E, which covered the mouth of the cistern H, the whole was cemented together, and the air was exhausted from the inside of the cistern thro' a valve C in the brass cap E just mentioned; which producing a perfect vacuum in the gage, afforded an instrument peculiarly well adapted for experiments of this kind. Things being thus adjusted, (a small wire F having been previously fixed on the inside of the cistern to form a communication between the brass cap E and the mercury G, into which the gage was inverted), the coated end was applied to the conductor of an electrical machine; and notwithstanding every effort, neither the smallest ray of light, nor the slightest charge, could ever be procured in this exhausted gage. I need not observe, that if the vacuum on its inside had been a conductor of electricity, the latter at least must have taken place; for it is well known, that if a glass tube be exhausted by an air-pump, and coated on the outside, both light and a charge may very readily be procured. If the mercury in the gage be imperfectly boiled, the experiment will not succeed; but the colour of the electric light, which, in air rarefied by an exhausteur, is always violet or purple, appears in this case of a beautiful green; and what is very curious, the degree of the air's rarefaction may be nearly determined by this means: for I have known instances, during the course of these experiments, where a small particle of air having found its way into the tube B, the electric light became visible, and as usual of a green colour; but the charge being often repeated, the gage has at length cracked at its sealed end, and in consequence the external air, by being admitted into the inside, has gradually produced a change in the electric light from green to blue, from blue to indigo, and so on to violet and purple, till the medium has at last become so dense as no longer to be a conductor of electricity. I think there can be little doubt from the above experiments, of the non-conducting power of a perfect vacuum; and this fact is still more strongly confirmed by the phenomena which appear upon the admission of a very minute particle of air into the inside of the gage. In this case the whole becomes immediately luminous upon the slightest application of electricity, and a charge takes place, which continues to grow more and more powerful in proportion as fresh air is admitted, till the density of the conducting medium arrives at its maximum, which it always does when the colour of the electric light is indigo or violet. Under these circumstances the charge may be so far increased as frequently to break the glass. In some tubes, which have not been completely boiled, I have observed that they will not conduct the electric fluid when the mercury is fallen very low in them; yet upon letting in air into the cistern, so that the mercury shall rise in the gage, the electric fluid, which was before latent in the inside, shall now become visible; and as the mercury continues to rise, and of consequence the medium is rendered less rare, the light shall grow more and more visible, and the gage shall at last be charged, notwithstanding it has not been near an electrical machine for two or three days. This seems to prove, that there is a limit even in the rarefaction of air, which sets bounds to its conducting power; or, in other words, that the particles of air may be so far separated from each other as no longer to be able to transmit the electric fluid; that if they are brought within a certain distance of each other, their conducting power begins, and continually increases till their approach also arrives at its limit, when the particles again become so near as to resist the passage of the fluid entirely, without employing violence, which is the case in common and condensed air, but more particularly in the latter.

"It is surprising to observe how readily an exhausted tube is charged with electricity. By placing it at 10 inches from the conductor, the light may be seen pervading its inside, and as strong a charge may sometimes be procured as if it were in contact with the conductor; nor does it signify how narrow the bore of the glass may be; for even a thermometer tube, having the minutest perforation possible, will charge with the utmost facility; and in this experiment the phenomena are peculiarly beautiful.

"Let one end of a thermometer tube be sealed hermetically; let the other end be cemented into a brass cap with a valve, or into a brass cock, so that it may be fitted to the plate of an air-pump. When it is exhausted, let the sealed end be applied to the conductor of an electrical machine, while the other end is either held in the hand or connected to the floor. Upon the slightest excitation the electric fluid will accumulate at the sealed end, and be discharged through the inside in the form of a spark, and this accumulation and discharge may be incessantly repeated till the tube is broken. By this means I have had a spark 42 inches long; and had I been provided with a proper tube, I do not doubt but that I might have had a spark of four times that length. If, instead of the sealed end, a bulb be blown at that extremity of the tube, the electric light will fill the whole of that bulb, and then pass through the tube in the form of a brilliant spark, as in the foregoing experiment; but in this case I have frequently been able to repeat the trials above three or four times before the charge has made a small perforation in the bulb. If, again, a thermometer filled with mercury be inverted into a cistern, and the air exhausted in the manner I have described for making the experiment with the gage, a Torricellian vacuum will be produced; and now the electric light in the bulb, as well as the spark in the tube, will be of a vivid green; but the bulb will not bear a frequent repetition of charges before it is perforated in like manner as when it has been exhausted by an air-pump. It can hardly be necessary to observe, that in these cases the electric fluid assumes the appearance of a spark (b), from the narrowness of the passage through which it forces its way. If a tube 40 inches long be fixed into a globe 8 or 9 inches in diameter, and the whole be exhausted, the electric fluid, after passing in the form of a brilliant spark throughout the length of the tube, will, when it gets into the inside of the globe, expand itself in all directions, entirely filling it with a violet and purple light, and exhibiting a striking instance of the vast elasticity of the electric fluid.

"I cannot conclude this paper without acknowledging my obligations to the ingenious Mr Brook of Norwich, who by communicating to me his method of boiling mercury, has been the chief cause of my success in these experiments (e). I have lately learned from him, that he has also ascertained the non-conducting power of a perfect vacuum; but what steps he took for that purpose, I know not. Of his accuracy, however, I am so well convinced, that had I never made an experiment myself, I should, upon his testimony alone, have been equally assured of the fact.

To most of the preceding experiments Dr Price, Mr Laue, and some others of my friends, have been eye-witnesses; and I believe that they were as thoroughly satisfied as myself with the results of them. I must beg leave to observe to those who wish to repeat them, that the first experiment requires some nicety, and no inconsiderable degree of labour and patience. I have boiled many gages for several hours together without success, and was for some time disposed to believe the contrary of what I am now convinced to be the truth. Indeed, if we reason a priori, I think we cannot suppose a perfect vacuum to be a perfect conductor without supposing an absurdity: for if this were the case, either our atmosphere must have long ago been deprived of all its electric fluid, by being everywhere surrounded by a boundless conductor, or this fluid must pervade every part of infinite space; and consequently there can be no such thing as a perfect vacuum in the universe. If, on the contrary, the Miscellanea truth of the preceding experiments be admitted, it will follow, that the conducting power of our atmosphere increases only to a certain height, beyond which this power begins to diminish, till at last it entirely vanishes; but in what part of the upper regions of the air these limits are placed, I will not presume to determine. It would not perhaps have been difficult to have applied the results of some of these experiments to the explanation of meteors, which are probably owing to an accumulation of electricity. It is not, however, my present design to give loose to my imagination. I am sensible, that by indulging it too freely, much harm is done to real knowledge; and therefore, that one fact in philosophy well ascertained, is more to be valued than whole volumes of speculative hypotheses."

A fact so contrary to the generally received opinion of the conducting powers of a vacuum, could not but excite a general surprize, and attempts to repeat the experiment would no doubt be ardently wished for. Unfortunately, however, the experiment itself, as must evidently appear from the account given of it by Mr Morgan, is of such a precarious nature, as must undoubtedly discourage any ordinary electrician from attempting it; for in the first place, there is no hope of success without a very tedious boiling of mercury in a tube for several hours; and even when this is done, the instrument will not remain in a state of perfection for any length of time. Mr Cavallo, who has greatly improved the air-pump, gives an account of some very curious experiments made with this instrument, in order to ascertain the truth of Mr Morgan's position; which we shall likewise give in his own words, with the conclusions he draws from them.

"I. In a glass receiver, of six inches diameter and nine inches in height, having a brass cap, a brass wire of two-tenths of an inch in diameter was fixed to its cap, and proceeding through the middle of the receiver, its lower extremity was five inches distant from the aperture of the receiver, and of course of the plate of the earth."

(b) "By cementing the string of a guitar into one end of a thermometer tube, a spark may be obtained as well as if the tube had been sealed hermetically."

(e) "Mr Brook's method of making mercurial gages is nearly as follows: Let a glass tube L, (fig. 81.) sealed hermetically at one end, be bent into a right angle within two or three inches of the other end. At the distance of about an inch or less from the angle, let a bulb K, of about ¼ths of an inch in diameter, be blown in the curved end, and let the remainder of this part of the tube be drawn out T, so as to be sufficiently long to take hold of when the mercury is boiling. The bulb K is designed as a receptacle for the mercury, to prevent its boiling over; and the bent figure of the tube is adapted for its inversion into the cistern: for by breaking off the tube at M within ⅓th or ¼th of an inch of the angle, the open end of the gage may be held perpendicular to the horizon when it is dipped into the mercury in the cistern, without obliging us to bring our finger or any other substance into contact with the mercury in the gage, which never fails to render the instrument imperfect. It is necessary to observe, that if the tube be 14 or 15 inches long, I have never been able to boil it effectually for the experiments mentioned in this paper in less than three or four hours, although Mr Brook seems to prefer a much shorter time for the purpose; nor will it even then succeed, unless the greatest attention be paid that no bubbles of air lurk behind, which to my own mortification I have frequently found to have been the case: but experience has at length taught me to guard pretty well against this disappointment, particularly by taking care that the tube be completely dry before the mercury is put into it; for if this caution be not observed, the instrument can never be made perfect. There is, however, one evil which I have not yet been able to remedy; and that is, the introduction of air into the gage, owing to the unboiled mercury in the cistern: for when the gage has been a few times exhausted, the mercury which originally filled it becomes mixed with that into which it is inverted, and in consequence the vacuum is rendered less and less perfect, till at last the instrument is entirely spoiled. I have just constructed a gage so as to be able to boil the mercury in the cistern, but have not yet ascertained its success." of the air-pump, when the receiver was placed upon it. A fine linen thread was fastened towards the top of the wire, and about four inches of it hanged freely along the brafs wire, and almost in contact with it. The extremity of the wire, which passing through the brafs cap projected out of the receiver, was furnished with a ball. Thus prepared, the receiver was placed upon the plate of the pump, without any leather, or anything else besides a little oil on its outside edge, which must be always understood in all the other experiments related in the course of this chapter. Then the exhaustion was commenced, and at intervals some electricity was communicated, either by the approach of the conductor of an electrical machine, or the knob of a charged jar, to the brafs ball of the wire, in order to observe the strength of the repulsion of the thread from the wire in different degrees of rarefaction; which degrees were ascertained by the short barometrical gage. Proceeding in this manner, it was observed, that till the rarefaction did not exceed one hundred, to wit, till the air remaining within the receiver was not less than the hundredth part of the original quantity, whenever the electricity was communicated to the brafs ball, the thread first adhered to the wire, and then was repelled by it; though this repulsion became smaller and smaller, according as the exhaustion came nearer to the above mentioned degree. The clinging of the thread to the wire first, was because being dry, it required some time before it acquired a sufficient quantity of electricity from the wire, and consequently it was not immediately repelled. When the air within the receiver was exhausted above 100 times, the thread was not first attracted and then repelled as before, but only vibrated a little backwards and forwards, and then remained in the situation in which it stood when electricity was not concerned. By exhausting the receiver still farther, the vibration of the thread when electrified was gradually diminished; so that when the degree of rarefaction was above 500, sparks and the discharge of a jar only made the thread vibrate in a manner just sensible; but this vibration, however small, did never become quite insensible, even when the receiver was exhausted to the utmost power of the pump, which was very near 1000. After this the air was gradually admitted into the receiver, and at various intervals the ball of the brafs wire was electrified, in order to observe whether the same phenomena appeared at the different degrees of exhaustion as had done before; and they were found to agree with sufficient exactness.

II. The brafs wire within the same glass receiver was made very short, and from its extremity a fine linen thread, fixes inches long, was suspended; and upon the plate of the pump a small brafs stand with a brafs pillar was placed: so that when the receiver was put upon the plate, and over the brafs stand, about one inch length of the thread stood parallel to, and at various required distances from, the brafs pillar (f). In this disposition of the apparatus, whenever any the least quantity of electricity was communicated to the knob of the brafs wire, the thread was immediately attracted by the brafs pillar, and adhered to it some time, because, being dry, it did not immediately part with the acquired electricity. At various degrees of exhaustion, the electricity being communicated to the brafs ball of the wire, it was found, that the thread was always attracted by the brafs pillar, though from a greater or less distance, according as a greater or less quantity remained within the receiver. Thus when the air was rarefied about 100 times, the thread was attracted from about one inch; when the air was rarefied 200 times, it was attracted from about \( \frac{1}{3} \) th of an inch; when the air was rarefied 300 times, it was attracted from about \( \frac{1}{6} \) th; and after this it was always attracted from about \( \frac{1}{12} \) th, even when the air within the receiver was rarefied about 1000 times. It is remarkable, that when the air in the receiver is rarefied about 300 times, if a jar is discharged through the vacuum, by touching its knob with the ball of the wire on the receiver, the thread is not in consequence of it attracted by the brafs pillar; the reason of which seems to be, because that large quantity of electricity opens a way through the vacuum, and passes through every part of it; whereas a small quantity of electricity, even the action of a small electrical machine in the same room, at no very great distance from the apparatus, will cause the thread being attracted by the brafs pillar.

III. "The brafs stand, with the pillar, and the thread which proceeded from the wire, being removed from under the receiver, a very sensible electrometer was fastened, instead of the thread, to the extremity of the brafs wire. This electrometer consisted of two very fine silver wires, each about one inch long, and having a small cone of cork at its extremity. The sensibility of such an electrometer is really surprising; for even the electricity of a single hair excited, does sensibly affect it; and, as its suspension is almost without any friction or other impediment, it never deceives one by appearing to be electrified when in reality it is not so. With this preparation, the receiver being placed upon the plate of the air-pump, the air was gradually exhausted, and at intervals some electricity was communicated to the ball on the outside of the receiver, either by an excited electric or by a charged jar; and it was found that the corks of the electrometer were always made to diverge by it, even when the air was exhausted as much as possible. Indeed their divergency was smaller and smaller, and lasted a shorter time, according as the air was more exhausted, but it was visible to the last.

"In this experiment, analogous to what has been observed in the preceding, when the air was exhausted above 300 times, if a jar was discharged through the vacuum, or a strong spark was given to the knob on the top of the receiver, the corks of the electrometer diverged very little indeed, and but for an instant; whereas a small quantity of electricity made them diverge more, and remain much longer in that state.

"It seems deducible from those experiments, that His celestial attraction and repulsion take place in every degree of rarefaction, from the lowest to about 1000, but that the power diminishes in proportion as the air

(f) This distance was altered by turning the brafs wire which passed through a collar of leather in the brafs cap of the receiver. air is more and more rarefied; and by following the law, we may perhaps conclude with F. Beccaria, that there is no electric attraction nor repulsion in a perfect vacuum; though this will perhaps be impossible to be verified experimentally; because when in an exhausted receiver no attraction or repulsion is observed between bodies to which electricity is communicated, it will be only suspected, that those bodies are not sufficiently small and light. But if we consult reason, and which alone ought to assist us when decisive experiments are not practicable, it seems likely that electric attraction and repulsion cannot take place in a perfect vacuum, by which I only mean a perfect absence of air; because either this vacuum is a conductor or a non-conductor of electricity. If a conductor, and nearer to perfection as it becomes more free from air, it must be a perfect conductor at the same time that it becomes a perfect vacuum; in which case electric attraction or repulsion cannot take place amongst bodies inclosed in it: for, according to every notion we have of electricity, those notions indicate or are the consequence of the intervening space in some measure obstructing the free passage of the electric fluid. And if the perfect vacuum is a perfect non-conductor, then neither electric attraction nor repulsion can happen in it.

IV. In my former experiments, having always observed the electric light in the receiver of the air-pump, even when the air was rarefied to the utmost power of that machine, I thought proper to repeat that experiment with receivers of various sizes; and accordingly have used receivers of above two feet in height, and some of as large a diameter as the plate of the pump could admit, which is about 14 inches; but the light in it was always visible, only with different colours in different degrees of exhaustion, and always more diffused, and at the same time less dense, when the air was more rarefied; which seems to render it probable, that when the air is quite removed from any space, the electric light is no longer visible in it, as must have been the case with the experiment of Mr Walsh's double barometer; for it is a maxim very well established in electricity, that the electric light is only visible when the electric fluid, in passing from one body to another, meets with some opposition in its way; and according to this proposition, when the air is entirely removed from a given receiver, the electric fluid passing through that receiver cannot show any light, because it meets with no opposition; but this will not account for the receiver ever becoming a non-conductor.

Having just mentioned, that according as the air is more and more rarefied in a receiver, so the electric light becomes gradually more faint, it will be proper to add, that the electric light is more diffused and less bright in an exhausted receiver than in air: Thus, when the receiver is not exhausted, the discharge of a jar through some part of it will appear like a small globe exceedingly bright; but when the receiver is exhausted, the discharge of the same jar will fill the whole receiver with a very faint light: whereas some persons, by seeing the whole receiver illuminated, are apt to say that the light of electricity is rendered stronger and greater by the exhaustion.

V. It is mentioned by Mr Nairne, in the 67th vol. of the Phil. Trans. that having put a piece of leather, just as it comes from the leather-sellers, into the receiver of an air pump, and afterwards having rarefied the air in it 148 times, the electric light appeared very faint in it; whereas, without the leather, and even when the air was much more rarefied, the light of the electric fluid, when made to pass through the receiver, was much more apparent. In consequence of this observation, I suspected that a little moisture in the receiver, or some other effluvia of substances might perhaps prevent the appearance of the electric light in rarefied air; and with this view I began to put various substances successively into the receiver; and after rarefying the air by working the pump, some electric fluid was made to pass through the receiver.

When a piece of moist leather was put into the receiver, the air could not be rarefied above 100 times, and the electric light appeared divided into a great many branches; though at the same time another sort of faint light filled up the whole cavity of the receiver.

When a linen rag, moistened with a mixture of spirit of wine and water, was put into the receiver, the pump could not exhaust above 40 times, and the light of electricity appeared divided into many branches.

A wine-glass full of olive oil placed under the receiver, prevented very little the exhaustion of the pump, the air being rarefied above 400 times. The electric light appeared exactly as it usually does in the same degree of rarefaction when no oil is under the receiver, viz. a uniform faint light inclining to purple or red.

Concentrated vitriolic acid placed in a glass under the receiver, produced no particular effect. As for the other mineral acids, they were not tried, because, being volatile, they would have damaged the pump.

Dry solids, that had a considerable smell, as sulphur, aromatic woods previously made very dry, and some resins, produced no particular effect, any more than some of them prevented a very great degree of exhaustion, owing to some moisture which still adhered to them.

From these experiments it appears, first, that in the light, the utmost rarefaction that can be effected by the attraction of the air-pump, which amounts to about 1000, both the attraction and repulsion, diminishes; secondly, that the attraction and repulsion of electricity become weaker in proportion as the air is more rarefied, and in the same manner the intensity of the light is gradually diminished. Now by reasoning on this analogy we may conclude, that both the attraction and the light will cease in a perfect absence of air; but this will never account for this perfect vacuum ever becoming a non-conductor of electricity; for since the electric fluid is very elastic, and expands itself with more and more freedom in proportion as the resistance of the air is removed, it seems unnatural that it should be incapable of pervading a perfect vacuum: however, the fact seems to be fully ascertained by Mr Walsh and Mr Morgan; and the only thing that remains to be done is to investigate the cause of so remarkable a property." With regard to the power of the electric fluid, we have already had occasion to speak in various parts of this treatise, and particularly to mention the machine in Teyler's Museum at Haarlem, as that which was capable of accumulating the greatest quantity of electricity that had ever been done artificially. Some of the effects of this machine, without any battery, have already been described; and those which follow are equally calculated to give an idea of its vast power. A battery of 135 vials, containing among them about 130 square feet of coated surface, was charged by about 100 turns of the glass plates; the discharge of which melted an iron wire 15 feet long and \( \frac{1}{4} \) of an inch diameter; and another time they melted a wire of the same metal 25 feet long and \( \frac{1}{4} \) of an inch in diameter. With such an extraordinary power they tried to give polarity to needles made out of watch-springs of three and even six inches in length, and likewise to steel bars nine inches long, from a quarter to half an inch in breadth, and about the twelfth-part of an inch in thickness. The result was, that when the bar or needle was placed horizontally in the magnetic meridian, whichever way the shock entered, the end of the bar that stood towards the north acquired the north polarity, or the power of turning towards the north when freely suspended, and the opposite end acquired the south. If the bar, before it received the shock, had some polarity, and was placed with its poles contrary to the usual direction, then its natural polarity was always diminished, and often reversed; so that the extremity of it, which in receiving the shock looked towards the north, became the north pole, &c.

When the bar or needle was struck standing perpendicularly, its lowest end became the north pole in any case, even when the bar had some magnetism before, and was placed with the south pole downwards. All other circumstances being alike, the bars seemed to acquire an equal degree of magnetic power, whether they were struck whilst standing horizontally in the magnetic meridian, or perpendicular to the horizon.

When a bar or needle was placed in the magnetic equator, whichever way the shock entered, it never gave it any magnetism; but if the shock was given through its width, then the needle acquired a considerable degree of magnetism, and the end of it which lay towards the west became the north pole, and the other end the south pole.

If a needle or bar, already magnetic, or a real magnet, was struck in any direction, its power was always diminished. For this experiment, they tried considerably large bars; one being 7½ inches long, 0.26 broad, and 0.05 thick.

When the shock was so strong, in proportion to the size of the needle, as to render it hot, then the needle generally acquired no magnetism at all, or very little.

The experiments lastly tried with this very powerful battery were concerning the calcination of metallic substances, and the revivification of their calces. It appears that the electric shock produced both these apparently contradictory effects.

The metallic calces used in those experiments were of the purest sort; they were confined between glasses whilst the shock was passed over them. By this means the calces were so far revivified as to exhibit several grains of the metal, large enough to be discerned by the naked eye, and to be easily separated from the refractory matter.

As to the calcination of metals, whenever a shock was employed much greater than that which was necessary to fuse the metal, part of the metal was calcined, and dispersed into smoke. It is remarkable, that this calcination or smoke generally produced several filaments, of various lengths and thicknesses, which swam in the air. It was farther observed, that those flying filaments of metallic calx, if a conductor was presented to them, were soon attracted by it; but after the first contact, they were instantly repelled, and generally broke into diverse parts.

Even this vast power was not the utmost effect of the machine. Dr Van Marum, whom we have already mentioned as principally concerned in making the experiments, thinking that it was capable of charging a larger surface, added to it 90 jars, each of the same size with the former; so that his grand battery is now a square of 15 jars every way, and contains 225 square feet of coated glass. To ascertain the degree of the charge, he uses the electrometer invented by Mr Brook, to be afterwards described, which is fixed in the centre of the battery, at the height of four feet above the knobs of the jars.

His first object was to try whether this battery could be fully charged by the machine, and whether its increase of power was proportional to the augmentation of its surface. In these respects, his expectations were fully answered. The former battery discharged itself over the uncoated part of the jars after 96 revolutions; and the present did the same after 160 turns of the machine. With the former battery, the Doctor had split a cylinder of box three inches in diameter and three inches in length, the section of the force which, through its axis, contained nine square inches, of its explosion. With the 225 jars, he split a similar cylinder, four inches in diameter and four inches in height, the section of which was 16 square inches. He found that to split a square inch of this wood in the same direction, required a force equal to 615 pounds; and hence calculates that the power of this explosion was not less than 9840 pounds.

The apparent resemblance between the effects of electricity and of fire, especially in melting metals, has led many to suppose that they act upon bodies in a similar manner. In order to examine whether this supposition be just, Dr Van Marum caused wires of different metals to be drawn through the same hole, of one thirty-eighth part of an inch in diameter, and observed how many inches of each could be melted by the explosion of his battery; taking care, in all these experiments, to charge it to the same degree as ascertained by his electrometer. The results were as follow:

- Of lead he melted 120 inches. - Of tin 120 inches. - Of iron 5 inches. - Of gold \( \frac{3}{4} \) inches. - Of silver, copper, and brass, not quite a quarter of an inch.

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

| Lead | 230 | | Silver | 430 | | Gold | 563 | | Copper | 630 | | Iron | 696 |

Thus tin and lead appear to be equally fusible by electricity, but not by fire: and iron, which by fire is less fusible than gold, is much more so by the electrical explosion. From these and other experiments of the same kind, Dr Van Marum concludes, that, in melting metals, the electrical fluid acts upon them in a manner very different from the action of fire, and that the supposed analogy between these two powerful agents cannot be proved, either from the fusion of metals, or the ignition of combustible substances.

By these experiments on the fusibility of metals, Dr Van Marum was induced to make trial of the comparative efficacy of lead, iron, brass, and copper, as conductors to preserve buildings from lightning. In this respect, he found that a leaden conductor ought to be four times the size of one of iron, in order to be equal in point of safety. He has also fully proved the superiority of rods to chains, and of copper to iron, for this important use.

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

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

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

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

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

From this phenomenon, Dr Van Marum conjectures, that when the globules approach the paper on which they fall, the matter issuing from their lower part strikes against its surface, and being elastic, forces them upwards again by its reaction. The clouded stain immediately under the wire, the Doctor attributes to the instantaneous calcination of its surface; whereas the remainder of the metal is melted into globules, which, while they retain their glowing heat, continue to be superficially calcined, and, during the process, part with this calcareous vapour.

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

Miscellaneous experiments.

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

Though Dr Van Marum was convinced, by M. Lavoisier's experiments, that metals, calcined in atmospheric air, afford from it that principle which renders it fit for respiration; yet he resolved further to investigate this point, by trying what would be the effect of a discharge of the battery through a piece of wire confined in phlogisticated air. For this purpose, he took air, in which a burning coal had been extinguished, and which had afterwards stood eight days upon water, that it might be entirely cleared from fixed air; with this he filled a glass cylinder, four inches in diameter, and fix inches high, closed at the upper end with a brass-plate; from the centre of this plate the wire was suspended, on which the experiment was made. The cylinder was let in a pewter dish filled with water; and, to prevent its being broken by the expansion of the air, its lower edges were supported by two pieces of wood half an inch high. The lower end of the wire rested on the dish, which was connected with the outside coating of the battery.

On transmitting the charge, in this manner, through wires of lead, tin, and iron, of only half the length of those which were calcined by an equal explosion in atmospheric air, no calcination took place. The first was reduced to a fine powder, which, upon trial by spirit of nitre, appeared to be merely lead; the two other metals were melted into small globules.

The Doctor then tried the same experiment in pure or dephlogisticated air, obtained from red precipitate; thinking that, in this, the metals would be more highly calcined than in common air. His expectation was answered only by the lead, which was entirely reduced to a yellow calx, perfectly resembling magnesia. The other metals were not more highly calcined in this than in common air; but the globules of iron acquired so great a heat, as to retain it for some seconds, even in the water, and to melt holes in the pewter dish into which they fell.

In nitrous air, calcination took place as easily as in common or in dephlogisticated air. This was contrary to Dr Van Marum's expectation; but he accounts for it, by observing that, from the experiments of Mr Cavendish and of M. Lavoisier, pure air appears to be one of the component parts of the nitrous acid.

In order to illustrate M. Lavoisier's theory, Dr Van Marum resolved to examine the phenomena resulting from the calcination of metals in water. This he tried with both iron and lead; and found that, in the moment of the explosion, a number of air-bubbles appeared on the surface, and the calx rose, like a cloud, through the water. This, he thinks, is not so easily accounted for by the theory of Stahl as by that of M. Lavoisier; because, according to the former, water does not readily either receive or part with phlogiston; whereas the latter supposes this fluid to be composed of the oxygenous principle, united with that of inflammable air. If this be true, nothing more is necessary to calcination, than that the metal should acquire a greater affinity with the oxygenous principle, that subsists between this and that of inflammable air, united with it in the composition of water. To collect the air generated by these calcinations was no easy matter; as the violence of the shock broke the glass receivers employed for this purpose; at last, however, the Doctor contrived a method of receiving it in a glazed stone basin. From the first calcination of lead, about a quarter of a cubic inch of air was produced, which showed no signs of inflammability; but, on every repetition of the experiment, a less quantity of air was generated; and on an accurate trial of that produced by the fourth calcination in the same water, it was found to consist of one part of inflammable and three of atmospheric air. Our author designs to repeat these experiments with water deprived of its air, by being boiled.

In order to imitate the phenomena of earthquakes, this ingenious philosopher followed Dr Priestley's method, and made the electrical explosion pass over a quakes imitation, floating on water, on which several columns of wood were erected; but this succeeded only once. Reflecting that the electric explosion exerts the greatest lateral force when it passes through imperfect conductors, and that water is probably its principal subterranean conductor, he laid two smooth boards upon each other, moistening the sides in contact with water; upon the uppermost, he placed pieces of wood, in imitation of buildings, the bases of which were 3 inches long and 1½ broad. When the charge of the battery was transmitted between the boards, all these were thrown down by the tremulous and undulatory motion of that on which they stood.

Mr Brookes, electrician at Norwich, has made a great number of experiments, with a view to determine exactly the force of batteries of an inferior size in melting fine wires of different kinds. In these he was particularly careful to ascertain the degree to which his batteries were charged; and this he did by the method which shall afterwards be shown to be the best, viz. that of determining the power of the electricity by the weight which it was capable of raising by its repulsive power; and therefore, in the following experiments, the phrase of batteries being charged to so many grains, implies that the repulsive power of the knob of the battery was able to raise that weight. Some of the most remarkable of these experiments were as follow:

1. With a battery of nine bottles, containing about 16 square feet of coated surface, charged to 32 grains of repulsion, which charge was sent through a piece of steel wire 12 inches long and \( \frac{1}{75} \)th of an inch thick 11 times; the wire was shortened one inch and a half, being then about 10 inches and an half long; the 12th time, the wire was melted to pieces.

2. A charge, with the same nine bottles, to 32 grains of repulsion, being sent through a piece of steel wire 12 inches long and \( \frac{1}{75} \)th of an inch thick, the first time melted the whole of it into small globules.

3. A charge of the same nine bottles charged to 32 grains, being sent through a piece of brafs wire 12 inches long, \( \frac{1}{75} \)th of an inch thick, the whole of it was melted, with much smoke, almost like gunpowder; but the metallic part of it, after it was melted, formed itself, in cooling, chiefly into concave hemispherical figures of various sizes.

4. With only eight of the above bottles charged to 32 grains, the charge did but just melt 12 inches of the steel wire, \( \frac{1}{75} \)th of an inch thick, so as to fall into several pieces; which pieces in cooling formed themselves into oblong lumps joining to each other by a very small part of the wire between each lump, which was not melted enough to separate, but appeared like oblong beads on a thread at different distances.

5. The same eight bottles charged to 32 grains, so perfectly heated 12 inches of brafs wire, about \( \frac{1}{75} \)th of an inch thick, as to melt it, or soften it enough for it to fall down by its own weight (from the forceps with which it was held at each end) upon a sheet of paper placed under to catch it; and when it fell down, it was so perfectly flexible, that by falling, it formed itself into a bent, or rather vermicular shape, and remained entire its whole length, i.e., about 12 inches when it was put into the forceps; but after it was fallen on the paper, it flagged so much as to be stretched by its own weight from 12 to about 15 inches long; and by falling on the paper it flattened itself the whole length of it, so that when it was examined with an half inch magnifier, it appeared about five or six times broader than it was in thicknesses.

6. With nine bottles again, charged only to 20 grains, the charge was sent through 12 inches of steel wire \( \frac{1}{75} \)th of an inch thick, which heated it enough to melt it so as to be separated in many places; and the pieces formed themselves into string-bead-like shapes, as in experiment 4.

7. With the same nine bottles charged to 20 grains, the charge was sent through 10 inches of brafs wire \( \frac{1}{75} \)th of an inch thick; the wire was heated so red hot as to be very flexible, yet it did not separate, but was shortened near \( \frac{1}{8} \)ths of an inch.

8. A charge of nine bottles, charged to 20 grains, sent a second time through the last piece of wire, melted it aflutter in three places.

9. Nine bottles charged to 30 grains, and the charge sent through 12 inches of brafs wire \( \frac{1}{75} \)th of an inch thick, treated it nearly as in experiment 3, except that it was separated in two places, and the pieces measured about 16 inches and an half long; but perfectly flattened by its fall on the paper, as before.

10. Nine bottles charged to 30 grains, and the charge being sent through eight inches and a half of brafs wire the size of the last, wholly dispersed it in smoke, and left nothing remaining to fall on the sheet of paper placed under it.

11. With 12 bottles, charged to 20 grains, the charge was sent through ten inches of steel wire one-hundredth of an inch thick, which made the wire red hot, but did not melt it.

12. A second charge, the same as the last, was sent through the same piece of wire, which heated it red hot as the first did, but it was not separated; this piece of wire was now shortened five-sixteenths of an inch.

13. A charge to 25 grains, with the same 12 bottles, was sent through the last piece of wire, which melted it into many pieces, and many globules of calcined metal.

14. A charge of 15 bottles, charged to 25 grains, was sent through ten inches of steel wire one-hundredth of an inch thick, which melted it the first time, and dispersed a great part of it about the room.

15. A charge with the last 15 bottles, charged to 20 grains, just melted ten inches of steel wire the size of the former, so as to run into beautiful globules, nearly as in exp. 13.

16. A charge of 15 bottles, charged to 15 grains, being sent through ten inches of steel wire the size of the last, it was barely made red hot; but it was shortened one-tenth of an inch by the stroke passing through it.

17. The last piece of wire having a charge of 15 bottles, charged to twelve and a half grains, sent through it, was not made red hot.

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

19. Four bottles, charged to 30 grains, just melted three inches of steel wire one hundred and seventieth of an inch thick, so as to fall into pieces.

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

21. Eight bottles, charged to 15 grains, melted three inches of steel wire one hundred and seventieth of an inch thick, similar to the five in the last experiment; so nearly alike both in appearance and effect, that it might have been said to be the same.

22. Ten bottles charged to twelve and a half grains, rather exceeded exp. 19, but scarcely came up to exp. 20 and 21.

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

24. A charge to 20 grains, with the last eight bottles, very finely melted six inches of steel wire one hundred and seventieth of an inch thick. With two bottles, charged to 45 grains, the charge was sent through one inch of such sized steel wire as the last, which only changed its colour.

Three bottles, with a 40 grains charge, differed one inch and a half of steel wire, the size of the last, all about the room.

As a steel wire of one-hundredth of an inch thick has nearly double the quantity of metal of a wire one hundred and seventieth of an inch thick, so I took three inches of the former, and sent a 25 grains charge with ten bottles through it, which melted it just as the five bottles did in exp. 20.

Twenty bottles, charged to twelve grains and a half, melted three inches of steel wire, the size of the last, exactly similar to the foregoing experiment.

As a steel wire of one-eightieth of an inch thick contains nearly twice the quantity of metal in the same length as a steel wire of one-hundredth, or four times the quantity of a steel wire of one hundred and seventieth of an inch thick; so it might, from the foregoing experiments, be expected that 20 bottles, charged to 25 grains, would melt three inches of steel wire one-eightieth of an inch thick; but on a great many trials 20 bottles could not be procured that would bear the discharge, when charged to 25 grains: for at the discharge there would be always one or more bottles broken or perforated. I was now reduced to the necessity of being content with getting bottles of any size that would bear the required charge, from one to three gallons each, or that contained from about 150 to 300, or more, square inches of coated surface, each; but all in vain, my only resource left (as I was not near any glass-house), was to increase the quantity of surface, and not to charge so high, and to proportion the one to the other: a third part was concluded on to be tried; that is, instead of about 36 feet of coating, I added one third, or 12 feet, which made it 48 feet: and that, instead of charging to 25 grains, or 24 grains, which divides by 3 better, to omit one-third of the height of the charge, which leaves 16 grains: and thus I succeeded perfectly well; for 3 inches of steel wire one-eightieth of an inch thick was as curiously melted with 48 feet of coated surface, charged to 16 grains, as any of the former.

These bottles, thus broken in large discharges, seem always to break, or to be struck through, nearly in the thinnest, but never in the thickest place, which shows the necessity of the substance in the glas.

As in exp. 19. and 21. where the former is but half the quantity of coated surface of the latter, charged to 30, and the latter to 15 grains, to know how high 48 feet of coating must be charged to produce the same effect exactly: and as the quantity of coating in four bottles, consisting of a little more than six feet and a half, is contained in 48 feet a little more than seven times; so I tried by charging 48 feet only to a little more than four grains, or only about one seventh part so high, as four times seven is 28; that is, but two less than 30: and this had exactly the same effect on the wire, which was one hundred and seventieth of an inch thick, and three inches long, as the former.

As the last experiment agreed so exactly with exp. 19. and 20. the next thing tried was to see the effect of 48 feet of coated surface charged to a little more than four grains, upon six inches of steel wire, the size of the last; but this was only made very faintly red.

A repetition of the last experiment with the same length of the same wire, to see how often the same charge might be sent through before it would be melted, and to observe the appearance of the wire after each stroke; the eighth stroke melted it into several pieces. After the first stroke, the redness grew less every time, even the last time, when it was separated. The first stroke, though little more than fairly red, made it so flexible, that by a little more than its own weight (about a penny-weight more), it was apparently made perfectly straight when it was cooled: about the third or fourth stroke it began to appear zigzagged; after the fifth stroke the surface of it appeared rough; after the seventh stroke the surface was very roughly scoriated or scaly; and some of the scales had fallen upon a piece of white paper, placed under it, at about half an inch distance below it. The eighth stroke melted it in three places; and at those places where the angles appeared the sharpest or most acute, a great number of the scaly appearances were driven off about the paper, which appeared like splinters (see exp. 18.); some of them were almost one-tenth of an inch long, and some of them about a third or a fourth part of the diameter of the wire in breadth, and very thin: after the seventh stroke it was shortened seven-sixteenths of an inch: the wire was one hundred and seventieth of an inch thick.

Repeating exp. 31. again with the same size and length of wire, and the same battery charged the same, in order to observe the method of the wire shortening, having fixed an inflated gage parallel to and about a quarter of an inch distant from it: after the first stroke, which made the wire fairly red, (it being fixed at one end, that the shortening might appear all at the other, which was held so as either to contract or dilate), I observed that it shortened considerably as it cooled; repeating the stroke, it did the same, and so on till it was melted, which was by the eighth stroke, as before. At the instant that the stroke passed thro' the wire it appeared to dilate a little, and after it was at its hottest, it gradually contracted after every stroke as it cooled, about one-sixteenth of an inch each time; the dilating was so very little, as to bear but a very small proportion to its contraction, and sometimes it was doubtful whether or not it did dilate at all; but after all the observations it appeared oftener as if it did dilate, than as if it did not.

The same 48 feet, negatively charged to a little more than four grains, melted three inches of steel wire one hundred and seventieth of an inch thick, the same as the positive charge, did in exp. 30.

The same battery of 48 feet of coated surface, charged to a little more than eight grains, melted three inches of steel wire one-hundredth of an inch thick. This is very nearly in proportion to exp. 27. but here the charge was negative, and the fusion was the most pleasing of any I have hitherto had; probably owing to the charge, by chance, happening to be so well adjusted as to be exactly sufficient to melt the wire and no more: it held hot the longest, and the fused metal ran into the largest globules: probably the length length of the time that the heat continued, was owing to the charge being just sufficient, and to the size of the lumps that the fated metal formed itself into.

"36. A repetition of exp. 1. with twelve inches of steel wire, one-hundredth of an inch thick, but with this difference, that as then I used only nine bottles, containing about 16 square feet of coated surface charged to 32 grains, I here used 18 bottles containing about 32 square feet of coating charged to only 16 grains. This was done, to observe the progress of the destruction of the wire, as in exp. 32, as well as to prove the similarity of the effect. The wire being the same size, sort of metal, and length, as recited just above; the first stroke made it fairly red-hot the whole length of it with smoke and smell, changed its colour to a kind of copperish hue, and shortened it considerably; the second stroke made it of a fine blue, but it did not appear red, and shortened it more; at the third stroke, it became zigzagged, many radii were very visible at the bendings, and continued to shorten till the eleventh stroke, when one of the bottles in the second row of the battery was struck through: the fracture was covered over with common cement, its place supplied by changing place with one in the third row, supposing the mended one to be the weakest; and thus, with the battery in this state, I made the twelfth stroke, which separated the wire, as in exp. 1, but this wire was shortened only one inch.

"37. A charge of 48 feet to eight grains, sent through three inches of copper wire, one hundred and seventieth of an inch thick, seven times, made it zigzagged, but not much shorter; the eighth stroke separated it at one end, close to the forceps which held it, but it did not appear to be made sensibly red-hot at all, notwithstanding it must have been often so at the place where it was melted: which space was so very small as barely to be perceptible, like as when a point is set upon any flat surface of iron, and a stroke from a pound phial being sent through, both the point and the flat surface where the point refted, if examined with a magnifying glass, will be found to have been melted; and a speck may be seen; but the radius of the metal will scarcely be visible.

"38. A charge of 48 feet, to 16 grains, was sent through six inches of lead wire, one-fiftieth of an inch thick, which melted it into many pieces.

"39. A charge of 48 feet, to 15 grains, was sent through six inches of wire like the last, which did not separate it, but made it smoke.

"40. A charge like the last was sent through the last piece of wire a second time; which melted it into several pieces.

"The law by which wires resist destruction, in proportion to the thickness of the wire, does not seem to be so equable, by much, as in the lead as in the steel wire. For a charge of four grains, in exp. 34, melted three inches of lead wire one sixty-fifth of an inch thick; but it took a charge of about three times that power to destroy three inches of lead wire one-fiftieth of an inch thick; which is about double the quantity of metal in the same length as in that of one sixty-fifth of an inch thick. Thus it is easy to find, what different resistance a wire of any of the foregoing metals, of equal size and length, will make to the electrical stroke or to lightning.

"The length of the electric circuit, in which the different wires were placed, in the foregoing experiments, from the nearest part of the inside to the nearest part of the outside of the battery, exclusive of the length of the said wires, was about eight feet.

"Notwithstanding the easy destruction of the lead wire by the electrical stroke, it seems greatly to be doubted, whether any thunder strokes happen in any place whatever, strong enough to destroy a strip of lead four inches broad and of the thickness of about eight pounds to the foot. Whence it may be presumed, that such a strip of lead may be perfectly safe for conductors through buildings of any kind whatever; as it is not much subject to decay in any common exposure.

"41. Two gentlemen coming in to see a piece of wire violently melted by electricity, I proceeded to show it them, plugging it by fixing 12 inches of steel wire one-hundred and seventieth of an inch thick, in the forceps, and then (supposing the electrometer and all other things ready placed) to charge the battery, but the electrometer did not move; nevertheless I continued charging as I supposed; but still the electrometer remained as it was, although I had been charging much longer than would have been necessary, contrary to my design, which was to take a small wire, that a small charge might be sufficient. Having been charging a long time, I left off to look about the apparatus, in order to see if anything was not right: as I was looking, I found there was no communication to the electrometer, and heard a small crackling in the battery, which convinced me that it was charged. Accordingly I made the discharge, expecting nothing unusual; but the wire was dispersed seemingly in a very violent manner. The report was so very loud that our ears were stunned, and the flash of light so very great, that my sight was quite confused for a few seconds. The singularity of the appearances attending this experiment led me to insert it."

Though from what has been said under section VI., the direction of the electric fluid outwards from a body positively electrified, and inwards from one negatively so, seems to be sufficiently ascertained, yet some experiments related by Mr Nicholson in the last volume of the Philosophical Transactions, which seem to militate against this doctrine, require a particular consideration; and for this reason we shall here not only give an account of these, but of some others made on Milner's subject of excitation, and the state of a charged account of phial in general, which seem to throw some light upon the Leyden the subject. Mr Milner, who has been at great pains to inquire into this matter, makes the following observations:

"I. In the charged phial, when the inside has either kind of electricity communicated to it, the outside is found to possess a contrary power. It appears also from the preceding experiments, that either kind of electricity always produces the other on any conducting substance placed within the sphere of influence. And as the same effect is also produced on electrics themselves, in the same situation, and as some portion of the air, supposing no other substance to be near enough, must be unavoidably exposed to such influence, it necessarily follows, that neither power can exist without the other; and therefore, in every possible case, positive..." positive and negative electricity are inseparably united.

"II. A phial cannot be fully charged, by which the outside acquires a contrary electricity, unless the external coating has a communication by some conductor with the earth. In the same manner, a full charge of the contrary electricity cannot readily be procured in these experiments without a similar communication.

"III. In both cases the interposition of an electric body between the contrary powers is absolutely necessary. In one case that body is glass, in the other it is air; and the experiment will not succeed in either, unless both the glass and the air be tolerably free from moisture.

"IV. It appears from the 18th experiment, that the influence of electricity acts in the same manner through glass as it does through the air, and produces a contrary power in both cases.

"V. A communication of the electric matter is more easily made through the fluid yielding substance of the air than through glass; which is so hard and solid a body, as to require a very considerable degree of power to separate its component particles: this, however, sometimes happens, and a hole is made through the glass itself, without design, in attempting to charge a very thin phial as high as possible, in the most favourable state of the atmosphere.

"VI. A conducting body receives the strongest charge of the contrary electricity, in these experiments when it is brought as near as possible to the electric power, without being within the communicating distance. And it is well known that the thinnest phial, if it be strong enough to prevent a communication between the two surfaces, will always receive the highest charge.

"VII. The electricity of the external surface of the charged phial cannot be destroyed, so long as the internal surface remains in force, and continues to exert its influence through the glass; because this influence was the cause of the contrary electricity on the external surface, and must therefore preserve it.

"VIII. If part of the course which the electric matter takes in discharging a phial be through the air, a small part of the charge will always remain; because the whole of the redundancy on one surface is not capable of forcing a passage through the resisting medium of the air, in order to supply the deficiency on the other surface. But if every part of the circuit, from the internal to the external coating, consists of the best conductors, and if the coated surfaces be nearly equal, and directly opposite to each other, the phial will then appear to have retained no part of the charge, so far as it is covered with tin-foil; but the parts of it above the coating on both sides will, however, still retain the contrary electricities, after the circuit has been completed (c). A residue of the charge may also be observed in every other instance of electrification, in which the degree of electricity is sufficient to force a communication between the electrified body and a conductor not insulated, through a small portion of the air: and if the experiment be carefully made, it will appear, that the whole of the redundancy is not capable of passing through the resisting intermediate air, in any case, and therefore a part of the charge must always remain. This may be conveniently shown by using a well excited electrophorus of about five inches diameter, the metal cover of which may be so strongly electrified, as to force a communication through the air, to any good conductor not insulated, at the distance of three quarters of an inch. After this, a second communication much weaker than the first may be made at the distance of about the twelfth-part of an inch, which is the residue of the charge, or rather a part of it: for if the second communication be carefully made through the air, without touching the cover, it will be found still to have retained enough of the first charge to electrify a pair of vertical needles.

"As it appears from this view, that both these cases are similar in so many remarkable particulars, it follows, that they are essentially the same, notwithstanding they differ in the degree of power and some other circumstances, which may alter the form of an experiment without changing its nature. It is apprehended, therefore, that the above mentioned distinction will not only appear to be unnecessary, but also that either power cannot possibly exist without the other, as it has been shown under the first particular, that positive and negative electricity were inseparably united. But here it will be proper to examine more particularly the nature of charged glasses.

"1. When a plate of coated glass has been charged, and the circuit between the coatings has been completed, by the mediation of a good conducting substance, no part of the coated surface is supposed to retain any part of the charge; but, according to the commonly received doctrine, the whole of it is said to be discharged; or in other words, to be brought into its natural state. This, however, is not really the case, as will evidently appear from the following experiment; the design of which is to show the effects produced by charging and discharging a plate of glass.

"2. Let the middle of a piece of crown window-glass, seven inches square, be placed between two circular plates of brass, about the tenth part of an inch thick, and five inches in diameter. In order to enable these plates to retain a greater degree of power, it will be proper to terminate each of them with a round bead the third part of an inch thick; and the whole of the bead should be formed on one side of the plate, that the other side may remain quite flat, and apply well to the surface of the glass. Let the whole be inflated.

"(c) The whole remainder of the charged phial must not, however, be ascribed to the cause above mentioned: for after taking away that part of it belonging to the coated surface, which could not force a passage through the air, if the phial be allowed to stand a short time on the table, the coated surface will again gradually acquire some power, which must be derived from the charge of the phial above the coating. Another source of the residuum will appear in the next experiment."

Sect. IX.

Miscellaneous Experiments.

Lated about four inches above the table, and in an horizontal position, by fastening one end of a cylindrical piece of some good insulating substance to the middle of the under plate, the other end of it being fixed in any convenient stand. Let a like insulating stem be fastened to the middle of the upper plate. Let a brass chain, which may easily be removed, reach from the under plate to the table. In the last place, bend a piece of brass wire into such a shape that it may stand perpendicularly on the upper plate; and let the upper extremity of this wire be formed into an hook, that it may be removed at any time by the assistance of a silk string, without destroying the insulation of the plate.

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

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

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

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

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

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

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

10. The whole quantity of electric matter added to the glass in charging it, is evidently distinguished into two parts in this experiment. The first part, which is by far the most considerable, appears to have been readily communicated from one surface of the glass to the other, along the bent wire, when it was first brought into contact with both coatings at the same time. The second part of the charge appears to be more permanent, and remains still united with the glass, notwithstanding the circuit has been completed (1). This permanent electricity, as well as the other, must be positive on the upper surface, and negative on the lower surface; because, in the present experiment, the charge was given by a smooth glass tube excited with a silk rubber. Now, the influence of the opposite and permanent powers on the different sides of the glass (each side having a tendency to bring the coating in contact with it into a state of electricity contrary to its own) must assist each other, in causing part of the electric matter naturally belonging to the upper coating to pass away from it to the under coating, along the discharging wire, and at the same time the surcharge to pass the same way. The upper coating, therefore, by losing some part of its natural quantity, must be negatively electrified; and the under coating, by receiving an addition to its natural quantity, must be positively electrified. The whole quantity of electric matter, which the influence of the permanent electricity of the glass is capable of taking from one coating and of adding to the other, bears but a small proportion to the whole charge; and therefore the second and every subsequent discharge must be considerably weaker than the first.

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

12. It appears from this experiment, that the ingenious professor Volta's electrophorus is, in reality, a refined plate charged with permanent electricity by friction; and because there is a less disposition in a body of this kind to attract moisture from the atmosphere than there is in glass; it will retain the power better, and consequently be the longer capable of producing a contrary electricity in the insulated metal cover. If it should be thought necessary to support this observation by a direct experiment, it may easily be done by making a thin flat plate of any refined electric substance, and larger than the insulated cover, but without fastening a

---

(H) The whole of this effect must not be ascribed to the attraction of electricity. Perhaps the passage of electric matter between the coating and the glass may help to exclude the air; and then the attraction of cohesion, and the pressure of the external air both above and below, may be supposed to have the most considerable share in producing this effect.

(1) Some new terms seem to be wanted in order to express with precision the different parts of the charge. And if that part of it which cannot be destroyed by completing the circuit, should be called the permanent part of the charge, or more simply the charge; then might the other part, or that which may be destroyed by completing the circuit, be named the surcharge. again placed on the middle of the glass, and let the bottom be again carried in contact over the middle of the glass, holding the wire in one hand, while the other has a proper communication with the tin-foil coating.

Let the phial be again returned to the tin-foil fillet as before, and let the insulated cover be applied repeatedly to the wire, immediately after every separation from the glass; and a brighter spark, together with a weak snapping, will now attend each application, if it be carefully observed to touch the cover with one hand before every separation, while the other hand rests on the fillet of tin-foil. By proceeding in this manner, after the third application of the phial to the glass, a very weak shock will be felt in those fingers which are used in completing the circuit of the glass; and after repeating two rounds more in the manner before mentioned, the phial will be fully charged. By applying the coating of the phial when it is in full force to the upper surface as before, the glass plate will get the greatest power it is thus capable of receiving, and will then give a shock as high as the elbows. After this, on attempting to lift the insulated cover, the glass itself will generally be lifted at the same time, with the tin-foil coating adhering to the under surface; but by continuing the separations of the cover from the glass, a succession of strong negative sparks may be produced by the influence of the upper surface; and by turning the glass over, and leaving the tin-foil coating on the baize, a succession of strong positive sparks may be produced by the influence of the other side.

This experiment may be performed more steadily by placing the glass, together with the tin-foil coating and baize, on a plate of metal about \( \frac{1}{4} \)th of an inch thick, and of the same square as the glass. The whole may be fastened together by two small holdfasts placed at the opposite corners, which will prevent the glass from being lifted. This plate of metal will be useful in another view; for after it has been sufficiently warmed, by retaining heat well, it will help to keep the glass dry, and consequently fit for use so much the longer. But when it shall be required to show the contrary powers of the opposite sides of the glass, it will be more convenient not to fasten the parts together, and the whole may be kept sufficiently steady, by the operator's keeping down one corner of the glass with a finger, and by placing a proper weight on the opposite corner.

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

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

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

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

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

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

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

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

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

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

In Sect. vi. of this treatise we have related some experiments, tending to show, that in the act of charging Brookes's phial with positive electricity, both became positive; and in the act of charging one negatively, both became negative. These were inferred in the former edition of this work; since which time Mr. Brookes at Norwich has published a treatise; in which he not only adopts the opinion, but lays claim to it as his discovery, from some letters wrote in the year 1775. His experiments are extremely well adapted to elucidate the point intended; and the most remarkable of them are as follow: Miscellaneous.

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

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

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

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

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

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

In the last part of the Philosophical Transactions for 1789, we have the following experiments by Mr Nicholson, on an improved method of excitation, as well as the action of points, and the direction of the fluid &c., in positive and negative electricity.

1. A glass cylinder was mounted, and a cushion applied with a silk flap, proceeding from the edge of the cushion over its surface, and thence half round the cylinder. The cylinder was then excited by applying an amalgamated leather in the usual manner. The electricity was received by a conductor, and passed off in sparks to Lane's electrometer. By the frequency of these sparks, or by the number of turns required to cause spontaneous explosion of a jar, the strength of the excitation was ascertained.

2. The cushion was withdrawn about one inch from the cylinder, and the excitation performed by the silk only. A stream of fire was seen between the cushion and the silk; and much fewer sparks passed between the balls of the electrometer. 3. A roll of dry silk was interposed, to prevent the stream from passing between the cushion and the silk. Very few sparks then appeared at the electrometer.

4. A metallic rod, not insulated, was then interposed instead of the roll of silk, so as not to touch any part of the apparatus. A dense stream of electricity appeared between the rod and the silk, and the conductor gave very many sparks.

5. The knob of a jar being substituted in the place of the metallic rod, it became charged negatively.

6. The silk alone, with a piece of tin-foil applied behind it, afforded much electricity, though less than when the cushion was applied with a light pressure. The hand being applied to the silk as a cushion, produced a degree of excitation seldom equalled by any other cushion.

7. The edge of the hand answered as well as the palm.

8. When the excitation by a cushion was weak, a line of light appeared at the anterior part of the cushion, and the silk was strongly disposed to receive electricity from any uninsulated conductor. These appearances did not obtain when the excitation was by any means made very strong.

9. A thick silk, or two or more folds of silk, excited worse than a single very thin flap. I use the silk which the milliners call Persian.

10. When the silk was separated from the cylinder, sparks passed between them; the silk was found to be in a weak negative, and the cylinder in a positive, state.

The foregoing experiments show that the office of the silk is not merely to prevent the return of electricity from the cylinder to the cushion, but that it is the chief agent in the excitation; while the cushion serves only to supply the electricity, and perhaps increase the pressure at the entering part. There likewise seems to be little reason to doubt but that the disposition of the electricity to escape from the surface of the cylinder is not prevented by the interposition of the silk, but by a compensation after the manner of a charge; the silk being then as strongly negative as the cylinder is positive; and, lastly, that the line of light between the silk and cushion in weak excitations does not consist of returning electricity, but of electricity which passes to the cylinder, in consequence of its not having been sufficiently supplied during its contact with the rubbing surface.

11. When the excitation was very strong in a cylinder newly mounted, flashes of light were seen to fly across its inside, from the receiving surface to the surface in contact with the cushion, as indicated by the brush figure. These made the cylinder ring as if struck with a bundle of small twigs. They seem to have arisen from part of the electricity of the cylinder taking the form of a charge. This appearance was observed in a 9-inch and a 12-inch cylinder, and the property went off in a few weeks. Whence it appears to have been chiefly occasioned by the rarity of the internal air produced by handling, and probably restored by gradual leaking of the cement.

12. With a view to determine what happens in the inside of a cylinder, recourse was had to a plate machine. One cushion was applied with its silken flap. The plate was 9 inches in diameter and 1/8ths of an inch thick. During the excitation, the surface opposite to the cushion strongly attracted electricity, which it gave out when it arrived opposite to the extremity of the flaps: so that a continual stream of electricity passed through an insulated metallic bow terminating in balls, which were opposed, the one to the surface opposite the extremity of the silk, and the other opposite to the cushion; the former ball showing positive and the latter negative signs. The knobs of two jars being substituted in the place of these balls, the jar applied to the surface opposed to the cushion was charged negatively, and the other positively. This disposition of the back surface seemed, by a few trials, to be weaker the stronger the action of the cushion, as judged by the electricity on the cushion side.

Hence it follows, that the internal surface of a cylinder is so far from being disposed to give out electricity during the friction by which the external surface acquires it, that it even greedily attracts it.

13. A plate of glass was applied to the revolving plate, and thrust under the cushion in such a manner as to supply the place of the silk flap. It rendered the electricity stronger, and appears to be an improvement of the plate machine; to be admitted if there were not essential objections against the machine itself.

14. Two cushions were then applied on the opposite surfaces with their silk flaps, so as to clasp the plate between them. The electricity was received from both by applying the finger and thumb to the opposite surfaces of the plate. When the finger was advanced a little towards its correspondent cushion, so that its distance was less than between the thumb and its cushion, the finger received strong electricity, and the thumb none; and, contrariwise, if the thumb were advanced beyond the finger, it received all the electricity, and none passed to the finger. This electricity was not stronger than was produced by the good action of one cushion applied singly.

15. The cushion in experiment 12 gave most electricity when the back surface was supplied, provided that surface was suffered to retain its electricity till the rubbed surface had given out its electricity.

From the two last paragraphs it appears, that no advantage is gained by rubbing both surfaces; but that a well managed friction on one surface will accumulate by rubbing as much electricity as the present methods of excitation seem capable of collecting; but that, when the excitation is weak, on account of the electric matter not passing with sufficient facility to the rubbed surface, the friction enables the opposite surface to attract or receive it; and if it be supplied, both surfaces will pass off in the positive state; and either surface will give out more electricity than is really induced upon it, because the electricity of the opposite surface forms a charge.

It may be necessary to observe, that I am speaking of the facts or effects produced by friction; but how the rubbing surfaces act upon each other to produce them, whether by attraction or otherwise, we do not here enquire.

It will hereafter be seen, that plate machines do not collect more electricity than cylinders (in the hands of the electrical operators of this metropolis) do with half the rubbed surface; which is a corroboration of the inference here made.

16. When a cylinder is weakly excited, the appearances mentioned (par. 8.) are more evident the more rapid In this case, the avidity of the surface of the cylinder beneath the silk is partly supplied from the edge of the silk, which throws back a broad cascade of fire, sometimes to the distance of above 12 inches. From these causes it is that there is a determinate velocity of turning required to produce the maximum of intensity in the conductor. The stronger the excitation, the quicker may be the velocity; but it rarely exceeds five feet of the glass to pass the cushion in a second.

"If a piece of silk be applied to a cylinder, by drawing down the ends so that it may touch half the circumference, and the cylinder be then turned and excited by applying the amalgamated leather, it will become very greedy of electricity during the time it passes under the silk. And if the entering surface of the glass be supplied with electricity, it will give it out at the other extremity of contact; that is to say, if insulated conductors be applied at the touching ends of the silk, the one will give, and the other receive, electricity, until the intensities of their opposite states are as high as the power of the apparatus can bring them; and these states will be instantly reversed by turning the cylinder in the opposite direction.

"As this discovery promises to be of the greatest use in electrical experiments, because it affords the means of producing either the plus or minus states in one and the same conductor, and of instantly repeating experiments with either power, and without any change of position or adjustment of the apparatus, it evidently deserves the most minute examination.

"There was little hope (par. 6.) that cushions could be dispensed with. They were therefore added; and it was then seen, that the electrified conductors were supplied by the difference between the action of the cushion which had the advantage of the silk, and that which had not; so that the naked face of the cylinder was always in a strong electric state. Methods were used for taking off the pressure of the receiving cushion; but the extremity of the silk, by the construction, not being immediately under that cushion, gave out large flashes of electricity with the power that was used. Neither did it appear practicable to present a row of points or other apparatus to intercept the electricity which flew round the cylinder; because such an addition would have materially diminished the intensity of the conductor, which in the usual way was such as to flash into the air from rounded extremities of four inches diameter, and made an inch and half ball become luminous and blow like a point. But the greatest inconvenience was, that the two states with the backward and forward turn were seldom equal; because the deposition of the amalgam on the silk, produced by applying the leather to the cylinder in one direction of turning, was the reverse of what must take place when the contrary operation was performed.

"Notwithstanding all this, as the intensity with the two cushions was such as most operators would have called strong, the method may be of use, and I still mean to make more experiments when I get possession of a very large machine which is now in hand.

"The more immediate advantage of this discovery is, that it suggested the idea of two fixed cushions with a moveable silk flap and rubber. Upon this principle, which is so simple and obvious, that it is wonderful it should have been so long overlooked, I have constructed a machine with one conductor, in which the two opposite and equal states are produced by the simple process of loosening the leather-rubber, and letting it pass round with the cylinder (to which it adheres) until it arrives at the opposite side, where it is again fastened. A wish to avoid prolixity prevents my describing the mechanism by which it is let go and fastened in an instant, at the same time that the cushion is made either to press or is withdrawn, as occasion requires.

"Although the foregoing series of experiments naturally lead us to consider the silk as the chief agent in excitation; yet as this business was originally performed by a cushion only, it becomes an object of enquiry to determine what happens in this case.

"The great Beccaria inferred, that in a simple cushion, the line of fire, which is seen at the extremity manner extends from which the surface of the glass recedes, is caused by returning electricity; and Dr Nooth grounded his happy invention of the silk flap upon the same supposition. The former affirms, that the lines of light without both at the entering and departing parts of the surface silk flap, are absolutely similar; and thence infers, that the cushion receives on the one side, as it certainly does on the other. I find, however, that the fact is directly contrary to this assertion; and that the opposite inference ought to be made, as far as this indication can be reckoned conclusive: for the entering surface exhibits many luminous perpendiculars to the cushion, and the departing surface exhibits a neat uniform line of light. This circumstance, together with the consideration that the line of light behind the silk in par. 8. could not consist of returning electricity, showed the necessity of farther examination. I therefore applied the edge of the hand as a rubber, and by occasionally bringing forward the palm, I varied the quantity of electricity which passed near the departing surface. When this was the greatest, the sparks at the electrometer were the most numerous. But as the experiment was liable to the objection that the rubbing surface was variable, I pasted a piece of leather upon a thin flat piece of wood, then amalgamed its whole surface, and cut its extremity off in a neat right line close to the wood. This being applied by the constant action of a spring against the cylinder, produced a weak excitation; and the line where the contact of the cylinder and leather ceased (as abruptly as possible) exhibited a very narrow fringe of light. Another piece of wood was prepared of the same width as the rubber, but one quarter of an inch thick, with its edges rounded, and its whole surface covered with tin-foil. This was laid on the back of the rubber, and was there held by a small spring, in such a manner as that it could be slid onward, so as occasionally to project beyond the rubber, and cover the departing and excited surface of the cylinder without touching it. The sparks at the electrometer were four times as numerous when this metallic piece was thus projected; but no electricity was observed to pass between it and the cylinder. The metallic piece was then held in the hand to regulate its distance from the glass; and it was found, that the sparks at the electrometer increased in number as it was brought nearer, until light appeared between the metal and the cylinder; at which time they became fewer the nearer it was brought, and at last ceased when it was in contact." The following conclusions appear to be deducible from these experiments. 1. The line of light on a cylinder departing from a simple cushion consists of returning electricity; 2. The projecting part of the cushion compensates the electricity upon the cylinder, and by diminishing its intensity prevents its striking back in such large quantities as it would otherwise do; 3. That if there was no such compensation, very little of the excited electricity would be carried off; And, 4. That the compensation is diminished, or the intensity increased, in an higher ratio than that of the distance of the compensating substance; because if it were not, the electricity which has been carried off from an indefinitely small distance, would never fly back from a greater distance and form the edge of light.

"22. I hope the considerable intensity I shall speak of will be an apology for describing the manner in which I produce it. I wish the theory of this very obscure process were better known; but no conjecture of mine is worth mentioning. The method is as follows:

"Clean the cylinder, and wipe the silk.

"Graze the cylinder by turning it against a greased leather till it is uniformly obscured. I use the tallow of a candle.

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

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

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

"My rubber, as before observed, consists of the silk flap pasted to a leather, and the cushion is pressed against the silk by a slender spiral spring in the middle of its back. The cushion is loosely retained in a groove, and rests against the spring only, in such a manner that by a sort of vibration upon it as a fulcrum, it adapts itself to all the irregularities of the cylinder, and never fails to touch it in its whole length. There is no adjustment to vary the pressure, because the pressure cannot be too small when the excitation is properly made. Indeed, the actual withdrawing of the cushion to the distance of \( \frac{1}{8} \)th of an inch from the silk, as in par. 2, will not materially affect a good excitation.

"The amalgam is that of Dr Higgins, composed of zinc and mercury. If a little mercury be added to melted zinc, it renders it easily pulverizable, and more mercury may be added to the powder to make a very soft amalgam. It is apt to crystallize by repose, which seems in some measure to be prevented by triturating it with a small proportion of grease; and it is always of advantage to triturate it before using.

"Avery strong excitation may be produced by applying the amalgamed leather to a clean cylinder with a clean silk; but it soon goes off, and is not so strong as the foregoing, which lasts several days.

"23. To give some distinctive criterions by which other electricians may determine whether the intensity they produce exceeds or falls short of that which this method affords, I shall mention a few facts.

"With a cylinder 7 inches diameter and cushion 8 inches long, three brushes at a time constantly flew out of a 3-inch ball in a succession too quick to be counted, and a ball of \( \frac{1}{2} \) inch diameter was rendered luminous, and produced a strong wind like a point. A 9-inch cylinder with an 8-inch cushion occasioned frequent flashes from the round end of a conductor 4 inches diameter: with a ball of \( \frac{1}{2} \) inches diameter the flashes ceased now and then, and it began to appear luminous; a ball of \( \frac{1}{2} \) inch diameter first gave the usual flashes; then, by quicker turning, it became luminous with a bright speck moving about on its surface, while a constant stream of air rushed from it; and, lastly, when the intensity was greatest, brushes of a different kind from the former appeared. These were less luminous but better defined in the branches; many started out at once with a horse sound. They were reddish at the stem, sooner divided, and were greenish at the point next the ball, which was brass. A ball of \( \frac{1}{8} \)ths of an inch diameter was surrounded by a steady faint light, enveloping its exterior hemisphere, and sometimes a flash struck out at top. When the excitation was strongest, a few flashes struck out sideways. The horizontal diameter of the light was longest, and might measure one inch, the stem of the ball being vertical.

"This last phenomenon is similar to a natural event related by M. Loammi Baldwin*, who raised an electrical kite in July 1771 during the approach of a fine rain, surrounded by a rare medium of fire, which as the cloud rose nearer the zenith, and the kite rose higher, continued to extend itself with some gentle faint flashes, Mr Baldwin felt no other effect than a general weakness, pains in his joints and limbs, and a kind of little feel going; all which he observes might possibly be the effect of surprise, though it was sufficient to discourage him from persisting in any farther attempt at that time. He therefore drew in his kite, and retired to a shop till the storm was over, and then went to his house, where he found his parents and friends much more surprized than he had been himself; who, after expressing their astonishment, informed him, that he appeared to them (during the time he was raising the kite) to be in the midst of a large bright flame of fire, attended with flashings: and that they expected every moment to see him fall a sacrifice to the flame. The same was observed by some of his neighbours, who lived near the place where he stood.

"This fact is similar to another observed by M. de Saussure on the Alps, and both are referable to my luminous ball with the second kind of brush. The cloud must have been negative.

"With a 12-inch cylinder and rubber of \( \frac{1}{2} \) inches, a five-inch ball gave frequent flashes, upwards of 14 inches long, and sometimes a 6-inch ball would flash. I do not mention the long spark, because I was not provided with a favourable apparatus for the two larger cylinders. The 7-inch cylinder affords a spark of \( \frac{1}{2} \) inches at best. The 9-inch cylinder, not having its conductor insulated on a support sufficiently high, afforded flashes to the table which was 14 inches distant. And the 12-inch cylinder, being mounted only as a model or trial for constructing a larger apparatus, is defective in several respects which I have not thought fit to alter. When the five-inch ball gives flashes, the cylinder is enveloped on all sides with fire which rushes from the receiving part of the conductor." I never use points, but in a simple machine bring the conductor almost in contact with the cylinder. In this apparatus that cushion to which the rubber is not applied serves that purpose.

"The marks exhibit the intensity as deduced from simple electrifying. I will now mention the rate of charging, which was nearly the same in all the three cylinders.

"A large jar of 350 square inches, or near 2½ square feet, with an uncoated varnished rim of more than four inches in height, was made to explode spontaneously over the rim. The jar, when broken, proved to be 0.082 inches thick on an average; and the number of square feet of the surface of the cylinder which was rubbed to produce the charge of one foot, was, when least, 18.03, and when most, with good excitement, 19.34. The great machine at Haarlem charges a single jar of one foot square by the friction of 66.6 square feet, and charges its battery of 225 square feet at the rate of 94.8 square feet rubbed for each foot. The intensity of electricity on the surface of the glass is therefore considerably less than ¼th of that here spoken of; but if we take the most favourable number 66.6 at the commencement of turning, and halve it on account of the unavoidable imperfection of a plate machine (as shown in par. 14.), it will be found, that the management applied to that machine would cause a cylinder to charge one square foot by the friction of 33½ square feet. It must be observed, however, that Mr. Van Marum's own machine, consisting of two plates 33 inches diameter, has only half the intensity, though he reckons it a very good one. This machine is about equal in absolute power to my 9-inch cylinder, with its short rubber; but it is near 30 times as dear in price. In all these deductions I omit the computations, for the sake of brevity, and because they are easily made. The data are found in the description of the Leydenian machine, and its continuation published at Haarlem in the years 1785 and 1787.

"I shall here take the liberty of observing, that the action of the cylinder, by a simple cushion or the hand, which excited the attention of all Europe, in the memory of our contemporaries, was first improved by the addition of a leather flap; then by moistening the rubber; afterwards by applying the amalgam; and, lastly, by the addition of a silk flap. Now I find by experiment, that we at present obtain upwards of 40 times the intensity which the bare hand produces; and consequently, that, since 18 times our present intensity will equal the utmost we can now condense on strong glass even in the form of a charge, we have a less step to take before we arrive at that amazing power than our immediate predecessors have already made. My 9-inch cylinder, when broken, proved to be ¼ inch thick.

"Some of the luminous appearances with balls in the positive state, have been slightly noticed as criteria of intensity. I shall here add, that the escape of negative electricity from a ball is attended with the appearance of straight sharp sparks with a boar's chirping noise. When the ball was less than two inches in diameter, it was usually covered with short flames of this kind, which were very numerous.

"When two equal balls were presented to each other, and one of them was rendered strongly positive, while the other remained in connection with the earth, the positive brush or ramified spark was seen to pass from the electrified ball: when the other ball was electrified negatively, and the ball, which before had been positive, was connected with the ground, the electricity (passing the same way according to Franklin) exhibited the negative flame, or dense, straight, and more luminous spark, from the negative ball; and when the one ball was electrified plus and the other minus, the signs of both electricities appeared. If the interval was not too great, the long zig-zag spark of the plus ball struck the straight flame of the minus ball, usually at the distance of about ¼d of the length of the latter from its point, rendering the other ¼ds very bright. Sometimes, however, the positive spark struck the ball at a distance from the negative flame. These effects are represented in Plate CLXXVIII. fig. 86, 87, 88.

"Two conductors of three-quarters of an inch diameter, with spherical ends of the same diameter, were laid parallel to each other, at the distance of about two inches, in such a manner as that the ends pointed in opposite directions, and were six or eight inches asunder. These, which may be distinguished by the letters P and M, were successively electrified as the balls were in the last paragraph. When one conductor P was positive, fig. 90, it exhibited the spark of that electricity at its extremity, and struck the side of the other conductor M. When the last mentioned conductor M was electrified negatively, fig. 91, both signs appeared at the same time, and continual streams of electricity passed between the extremities of each conductor to the side of the other conductor opposed to it. In each of these three cases, the current of electricity, on the hypothesis of a single fluid, passed the same way.

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

"From comparing some experiments made by myself many years ago with the present set, I considered a point as a ball of an indefinitely small diameter, and constructed an instrument consisting of a brass ball of six inches diameter, through the axis of which a stem, carrying a fine point, was screwed. When this..." flem is fixed in the prime conductor, if the ball be moved on its axis in either direction, it causes the fine point either to protrude through a small hole in its external surface, or to withdraw itself; because by this means the ball runs along the stem. The disposition of the point to transmit electricity may thus be made equal to that of any ball whatever, from the minutest size to the diameter of six inches. See fig. 92. A.

"30. The action of pointed bodies has been a subject of discussion ever since it was first discovered, and is not yet well explained. To those who ascribe this effect to the figure of electric atmospheres, and their disposition to fly off, it may be answered, that they ought first to prove their existence, and then show why the cause which accumulated them does not prevent their escape; not to mention the difficulty of explaining the nature of negative atmospheres. If these be supposed to consist of electrified air, it will not be easy to show why a current of air passing near a prime conductor does not destroy its effects. The opinion supported by the celebrated Volta and others, that a point is the coating to an infinitely small plate of air, does not appear better founded: for such a plate must be broken through at a greater distance only because higher charged; whence it would follow, that points should not act but at high intensities. I must likewise take notice, as a proof that the charge has little to do here, that if a ball be presented to the prime conductor, at the same time that a point proceeds from the opposite side of the ball, the electricity will pass by the point, though it is obliged to go round the ball for that purpose; but it can hardly be doubted, that whatever charge obtains in this case is on the surface of the ball next the conductor, and not on the remote side to which the electricity directs its course.

"31. The pointed apparatus described (par. 29.) shows that the effect of points depends on the remoteness of their extremities from the other parts of the conductor. This leads to the following general law: In any electrified conductor, the transition or escape of electricity will be made chiefly from that part of the surface which is the most remote from the natural state. Thus in the apparatus of the ball and stem, the point having a communication with the rest of the whole conductor, constantly possesses the same intensity; but the influence of the surrounding surface of the ball diminishes its capacity. This diminution is less the farther the ball is withdrawn, and consequently the point will really possess more electricity, and be more disposed to give it out when it is prominent than when deprived. The same explanation serves for negative electricity.

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

"For the sake of conciseness, I pass over many facts which have presented themselves in the course of my experiments on the two electricities, and content myself with observing, that there is scarcely any experiment made with the positive power, which will not afford a result worthy of notice, if repeated with the negative."

With regard to the direction of the electric fluid, we shall only farther take notice of two experiments,

No 113. Place the jar to the conductor as usual; and, when charging, a luminous speck will appear upon the upper point of the wire at P, clearly showing, according to the commonly received opinion, that the point is then receiving the electric fluid. From the upper ring of coating B, on the outside of the jar, a fine stream or pencil of rays will at the same time fly off, beautifully diverging from the lower point of the wire F upon the bottom ring of the coating of the jar. When the appearances cease, which they do when the jar is charged, let a pointed wire be presented towards the conductor: this will soon discharge the jar silently; during which the point will be illuminated with a small spark, while the upper point of the wire will throw off a pencil of rays diverging towards the upper ring of the coating.

We shall conclude this section with an account of some effects of the electrical fluid upon various elastic vapours. These were tried to the greatest advantage by Dr Van Marum with the great machine already mentioned; and for this purpose he used a cylindrical glass receiver five inches long and an inch and a quarter in diameter, into which different sorts of elastic fluids were successively inserted, and were confined by quicksilver or water. To a hole made in the bottom of the inverted glass receiver an iron wire was fastened, the external part of which communicated with a conductor, which being presented to the prime conductor of the machine, received the sparks from it. In this disposition of the apparatus it evidently appears, that the sparks passed through the elastic fluid contained in the receiver, by going from the inner extremity of the wire to the quicksilver or water in which the receiver was inverted. With this apparatus it was found, that deploglificated air, obtained from mercurial red precipitate, lost 1/36th of its bulk; but its quality was not sensibly altered, as it appeared from examining it with the eudiometer. This experiment being repeated when the receiver was inverted in lime-water, and likewise in the infusion of turnsole, there ensued no precipitation, no change of colour, nor any phlogistication of the air. On pouring out this air, the usual smell of the electric fluid was perceived very sensibly.

Nitrous air was diminished of more than the half of its original bulk; and in that diminished state, being mixed with common air, it occasioned no red colour, nor any sensible diminution. It had lost its usual smell, and it extinguished a candle. In passing the sparks through the nitrous air, a powder is formed on the surface of the quicksilver, which is a part of that metallic substance dissolved by the nitrous acid.

Inflammable air, obtained from iron and diluted vitriolic acid, communicated a little redness to the tincture of turnsole. The stream of electric fluid through this air appeared more red, and much larger, than in common air, being everywhere surrounded by a faint blue light.

The inflammable air, obtained from spirit of wine and vitriolic acid, was increased to about three times its original bulk, and lost a little of its inflammability. Fixed air, from chalk and vitriolic acid, was a little increased in bulk by the action of electricity; but it was rendered less absorbable by water.

VOL. VI. Part II.

Vitriolic acid air, obtained from vitriolic acid and charcoal, was diminished a little, and black spots were formed on the inside of the glass receiver. Afterwards it was observed, that only one-eighth part of the electrified elastic fluid was absorbed by water. It extinguished a candle, and had very little smell.

Marine acid air seemed to oppose in great measure the passage of the electric fluid; since the sparks would not pass through a greater length than 2/3rd inches of this air. It was considerably diminished, but the rest was readily absorbed by water.

Spatious air was neither diminished, nor any other way sensibly altered, by the electric sparks.

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

Common air was lastly tried, and it was found to give a little faint redness to the tincture of turnsole; becoming at the same time sensibly phlogisticated. The experiment was repeated thrice at different times, and in each time after the electrization it was examined by the admixture of nitrous air in Mr Fontana's eudiometer, and it was compared with the same air not electrified; the latter always suffering the greatest diminution. In the first experiment the diminutions were 1/45 and 1/75; in the second, 1/58 and 1/95; and in the last, 1/45 and 1/95.

On attempting to repeat Mr Cavendish's experiment, in which he produced the nitrous acid by a See Aerolite mixture of pure with phlogisticated air; instead of a syphon, the Doctor made use of a glass tube 1/4th part of an inch in diameter, closed at one end, into which an iron wire, 1/4th of an inch in diameter, had been inserted: into this tube, filled with mercury, and fixed in a vertical position, was introduced the air with which the experiment was to be tried. The deploglificated air was obtained from red precipitate, and had been thoroughly purified by alkaline salts, from any acid it might have contained. With a mixture of 5 parts of this and 3 of common air, the tube was filled to the height of 3 inches, to which was added 1/5ths of an inch of lixivium, of the same kind with that used by Mr Cavendish. The result was, that, after transmitting through the tube a continued stream of the electrical fluid during 15 minutes, 2 inches of the air were absorbed by the lixivium: more air being introduced into the tube till it was filled to the height of 3 inches, when it was again electrified. This process was repeated till 3/4ths inches of air had been absorbed by the lixivium; this was now examined, and found to be, in some degree, impregnated with the nitrous acid; but it was very far from being saturated. With the same lixivium, of which a quarter of an inch remained in the tube, the experiment was continued till 14 inches more of air had been absorbed; but its diminution was not perceived to decrease, though the lixivium had now absorbed 77 measures of air, each equal to its own; whereas, in the experiment related by Mr Cavendish, only 38 measures of air were absorbed by the alkali. But notwithstanding this greater greater absorption, the lixivium was yet far from being saturated.

The experiment was repeated with pure air, produced by minium, moistened with the vitriolic acid, and deprived of its fixed air; seven parts of this were mixed with three of phlogisticated air, and lixivium added to the height of \( \frac{1}{4} \)th of an inch. Here, as in the former experiment, the diminution continued without any decrease; and the lixivium, after it had absorbed 22 \( \frac{1}{4} \)th inches, and consequently 178 times its own measure of air, was very far from being saturated with the nitrous acid.

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

Our author then goes on to some experiments made by suffering the electric fluid to pass in a continued stream through various kinds of air, inclosed for this purpose in the little glass tube used in the last experiments.

Pure air obtained the week before from red precipitate, being placed over mercury, and electrified for 30 minutes, was diminished by \( \frac{1}{4} \)th, the surface of the quicksilver soon began to be calcined, and towards the end of the experiment the glass tube was lined with the calx as to cease to be transparent. By introducing a piece of iron, the electric stream was made to pass through the air without immediately touching the mercury; yet this was equally calcined. This phenomenon the Doctor ascribes solely to the dissolution of the pure air, the principle of which unites itself with the metal; as in these experiments the mercury had not acquired any sensible heat. Two inches and \( \frac{3}{4} \) quarters of the same kind of air being placed over water, and electrified in the same manner during half an hour, lost a quarter of an inch; and being suffered to stand 12 hours in the tube, was found to have lost \( \frac{1}{4} \)th of an inch more. This was very nearly the same diminution of the air that had taken place when it was electrified over mercury; but, in this case, the process appears to be more slow, and the detached principle not so readily absorbed. The air remaining after these experiments, being tried by the eudiometer, did not differ from unelectrified pure air taken from the same receiver.

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

Three inches of phlogisticated air being electrified, during the first 5 minutes were augmented to \( \frac{3}{4} \)th inches, and in the next 10 minutes to \( \frac{3}{4} \)th inches: some lixivium was then introduced to try whether this would absorb it; but upon being electrified 15 minutes, the column rose to the height of 3 \( \frac{1}{4} \)th inches. It was suffered to stand in the tube till the next day, when it was found to have sunk to its original dimensions.

Nitrous air, confined by lixivium, being electrified during half an hour, lost 3 quarters of its bulk; the lixivium appeared to have absorbed a great deal of nitrous acid; and the air remaining in the tube did not seem to differ from common phlogisticated air. Some of the same nitrous air, confined by lixivium, was, by standing 3 weeks, diminished to half its bulk, and this residuum also proved to be phlogisticated air. Thus electricity very speedily effects that separation of the nitrous acid from nitrous air, which is slowly produced by the lixivium alone.

Inflammable air obtained from steel-silings and the diluted vitriolic acid, being confined by an infusion of turnsole, was electrified for 10 minutes without any change of colour in the infusion, or any alteration in the bulk of the air. The tube being filled with the same air to the height of 2 \( \frac{1}{4} \) inches, and placed in diluted vinegar of lead, was exposed to the electric stream during 12 minutes, in which time the inclosed air rose to 5 inches; but the vinegar remained perfectly clear. Three inches of inflammable air obtained from a mixture of spirits of wine with oil of vitriol, on being electrified for 15 minutes, rose to 10 inches; thus dilated, it lost all its inflammability, and when nitrous air was added, no diminution ensued.

A column of alkaline air obtained by heat from spirit of sal ammoniac, 3 inches high, was electrified 4 minutes, and rose to 6 inches, but did not rise higher when electrified 10 minutes longer. It appears that this air is not expanded more by the powerful electric stream from this machine than by the common spark. Water would not absorb this electrified air, which was in part inflammable.

The tube, being filled to the height of an inch with spirit of sal ammoniac, and inverted in mercury, was electrified 4 minutes; in which time the tube was filled with 8 inches of air, which proved to be equally inflammable, and as little absorbed by water as the alkaline air. Hence Dr Van Marum conjectures that this air is only the volatile alkali rendered elastic.

The following experiment is very curious, and may serve to illustrate some phenomena observed in thunderstorms. Two balloons, made of the allantoides of a calf, were filled with inflammable air, of which each contained about 2 cubic feet. To each of these was suspended, by a filken thread about 8 feet long, such a weight as was just sufficient to prevent it from rising higher in the air; they were connected, the one with the positive, the other with the negative conductor, by small wires about 30 feet in length, and being kept near 20 feet asunder, were placed as far from the machine as the length of the wires would admit. On being electrified, these balloons rose up in the air as high as the wire allowed, attracted each other, and uniting as it were into one cloud, gently descended. The rising of these artificial clouds is ascribed to the expansion of the air they contained, in consequence of the repulsive force communicated to its particles by elec- electricity: when in contact, their opposite electrical powers destroyed each other, and they recovered their specific gravity by losing the cause of its diminution.

In order to render this experiment more perfectly imitative, the Doctor suspended to the balloon which was connected with the negative conductor, a bladder filled with a mixture of inflammable and atmospheric air, which, being kindled by the spark that took place on the union of these clouds, gave a considerable explosion. From these experiments, the Doctor explains the sudden elevation of the clouds, and the violent showers of rain and hail, which often accompany thunderstorms.

In the course of his experiments upon air and electric fluid, Dr Priestley found, that, by means of the spark, he was able to turn vegetable blues to a red colour; though we are not to imagine that this was any indication of the acidity of the electric fluid, but merely of the decomposition of the air, and its conversion into fixed air or aerial acid. The instrument used in this experiment is a glass tube about 4 or 5 inches long and 1 or 2 inches of an inch diameter in the inside; a piece of wire is put into one end of the tube, and fixed there with cement; a brass ball is placed on the top of this wire; the lower part of the tube is to be filled with water, tinged blue with a piece of turnip or archil. This is easily effected, by setting the tube in a vessel of the tinged water, then placing it under a receiver on the plate of the air-pump; exhaust the receiver in part, and then, on letting in the air, the tinged liquor will rise in the tube, and the elevation will be in proportion to the accuracy of the vacuum; now take the tube and vessel from under the receiver, and throw strong sparks on the brass ball from the prime conductor.

When Dr Priestley made this experiment, he perceived, that after the electric spark had been taken between the wire and the liquor about a minute, the upper part of it began to look red; in 2 minutes it was manifestly so, and the red part did not readily mix with the liquor. If the tube was inclined when the sparks were taken, the redness extended twice as far on the lower side as on the upper. In proportion as the liquor became red, it advanced nearer to the wire, so that the air in which the sparks were taken was diminished; the diameter amounted to about 1/3th of the whole space; after which, a continuance of the electrification produced no sensible effect.

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

In passing the electric spark through different elastic fluids, it appears of different colours. In fixed air, the spark is very white; in inflammable and alkaline air, it appears of a purple or red colour.

From hence we may infer, that the conducting power of these airs is different, and that fixed air is a more perfect non-conductor than inflammable air.

The spark was not visible in air from a caustic alkali made by M. Lane, nor in air from spirit of salt; so that they seem to be more perfect conductors of electricity than water or other fluid substances.

The electric spark, taken in any kind of oil, produces inflammable air. Dr Priestley tried it with ether, oil of olives, oil of turpentine, and essential oil of mint, taking the electric spark in them without any air to begin with; inflammable air was produced in them all.

Dr Priestley found, that on taking a small electric explosion for an hour, in the space of an inch of fixed air, confined in a glass tube 1/3th of an inch diameter, when water was admitted to it, only 1/4th of the air was imbibed. Probably the whole would have been rendered immiscible in water, if the electrical operation had been continued a sufficient time.

The electric spark, when taken in alkaline air, appears of a red colour; the electric explosions, which pass through this air, increase its bulk; so that, by making about 200 explosions in a quantity of it, the original quantity will be sometimes increased 1/4th. If water is admitted to this air, it will absorb the original quantity, and leave about as much elastic fluid as was generated by the electricity, and this elastic fluid is a strong inflammable air.

Dr Priestley found, when the electric spark was taken in vitriolic acid air, that the inside of the tube in which it was confined was covered with a blackish substance. He seems to think, that the whole of the vitriolic acid air is convertible into this black matter, not by means of any union which it forms with the electric fluid, but in consequence of the concussion given to it by the explosion; and that, if it be the calx of the metal which supplied the phlogiston, it is not to be distinguished from what metal, or indeed from what substance of any kind, the air had been extracted.

Dr Priestley made 150 explosions of a common jar in about a quarter of an ounce measure of vitriolic acid air from copper, by which the bulk was diminished about 1/3, and the remainder seemingly not changed, being all absorbed by water. In the course of this process, the air was carefully transferred three times from one vessel to another; and the last vessel, in which the explosions were made, was, to all appearance, as black as the first; so that the air seems to be all convertible into this black substance.

Thinking this diminution of the vitriolic acid air might arise from its absorption by the cement with which the glass tubes employed in the last experiment were closed, he repeated it with the air from quicksilver, in a glass syphon confined by quicksilver, and the result was the same.

That this matter comes from the vitriolic acid air only, and not from any combination of the electric matter with it, will appear from the following experiment.

He took the simple electric spark from a conductor of moderate size, for the space of 5 minutes without interruption, in a quantity of vitriolic acid air, air, without producing any change in the inside of the glass; when immediately after, making it only two explosions of a common jar, each of which might be produced in less than a quarter of a minute with the same machine in the same state, the whole of the inside of the tube was completely covered with the black matter. Now, had the electric matter formed any union with the air, and this black matter had been the result of that combination, all the difference that would have arisen from the simple spark or the explosion, could only have been a more gradual or a more sudden formation of that matter.

A large phial, about an inch and a half wide, being filled with this air, the explosion of a very large jar, containing more than 2 feet of coated surface, had no effect upon it; from which it should seem, that in these cases the force of the shock was not able to give the quantity of air such a concussion as was necessary to decompose any part of it.

He had generally made use of copper, but afterwards he procured this air from almost every substance from which it could be obtained; the electric explosion taken in it produced the same effect. But as some of the experiments were attended with peculiar circumstances, he briefly mentions them as follows.

When he endeavoured to get vitriolic acid air from lead, putting a quantity of leaden shot into a phial containing oil of vitriol, and applying only the usual degree of heat, a considerable quantity of heat was produced; but afterwards, though the heat was increased till the acid boiled, no more air could be got. He imagined, therefore, that in this case the phlogiston had in fact been supplied by something that had adhered to the shot. However, in the air so produced, he took the electric explosion; and in the first quantity he tried, a whitish matter was produced, almost covering the inside of the tube; but in the succeeding experiments, with air produced from the same shot or from something adhering to it, there was less of the whitish matter; and at last nothing but black matter was produced, as in all the other experiments. Water being admitted to this air, there remained a considerable residuum, which was very slightly inflammable.

Vitriolic acid air is easily procured from spirit of wine, the mixture becoming black before any air is yielded. The electric explosion taken in this air also produced the black matter.

The experiments made with ether seem to throw most light upon this subject, as this air is as easily procured from ether as any other substance containing phlogiston. In the air procured by ether the electric explosion tinged the glass very black, more so than in any other experiment of the kind; and when water had absorbed what it could of this air, there was a residuum in which a candle burned with a lamplike blue flame. But what was most remarkable in this experiment was, that besides the oil of vitriol becoming very black during the process, a black substance, and of a thick consistence, was formed, which swam on the surface of the acid.

It is very possible, that the analysis of this substance may be a means of throwing light upon the nature of the black matter formed by electric explosions in vitriolic acid air, as they seem to resemble one another very much.

The electric spark or explosion taken in common air, confined by quicksilver in a glass tube, covers the inside of the tube with a black matter, which, when heated, appears to be pure quicksilver. This, therefore, may be the case with the black matter into which he supposed the vitriolic acid air to be converted by the same process, though the effect was much more remarkable than in the common air. The explosion will often produce the diminution of common air in half the time that simple sparks will do it, the machine giving the same quantity of fire in the same time: also, the blackness of the tube is much sooner produced by the shocks than by the sparks. When the tube considerably exceeds 1/5th of an inch in diameter, it will sometimes become very black, without any sensible diminution of the quantity of air.

**Sect. X. Of the Methods of measuring Electricity both artificial and natural; of condensing and doubling it, so that the smallest Quantity may be made perceptible; of distinguishing the two Kinds of Electricity from one another, &c.**

We have already had occasion to mention, and in part to explain, the instruments for this purpose named electrometers. When the electricity is very evident, many obvious contrivances may be fallen upon to determine its quality and strength, when compared with that of any other body electrified also to a considerable degree. But in many cases the quantity of electricity is so small that it does not discover itself by any of the ordinary electrometers; and in others, though the quantity be very great, yet we are destitute of any proper standard which might enable us to compare it with another of apparently the same height, or which might determine the degrees of charge which the electrified substance progressively receives.

In the former case, Dr Priestley recommends a single thread of silk as it comes from the worm; which being of various extremely light and flexible, very readily discovers the electrometric properties of any body, by being first attracted and then repelled by it: and, as this substance at the same time has a power of retaining its electricity very strongly, we have thus an opportunity of determining whether the body from which it received the electricity was positive or negative. Even this electrometer has not been found to be endowed with all the sensibility to be wished for; so that others have been contrived which answer to a still greater degree of exactness. For ordinary purposes the following instruments are most commonly made use of.

Fig. 13. represents a stand supporting the electrometers DD, CC. B is the base of it, made of common wood. A is a pillar of wax, glass, or baked wood. To the top of the pillar, if it be of wax or glass, a circular piece of wood is fixed; but if the pillar be of baked wood, that may constitute the whole. From this circular piece of wood proceed four arms of glass, or baked wood, suspending at their ends four electrometers, two of which DD are silk threads about eight inches long, suspending each a small downy feather. Methods of feather at its end. The other two electrometers CC measuring are those with very small balls of cork, or of the pith Electricity, of alder; and they are constructed in the following manner. \(ab\) is a stick of glass about six inches long, covered with sealing-wax, and shaped at top in a ring; from the lower extremity of this stick proceed two fine linen threads (\(x\)) \(cc\) about five inches long, each suspending a cork or pith-ball \(d\) about \(\frac{1}{8}\)th of an inch in diameter. When this electrometer is not electrified, the threads \(cc\) hang parallel to each other, and the cork-balls are in contact; but when electrified, they repel one another, as represented in the figure. When it happens to be inconvenient to use the insulating stand \(AB\), the electrometers may be easily supported by a glass rod or tube.

Another species of the above electrometer is represented in fig. 14, which consists of a linen thread, having at each end a small cork-ball. This electrometer is suspended by the middle of the thread on any conductor proper for the purpose, and serves to show the kind and quantity of its electricity.

Fig. 15 represents Mr Henry's quadrant electrometer fixed upon a small stand, from which it may be occasionally separated and fixed upon the prime conductor, or in any other place, at pleasure. This electrometer consists of a perpendicular stem formed at the top like a ball, and furnished at its lower end with a brass ferrule, by which it may be fixed in one of the holes of the prime conductor, or in its proper stand, as occasion requires. To the upper part of the stem or pillar, a graduated ivory semicircle is fixed; about the middle of which is a brass arm, which contains a pin, or the small axis of the index. The index consists of a very slender stick, which reaches from the centre of the graduated semicircle to the brass ferrule, and at its lower extremity is fastened a small cork-ball, nicely turned in a lathe.

When this electrometer is not electrified, the index hangs parallel to the pillar; but when it is electrified, the index recedes more or less, according to the quantity of electricity from the stem. See \(FGDI\), in fig. 14, and \(ab\) in fig 6, both of which are new and improved ways of applying it; by which the quantity of the shocks are regulated in the most convenient manner, as will be more particularly explained under Medical Electricity, Sect. XII.

Fig. 16 and 17 represent an electrometer nearly similar to that contrived by Mr Brooke. The two instruments are sometimes combined in one, or used separately, as in these figures. The arms \(FHFk\), fig. 17, when in use, are to be placed as much as possible out of the atmosphere of a jar, battery, prime conductor, &c. The arm \(FH\) and the ball \(K\) are made of copper, and as light as possible. The divisions on the arm \(FH\) are each of them exactly a grain. They are ascertained at first by placing grain weights on a brass ball which is within the ball \(L\) (this ball is an exact counterbalance to the arm \(FH\) and the ball \(K\) when the small slide on this arm is at the first division); and then removing the slide till it, together with the ball \(K\), counterbalances the ball \(L\) and the weight laid on it.

\(A\), fig. 16, is a dial-plate, divided into 90 equal parts. The index of this plate is carried once round, when the arm \(BC\) has moved through 90 degrees, or a quarter of a circle. That motion is given to the index by the repulsive power of the charge acting between the ball \(D\) and the ball \(B\).

The arm \(BC\) being repelled, shows when the charge is increasing, and the arm \(FH\) shows what this repulsive power is between two balls of this size in grains, according to the number the weight rests at when lifted up by the repulsive power of the charge; at the same time the arm \(BC\) points out the number of degrees to which the ball \(B\) is repelled; so that, by repeated trials, the number of degrees answering to a given number of grains, may be ascertained, and a table formed from these experiments, by which means the electrometer, fig. 16, may be used without that of fig. 17.

Mr Brookes thinks, that no glass charged (as we call it) with electricity, will bear a greater force than that whose repulsive power, between two balls of the size he used, is equal to 60 grains; that in very few instances it will stand 60 grains weight; and he thinks it hazardous to go more than 45 grains.

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

Mr Brookes thinks, that he is not acquainted with all the advantages of this electrometer; but that it is clear, it speaks a language which may be universally understood, which no other will do; for though other electrometers will show whether a charge is greater or less, by an index being repelled to greater or smaller distances, or by the charge exploding at different distances, yet the power of the charge is by no means ascertained; but this electrometer shows the force of the repulsive power in grains; and the accuracy of the instrument is easily proved, by placing the weights on the internal ball, and seeing that they coincide with the divisions on the arm \(FH\), when the slide is removed to them.

Mr Achard has shown clearly, that if the scale of an electrometer is divided into equal parts (degrees for example), the angle at which the index is held suspended by the electric repulsion will not be a true measure of the repulsive force; to estimate which truly, he demonstrates that the arc of the electrometer should be divided according to a scale of arcs, the tangents of which are in arithmetical progression.

The electrometer of which this is an imitation was invented by Mr Brookes, and described in his treatise already quoted. An account of it is given in that treatise, along with a very full representation of it by plates; but as these are somewhat difficult to be understood, we must for further particulars refer to the treatise itself. On this electrometer, however, we must observe, that it is constructed on the only true principle on which machines for measuring the quantity of electricity can be made. The mere attraction of any light body shows indeed that the substance which attracts it is electrified; but this property is by no means calculated to discover the comparative strength of it, on account

(k) These threads should be wetted in a weak solution of salt. Methods of count of its continual variation. Thus, if we hold any nefarious body within the electrified atmosphere of another, Electricity, &c.

The electricity of the atmosphere particularly, has engaged the attention of philosophers; and by reason of its infinite variety, requires the most delicate instru- ments to observe its minutiae. Besides the kite formerly described, which was an invention of Dr Franklin's, Mr Cavallo has invented several others. Fig. 61. repre- sents a portable atmospherical electrometer, the principal part of which is a glass tube $CDMN$, cemented at the bottom into the brass piece $AB$, by which part the instrument is to be held when used for the atmosphere; and it also serves to screw the instru- ment into its brass case $ABO$, fig. 69. The upper part of the tube $CDMN$ is shaped tapering to a small extremity, which is entirely covered with sealing-wax; to this tapering part a small tube is cemented; the lower extremity, being also covered with sealing-wax, pro- jects a small way within the tube $CDMN$; into this smaller tube a wire is cemented, which with its under extremity touches the flat piece of ivory $H$, fastened to the tube by means of a cork; the upper extremity of the wire projects about a quarter of an inch above the tube, and screws into the brass cap $EF$, which cap is open at the bottom, and serves to defend the waxed part of the instrument from the rain, &c.

$IM$ and $KN$ are two narrow slips of tin-foil, stuck to the inside of the glass $CDMN$, and communicating with the brass bottom $AB$. They serve to convey that electricity which, when the balls touch the glass, is communicated to it, and being accumulated, might disturb the free motion of the balls.

To use this instrument for artificial electricity, elec- trify the brass cap by an electrified substance, and the divergence or convergence of the balls of the electro- meter, at the approach of an excited electric, will show the quality of the electricity. The best manner to electricity this instrument is, to bring excited wax so near the cap that one or both of the corks may touch the side of the bottle $CDMN$, after which they will soon collapse and appear unelectrified. If now the wax is removed, they will again diverge, and remain unelectrified positively.

When this electrometer is to be used to try the electricity of the fogs, air, clouds, &c., the observer is to do nothing more than to unscrew it from its case, and hold it by the bottom $AB$ to present it to the air a little above his head, so that he may conveniently see the ball $P$, which will immediately diverge if there is any electricity; i.e. whether positive or negative may be ascertained, by bringing an excited piece of sealing- wax or other electric towards the brass cap $EF$.

An improvement of Mr Cavallo's electrometer has been made by M. Sauffure. The principal circum- stances in which they differ are, 1. The fine wires by which the balls are suspended, should not be long e- nough to reach the tin-foil which is pasted on the in- side of the glass; because the electricity, when strong, will cause them to touch this tin-foil twice consecutively, and thus deprive them in a moment of their elec- tricity. To prevent this defect, and yet give them a measur- ing sufficient degree of motion, it is necessary to use larger glass than those that are generally applied to Mr Ca- vallo's electrometer; two or three inches diameter will be found to answer the purpose very well. But as it is necessary to carry off the electricity which may be communicated to the inside of the glass, and thus be confounded with that which belongs to those substances that are under examination; four pieces of tin-foil should be pasted on the inside of the glass; the balls should not be more than $\frac{1}{25}$th of an inch diameter, ful- suspended by silver wire, moving freely in holes nicely rounded. The bottom of the electrometer should be of metal; for this renders it more easy to deprive them of any acquired electricity, by touching the bottom and top at the same time.

This electrometer may be used instead of the con- denser of M. Volta, by only placing it on a piece of lead of M. oiled silk, somewhat larger than the base of the instru- ment; but in this case it is the base and not the top of the instrument, which must be brought into con- tact with the substance whose electricity is to be ex- plored.

By this instrument, it is easy to ascertain the degree of conducting power in any substance. For example, the conduc- tor is placed on an imperfect conductor, as dry wood or marble, and if the instrument is electrified strongly, substances, and afterwards the top is touched, the electricity will appear to be destroyed; but on lifting up the instrument by the top, the balls will again open, because the im- perfect conductor formed with the base a kind of elec- trophorus, by which the electric fluid was condensed, and lost its tension, till the perfect conductor was se- parated from the imperfect one; whereas, if the con- ductor had been more perfect, it would have been de- prived of its electricity immediately on the application of the hand.

It is easy to discover also, by this instrument, the electricity of any substance, as of cloaths, hair of dif- ferent animals, &c. For this purpose, it must be held by the base, and the substance rubbed briskly (only once) by the ball of the electrometer; the kind of electricity may be ascertained in the usual manner. It is proper, however, to observe here, that as the top of the electrometer acts in this case as an inflated rubber, the electricity it acquires is always contrary to that of the rubbed body.

In order to collect a great quantity of electricity How to from the air, the electrometer is furnished with a point-collect a ed wire 15 inches or two feet long, which unscrews great quan- in three or four pieces, to render the instrument more portable; see fig. 62. When it rains or snows, the cal elec- tric small cover, fig. 63, is to be screwed on the top of city, the instrument, as by this its insulation is preserved, notwithstanding the rain.

This instrument indicates not only the electricity Or to af- fogs, but that also of serene weather, and enables us certain the to discover the kind of electricity which reigns in the kind of it. atmosphere; and to a certain degree to form an esti- mate of its quantity, and that under two different points of view, the degree of intensity, and the dis- tance from the earth at which it first begins to be sensible.

A conductor exhibits signs of electricity only when Methods of the electric fluid is more or less condensed in the air than in the earth. Though the air resists the passage of the electric fluid, it is not absolutely impermeable to it; it suffers it to pass gradually, and generally with more ease in proportion as its mass or thickness is less.

It is therefore interesting to discover at what height it is necessary to be elevated in order to find a sensible difference between the electricity of the earth and that of the air. A very sensible difference may be generally discovered by this instrument at the distance of four or five feet from the ground; sometimes it may be seen if the instrument is placed even on the ground, while at others it must be raised seven or more feet before the balls will open; sometimes, though seldom, this height is not sufficient. This distance is generally greater when the electricity is strongest, though necessarily modified by a variety of circumstances, some of which are known, as the degree of dryness or humidity of the air, and others are unknown.

The degree of intensity, at a given height, may be discovered thus: raise the electrometer, and judge by the divisions which are placed on the edge thereof the degree of their divergence. To find the relation between this degree of divergence and the force of the electricity, M. Saussure took the following method: As he could not with certainty double or triple a given quantity of electricity; yet as a given force may be reduced one half, a fourth, or eighth, &c. by dividing it between two equal and similar bodies, the electricity contained in one; he took two of his unarmed electrometers, which were as similar as possible, and electrified one of them, so that the balls separated precisely 6 lines: he then touched the top thereof by the top of that which was not electrified; in an instant the electricity was equally divided between them, as was evident by the divergence of the balls, which was 4 lines in each; consequently, a diminution of half the density had only lessened the divergence one third. One of these electrometers was then deprived of its electricity, and was afterwards brought in contact with the other, as before; the remaining electricity divided itself again between them, and the balls fell from 4 to 28 lines, nearly in the same proportion as before; in the third operation they fell to 19; in the fourth to one, where he was obliged to stop, as there was not now sufficient force in the fluid to pass from one electrometer to the other, and distribute itself uniformly between them. The same experiment repeated several times gave very nearly the same results. Negative electricity decreased also in the same proportion as the positive. The following table may therefore be considered as giving a general, though not exact, idea of the increase in force, which corresponds to different degrees of divergence in the balls; it is only calculated to every fourth of a line; the force of electricity is always expressed by whole numbers, as it would be ridiculous to put a greater degree of accuracy in the numbers than is to be found in the experiments which form the basis of the calculation (L).

| Distance of the balls in fourths of a line | Corresponding 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 | | 13 | 23 | | 14 | 26 | | 15 | 29 | | 16 | 32 | | 17 | 36 | | 18 | 40 | | 19 | 44 | | 20 | 48 | | 21 | 52 | | 22 | 56 | | 23 | 60 | | 24 | 64 |

Those who are desirous to carry this measure of the electric force further, may do it by having similar electrometers constructed, but made upon a larger scale, and with heavier balls, which would only separate one line, with the degree of electricity that makes the smaller ones diverge 6 lines; these would consequently measure a force 1024 times greater than that which forms the unity of the preceding table; and thus by degrees we may be enabled to discover the ratio of the strongest discharge of a great battery, or perhaps even of thunder itself, to that of a piece of amber, which only attracts a bit of straw or any other light substance. (M)

---

(L) M. Saussure, in a long note, anticipates the objections that may be made to the foregoing method of estimating the force of electricity; but as at the most they only show that this science is at present in a state of considerable imperfection, it will be unnecessary to take notice of them here.

(M) The consideration of the repulsive force is not sufficient to discover the absolute force of an explosion or electrical discharge: for M. Volta has shown, that the force of a discharge depends principally on the quantity of the electric fluid which passes from one body to another. Now the repulsive force of the electrometer only indicates the ratio of this quantity in equal and similar bodies, and which are also similarly situated. If equal quantities of the electric fluid were imparted to two unequal and separate conductors, the electric fluid being less condensed on the largest, would act with the least force on the electrometer; though it is probable, the force of the discharge in the two conductors would be equal. The repulsive force serves, however, to show what M. Volta calls the electrical capacity of a body, the quantity of the electric fluid it actually contains, or is capable of containing. To effect this, and have points of comparison, we should use light metallic balls, of different sizes, suspended by silk thread. One of these balls, unelectrified, being brought into contact with the substance... In order to observe the electricity of the atmosphere with this instrument, we must first bring the electric fluid contained in the electrometer to the same degree of density with that at the surface of the earth; this is easily done by letting the bottom and top touch the ground at the same time; then raise the point, keeping the bottom still in contact with the ground, from whence it may be lifted up in a vertical position till the balls are level with the eye.

The second circumstance is to render the divergence of the balls, which is occasioned by the electricity of the air, permanent. This is effected by touching the top of the electrometer with the finger; but here the acquired electricity becomes contrary to that of the body by which they are electrified. Let us suppose, for example, that the electrometer is at five feet from the ground, and the balls diverging; touch the top of the electrometer with the finger, and the balls will close; but they will again open if the electrometer is withdrawn from the influence of the electricity of the air, by being brought nearer the ground, or into the house. M. Saufleur only employed this method when the electricity was so weak that he could not perceive any until the electrometer was raised considerably above his eye: as in this case he could not perceive the divergence of the balls, he always endeavoured to obtain a permanent electricity in the foregoing manner.

To know whether the balls separate with positive or negative electricity, bring a piece of excited wax gradually near the top of the electrometer; if the balls separate further on the approach of the wax, they are negatively electrified, or of the same nature with the electricity of the wax; if on the other hand they come nearer together on the approach of the wax, then the electricity is positive, or in a contrary state to that of the wax. If glass is used, the results will be exactly the reverse of the preceding.

The following example will render the use of the foregoing observations more familiar. Choose an open situation free from trees and houses, secure 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 elevate it to the height of the eye, and observe the quantity of lines, or fourths of a line, that the balls have diverged; now lower it till the balls almost touch each other, and observe at what distance the top of the conductor is from the ground; and this is the height from the ground at which the electricity of the air begins to be sensible. If the electricity of the air is sufficiently strong to make the balls diverge when it stands upon the ground, one of the lengths of the electrometer must be unscrewed from it. If the balls however still diverge, the other parts of the conductor should also be unfixed, and you may mark down, that the electricity is sensible at zero, or on the surface of the earth. If, on the contrary, the electricity is so weak, as not to cause the balls to diverge when they are even with the eye, and consequently when the conductor is two feet higher, or seven feet from the ground, you should then raise it a foot higher; while it is thus elevated, touch the top with the other hand; when this hand is taken away, lower the electrometer, and if it is electrified you may say the electricity is sensible at eight feet; if it is not, raise it as high as the arm can reach, and repeat the same operation; if any electricity is found, write down electricity sensible at nine feet; if not, mark o, or no electricity relative to this instrument, and this mode of employing it; for signs of electricity may still be obtained, by throwing a metallic ball 50 or 60 feet into the air, which is at the same time connected with the electrometer by a metallic thread.

One advantage of this instrument is, that it will often exhibit signs of electricity when none can be obtained from a conductor of 100 feet in height, because it can more easily be preserved from humidity, &c., which destroy the insulation of the large conductors.

Aerial electricity varies according to the situation; it is generally strongest in elevated and insulated situations, not to be observed under trees, in streets, in houses, or any inclosed places; though it is sometimes to be found pretty strong on quays and bridges. It is also not so much the absolute height of the places as their situation; thus a projecting angle of a high hill will often exhibit a stronger electricity than the plain at the top of the hill, as there are fewer points in the former to deprive the air of its electricity.

The intensity of the atmospheric electricity is varied by a great many circumstances, some of which may be easily accounted for, others with more difficulty. When the weather is not serene, it is impossible to assign any rule for their variation, as no regular correspondence can then be perceived with the different hours of the day, nor with the various modifications of the air. The reason is evident; when contrary and variable winds reign at different heights, when clouds are rolling over clouds, these winds and clouds, which we cannot perceive by any exterior sign, influence however the strata of air in which we make our experiments, produce these changes of which we only see the result, without being able to assign either the cause or its relation.

Substance whose electricity is to be explored, will diminish the tension or repulsive force of this substance; and the quantity diminished by the contact of the ball will give the ratio of the capacity of this substance with that of the ball. Let us suppose a Leyden phial uninflated, but so concealed, that only the knob is visible, and we are therefore ignorant of its size, and the strength of the shock it will give. Let the top of M. Saufleur's electrometer be in contact with the knob of the bottle, and the balls of the electrometer separate 6 lines,—from this solitary fact, we shall gain no information relative to the force of the shock; because, if the jar is very large, this degree of tension will give a very painful sensation; when, if it is very small, with the same indicated tension, the sensation may be almost imperceptible. But if we bring a ball of a foot diameter, in contact with the knob of the bottle, and after having thus taken a part of the fluid therefrom, the electrometer is again put in contact with the knob thereof, the remaining quantity of repulsive force will show the relation between its contents and that of the globe of metal, and by this means the intensity of its charge. Thus, in stormy weather, we see the electricity strongly felt; then null, and in a moment after arise to its former force; one instant positive, the next negative, without being able to assign any reason for these changes. M. Sauffure says, that he has seen these changes succeed with such rapidity, that he had not time to note them down.

When rain falls without a storm, these changes are not so sudden; they are, however, very irregular, particularly with respect to the intensity of force; the quality thereof is more constant. Rain or snow almost uniformly gives positive electricity.

In cloudy weather, without rain or storms, the electricity follows generally the same laws as in serene weather.

Strong winds generally diminish its intensity; they mix together the different strata of the atmosphere, and make them pass successively towards the ground, and thus distribute the electricity uniformly between the earth and the air. M. Sauffure has observed a strong electricity with a strong north wind.

The state of the air in which the electricity is strongest, is foggy weather; this is always accompanied with electricity, except when the fog is going to resolve into rain.

The most interesting observations, and those which throw the greatest light upon the various modifications of electricity in our atmosphere, are those that are made in serene weather. In winter (during which most of M. Sauffure's observations were made), and in serene weather, the electricity was generally weakest in an evening, when the dew had fallen, until the moment of the sun's rising; its intensity afterwards augmented by degrees, sometimes sooner and sometimes later; but generally before noon, it attained a certain maximum, from whence it again declined, till the fall of the dew, when it would be sometimes stronger than it had been during the whole day; after which, it would again gradually diminish during the whole night; but it is never quite destroyed, if the weather is perfectly serene.

Atmospherical electricity seems, therefore, like the sea, to be subject to a flux and reflux, which causes it to increase and diminish twice in 24 hours. The moments of its greatest force are some hours after the rising and setting of the sun; those when it is weakest, precede the rising and setting thereof.

M. Sauffure has given an instance of this periodic flux in electricity: On the 23rd of February 1785, (one of the coldest days ever remembered at Geneva), the hygrometer and thermometer were suspended in the open air on a terrace exposed to the south-west; the electrometer, from its situation, indicated an electricity equal to what it would have shown if it had been placed on an open plain. The height of the barometer was reduced to what it would have been if the mercury had been constantly at the temperature of 10 degrees of Reamur's thermometer. The place of observation was elevated 60 feet above the level of the lake. The observations of the day preceding and following this great cold were marked down by him because it is pleasing to have these which precede and follow any singular phenomena. There was a weak S.W. wind during the whole three days; and it is rather remarkable, that most of the great colds, which have been observed at Geneva, were preceded by, or at least accompanied with, a little S.W. breeze.

From the first 18 observations made during these three days, when the sky was quite serene, we learn that the electricity was pretty strong at nine in the morning; that from thence it gradually diminished till towards six in the evening, which was its first minimum; after which it increased again till eight, its second maximum; from whence it again gradually declined till six the next morning, which was the time of its second minimum; after which, it again increased till ten in the morning, which was the first maximum of the following day; as this was cloudy, the electric periods were not so regular.

The electricity of serene weather is much weaker in summer than in winter, which renders it more difficult to observe these gradations in summer than in winter; besides a variety of accidental causes, which at the same time render them more uncertain. In general, in summer, if the ground has been dry for some days, and the air is dry also, the electricity increases from the rising of the sun till three or four in the afternoon, when it is strongest; it then diminishes till the dew begins to fall, which again reanimates it; though after this it declines, and is almost extinguished during the night.

But the serene days that succeed rainy weather in summer, generally exhibit the same diurnal periods or states of electricity, as are to be observed in winter.

The air is invariably positive in serene weather, both in winter and summer, day and night, in the sun or in positively the dew. It would seem, therefore, that the electricity of the air is essentially positive; and that whenever it appears to be negative, in certain rains or in storms, it probably arises from some clouds, which have been exposed to the pressure of the electric fluid contained in the upper part of the atmosphere, or to more elevated clouds that have discharged a part of their fluid upon the earth, or upon other clouds.

In order to find out the cause of these phenomena, M. Sauffure instituted a set of experiments on evaporation, avoiding the use of M. Volta's condenser.

To produce a strong evaporation, he threw a mass of red-hot iron into a small quantity of water, which was contained in a coffee-pot with a large mouth, and determine suspended by silk strings; by this he obtained a strong positive electricity; though, according to M. Volta's experiment, it ought to have been negative: the experiment was repeated several times, varying some of the circumstances, but the result was always the same.

As it was not easy to think so able a philosopher as M. Volta was deceived, it was necessary to try the experiment in a manner more analogous to that of M. Volta. A small chafing-dish was therefore infibulated with silk cords, and the coffee-pot, with a small quantity of water, placed on it; one electrometer was connected with the coffee-pot and another with the chafing-dish; the fire was raised by a pair of bellows; when the water had boiled strongly for a few minutes, both electrometers exhibited signs of electricity, which, on examination, was found to be negative; proving the truth of M. Volta's experiment. Methods of measuring Electricity.

The evaporation produced by the effervescence of iron in the vitriolic acid, and by that of chalk in the same acid, gave also negative electricity.

It was now necessary to inquire, why the vapour excited by the heated iron, produced positive electricity; while that from boiling water in any other way produced a negative electricity?

M. Sauflure suspected, that the intensity of heat to which the water is exposed, by the contact of a body in a state of incandescence, was the cause of the electricity produced by its evaporation; and that a combination was then formed, by which a new quantity of the electric fluid was produced. This conjecture may at first sight seem improbable; but the quantity of electricity produced by this experiment will astonish those that repeat it: and this quantity is the more surprising, because, if it is true, according to the system of M. Volta, that the waters absorb, while they are forming a quantity of the electric fluid, there must, therefore, be enough developed in this experiment for the formation of the great quantity of vapours produced by the heated iron, and afterwards a sufficient quantity to electrify strongly the apparatus, and all these vapours.

This experiment shows clearly the cause of that prodigious quantity of electricity which is unfolded in the eruption of volcanos; as it is probable that the water in these, from many circumstances, acquires a much greater degree of heat than is given to it in our experiments.

To verify this conjecture, that it was in some measure the combustion of the water or the iron that produced the positive electricity, it was proper to try whether, by a regular moderation of the heat of the iron, positive electricity would always be obtained. This was essayed in the following manner: A large iron crucible, five inches high, four in diameter, and six lines thick, was heated red hot, then insulated; after which, small quantities of water were thrown into it, each projection of the water cooling more and more the crucible; thus descending by degrees till there was only sufficient heat to boil the water; carefully observing, and then destroying, the electricity produced at each projection. The electricity was always positive or null; at the first projections it was very strong; it gradually diminished to the twelfth, when it was scarce sensible, though always with a tendency to be positive.

On repeating this experiment, and varying it in different ways, a remarkable circumstance was observed: When a small quantity of water was thrown into the crucible, the moment it was taken from the fire, while it was of a pale red, approaching what is called the white heat, no electricity was obtained.

This fact seemed to have some connection with another mentioned by Muffchenbroek, that water evaporates more slowly on a metal, or any other incandescent body, than on the same body, heated only a small degree above boiling water. To examine this relation, and to find whether there was any between the periods of evaporation and the production of electricity, M. Sauflure made a great number of experiments, which are most accurately described in his work; but as the detail would be much too long, we shall only present the reader with the heads thereof, and a description of the apparatus.

The apparatus consisted of a pot of clay, well baked or annealed, fifteen lines thick and four inches diameter; this was insulated by a dry glass goblet; upon this pot was placed the crucible, or any other heated substance on which the water was to be thrown, in order to be reduced into vapour; the crucible was contiguous to a wire connected with an electrometer; a measure, containing fifty-four grains weight of distilled water, was thrown upon the heated crucible: the time employed in the evaporation thereof was observed by a second watch; the electricity produced by this evaporation was noted. When this measure of water was reduced into vapour, the electricity of the apparatus is destroyed, and a fresh measure of water is thrown into the crucible, proceeding in the same manner till the crucible is almost cold.

The first experiment was with an iron crucible, from which it was found that Muffchenbroek was not right in saying that the evaporation was slowest when the iron was hottest; for at the instant it was taken from the fire, it required nineteen seconds to evaporate the water, and took more time till the third projection, when it took thirty-five seconds, though from that period it employed less time, or in other words, the evaporation accelerated in proportion as the iron cooled.

With respect to the electricity, it was at first positive, afterwards negative, then positive, and afterwards positive to the end of the experiment. The vapour was not visible till the seventh projection.

In the second experiment with the same crucible, though every endeavour was made use of to render them as similar as possible, the electricity was constantly positive.

The third experiment was with a copper crucible: here also the electricity was positive; and the longest time employed in evaporation was not the instant of the greatest heat. It was very curious to see the water endeavouring to gather itself into a globule, like mercury on glass, to be sometimes immovable, and then to turn on itself horizontally, with great rapidity; sometimes throwing from some of its points a little jet, accompanied with a hissing noise.

The fourth experiment was with the same crucible: the electricity was at first negative, then constantly positive.

The fifth was with a crucible of pure silver: a considerable time was employed here in evaporating the same quantity of water; even in the instant of the greatest heat it took five minutes six seconds; the electricity was weak; three times no electricity was perceived; five times negative electricity was discovered.

In a sixth experiment with the same crucible, a positive electricity was obtained at the second projection, after which none of any kind was perceived.

The seventh with the same, gave at first a strong negative electricity; the second and third projection gave a weak positive electricity.

The eighth was made with a porcelain cup: here the evaporation was slower at the second than the first projection; but from this it took longer time till it was cold. Methods of cold, contrary to what happened with the metals; the measuring electricity was always negative.

The ninth and tenth experiments with the same cup produced similar effects.

The eleventh experiment was with spirits of wine in a silver crucible; there was no electricity produced at the two first projections, and what was afterwards obtained was negative.

Twelfth experiment with ether: here the electricity was also negative. These two inflammable fluids, in evaporating, followed the same laws as water, being distillated at first most rapidly in the greatest heat, afterwards taking a longer and longer time before they were evaporated to a certain period, then employing less time, or evaporating quicker, till the crucible was nearly cold.

Now as china and silver always produced negative electricity, while iron and copper have generally given positive electricity, we may conclude, that electricity is positive with those bodies that are capable of decomposing water, or of being decomposed themselves by their contact with the water; and negative with those which are not at all decomposed or altered.

From hence M. Sauflure conjectures, that the electric fluid may be looked upon as formed by the union of fire with some unknown principle, perhaps a fluid analogous to inflammable air, but exceedingly more fugitive. This analogy seems to him sufficiently proved by the inflammation of the electric fluid, and by the diminution of the air in which this inflammation is made. Though many doubts have been attempted to be thrown on this inflammation, there seems to be one reason which forces us to admit it, which is the loss of a quantity of this fluid at every spark; we may diminish at pleasure any quantity of this fluid by taking a number of sparks from it. From whence also it may be inferred, that a considerable quantity is destroyed every day by thunder.

According to this system, when the operation, which converts water into vapour, produces at the same time a decomposition, it then generates the electric fluid. A part of this fluid combines itself immediately with these vapours, and serves even to form them. The vessel in which this operation is performed, will acquire a positive electricity, none at all, or a negative, according as the quantity of the fluid generated is superior, equal, or inferior to that which the formation of the vapour confines. When no decomposition accompanies the evaporation, the electricity ought to be constantly negative, because there is nothing to replace the quantity of this fluid which is employed in forming the vapour.

If in the foregoing experiments, those substances which were susceptible of calcination had constantly given a positive electricity, and those which do not calcine had always given the negative, everything would have been explained by these principles, and they would thence have acquired a greater degree of probability; but the phenomena have not always followed this law. We have seen iron and copper sometimes give a negative electricity, and silver the positive. The first case is not difficult to account for; it is well known with what facility iron and copper calcine in a brisk fire; they become covered with a scaly crust, which is not susceptible of any further alteration with the same heat. If the bottom of the crucible acquires this crusty coating, the drop of water placed thereon will be no longer in contact with a calcinable substance; there will be no farther decomposition, no generation of the electric fluid: the vapours, however, which are still formed, will absorb a part of the fluid naturally contained in the apparatus, and this will therefore be electrified negatively. If some of the scales should be too far detached, that the water may gain some points of contact, the quantity thus generated may compensate for what is absorbed by the vapours, and thus the electricity will be null. If more are detached, it will superabound and be positive. For the same reasons, a large mass of water, by attacking the iron in a greater number of points, always gives positive electricity; and hence, also, a strong positive electricity is obtained, by throwing a piece of red-hot iron into a mass of water.

It is not so easy to explain why silver gives sometimes a positive electricity, but by supposing it to have been mixed with some substances capable of calcination; and this the more, as the white porcelain always gave negative electricity. This supposition was verified by some subsequent experiments, in which the same silver, when purified, always gave a negative electricity.

M. Sauflure owns himself incapable of explaining why heated charcoal always gives negative electricity; unless it can be attributed to the promptitude with which so rare a substance loses its heat by the contact of water.

One fact astonished him, namely, that by combustion properly so called, although it is an evaporation, of electricity, the highest degree of evaporation, he never obtained any signs of electricity, though he tried to obtain it in a variety of ways. Probably the current produced by the flame disperses and dissipates the electricity as soon as it is formed. The case, however, must not be looked upon as general, because M. Volta obtained signs of electricity from bodies in combustion by means of his condenser.

Another singular fact was, his not being able to obtain electricity without ebullition, though he endeavoured to compensate by the quantity of surface for the quantity of vapours that were elevated by boiling water; and indeed, the same quantity of water, if extended over too large a surface, will not give any electricity.

But of all the instruments by which it hath been attempted to measure electricity, none have been found to answer the purpose equally well with that invented by Mr Bennet, of which an account is given in the 77th volume of the Philosophical Transactions, and which is represented fig. 64. It consists of two slips of leaf gold, a a, suspended in a glass cylinder b. The foot c may be made of wood or metal, and the cap d of metal; the latter being made flat at top for the convenience of putting anything upon it that is to be electrified. The cap is about an inch wider than the diameter of the glass, and its rim about three quarters of an inch broad, hanging parallel to the glass to keep it sufficiently insulated, and to turn off the rain. Within this is another circular rim about half as broad as the former, lined with silk or velvet, so that... Methods of it may be made to fit the outside of the glass exactly, measuring while the cap may be easily taken off to repair any accident happening to the gold-leaf. From the centre of the cap hangs a tin tube somewhat longer than the depth of the inner rim, in which a small peg f is placed, which may be taken out occasionally. To this peg, which is rounded at one end and flat at the other, two slips of leaf-gold are fastened with paste, gum-water, or varnish. These are about a fifth-part of an inch broad, and two inches long, tapering to a sharp point. In one side of the cap is a small tube g, to place wires in; h h are two long pieces of tin-foil fastened with varnish on opposite sides of the internal surface of the glass, where the leaf-gold may be expected to strike, and in connection with the foot. The upper end of the glass is covered and lined with sealing-wax as low as the outermost rim, to make the insulation more perfect. An improvement on this electrometer is to make the cylinder pretty long, and to have a small additional tube of gum-lac on the end of it. The slips of tin-foil reach almost to the edge of the outer rim, and are sharp pointed at top; widening in the middle, and decreasing in breadth again as they descend.

The sensibility of this electrometer is extreme, as appears from the following examples.

1. On putting powdered chalk into a pair of bellows, and blowing it upon the cap, the latter was electrified positively when the nozzle of the bellows was about six inches from it; but at the distance of three feet from the nozzle, the same stream electrified it negatively. Thus it appears that the electricity may be changed from positive to negative from the mere circumstance of the wider diffusion of this stream of chalk in the air. It may also be changed by placing a bunch of fine wire, silk, or feathers, in the nozzle of the bellows; and it is likewise negative when blown from a pair of bellows without their iron-pipe, so that it may come out in a larger stream: but this last experiment was found to answer best in wet weather. There is likewise a remarkable difference between the experiment in which the electricity is positive and that in which it is negative; the former being communicated with some degree of permanency to the cap, so that the gold-leaf continues for some time to diverge; but the latter being only momentary, and the gold-leaf collapsing as soon as the cloud of chalk is dispersed. The reason why the former continues is, that the chalk sticks to the cap.

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

3. Powdered chalk blown with the mouth or bellows from a metal plate placed upon the cap, electrifies it permanently positive. Or if the chalk is blown from the plate, either insulated or not, so that the powder may pass over the cap, if not too far off, it is also positive. Or if a brush is placed upon the cap, and a piece of chalk drawn over it, when the hand is withdrawn, the leaf gold gradually opens with positive electricity as the cloud of chalk disperses.

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

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

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

If a metal cup be placed upon the cap with a red-hot coal in it, a spoonful of water thrown in electrifies the cap negatively; and if a bent wire be placed in the cap, with a piece of paper fastened to it to increase its surface, the positive electricity of the ascending vapour may be tried by introducing the paper into it. Perhaps the electrification of fogs and rain is well illustrated by pouring water through an insulated cullender, containing hot coals, where the ascending vapour is positive and falling drops negative.

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

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

In very clear weather, when no clouds were visible, the electrometer has been often applied to the insulation to electric string of kites without metal, and their positive electrical electricity caused the leaf-gold to strike the sides; but kites, when a kite was raised in cloudy weather with a wire in the string, and when it gave sparks about a quarter of an inch long, the electricity was sensible by the electrometer at the distance of ten yards or more from the string; but when placed at the distance of six feet, the leaf-gold continued to strike the sides of the electrometer for more than an hour together, with a velocity increasing and decreasing with the density or distance of the unequal clouds which passed over.

Sometimes the electricity of an approaching cloud has... Methods of has been sensible without a kite, though in a very unfavourable situation for it, being in a town surrounded with hills, and where buildings encompassed the wall on which the electrometer was placed. A thundercloud passing over, caused the leaf-gold to strike the sides of the glass very quick at each flash of lightning.

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

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

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

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

To the experiments on blowing powders from a pair of bellows it may be added, that if the powder is blown at about the distance of three inches upon a plate which is moistened or oiled, its electricity is contrary to that produced by blowing upon a dry plate. This shows that the electricity of the streams of powder issuing out of the bellows is only contrary to the more expanded part, because it is within the influence of its atmosphere; for when this is destroyed by the adhesion of the powder to the moistened plate, it is negative when the bellows are positive, as it was before positive when the more expanded cloud was negative. The experiments on evaporation of water may be tried with more ease and certainty of success by heating the small end of a tobacco-pipe, and pouring water into the head; which running down to the heated part, is suddenly expanded, and will show its electricity when projected upon the cap of the electrometer more sensibly than any other way that has been tried. If the pipe be fixed in a cloven stick, and placed in the cap of one electrometer whilst the steam is projected upon another, it produces both electricities at once. Spirit of wine and ether are electrified like water. Oil and vitriolic acid produced smoke without any change of electricity. In these experiments a long pipe is better than a short one.

Besides these instruments, there are several others invented by Mr Cavall which answer the purpose of observing the electricity of the atmosphere extremely well, methods of those not with such great accuracy as that just now described; and of which he gives the following account.

"Fig. 67. represents a very simple instrument for making experiments on the electricity of the atmosphere; and which, on several accounts, seems to be the most proper for that purpose. AB is a common jointed metal fishing-rod, without the last or smallest joint. From observing the extremity of this rod proceeds a slender glass tube the electric C, covered with sealing-wax, and having a cork D at its end, from which a pith-ball electrometer is suspended. HGI is a piece of twine fastened to the other extremity of the rod, and supported at G by a small string FG. At the end (I) of the twine a pin is fastened; which when pushed into the cork D, renders the electrometer E uninsulated. When I would observe the electricity of the atmosphere with this instrument, I thrust the pin (I) into the cork D; and holding the rod by its lower end A, project it out from a window in the upper part of the house, into the air, raising the end of the rod with the electrometer, so as to make an angle of about 50° or 60° with the horizon. In this situation I keep the instrument for a few seconds; and then pulling the twine at H, the pin is disengaged from the cork D: which operation causes the string to drop in the dotted situation KL, and leaves the electrometer insulated, and electrified with an electricity contrary to that of the atmosphere. This done, I draw the electrometer into the room; and examine the quality of the electricity without obstruction either from wind or darkness. With this instrument I have made observations on the electricity of the atmosphere several times in a day for several months."

His electrometer for rain is shown Plate CLXXVII. His electrometer fig. 70. and of this he gives the following description.

"A B C I is a strong glass tube about two feet and a half long, having a tin funnel D E cemented to its extremity, which funnel defends part of the tube from the rain. The outside surface of the tube from A to B is covered with sealing-wax; so also is the part of it which is covered by the funnel. FD is a piece of cane, round which brass wires are twisted in different directions, so as to catch the rain easily, and at the same time to make no resistance to the wind. This piece of cane is fixed into the tube; and a slender wire proceeding from it goes through the bore of the tube, and communicates with the strong wire AG, which is thrust into a piece of cork fastened to the end A of the tube. The end G of the wire AG is formed in a ring, from which I suspend a more or less sensible pith-ball electrometer as occasion requires. This instrument is fastened to the side of the window-frame, where it is supported by strong brass hooks at CB; which part of the tube is covered with a silk lace, in order to adapt it better to the hooks. The part FC is out of the window, with the end F elevated a little above the horizon. The remaining part of the instrument comes through a hole in one of the lights of the sash within the room, and no more of it touches the side of the window than the part CB. When it rains, especially in passing showers, this instrument, standing in the situation above described, is frequently electrified; and, by the diverging of the electrometer, the quantity and quality of the electricity of the rain may be observed without any danger of a mistake. With this instrument I have observed..." Methods of served, that the rain is generally, though not always, electrified negatively; and sometimes so strongly, that I have been able to charge a small coated phial at the wire AG. This instrument should be fixed in such a manner that it may be easily taken off from the window and replaced again as occasion requires; for it will be necessary to clean it very often, particularly when a shower of rain is approaching.

Notwithstanding the great accuracy of these instruments, however, there are still many degrees of electricity too small to be observed by any of them. To be able to collect these, it is necessary to have one capable of retaining the electricity it receives for a considerable time, and of allowing it to accumulate till it becomes capable of being measured by some of the common methods. Upon instruments of this kind Mr Cavallo gives the following dissertation.

Besides the way of ascertaining small quantities of electricity by means of very delicate electrometers, two methods have been communicated to the philosophical world, by which such quantities of electricity may be rendered manifest as could not be perceived by other means. The first of those methods is an invention of M. Volta, the apparatus for it being called the condenser of electricity, and is described in the Philosophical Transactions, Vol. LXXII. The second is a contrivance of the above-mentioned Mr Bennet, who calls the apparatus the doubler of electricity. A description of it is inserted in the Philosophical Transactions, Vol. LXXVII.

M. Volta's condenser consists of a flat and smooth metal plate, furnished with an insulating handle, and a semiconducting, or imperfectly insulating, plane. When one wishes to examine a weak electricity with this apparatus, as that of the air in calm and hot weather, which is not generally sensible to an electrometer, he must place the above-mentioned plate upon the semiconducting plane, and a wire, or some other conducting substance, must be connected with the metal plate, and must be extended in the open air, so as to absorb its electricity; then, after a certain time, the metal plate must be separated from the semiconducting plane; and being presented to an electrometer, will electrify it much more than if it had not been placed upon the above-mentioned plane.

The principle on which the action of this apparatus depends is, that the metal plate, whilst standing contiguous to the semiconducting plane, will both absorb and retain a much greater quantity of electricity than it can either absorb or retain when separate, its capacity being increased in the former and diminished in the latter case.

Whoever considers this apparatus, will easily find, that its office is not to manifest a small quantity of electricity, but to condense an expanded quantity of electricity into a small space; hence, if by means of this apparatus one expected to render more manifest than it generally is, when communicated immediately to an electrometer, the electricity of a small tourmalin, or of a hair when rubbed, he would find himself mistaken.

It is Mr Bennet's doubler that was intended to answer that end; viz. to multiply, by repeated doubling, a small, and otherwise unperceivable, quantity of electricity, till it became sufficient to affect an electrometer, to give sparks, &c. The merit of this invention is certainly considerable; but the use of it is far from precise and certain.

This apparatus consists of three brass plates, which we shall call A, B, and C; each of which is about three or four inches in diameter. The first plate A is placed upon the gold-leaf electrometer, or it may be supported horizontally by any other insulating stand, and its upper part only is thinly varnished. The second plate B is varnished on both sides, and is furnished with an insulating handle, which is fastened laterally to the edge of it. The third plate C is varnished on the under side only, and is furnished with an insulating handle, which is perpendicular to its upper surface.

This apparatus is used in the following manner. The plate B being laid upon the plate A, the small quantity of electricity, which is required to be multiplied, is communicated to the under part of the plate A, and at the same time the upper part of B is touched with a finger; then the finger is first removed; the plate B is afterwards removed from over the plate A. The plate C is now laid upon B, and its upper surface is touched, for a short time, with a finger. By this operation it is clear, that if the electricity communicated to the plate A is positive, the plate B must have acquired a negative electricity, and the plate C must have acquired the positive, viz. the same of the plate A. Now the plate B, being separated from C, is laid as before upon A; the edge of C is brought into contact with the under part of the plate A, and at the same time the upper part of B is touched with a finger; by which means the plate B, being acted upon by the atmospheres of both the plates A and C, will acquire nearly twice as much electricity as it did the first time, and of course will render the plate C, when that is laid upon it, proportionally more electrified than before; thus, by repeating this operation, the electricity may be increased to any required degree.

The varnish on those surfaces of the plates which are to lie contiguous to each other, serves to prevent the metal of one touching the metal of the other; for in that case, instead of one plate causing a contrary electricity in the other, the electricity of the first would be gradually communicated to the others, and would be dissipated.

As soon as I understood the principle of this contrivance, I hastened to construct such an apparatus, in order to try several experiments of a very delicate nature, especially on animal bodies and vegetables, which could not have been attempted before, for want of a method of ascertaining exceedingly small quantities of electricity; but after a great deal of trouble, and many experiments, I was at last forced to conclude, that the doubler of electricity is not an instrument to be depended upon, for this principal reason, viz. because it multiplies not only the electricity which is willingly communicated to it from the substance in question; but it multiplies also that electricity which in the course of the operation is almost unavoidably produced by accidental friction; or that quantity of electricity, however small it may be, which adheres to the plates in spite of every care and precaution.

Having found, that with a doubler constructed in the above-described manner, after doubling or multiplying Methods of multiplying 20 or 30 times, it always became strongly electrified, though no electricity had been communicated to it before the operation, and though every endeavor of depriving it of any adhering electricity had been practised; I naturally attributed that electricity which appeared after repeatedly doubling, to some friction given to the varnish of the plates in the course of the operation. In order to avoid entirely this source of mistake, or at least of suspicion, I constructed three plates without the least varnish, and which, of course, could not touch each other, but were to stand only within about one-eighth of an inch of each other. To effect this, each plate stood vertical, and was supported by two glass sticks, which were covered with sealing-wax. These were inserted into a wooden pedestal 7½ inches long, 2½ broad, and 1½ inches thick, being kept fast by cement both to the pedestal and likewise to another piece of wood fastened to the back of the plate. The plate itself is of strong tin, and measures about eight inches in diameter. The stand projects very little before the plate; by which means, when two of those plates are placed upon a table facing each other, the wooden stands will prevent their coming into actual contact.

"I need not describe the manner of doubling or of multiplying with those plates; the operation being essentially the same as when the plates are constructed according to Mr Bennet's original plan, excepting that, instead of placing them one upon the other, mine are placed facing each other; and in performing the operation they are laid hold of by the wooden stand AB; so that no friction can take place either upon the glass legs or upon any varnish; for these plates have no need of being varnished. Sometimes, instead of touching the plates themselves with the finger, I have fixed a piece of thin wire to the back of the plate, and have then applied the finger to the extremity of the wire, suspecting that some friction and some electricity might possibly be produced when the finger was applied in full contact to the plate itself.

"It is evident, that as the plates do not come so near to each other in this as they do in the other construction, the electricity of one of them cannot produce so great a quantity of the contrary electricity in the opposite plate; hence, in this construction, it will be necessary to continue the operation of doubling somewhat longer; but this disadvantage is more than repaid by the certainty of avoiding any friction.

"Having constructed those plates, I thought that I might proceed to perform the intended experiments without any further obstruction: but in this I found myself quite mistaken; for on trying to multiply with those new plates, and when no electricity had been previously communicated to any of them, I found, that after doubling 10, 15, or at most 20 times, they became so full of electricity as to afford even sparks. All my endeavors to deprive them of electricity proved ineffectual. Neither exposing them, and especially the glass sticks, to the flame of burning paper, nor breathing upon them repeatedly, nor leaving them untouched for several days, and even for a whole month, during which time the plates remained connected with the ground by means of good conductors, nor any other precaution I could think of, was found capable of depriving them of every vestige of electricity; so that they might show none after doubling 10, 15, or at most 20 times.

"The electricity produced by them was not always of the same sort; for sometimes it was negative for two or three days together; at other times it was positive for two or three days more; and often it changed in every operation. This made me suspect, that possibly the beginning of that electricity was derived from my body, and being communicated by the finger to the plate that was first touched, was afterwards multiplied. In order to clear this suspicion, I actually tried those plates at different times, viz. before and after having walked a great deal, before and after dinner, &c., noting very accurately the quality of the electricity produced each time; but the effects seemed to be quite unconnected, with the above mentioned concomitant circumstances; which independence was further confirmed by observing that the electricity produced by the plates was of a fluctuating nature; even when, instead of touching the plates with the finger, they had been touched with a wire, which was connected with the ground, and which I managed by means of an insulating handle.

"At last, after a great variety of experiments, which it is unnecessary to describe, I became fully convinced, that those plates did always retain a small quantity of electricity, perhaps of that sort with which they had been last electrified, and of which it was almost impossible to deprive them. The various quality of the electricity produced was owing to this, viz. that as one of those plates was possessed of a small quantity of positive electricity, and another was possessed of the negative electricity, that plate which happened to be the most powerful, occasioned a contrary electricity in the other plate, and finally produced an accumulation of that particular sort of electricity.

"Those observations evidently show, that no precise result can be obtained from the use of those plates; and of course, that when constructed according to the original plan, they are still more equivocal, because they admit of more sources of mistake.

"As those plates, after doubling or multiplying only four or five times, show no signs of electricity, none having been communicated to them before, I imagined that they might be useful to far only, viz. that when a small quantity of electricity is communicated to any of them in the course of some experiment, one might multiply it with safety four or five times, which would even be of advantage in various cases; but in this also my expectations were disappointed.

"Having observed, after many experiments, that, ceteris paribus, when I began to multiply from a certain plate, which we shall call A, the electricity which resulted was generally positive; and when I began with another plate B, viz. considered this plate B as the first plate, the resulting electricity was generally negative; I communicated some negative electricity to the plate A, with a view of destroying its inherent positive electricity. This plate A being now electrified negatively, but so weakly as just to affect an electrometer, I began doubling; but after having doubled three or four times, I found, by the help of an electrometer, that the communicated negative electricity in the plate was diminished instead of being increased; so that sometimes it vanished entirely, though by continuing the opera- Methods of measuring electricity often began to increase again after a certain period. This shows that the quantity of electricity, which however small it may be, remains in a manner fattened to the plates, will help either to increase or to diminish the accumulation or multiplication of the communicated electricity, according as it happens to be of the same or of a different nature.

"After all the above mentioned experiments made with those doubling or multiplying plates, we may come to the following conclusion, viz. that the invention is very ingenious, but their use is by no means to be depended upon. It is to be wished that they may be improved so as to obviate the weighty objections that have been mentioned; the first desideratum being to construct a set of such plates as, when no electricity is communicated, they will produce none after having performed the operation of doubling for a certain number of times.

"Upon the whole, the methods by which small quantities of electricity may be ascertained with precision are, as far as I know, only three. If the absolute quantity of electricity be small and pretty well condensed, as that produced by a small tourmalin when heated, or by a hair when rubbed, the only effectual method of manifesting its presence, and ascertaining its quality, is to communicate it immediately to a very delicate electrometer, viz. a very light one, that has no great extent of metallic or of other conducting substance; because if the small quantity of electricity that is communicated to it be expanded throughout a proportionably great surface, its elasticity, and of course its power of separating the corks of an electrometer, will be diminished in the same proportion.

"The other case is, when one wants to ascertain the presence of a considerable quantity of electricity, which is dispersed or expanded into a great space, and is little condensed, like the constant electricity of the atmosphere in clear weather, or like the electricity which remains in a large Leyden phial after the first or second discharge.

"To effect this, I use an apparatus, which in principle is nothing more than M. Volta's condenser, but with certain alterations, which render it less efficacious than in the original plan, but at the same time render it much less subject to equivocal results. I place two of the above described tin-plates upon a table, facing each other, and about 1/8th of an inch apart. One of those plates, for instance A, is connected with the floor by means of a wire, and the other plate B is made to communicate, by any convenient means, with the electricity that is required to be collected. In this disposition the plate B, on account of the proximity of the other plate, will imbibe more electricity than if it stood far from it, the plate A in this case acting like the semiconducting plane of M. Volta's condenser, though not with quite an equal effect, because the other plate B does not touch it; but yet, for the very same reason, this method is incomparably less subject to any equivocal result. When the plates have remained in the said situation for the time that

N° 113.

(n) This small plate is nearly of the size of a shilling, and the semiconducting plane is of wood covered with copal varnish. Methods of field, contains its proper share of electric fluid, which proportionate to its bulk, or to some other of its properties; and it is generally believed, that this equal or proportionate distribution of electric fluid takes place with the greatest part of natural bodies. However, the fact is far from being so; and I may venture to assert, that, strictly speaking, every substance is always electrified, viz., that every substance, and even the various parts of the same body, contain at all times more or less electric fluid than that quantity of it which it ought to contain, in order to be in an electrical equilibrium with the bodies that surround it.

"At first sight it may be thought quite immaterial to know, whether the electric fluid is dispersed in the just proportion among the various substances which are not looked upon as electrified, or whether it deviates in a small degree from that proportionate distribution; but it will hereafter appear, that one of those assertions will lead us to the explanation of an interesting phenomenon in electricity, whereas the other does not admit of it; besides, what is called a small difference of the proportionate distribution, insomuch as it does not affect our instruments, may be sufficient for several operations of nature, which it is our interest to investigate.

"If we inquire what phenomena evince this altered distribution, or the actually electrified state of all bodies, the preceding observations will furnish some very unequivocal ones; especially that of the doubling plates made after my plan, which showed to be electrified even after having remained untouched for a whole month, during which time they had been in communication with the ground; for if each of them had contained an equal share of electric fluid, the electric atmosphere of one of them could not possibly occasion a contrary electricity in the other, and consequently no accumulation of that power could have happened.

"A great number of instances are related in books on the subject of electricity, and in the Phil. Trans. of pieces of glass, of sulphur, of sealing-wax, &c., having remained electrified so far as to affect an electrometer for months after they had been excited, or even touched; but the following experiment will show, in a clearer manner, the great length of time that a quantity of electricity will remain upon a body.

"Having constructed a gold-leaf electrometer in the nicest manner I could, and which, on account of the non-conducting nature and construction of its upper part, could remain sensibly electrified for several hours together, I communicated some electricity to it, which caused the slips of gold-leaf to diverge with a certain angle; and as the electricity was gradually dissipated, the divergency diminished in the same proportion. Now, whilst this diminution of divergency was going on, I looked through a small telescope, and, by means of a micrometer, measured the chords of the angles of divergency, setting down the time elapsed between each pair of contiguous observations; and as the chord of the angle of divarication is in the direct simple proportion of the density of the electric fluid (A), I could by this means know how much electric fluid was lost by the electrometer in a certain time, and of course what portion of the electricity first communicated to Methods of the electrometer still remained in it. Let us make the measuring chord of the angle of divarication on first electrifying electricity, the electrometer, or rather when first observed, equal to 16; or let us conceive that quantity of electricity to be divisible into 16 equal parts.

"I observed, that, when the chord of the angle became equal to eight, the time elapsed between this and the first observation was one minute; when the chord became equal to four, the time elapsed between this point and the preceding observation was 3' 30"; when the chord became equal to two, the time elapsed since the preceding observation was 17'; and when the chord became equal to one, the time elapsed since the preceding observation was one hour and a quarter; after which the electrometer remained sensibly electrified for a long time.

"In repeating this experiment, the times elapsed between the corresponding observations did not follow strictly the same proportion of increase; nor did they increase regularly in the same experiments, which may be attributed in great measure to the inaccuracy in observing, and to the fluctuating state of the air; but it could be safely inferred from all the experiments, that the times required for the dispersion of the electricity were at least greater than the inverse duplicate proportion of the densities of the electricity remaining in the electrometer. And if we imagine, that they continue to diminish in the same proportion of increasing time, which is far from being an extravagant supposition, we shall find, by a very easy calculation, that about two years after the electrometer would still retain the $\frac{1}{125}$th part of the electricity communicated to it in the beginning of the experiment; and as we do not know how far a quantity of electricity is divisible, or to what extent it may be expanded, we may conclude with saying, that strictly speaking the electrometer would remain electrified for many years.

"It may be inferred from this, as well as from many other experiments, that the air, or in general any substance, is a more or less perfect conductor of electricity, according as the electricity which is to pass through it is more or less condensed; so that if a given quantity of electric fluid be communicated to a small brass ball, one may take it away by simply touching the ball with a finger; but if the same quantity of electric fluid be communicated to a surface of about 100 or 1000 square feet, the touching with the finger will hardly take away any part of it.

"If it be asked, what power communicates the electricity, or originally disturbs the equilibrium of the natural quantity of electric fluid in the various bodies of the universe? we may answer, that the fluctuating electric state of the air, the pallage of electrified clouds, the evaporation and condensation of fluids, and the friction arising from divers causes, are perpetually acting upon the electric fluid of all bodies, so as either to increase or diminish it, and that to a more considerable degree than is generally imagined.

"I shall conclude, with briefly proposing an explanation of the production of electricity by friction, which is dependent upon the above stated proposition,

(4) This proposition was first ascertained by F. Beccaria. See Philosophical Transactions, Vol. LVI. Methods of viz. that bodies are always electrified in some degree; measuring and likewise upon the well known principle of the capacity of bodies for holding electric fluid being increased by the proximity of other bodies in certain circumstances.

"It seems to me, that the cylinder of an electrical machine must always retain some electricity of the positive kind, though not equally dense in every part of its surface; therefore, when one part of it is set contiguous to the rubber, it must induce a negative electricity in the rubber. Now, when, by turning the cylinder, another part of it (which suppose to have a less quantity of positive electricity than the preceding) comes quickly against the rubber; the rubber being already negative, and not being capable of losing that electricity very quickly, must induce a stronger positive electricity in the former part which is now opposite to it; but this part cannot become more positively electrified, unless it receives the electric fluid from some other body, and therefore some quantity of electric fluid passes from the lowest part of the rubber to this part of the glass; which additional quantity of electric fluid is retained by it alone only whilst it remains in contact with the rubber; for after that, its capacity being diminished, the electric fluid endeavours to escape from it. Thus we may conceive how every other part of the glass acquires the electric fluid, &c. and what is said of the cylinder of an electrical machine may, with proper changes, be applied to any other electric and its rubber."

An instrument for observing very small quantities of electricity has likewise been invented by the same author, and described in the second part of the volume just quoted. The properties of this machine, which from its office may be called a collector of electricity, are, firstly, that when connected with the atmosphere, the rain, or in short with any body which produces electricity slowly, or which contains that power in a very rarefied manner, it collects the electricity, and afterwards renders both the presence and quality of it manifest, by communicating it to an electrometer. Secondly, This collecting power, by increasing the size of the instrument, and especially by using a second or smaller instrument of the like sort to collect the electricity from the former, may be augmented to any degree. Thirdly, It is constructed, managed, and preserved with ease and certainty; and it never gives, nor can it give, an equivocal result, as he has proved experimentally, and as will appear by considering its construction.

Plate CLXXVIII. exhibits two perspective views of this collector. Fig. 93 shows the instrument in the state of collecting the electricity; and fig. 94 shows it in the state in which the collected electricity is to be rendered manifest. An electrometer is annexed to each. The letters of reference indicate the same parts in both figures.

ABCD is a flat tin plate, 13 inches long and 8 inches broad; to the two shorter sides of which are soldered two tin tubes AD and BC, which are open at both ends. DE and CF are two glass sticks covered with sealing-wax by means of heat, and not by dissolving the sealing-wax in spirits. They are cemented into the lower apertures of the tin tubes, and also in the wooden bottom of the frame or machine at E and F; so that the tin plate ABCD is supported by those glass sticks in a vertical position, and is exceedingly well insulated. GHILKM and NOPV are two frames of wood, which being fastened to the bottom-board, measuring by means of brass hinges, may be placed so as to stand upright and parallel to the tin-plate, as shown in fig. 94, or they may be opened, and laid upon the table which supports the instrument, as shown in fig. 93. The inward surfaces of those frames from their middle upwards are covered with gilt paper XY; but it would be better to cover them with tin-plates hammered very flat. When the lateral frames stand straight up, they do not touch the tin-plate; but they stand at about one-fifth part of an inch afar. They are also a little shorter than the tin-plate, in order that they might not touch the tin-tubes AD, BC. In the middle of the upper part of each lateral frame is a small flat piece of wood S and T, with a brass hook; the use of which is to hold up the frames without the danger of their falling down when not required, and at the same time it prevents their coming nearer to the tin-plate than the proper limit. It is evident, that when the instrument stands as shown in fig. 94, the gilt surface of the paper XY, which covers the inside of the lateral frames, stands contiguous and parallel to the tin-plate.

When the instrument is to be used, it must be placed upon a table, a window, or other convenient support; a bottle electrometer is placed near it, and is connected, by means of a wire, with one of the tin tubes AD, BC; and by another conducting communication the tin-plate must be connected with the electrified substance, the electricity of which is required to be collected on the plate ABCD: thus, for instance, if it be required to collect the electricity of the rain or of the air, the instrument being placed near a window, a long wire must be put with one extremity into the aperture A or B of one of the tin-tubes, and with the other extremity projecting out of the window. If it be required to collect the electricity produced by evaporation, a small tin pan, having a wire or foot of about six inches in length, must be put upon one of the tin-tubes, so that, the wire going into the tube, the pan may stand about two or three inches above the instrument. A lighted coal is then put into the pan, and a few drops of water poured upon it will produce the desired effect. Thus far may suffice with respect to the mechanical description of the instrument: the power and use of it will be made apparent by the following experiments.

1. Communicate to the tin-plate ABCD a quantity of electricity, for instance, as much as would very sensibly affect a common cork-ball electrometer; then, if the lateral frames GHLM, NOP, stand upright, as in fig. 94, the electrometer W will show no diversity; but if the frames are opened and let down, as in fig. 93, the balls of the electrometer W will immediately repel each other, and by the approach of an excited piece of sealing-wax, the quality of the electricity may be easily ascertained after the usual manner. Put up the lateral frames again, and the electricity will apparently vanish; let them down, and the electricity will re-appear, and so on. If you touch any part of the tin-plate or tin-tubes with your finger, the electricity is thereby entirely removed, and that will be the case whether the lateral frames are up or down.

2. Take Methods of Electricity.

2. Take an extended piece of tin-foil, about four yards square, and, holding it by a silk thread, electrify it so weakly as not to be capable of affecting an electrometer; then bring it in contact with the tin-plate of the collector, whilst the lateral frames are up. This done, remove the tin-foil, let down the lateral frames one after the other; and on doing this the electrometer \( W \) will immediately manifest a considerable degree of electricity. But if the electrometer were to show no sensible degree of electricity, a smaller collector, viz. one having a tin-plate of about four square inches, must be brought into contact with the tin-plate of the large collector, whilst the lateral frames of the latter only are down; and then the small collector being removed from the large one, its lateral frames are opened, and its tin-plate is presented to an electrometer, which will thereby be electrified to a much greater degree than the electrometer \( W \) was by the large collector.

3. Let a common cork-ball electrometer be fastened to an insulated conductor, having about two or three square feet of surface, and communicate to it such a quantity of electricity as may be sufficient to let the balls of the electrometer stand at about one inch asunder. In this state bring the conductor in contact with the tin-plate of the collector for a very short time, and it will be found, that the balls of its electrometer will immediately approach and touch each other, showing that the electricity of the conductor is gone to the plate of the collector; and, in fact, if you let down the lateral frames, the balls of the electrometer \( W \) will immediately repel each other to a very great degree.

It seems, therefore, to be clearly shown by these experiments, that the tin-plate of this instrument can collect and retain a vast quantity of electricity, when the conducting surfaces of the lateral frames are contiguous to it, in comparison to that quantity which it can either collect or retain when those surfaces are removed from its vicinity.

The quantity of electricity which the tin-plate \( ABCD \) is capable of collecting, principally depends on three circumstances, viz. 1st, on the distance between the tin-plate and the conducting lateral surfaces; the smaller that distance is, the greater being the collecting power; 2dly, on the size of the instrument; and, 3dly, on the quantity of electricity possessed by the body from which it must be collected or taken away.

The principle upon which the action of this instrument depends, is the same as that of the electrophorus of M. Volta's condenser, and of many other electrical experiments; namely, that a body has a much greater capacity for holding electricity when its surface is contiguous to a conductor which can easily acquire the contrary electricity, than when it stands not in that situation.

The electrical air thermometer, fig. 34, is an instrument designed to show the power of electricity by its rarefaction of the air through which the fluid passes. But though this instrument in theory might be supposed capable of manifesting the very least degrees of electricity, the rarefaction of the air by its means is so very small, that until the power of electricity be very considerable, no expansion will be perceived. The instrument, however, certainly has its uses, and many curious experiments may be performed with it. \( AB \) represents a glass cylinder having a brass measuring cap, with a wire and knob passing through it, and &c., which is cemented on the open part of the glass. The under part is inverted into a small dish \( BC \), containing quicksilver or some other liquid, which may rise in the small tube \( AH \) by any expansion of the air in the cylinder \( AB \). \( CD \) is an insulating stand, which serves to sustain the whole; \( E \) is a hook by which a communication may be made to the ground; and \( F \) another for connecting the whole with the prime conductor of an electrical machine. The discharges of electricity made by the sparks between the knobs \( G \) and \( I \) expand the air, and force up the fluid into the small tube \( AH \); and its rise there is marked upon a graduated scale. This instrument will likewise answer for showing the diminution or increase of any kind of air by the electric spark, as well as its sudden expansion by a spark or shock of a phial. Mr Morgan has shown that the mercury in a common thermometer, if well made, may be raised by the electric blast.

In a treatise lately published by the Reverend Mr Abraham Bennet, he gives an account of the machine called the doubler of electricity, with some improvements upon it by Mr Nicholson; by which means the machine becomes less liable to the objections of Mr Cavallio above-mentioned. In its improved state, it consists of two insulated and immovable plates about two inches in diameter, and a moveable plate also insulated, which revolves in a vertical plane parallel to the two immovable plates, passing them alternately.

"The plate \( A \) is constantly insulated, and receives the communicated electricity. The plate \( B \) revolves; and when it is opposite the plate \( A \), the connecting wires at the end of the cross piece \( D \) must touch the pins of \( A \) and \( C \) at \( EF \), and a wire proceeding from the plate \( B \) must touch the middle piece \( G \), which is supported by a brass, wooden, or other conducting pillar in connection with the earth. In this position, if electricity be communicated to the plate \( A \), the plate \( B \) will acquire a contrary state; and passing forwards, the wires also moving with it by means of the same insulating axis, the plates are again insulated till the plate \( B \) is opposite to \( C \), and then the wire at \( H \) touches the pin in \( C \), connecting it with the earth, and communicating the contrary state of electricity to that of \( B \), but of the same kind with that of \( A \). By moving the handle still further, \( B \) is again brought opposite to \( A \); and the connecting wires joining \( A \) and \( C \), they both act upon \( B \), which is connected with the earth as before, and nearly double its intensity, whilst the electricity of \( C \) is absorbed into \( A \); because of the increased capacity of \( A \), whilst opposed to \( B \), capable by its connection with the earth of acquiring a contrary state sufficient to balance the influential atmospheres of both plates.

"Thus by continuing to revolve the plate \( B \), the process is performed in a very expeditious and accurate manner.

"The ball \( I \) is made heavier on one side than the other, and screwed upon the axis opposite to the handle, to counterbalance the plate \( B \), which may therefore be stopped in any part of its revolution.

"Yet notwithstanding the convenience and accuracy of this doubler, it always produced spontaneous electricity," Methods of city, even after all the refrinous substances used in its measuring construction had been melted over a candle, and after standing a long time with its plates in connection with the earth. I therefore conjectured that this spontaneous electricity was not owing to accidental friction, but to the increased capacity of approximating parallel plates which might attract and retain their charge tho' neither of them were insulated. To prove my hypothesis, I first endeavoured more effectually and speedily to deprive the instrument of the electricity last communicated, and that I might know whether this spontaneous charge, supposed to arise from the increased capacity of the parallel plates, would be always of the same kind.

"To effect this deprivation, I connected the plates A and C together by a wire hooked at each end upon two small knobs on the backs of the plates, the middle of the same wire touching the pillar which supports the doubler. Another wire was hooked at one end upon the back of the plate B, and at the other end to the brafs ball which counterbalances this plate. Thus all the plates were connected with the earth; and by turning the handle of the doubler, it might be discharged of electricity in every part of its revolution.

"After often trying this method of depriving the doubler, I observed that its spontaneous charge was almost always negative. I then touched A and C with a positively charged bottle, and turned the doubler till it produced sparks for a long time together; and after this strong positive charge, I hooked on the wires as above, and revolved the plate B about 100 times, which so deprived the doubler of its positive electricity, that when the wires were taken off, it produced a negative charge at about the same number of revolutions which it required before.

"The positively charged bottle was again applied; and the wires being hooked upon the plates as before, B was revolved only 50 times; yet this was found sufficient to deprive it of its positive charge, and in many experiments 5 or 6 revolutions were sufficient: but I never thought it safe to stop at so few, and have therefore generally turned the handle 40 or 50 times between every experiment.

"Left electricity adhering to the electrometer should obstruct the above experiments, I did not let it stand in contact with the doubler during its revolutions, but touched the plate A with the cap of the electrometer, after I supposed its electricity was become sufficiently sensible: but left even this contact should communicate any electricity, I made a cap for my electrometer of shell-lac, having a small tin tube in the centre, to which the gold leaf was suspended within the glass, and a bent wire was fixed to the top, which might easily be joined to the plate A of the doubler; and thus the gold-leaf was more perfectly insulated, and the electricity could not be diffused over so large a surface. The glass which insulates the plates and crofs piece of the doubler was also covered with shell-lac."

Fig. 66. shows an instrument invented by Mr Nicholson's instrument for distinguishing the two electricities. A and B are two metallic balls placed at a greater or less distance from each other by means of the joint at C; the two branches CA being made of varnished glass. From one of the balls B proceeds a short point towards the other ball A. If the two be placed in the course or current of the electric matter, so that it may pass through the air from one to the other, its direction will be known. For if the electric matter pass from A to B, there will be a certain distance of the balls dependent on the strength of electricity, within which the dense sparks will pass from the point; but if its course be in the contrary direction, no spark will be seen, unless the balls be almost in contact with the point.

We shall conclude this section with some observations on the electrical kite; which perhaps may after all be found, the only instrument that will certainly show the electricity of the atmosphere upon all occasions. The use of it, however, is very troublesome, as it obliges the observer always to go abroad, which sometimes must be disagreeable. By means of the apparatus represented fig. 72, this inconvenience may be avoided. AB represents the string of the kite, insulated by means of the silk cord C, tied about the foot of a table in the room where the experiments are to be made. This string passes out through a window of the room, and supports the kite; the electricity being conveyed by means of a small wire to the insulated conductor D, having a quadrant electrometer applied to it, as in the figure. G is a glass tube about 18 inches long, with a brafs wire and knob proceeding from it; by taking a small spark with which from the conductor, the quality of the electricity may be observed.

Fig. 68. 69. represent a pocket electrometer, which may be very conveniently used when the atmospheric electricity is collected in any quantity. The case or electrometer handle of this electrometer is formed by a glass tube about three inches long and 1/8th of an inch in diameter, half of which is covered with sealing-wax. From one extremity of this tube, viz. that without sealing-wax, a small loop of silk proceeds, which serves occasionally to hang the electrometer on a pin, &c. To the other extremity of the tube a cork is adapted, which, being cut tapering on both ends, can fit the mouth of the tube with either end. From one extremity of this cork two linen threads proceed, a little shorter than the length of the tube, suspending each a little cone of pith of alder. When this electrometer is to be used, that end of the cork which is opposite to the threads is pushed into the mouth of the tube; then the tube forms the insulated handle of the pith electrometer, as represented fig. 69. But when the electrometer is to be carried in the pocket, then the threads are put into the tube, and the cork stops it as represented fig. 68. The peculiar advantages of this electrometer are, its convenient small size, its great sensibility, and its continuing longer in good order than any other. Fig. 68. represents a case to carry the above described electrometer in. This case is like a common toothpick-case, except that it hath a piece of amber fixed on one extremity A, which may occasionally serve to electrify the electrometer negatively; and on the other extremity it hath a piece of ivory fastened upon a piece of amber BC. This amber BC serves only to infuse the ivory; which when insulated, and rubbed against woollen cloths, acquires a positive electricity, and is therefore useful to electrify the electrometer positively.

In making experiments with the kite, it is sometimes necessary to act with caution, on account of Sect. XI. ELECTRICITY.

Methods of the great quantity of electricity collected by it. Of this measuring we have already given some instances, to which we shall add the following from Mr Bennet; viz., that having on the 5th of July 1788 raised a kite with 200 yards of string, when it had been flying about an hour, a dark cloud appeared at a great distance, and changed the electricity from positive to negative. The electric power increased till the cloud became nearly vertical, when some large drops of rain fell; and our author attempting to secure the string from wet, received such a strong shock in his arm, as deprived it for a few seconds of sensation. The explosion was heard at the distance of about 40 yards, like the loud crack of a whip.

Sect. XI. Of the Effects of Electricity on Vegetation.

It is a very considerable time since electricians began to make experiments on this subject; and it was generally agreed that the electric fluid was favourable to the growth of vegetables. For a long time, however, such researches seem to have been laid aside; nor indeed did it seem very probable that any quantity of the fluid could be collected artificially sufficient to be of use. But in a late treatise the subject has been revived by the Abbé Bertholon; who not only shows a method of collecting the fluid from the atmosphere so as to be useful in ordinary practice, but endeavours to cure by means of this fluid some of those diseases to which plants are liable from insects, and which cannot be removed by any of the ordinary remedies.

"In the first place (says the Abbé), there is continually parts of the atmosphere (particularly in the upper regions) a considerable quantity of the electric fluid. On the mountains especially, it is always felt with most energy, and shows itself in greater abundance than on the plains. On the former, if you erect conductors, or launch electric paper-kites, in order to seek out and direct this fluid towards the surface of the earth, where several causes sometimes prevent its appearance; you will find it very soon subjected to your power, descend, as if at your command, from heaven itself, and creep at your feet to execute your orders. These are facts extremely well ascertained; but if one doubts of them, he has nothing to do but to erect a similar apparatus or set off electric kites to be convinced of the truth. These will immediately and at all times obtain an electricity so much the more strong as the height of the apparatus shall be the more considerable. Being informed, that in England this experiment was tried with the most convincing effect, I mention it, as it has hitherto not been published. Upon a high mountain there were launched two electric paper-kites, one of which was fixed to the inferior extremity of the other, thus gaining a double advantage in point of height; the consequence of which was, that the electric effects were incomparably greater than those produced by a single instrument. But I suppose it entirely useless to insist longer upon a fact so well demonstrated and universally admitted.

"This principle being granted; in order to remedy the deficiency of electric fluid which has already been proved to be hurtful to vegetation, we must erect in the spot which we want to fecundate the following new apparatus, which has had all possible success, and which I shall call by the name of the electro-vegetometer. This machine is as simple in its construction as efficacious in its manner of acting; and I doubt not but it will be adopted by all those who are sufficiently instructed in the great principles of nature.

"This apparatus is composed of a mast AB (Plate CLXXX. fig. 82.), or a long pole thrust just so far into the earth as to stand firm and be able to resist the winds. That part of the mast which is to be in the earth must be well dried at the fire; and you must take care to lay on it a good coat of pitch and tar after taking it from the fire, that the resinous particles may enter more deeply into the pores of the wood, which will then be dilated, at the same time that its humidity will be expelled by the heat. Care must likewise be taken to throw around that part fixed in the earth a certain quantity of coal-dust, or rather a thick layer of good cement, and to build besides a base of masonry of a thickness and depth proportionate to the elevation of the instrument, so as to keep it durable and solid. As to the portion of it above the ground, it will be sufficient to put upon it some coats of oil-paint, except one chooses rather to lay on a coat of bitumen the whole length of the piece.

"At the top of the mast there is to be put an iron-console or support C; whose pointed extremity you are to fix in the upper end of the mast, while the other extremity is to terminate in a ring, in order to receive the hollow glass-tube which is seen at D, and in which there is to be glued an iron rod rising with the point E. This rod, thus pointed at its upper extremity, is completely insulated, by reason of its keeping a strong hold of a thick glass-tube, which is filled with a quantity of bituminous matter, mixed with charcoal, brick-dust, and glass-powder; all together forming a sufficiently good and strong cement for the object in view.

To prevent rain wetting the glass tube, care must be taken to solder to the rod E a funnel of white-iron; which consequently is entirely insulated. From the lower extremity of the rod E hangs a chain G, which enters into a second glass-tube H, supported by the prop I. The lower end of the above-mentioned chain rests upon a circular piece of iron-wire, which forms a part of the horizontal conductor KLMN. In L is a breaker with a turning joint or hinge, in order to move to the right or left the iron-rod LMN; there is likewise another in Q, to give still greater effect to the circular movement. O and P are two supports terminating in a fork, where there is fixed a silk cord tightly stretched, in order to insulate the horizontal conductor; in N are several very sharp iron-points.

"In fig. 83, you see an apparatus in the main like the former, but with some difference in the construction, form of this instrument. At the upper extremity of the mast ab there is bored a hole into which enters a wooden cylinder c; which has been carefully dried before a great fire, in order to extract its humidity, dilate its pores, and saturate it with tar, pitch, or turpentine, applied at repeated intervals. The more heat the wood and bituminous matter receives, the more the substance penetrates, and the insulation will be the more complete. It is moreover proper to befurnish the circumference of the little cylinder with a pretty thick coat of bitumen. This preparation being made, we next insert the cylinder...

Effects of Electricity on Vegetation.

Into the hole b of the mast; and it is easy to join together these two wooden pieces in the most perfect manner.

"At the upper extremity of the cylinder c we strongly attach an iron-rod g f; which, instead of one, is terminated by several sharp points all of gilded iron. In e you see a branch of iron resembling the arm of an iron-crow, from whence hangs an iron chain h i, at the end of which there is hooked a piece of iron resembling a mason's square, and ending in a fork. The piece of iron l is a ring with a handle entering a little into the glass-tube m filled with mastic, in the same manner as does the iron-rod n. The conductor p o is to be considered as an additional piece to act in that marked p. There are likewise put iron-spikes in q: the support s resembles those of O and P in the former figure. In this new machine you can lengthen or shorten the horizontal conductor as you please; and as the iron-ring l turns freely in a circular gorge made in the mast, the conductor is enabled to describe the entire area of a circle.

"The construction of this electro-vegetometer once well understood, it will be easy for us to conceive its effects. The electricity which prevails in the aerial regions will soon be drawn down by the elevated points of the upper extremity. This effect of the points is proved by the most decisive experiments, and is called by philosophers the power of the points.

"The electric matter brought down by the point E, or by those marked f f f, will be necessarily transmitted both by the rod and chain; because the insulation produced at the upper extremity of the mast completely prevents its communication with the timber. The electric fluid passes from the chain to the horizontal conductor K M or n o; it then escapes by the points at P and q; because the same points that have the power of bringing down the electric fluid, have likewise that of pushing it forward; a thing well known by experience.

"The manner of using this instrument is not more difficult than the knowledge either of its conductor or effects. Suppose, for example, we are to place it in the midst of a kitchen-garden. By making the horizontal conductor turn round successively, you will be able to carry the electricity over the whole surface of the proposed ground. The electric fluid thus drawn down, will extend itself over all the plants you want to cultivate; and this at a time when there is little or no electricity in the lower regions near the surface of the earth.

"On the other hand, when it happens that the electric fluid shall be in too great abundance in the atmosphere, in order to take off the effect of the apparatus in K fig. 82, and in n fig. 83, you have only to hang to it an iron chain reaching to the ground, or else a perpendicular iron-rod, which will have the same effect, viz. that of destroying the insulation, and of insensibly transmitting the electric fluid in the same proportion as it is drawn by the points; so that there shall never be an overcharge of this fluid in the instrument, and its effect shall be either something or nothing, according as you add or remove the second chain or the additional rod.

"There will be nothing to fear from the spontaneous discharge of this apparatus, because it is terminated below by proper points in P and q of both machines; and it is a certain fact, that a pointed conductor makes no explosion, and that instead of flashes there are only luminous streams. However, it will be easy to furnish one, by means of which we may approach the apparatus with perfect security; it is only necessary to hold the hand before it. This has the form of a great C, and is of a height equal to the distance that takes place betwixt the horizontal conductor and the surface of the earth. This discharger near the middle must be furnished with a glass-handle; and at that extremity which is directed towards the conductor, there must hang an iron-chain made to trail on the ground. This instrument is an excellent safeguard. See fig. 84.

"By means of the electro-vegetometer just now described, one may be able to accumulate at pleasure advantages from this wonderful fluid, however diffused in the regions above, and conduct it to the surface of the earth, in these seasons when it is either scantily supplied, or its quantity is insufficient for vegetation; or although it may be in some degree sufficient, yet can never produce the effects of a multiplied and highly increased vegetation. So that by these means we shall have an excellent vegetable manure or nourishment brought down as it were from heaven, and that too at an easy expense; for after the construction of this instrument, it will cost you nothing to maintain it: It will moreover be the most efficacious you can employ, no other substance being so active, penetrating, or conducive to the germination, growth, multiplication, or reproduction of vegetables. This heavenly manure is that which nature employs over the whole habitable earth; not excepting even those regions which are esteemed barren, but which, however, are often fecundated by those agents which nature knows so well to employ to the most useful purposes. Perhaps there was nothing wanting to bring to a completion the useful discoveries that have been made in electricity, but to show this so advantageous an art of employing electricity as a manure; consequently, that all the effects which we have already mentioned, depend upon electricity alone; and lastly, that all these effects, viz. acceleration in the germination, the growth, and production of leaves, flowers, fruit, and their multiplication, &c. will be produced, even at a time when secondary causes are against it; and all this is brought about by the electric fluid, which we have the art of accumulating over certain portions of the earth where we want to raise those plants that are most calculated for our use. By multiplying these instruments, which are provided at no expense (since iron-rods of the thickness of one's finger, and even less, are sufficient for the purpose), we multiply their beneficial effects, and extend their use ad infinitum.

"This apparatus having been raised with care in the midst of a garden, the happiest effects were perceived, viz. different plants, herbs, and fruits, in greater forwardingness than usual, more multiplied, and of better quality. At the same time it was observable, that during the night, the points P and q, as well as the upper extremities, were often garnished with beautiful luminous sparks. These facts are analogous to an observation which I have often made, viz. that plants most vigorous best and are most vigorous near thunder-rods, thunder-rods where their situation favours their development. They ke- likewise serve to explain why vegetation is so vigorous in lofty forests, and where the trees raise their heads far from the surface of the earth, so that they seek, as it were, the electric fluid at a far greater height than plants less elevated; while the sharp extremities of their leaves, boughs, and branches, serve as so many points granted them by the munificent hand of nature, to draw down from the atmosphere that electric fluid, which is so powerful an agent in forwarding vegetation, and in promoting the different functions of plants.

"This electro-vegetometer may be set up not only in a kitchen-garden, but in an orchard, in a field of corn, olive-yard, &c. &c. Everywhere the same effects are produced, namely, fecundity in the soil, quickness of vegetation, increase of produce, superiority in the quality, &c. This machine is applicable to all kinds of vegetable productions, to all places, and all seasons; and if I am to believe the most enlightened and intelligent of my friends, the electro-vegetometer is one of the most noble and useful discoveries that have been made in the present century.

Besides the advantages of the electro-vegetometer, of which we have been speaking above, there is still another very important one, namely, that by applying it to a large electrometer or grand conductor, we may thus find out the electricity of the atmosphere. For this purpose we must take away the points HR (fig. 82, and r, fig. 83.) which are seen in Rr. This machine will likewise serve the purpose of a thunder-rod, if one takes care to thrust into the earth, to the depth of about 10 or 15 feet, a leaden tube, whose upper extremity may rise a few inches above the surface of the ground; and into this tube you are to pass the long iron chain or perpendicular rod set apart for destroying the insulation, and whose upper end is to be hooked to a chain in H, fig. 82, or in k, fig. 83. These two chains are very strong, and are fit for serving as an excellent conductor. Or if you choose, you may substitute in their room wools of white thread, or iron-wires, which will make no difference in the effects of the apparatus. In the figures we have preferred chains, that the distinction of the different parts may be the more sensibly perceived. With these additions the electro-vegetometer will be as good a thunder-rod as any that are ordinarily constructed.

It is not only by means of the electricity in the atmosphere, collected by the above apparatus, that one can supply the electric fluid, which is so necessary to vegetation; but the electricity named artificial answers the same purpose. However astonishing the idea may be, or however impossible it may appear to realize it, yet nothing will be found more easy upon trial. Let us suppose that one wants to augment the vegetation of trees in a garden, orchard, &c. without having recourse to the apparatus destined to pump down as it were the electricity from the atmosphere, it is sufficient to have a large insulating stool. This may be made in two ways; either by pouring a sufficient quantity of pitch and melted wax upon the above stool, whose borders being more raised than its middle, will form a kind of frame, or more simply, the stool (which is likewise called the insulator) shall only be composed of a plate longer than broad, supported by four glass-pillars, like those used for electrical machines. One must take care to place above the insulator a wooden tray full of water, and to cause mount upon the stool a man carrying a small pump in the form of a syringe. If you establish a communication between the man and an electrical machine put in motion (which is easily done by means of a chain that connects with the conductor of the machine), then the man thus insulated (as well as everything upon the stool) will be able, by pushing forward the sucker, to water the trees, by pouring upon them an electrical shower; and thus diffusing over all the vegetables under its influence a principle of fecundity that exerts itself in an extraordinary manner upon the whole vegetable economy; and this method has moreover this advantage, that at all times and in all places it may be practised and applied to all plants whatever.

Every one knows that the electricity is communicated to the water thus employed; and it would be easy to obtain the most ample conviction (if any one doubted it), by receiving upon his face or hand this electrical shower; he immediately feels small punctures or shocks, which are the effects of the sparks that issue from each drop of water. This is perceived most sensibly if there is presented a metal-dish to this electrical dew; for at the very instant of contact, brilliant flashes are produced.

That the electricity received by the man from the chain may be communicated to the tray, we must put a small cake of white-iron, upon the end of which he may place his foot. The tray filled with water is a kind of magazine or reservoir to serve as a continual supply to the pump. After watering one tree, you transport the stool to a second, a third, and so on successively; which is done in a short time, and requires very little trouble.

Instead of the chain, it is better to employ a cord or twist of pinchbeck or any other metal; by means of which there can be no loss of the electric matter, as there is in the case of the chain by the ring-points. Moreover, this metal cord or thread being capable of being untwisted and lengthened, there will be no occasion of transporting so often the electrical machine. It is almost needless to add, that this string or metallic chord, which is always insulated, may rest upon the same kind of supports with those which have been exhibited in OP and s of fig. 82. and 83. This method is simple, efficacious, and nowise expensive, and cannot be too much employed.

If one wants to water either a parterre or common garden-beds and platforms of flowers, or any other plots in which are sown grain or plants of different ages and kinds, no method is more easy and expeditious than the following: Upon a small carriage, with two wheels there is placed a framed insulator in form of a cake of pitch and rosin, as we have mentioned before in fig. 82. The carriage is drawn the whole length of the garden by a man or horse fixed to it. In proportion as you draw the carriage, the metallic cord winds itself upon a bobbin, which turns as usual. This last is insulated, either because the little apparatus that sustains the bobbin is planted in a mass of rosin (when you choose the axle to be of iron), or else because this this moveable axis is a tube of solid glass. There must also be a support, which serves to prevent the gold-thread or the metallic-cord from trailing on the ground, and thus dissipating the electricity; and, moreover, it serves as an insulator. To accomplish this last purpose, it is necessary that the ring into which it passes be of glass. One may likewise employ the insulators and supports marked OP and r, in figs. 82. and 83. If a gardener, mounted upon an insulator, holds in one hand a pump full of water, and with the other takes hold of a metallic-cord, in order to transmit the electricity which comes from the conductor; in this case, the water being electrified, you will have an electrical shower; which falling on the whole surface of the plants which you want to electrify, will render the vegetation more vigorous and more abundant. A second gardener is to give additional pumps full of water to him who is upon the insulator, when he shall have emptied those he holds; and thus in a little time you will be able to electrify the whole garden. This method takes hardly longer time than the ordinary one; and although it should be a little longer, the great advantages resulting from it will abundantly recompense the small additional trouble.

"By repeating this operation several days successively, either upon seed sown or plants in a state of growth, you will very soon reap the greatest advantages from it. This operation, equally easy with the preceding described upon the subject of watering trees, has been put in practice with the greatest success. Several other methods, answering the same purpose, might be devised; but they are all of them pretty similar to that just described.

"I cannot finish this article without mentioning another method relative to the present object, although it be much less efficacious than the preceding ones. It consists in communicating to water kept in basins, reservoirs, &c. (for the purpose of watering), the electric fluid, by means of a good electrical machine. To this end, one must plaster over with a bituminous cement all the interior surface of the basin destined to receive the water that serves for irrigation: the nature of this cement answering the purpose of insulation, will prevent the electric fluid that communicates with the water from being dissipated; and the water thus charged with electricity will be the more fitted for vegetation.

"The method just now laid down of electrifying water for the purpose of watering trees is both easy and cheap; the expense of the cement being inconsiderable, as it requires but once to be done, and as it prevents the water from filtrating and being lost, as well as from hurting the walls themselves, which would otherwise have occasion to be oftener repaired; consequently you are sufficiently indemnified by its utility for all the trouble you take. A machine applied to the extremity of the axle of the electric apparatus might communicate to it a rotatory movement, and still further diminish the expense of the operation.

"If the deficiency of the electric fluid, or rather a small quantity of it, is apt to be hurtful to vegetables, a too great abundance of this matter will likewise sometimes produce pernicious effects. The experiments made by Messrs Nairne, Banks, and other learned men of the Royal Society of London, prove sufficiently this truth. An electric battery, very strong, was discharged upon a branch of balsam still holding its trunk. Some minutes after, there was observed a remarkable alteration in the branch, of which the less woody parts immediately withered, drooped towards the ground, died next day, and in a short time entirely dried up; at the same time that another branch of the same plant that had not been put under the electric chain, was not in the smallest degree affected.

"This experiment repeated upon other plants showed the same effects; and it was remarked that the attraction, occasioned by a strong discharge of the electricity, produced an alteration different according to the different nature of the plants. Those which are less woody, more herbaceous, more aqueous, experience in proportion impressions that are stronger and much more speedy in their operation.

"A branch of each of the following plants, composing an electrical chain, it was observed by these able philosophers, that the balsam was affected by the discharge of the battery in a few moments after, and perished next day. The leaves of a marvell of Peru did not drop till the day following that; and the same phenomenon happened to a geranium. Several days elapsed before there was observed any fatal effect on the cardinal flower. The branch of a laurel did not show any symptoms till after the lapse of about 15 days, after which it died; but it was a full month before they perceived any sensible change on the myrtle; at the same time they constantly observed that the bodies of those plants and branches which had formed no part of the chain, continued to be fresh, vigorous, and covered with leaves in good condition.

"It hardly ever happens that the superabundance of the electric fluid existing in a small portion of the atmosphere where a plant is situated, can be so great as that which took place by the explosion of the strong battery of Mr Nairne, directed particularly upon one branch; or if this should happen, it can only be upon a few individual plants in very small number; as when blasting or lightning falls upon a tree, breaks it, strips it of its mildewy bark, or withers its leaves; or in the case of blasting or poled to be mildew in corn, which several farmers ascribe to the force of lightning. "This sentiment (says M. du Hamel) has acquired much probability since the discovery of the great effects of that electricity which is diffused so abundantly in the atmosphere when the weather is disposed to be stormy." (Elements d'Agric. Tom. I. p. 346.)

"It is not proposed here to prescribe the means of remedying the pernicious effects which may be produced upon this occasion; as there are none of them in circumstances exactly similar to that of the experiments of the philosopher just now quoted. But although this enormous excess of the electric fluid of which we have been speaking, never takes place through any great extent of space, nevertheless this excess, though even but inconsiderable, may be too great in several respects regarding the vegetable economy; and it is in this case that it is proper to seek the means of remedying it.

"Let us suppose that one has some plants or shrubs, or..." or some valuable trees or exotics that he wants to preserve, and is sensible that too great a quantity of electricity predominant in the atmosphere becomes hurtful to them, there are two methods that may serve to obviate the evil of which he is apprehensive. One is, to water plentifully these vegetables, so that their whole surface may be kept sufficiently wet; the consequence of which is, that the electricity prevailing in the atmosphere will be transmitted to the earth by the water adhering to the outside of the plants, as it is well known that water is an excellent conductor of the electric fluid: The other is, to place near these trees metallic points, which may be easily accomplished by simple lathes or wooden-poles; along which one must fallen by bandages plain iron-wires, so as to overtop them by some inches. These poles thus prepared, being thrust into the earth, will then draw down the electric fluid, and transmit it to the earth."

Our author now proceeds to consider of methods of destroying the insects which frequently infest and destroy vegetables; which, he thinks, may be obtained by means of the electric fluid.

"Experience (says he) proves, that in years when vegetation is most vigorous and abundant, insects, if nothing opposes them, will then be most multiplied; and in fact they are sometimes so to an astonishing degree. How great mischief they produce on these occasions, every body knows, and as ardently desires to find a remedy for the calamity. The damage is indeed so considerable, that people imagine it is not possible by any means to put a stop to it; but I am of opinion, it is one of those evils to which electricity may be applied with effect.

"It has been often remarked, that several species of worms or caterpillars are found in the heart of shoots, twigs, and even the trunks of trees, of shrubs, and of plants of different sorts. There are numbers, for example, in pear and other fruit trees. As soon as the animal has got to the inside of a branch, he forms a gallery according to the length of it; armed with strong scaly jaws, he soon reduces the woody substance to powder; and this same delicate caterpillar makes the wood, hard as it is, his favourite nourishment. Other insects generally show themselves in open day; but this one, like a pioneer, marches always in obscurity within; and we are apprised of his presence only by the mischief he produces, namely, by obliterating the tops of branches to wither, the leaves to fade and incline to the earth, and in fine the whole infected bough to decay and die away. In vain do you seek for this frail though terrible animal on the leaves; he enters the skin and penetrates the thickest bark of the surface; he goes even to the heart of the woody substance; and you can extirpate him only by cutting off the wood; and if this is a remedy, you must confess that it is at least equal to the mischief.

"This evil so much the more merits attention, that it extends itself particularly over a very great number of fruit-trees; in which, for the same reason, we are as particularly interested. Electricity, however, furnishes us with a remedy of the most efficacious sort to stop the progress of the evil, by attacking the enemy in his quarters, and destroying him in his own mine; which in this event is to become his tomb.

Vol. VI. Part II.

"The Leyden phial, by the mere force of its shock, Effects of which can be augmented gradually, is capable of destroying not only rabbits and pigeons, but bulls and oxen, especially when we employ electrical batteries of great size, and containing a great number of electrified jars. Of consequence then it may be employed even with little apparatus to kill a tender and delicate caterpillar, which, in order to shelter itself from the impressions of the air, is obliged to keep shock perpetually shut up in the heart of trees, or in that of twigs, branches, or trunks themselves.

"In order to succeed in killing these animals at the time when they begin to show their ravages, which mark likewise the place where the caterpillar is concealed, it is sufficient to make an electric chain with two plain iron-wires, and to place betwixt the two that part of the tree where it is supposed the insect resides. One need not be afraid of taking in even a larger space, for the experiment will succeed as well in a greater extent as in a small; and besides, one runs no risk of missing the enemy he wants to combat. Let us suppose, that one be afflicted from the aforementioned symptoms, that there is an insect in the tree; in this case you place iron-wires above and below the place where you suspect it to be lodged. Next, you must take care to make the one communicate with the exterior surface of an ordinary jar charged with electricity, and the other with the interior surface, which it is easy to do by bending the iron-wires so as to make them approach the electrical jar; then upon discharging this vessel where the electric fluid superabounds, the explosion is made to traverse the part where the animal lodges: the violence of the shock makes him die without recovery, and so destroys the evil in its source. If the ravage has not been carried to a high pitch, the tree recovers very soon, as I have often observed; but whatever be the result as to the re-establishment in certain circumstances, the evil proceeds no further; its progress stops; and it is always a great advantage to have arrested it in its march.

"Several experiments have convinced me of the success of this method. Upon cutting off several branches on which I discharged my jar or Leyden bottle, I constantly observed the animal dead; and you never fail of killing it when the distance betwixt the two extremities of the iron-wires is not too great, and when you take care to approach or remove them successively by repeating the shock several times.

"The bottle here employed cannot hurt the vegetable economy, because its dimensions are not too great, and no batteries are brought in play. The electric shock, given in certain bounds, is useful to animals; it therefore cannot be noxious to plants in the same circumstances.

"This operation is not tedious even when employed upon a great number of trees; but if one wants still further to abridge it, I here give him a method by which the experiment can be made in the same instant upon all the trees of an orchard, and will not be more tedious than if it were employed upon one at once.

You have only to provide a sufficient number of iron-wires, and to dispose them as was done for the first tree we spoke of just now, and in the same manner; by which means all these trees form an elec- trical chain, and the fluid in the explosion of the bottle will run over through the whole, supposing that you have discharged the bottle in the ordinary way, and at the same time taken care of what is very essential, that while the free extremity of the first wire touches the exterior surface of the electrical jar, the end of the other may communicate with the inside of the same charged phial.

"If the caterpillar be in the root, the operation is pretty much the same. By taking away, for an instant, a little earth, you easily put the affected roots within the chain; but if one is ignorant of the particular ramification of the root which is attacked, without uncovering the tree, you need only insert in the earth two wires opposite in their directions, and then perform the Leyden experiment, which is easily done. After having placed these two wires north and south, you may repeat the experiment by placing them east and west. You can hardly then miss the insect, especially if, in order to take in more space, you insert one of the wires farther than the other: for in this case the electric fluid will describe a diagonal, as we have already shown in regard to branches.

"This method serves not only to prevent the progress of the evil, but in some degree to anticipate it. In regard to these destructive insects there are epochs as for plants; both of them have marked times for their birth, their development, their growth, their multiplication, and that relative both to their genera and species. When the time is come that insects, caterpillars, and other animals attack plants, one must employ, by way of precaution, the method we have just now laid down; and by repeating the same from day to day for a certain space of time, we will at last succeed in preserving trees from the ravages of insects. The operation is neither tedious nor expensive; why then not recur to it for those curious and rare trees which come from afar at a great expense, and those valuable other trees that yield us yearly the most delicious fruits?

"The method just mentioned is the most effectual that can be imagined, since it pursues the enemy to his most concealed corners in the inmost texture of the wood, and is capable of killing him in the very heart of trees, under the bark when he is to be found there, in the branches, and in the heart of the roots themselves: all which we have made appear in the foregoing remarks. I may further add, that there is no other remedy known but by electricity; for how is it possible to find out under the bark of a tree one or more insects that gnaw and destroy it? Must we not in this case strip the entirely of their bark? and would not, therefore, the remedy be often worse than the disease? Besides, by what means could we penetrate into the heart of the tree? Would not the instrument employed to cut and lop it, rather add to the mischief, especially in the beginning of its progress? How again could we rummage to the inside of the roots? The tree thus uncovered, would it not suffer, especially in the great heats, when a perspiration more abundant must render necessary a nourishment, whose quantity ought at all times to be equal at least to the waste? Thus the celebrated Linnæus, struck with the calamities which fruit-trees in particular suffer from insects and their effects of caterpillars, cried out: "Who shall deliver us from Electricity this scourge?" Quis possit liberare arbores fructiferas larvis?"

On this subject we cannot help observing, that there is some reason to suppose that the Abbé has over-rated the power of his remedy with regard to the destruction of insects. There is not the least doubt that an insect will be destroyed by finding a shock of electricity through its body; but while this insect is defended by the vegetable which it has pierced, and in which it lodges, the vegetable will also receive a very considerable part; and thus the insect may still escape, unless the shock be augmented to such a degree as to injure the vegetable also. His other experiments, it is said, have been confirmed by the observations of modern electricians.

**Sect. XII. Effects of Electricity on Animals; of the Gymnotus, Torpedo, and other Electric Fishes; Medical Electricity.**

Soon after the discovery of the electrical shock, and the method of augmenting the power of electricity, it naturally became an object with philosophers to investigate the effects of it upon animal bodies. These were quickly found to be entirely similar to such as are produced upon any other conducting substances, viz. an emission of sparks, attraction, and repulsion, &c. By degrees it was found, that very strong signs of electricity were exhibited by some animals, even without the application of any artificial apparatus. The experiment of producing sparks by stroking some animal of a cat in frothy weather, readily showed that the electric fluid may exist in a very active state in the body of an animal without injuring any of its functions. From animals of the inferior kind a transition was made to the human species; and signs of electricity were discovered in them where it had not been suspected before. Some people have been remarkable for an extreme lustre of their eyes; and others have been so much electrified naturally, as to give evident signs of it when a sensible electrometer had been applied to them. Others have manifested an extreme sensibility of even the smallest degrees of electricity, inasmuch that they would be affected by a flash of lightning, though so remote that the thunder could not be heard. All this showed that the subtle fluid we treat of bears a very active part in the animal economy, and led to more important researches on the subject. One of the first discoveries was, that some creatures are so strongly electrified naturally as to have it in their power to give a strong shock at pleasure, capable of destroying any small animal that comes near them. Of these, however, only three, and those of the aquatic kind, have yet been observed, viz. the gymnotus electricus, the torpedo, and another called the *sturis electricus*.

The gymnotus* hath the astonishing property of giving the electric shock to any person, or number of persons, article either by the immediate touch with the hand, or by the Gymnotus, mediation of any metallic conductor; and a person who kept some of them told Dr Garden, that they had this property much stronger when first caught than afterwards. The person (says he) who is to receive the shock, must take the fish with both hands, at some considerable distance apart, so as to form the communication; otherwise he will not receive it, at least I never saw any one shocked from taking hold of it with one hand only; though some have assured me, that they were shocked by laying one hand on it. I myself have taken hold of the largest with one hand often without ever receiving a shock; but I never touched it with both hands, at a little distance apart, without feeling a smart shock. I have often remarked, that when it is taken hold of with one hand, and the other is put into the water over its body without touching it, the person received a smart shock; and I have observed the same effect follow when a number joined hands, the person at one extremity of the circle taking hold of or touching the fish, and the person at the other extremity putting his hand into the water over the body of the fish. The shock was communicated through the whole circle as smartly as if both the extreme persons had touched the fish. In this it seems to differ widely from the torpedo, or else we are much misinformed of the manner in which the numbing effect of that fish is communicated. The shock which the gymnus gives seems to be wholly electrical; and all the phenomena or properties of it exactly resemble those of the electric aura of our atmosphere when collected, as far as they are discoverable from the several trials made on this fish. This stroke is communicated by the same conductors, and intercepted by the interpolation of the same original electrics, or electrics per se as they used to be called. The keeper of this fish informs me, that he catches them in Surinam river, a great way up, beyond where the salt-water reaches; and that they are a fresh-water fish only. He says, that they are eaten, and by some people esteemed a great delicacy. They live on fish, worms, or any animal-food if it is cut small so that they can swallow it. When small fishes are thrown into the water, they first give them a shock, which kills or stupefies them, that they can swallow them easily and without any trouble. If one of these small fishes, after it is shocked, and to all appearance dead, be taken out of the vessel where the electrical fish is, and put into fresh water, it will soon revive again. If a larger fish than they can swallow be thrown into the water, at a time that they are hungry, they give him some smart shocks till he is apparently dead, and then they try to swallow or suck him in; but, after several attempts, finding he is too large, they quit him. Upon the most careful inspection of such fish, I could never see any mark of teeth, or the least wound or scratch on them. When the electrical fish are hungry, they are pretty keen after their food; but they are soon satisfied, not being able to contain much at one time. An electrical fish of three feet and upwards in length cannot swallow a small fish above three or at most three inches and a half long. I am told, that some of these have been seen in Surinam river upwards of 12 feet long, whose stroke or shock proved instant death to any person that unluckily received it.

Several other accounts of this fish have been published by different persons, but none of them so full and distinct as the above. They all agree that the electric virtue of the fish is very strong. Mr. Fermin, in his natural history of Surinam, published in 1765, tells us, that one cannot touch it with the hands, or even with a stick, without feeling a horrible numbness in the arms up to the shoulders; and he farther relates, that, making 14 persons grasp each other by the hands, while he grasped the hand of the last with one of his, and with the other touched the eel with a stick, the whole number felt so violent a shock, that he could not prevail on them to repeat the experiment. V. Vanderloot, in two letters from Rio de Janeiro, dated in 1761, makes two species, the black and the reddish; though he acknowledges, that, excepting the difference of colour and degree of strength, they are not materially different. In most experiments with these animals, he remarks a surprising resemblance between them and an electrical apparatus: nay, he observed, that the shock could be given to the finger of a person held at some distance from the bubble of air formed by the fish when he comes to the surface of the water to breathe; and he concluded, that at such times the electrical matter was discharged from his lungs. He mentions another characterizing circumstance, which able difference, that though metals in general were conductors of its electric property, yet some were found to be far more conducting powerfully than others for that purpose. Of this metals with property Dr. Priestley takes notice, and says, that a regard to gold ring is preferable to any thing else. The same shock is likewise observed by Linnaeus. Dr. Priestley adds, that the sensation is strongest when the fish is in motion, and is transmitted to a great distance; so that if persons in a ship happen to dip their fingers or feet in the sea, when the fish is swimming at the distance of 15 feet from them, they are affected by it. He also tells us, that the gymnus itself, notwithstanding all its electric powers, is killed by the lobster.

The surprising property of the torpedo* in giving a violent shock to the person who takes it in his hands, or feet, who treads upon it, was long an object of wonder. For some time it was in general reckoned to be entirely fabulous; but at last the matter of fact being ascertained beyond a doubt, philosophers endeavoured to find out the cause. M. Reaumur resolved it into which the action of a vast number of minute muscles, which by their accumulated force gave a sudden and violent stroke to the person who touched it. But solutions of this kind were quite unsatisfactory, because the stroke was found to be communicated through water, iron, wood, &c. When the phenomena of electricity began to be better known, it was then suspected that the shock of the torpedo was occasioned by a certain action of the electric fluid; but as not the least spark of fire, or noise, could ever be perceived, this too seemed insufficient. Of late, however, Mr. Walsh has with indefatigable pains, not only explained this surprising phenomenon on the known principles of electricity, but given a demonstration of his being in the right, by constructing an artificial torpedo, by which a shock resembling that of the natural one can be given.

The electric organs of the torpedo consist of two sets of very small cylinders lying under the skin, one of which is electrified positively and the other negatively, tively, seemingly at the pleasure of the fish. When a communication is made between the set of cylinders positively electrified and those which are negatively so, a discharge and shock ensue, like what happens in the case of the Leyden phial. The only difficulty now is to account for the total absence of a spark (which in the case of the torpedo never exists even in the smallest degree), and the impossibility of conducting the shock through the smallest interval of air. But this also is explained in a satisfactory manner by Mr. Walsh, and shown to be nothing else than what every day takes place in our electrical experiments. It is well known, that a small charge of electricity, if put into a little phial, will occasion a bright spark and loud noise when discharged; but if the same charge is put into a phial much larger, the spark and noise will be less in proportion; neither will the spark break through near such a space of air in the latter case as in the former; though the shock would in both cases be the same to a person who received it through his body. If, instead of a large phial, we suppose the charge to be diffused all over a large battery, the shock would still be the same, and yet the spark and noise attending it would be almost imperceptible. The case is just the same with the torpedo. Each of the electric organs is a battery composed of innumerable small cylinders, which discharging themselves all at once produce a formidable shock; but by reason of the smallness of the charge of each, the spark is imperceptible, and cannot break through the least space of air. The truth of this was exemplified in Mr. Walsh's artificial torpedo, which though it would give a very considerable shock through a conductor totally uninterrupted, yet on the least breach therein, even for the breadth of a hair, no shock was felt.

In every other respect the electricity of the torpedo agrees with that exhibited by the common electrical machines. An insulated person cannot receive a shock by touching one of the electric organs of the fish; but a violent stroke is given to the person, whether insulated or not, who lays one hand on the positive and the other on the negative organ. The fish, as is reasonable to imagine, seems to have this electric property in its own power; and appears sensible of his giving the shock, which is accompanied by a kind of winking of his eyes.

The third fish which is known to have the power of giving the shock, is found in the rivers of Africa, but we have a very imperfect account of its properties (o). This animal belongs to the order called in Willoughby's system filurus; hence it is commonly called filurus electricus. Some of these fishes have been seen even above 20 inches long. The body of the filurus electricus is oblong, smooth, and without scales; being rather large, and flattened towards its anterior part. The eyes are of a middle size, and are covered by the skin which envelopes the whole head. Each jaw is armed with a great number of small teeth. About the mouth it has six filamentous appendices, viz. four from the under lip and two from the upper; the two external ones, or farthermost from the mouth on the under lip, are the longest. The colour of the body is greyish, and towards the tail it has some blackish spots. The electric organ seems to be towards the tail, where the skin is thicker than on the rest of the body; and a whitish fibrous substance, which is probably the electric organ, has been distinguished under it. It is said that the filurus electricus has the property of giving a shock or benumbing sensation like the torpedo, and that this shock is communicated through substances that are conductors of electricity; but no other particular about it is known with any considerable degree of certainty.

An inquisitive mind will immediately ask, for what purpose has nature furnished these animals with so singular a property? But the present knowledge of the subject seems to furnish no other answer, except that they are endowed with the power of giving the shock for the sake of securing their prey, by which they must subsist, and perhaps of repelling larger animals which might otherwise annoy them.

The ancients considered the shocks given by the torpedo as capable of curing various disorders; and a modern philosopher will hardly hesitate to believe their assertions, after that electricity has been found to be a remedy for many diseases.

Besides these animals which manifest their electric power evidently by giving a strong shock, there are others in which the fluid seems to act by the emission of light. This indeed has not been proved by actual experiment, tho' it would certainly be well worth while to try whether by inflating a number of them, any more evident signs of electricity could be obtained. These creatures are of the insect tribe; some of them furnished with wings, as the shining flies in the warm countries; while others, as the glow-worm, crawl perpetually on the earth. It is most probable also, that the sparkling of sea-water is owing to the electricity of the insects which occasion it. Be this as it will, however, from the instances already adduced, it is certain that the electric fluid pervades at all times the whole body of every animal; whence, by exciting or diminishing its action, it is reasonable to suppose that many important changes might be made in the human body, and hence the foundation of Medical Electricity.

Though the effects of this fluid as a remedy for different cases fall particularly to be mentioned under the article Electricity, Medicine, we cannot help here taking notice, that a very strange uncertainty remains concerning what we should imagine to be its first and most obvious effects; namely, whether simple electrification has any effect in quickening and augmenting the pulse? This was laid to be the case by the first electricians, but denied by their successors; and even when the great machine at Haarlem is made use of, it still remains doubtful whether there be any effect of this kind or not.

The shock of the Leyden phial having been found effectual in removing some complaints, the use of it was introduced into the common practice of medicine; and is still continued, though a more gentle method of using the fluid is now generally preferred. The apparatus for the medical electrician, besides the machine already described, consists of the following parts. 1. An apparatus for the infusion of medicines.

(o) Meiss Adamson and Forskal make a short mention of it; and M. Broussonet describes it under the French name of le Trembleur in the Hilt, de l'Academie Royale des Sciences for the year 1782.

Medical Electricity.

Sect. XII.

Medical insulating stool with glass feet, or, what is much better, an arm chair, well rounded at the edges of the wooden parts, and fixed on a large stool with glass feet, which should be at least nine or ten inches in length; for the longer the feet are, the better will the insulation be. The inside part of the back of the chair should move on a hinge, that it may occasionally be let down to the stool, and so the back of the patient be electrified more conveniently; the arms of the chair should be made longer than ordinary. 2. A Leyden bottle with a discharging electrometer. 3. A pair of directors of considerable size, with glass handles and wooden points. 4. A large metallic ball of brass or copper, with a metallic handle to receive the sparks. The ball should be unsecured, and the wire long and sharp pointed to receive the stream of electric fire. 5. A few glass tubes of different bores, some of them with capillary points. 6. Several yards of brass wire or chain; or, which is much better, several lengths of wires with loops at the end; the part of the wire between these being covered with some non-conducting substance, as a silk ribbon, &c.

The directors are represented by fig. 29, the handles being of glass, one of them having a ball on its end represented by A; the other is without the ball, having its wire bent for the convenience of conducting the electric stream on the eye, &c. Either of the balls may be unsecured from the wires, and the wooden point B screwed in its place, or the pointed end of the brass wire used. The glass handles should be held as far from the brass work as possible. To convey the electric fluid to the ear or throat, glass tubes with sliding brass wires through them should be made use of, such as are represented in fig. 30.

Fig. 31, 32, represent the electric forceps, which is thought by some electricians to be more convenient for giving the shock than the directors. Fig. 33 is the medical jar, with an electrometer, that regulates the strength of the shock, and enables the operator to give a succession of them nearly equal force. On the upper part of a bent piece of glass C is cemented a brass socket D, which is fastened to a spring tube E; a wire F moves in this tube, so that the ball G may be set at any required distance from the ball H. The end I of the bent piece of glass is also cemented to a spring tube, which slides upon the wire K, communicating with the inside of the jar.

To use this medical jar, the ball H must be placed in contact with the conductor of the electrical machine, or at least be connected with it by a wire; after which it is to be charged in the usual manner. If a wire proceeds from the ball L to the outside coating, the jar will be immediately discharged, as the accumulation of the electric fluid is sufficiently powerful to pass through the space of air between the two balls; hence a shock may be communicated to the arm by means of the wires and directors as in the figure, and it will be stronger in proportion as the distance of the ball G from H is augmented. This electrometer acts in the manner of the common discharging rod, and therefore has received the name of the discharging electrometer.

In fig. 6, we have a representation of Mr Lane's electrometer applied to the machine for medical electricity. G, the lower part of which is inclosed in the pillar F, is made of wood baked and boiled in linseed oil, and bored cylindrically for two-thirds of its length. The brass work is fixed to the pillar by the screw H, and is moveable in the groove I, so that it may be raised higher or lower as the height of the jar D requires. A steel screw L passes through the brass work, having its threads about 1/4th of an inch distant from one another. To the end of this, and opposite to K, is fixed a hemispherical and well polished piece of brass; and a brass ball M, likewise well polished, is fixed to the prime conductor. To this screw is annexed a circular plate O, divided into 12 equal parts; and in every revolution of this screw pointing to the divisions of the scale N, each of which are equal to one turn of the screw. The use of this electrometer is to discharge the jar D, or any battery connected with the prime conductor, when the machine is not applied to medical purposes. If a person holds a wire fastened to the screw H in one hand, and another wire (fixed to E by a loop of brass) passing from the frame of the machine to a tin-plate on which the jar D stands, or the hook E connected with it, he will perceive no shock when K and M are in contact; and the degree of explosion, as well as the quantity of electricity accumulated in the jar, will be regulated by the distance of K and M from each other.

The improved way of applying the discharging electrometer to the conductor, is found to be much more convenient and ready than any other; as it has also the advantage of being useful to a jar or battery of any size. See fig. 6, where A represents the electrometer as applied to the conductor; C the improved medical jar suspended at a small distance from it. A small glass tube ef is fixed in this jar, a part of the lower end of which is coated. Two wires pass through the brass ball G on the top of this tube; one of which is connected with the bottom of the jar, and the other goes only to the internal coating of the small tube. The wires are moveable at pleasure, and the jar is suspended from the conductor by a brass ring; and a chain or wire must be fixed to the hook d at the bottom. From a bare inspection of the figure, it appears that the arm will receive the shock by the discharge of the jar acd; for, by turning the cylinder round, the jar soon becomes charged either with one or both wires in it; and directly as the charge becomes sufficiently strong to pass through the air, it will explode, and the fluid pass to the end of b next to it, going through the wire to the wrist, and from thence up to the other chain at the shoulder. By reversing the positions or the connections of the two wires, the progress of the shocks will be reversed, viz., from the shoulder to the wrist. If the short wire alone be left in the jar cd, and the discharging ball of the electrometer abe placed from a quarter of an inch to a whole one from the conductor, a most delicate small shock may be given, and repeated any number of times at pleasure. This is called the electrical vibrating shock.

Fig. 31, g represents the bottle director. It is hollow, and coated like a common jar, acting as such, and in some cases is looked upon as convenient. With this, as with the common director, it is proper to press the ends against the part where the shock is to be applied. Fig. 56 represents a small pocket electrical apparatus, which may sometimes be of use for medical purposes as well as others. It is packed up in a very small size, being only five inches long, two broad, and one deep. It is capable of a tolerable strong charge or accumulation of electricity, and will give a small shock to one, two, three, or a greater number of persons.

A is the Leyden phial or jar that holds the charge; B is the discharger to discharge the jar when required without electrifying the person that holds it; C is a silk ribbon prepared by a coating of varnish, so as to be excited, and communicate its electricity to the jar; D are two hair, &c., skin rubbers, which are to be placed on the first and middle fingers of the left hand, and serve to excite the ribbon C.

To charge the jar. Place the two finger-caps D on the first and middle finger of the left hand; hold the jar A at the same time at the joining of the red and black E on the outside between the thumb and first finger of the same hand; then take the ribbon in your right hand, and steadily and gently draw it upwards between the two rubbers D, on the two fingers, taking care at the same time the brafs ball of the jar is kept nearly close to the ribbon while it is passing through the fingers. By repeating this operation 12 or 14 times the electrical fire will pass into the jar, which will become charged; and by placing the discharger C against it, as in the plate, you will see a sensible spark pass from the ball of the jar to that of the discharger. If the apparatus is dry and in good order, you will hear the crackling of the fire when the ribbon is passing through the fingers, and the jar will discharge at some distance.

To electrify a person. You must desire him to take the jar in one hand, and with the other touch the knob of it; or, if diversion is intended, desire the person to smell at the knob A of it, in expectation of smelling the scent of a rose or a pink: this last mode has occasioned it to be sometimes called the magic smelling bottle.

The following are the principal methods by which electricity may be applied to the human body with a medical view.

1. By merely placing the patient in an insulated chair, and connecting him with the prime conductor.—When the machine is in action, he will thus be filled with the electric fluid, which will be continually dissipated from the points and edges of his clothes: and though the effects of this are probably too slow to be rendered very advantageous, yet a sedentary person might perhaps derive some benefit from sitting in an insulated chair, having before him an insulated table, the chair to be connected with the ball of a large charged jar or battery; by which means a small quantity of the fluid will be continually passing through those innumerable capillary vessels, on the right state of which our health so much depends.

2. By throwing the fluid upon, or extracting it from a patient, by means of a wooden point.—This may be effected in a twofold manner: 1st, By inflating the patient, and connecting him either with the cushion or the positive prime conductor, the operator presenting the point. 2nd, Let the patient stand upon the ground, and the wire of the director be connected either with the positive or negative parts of the machine. The sensation produced by the fluid when acting in this manner is mild and pleasing, resembling the soft breezes of a gentle wind; generating a genial warmth, and promoting the secretion and diffusion of tumors, inflammations, &c.

3. By the electric friction.—Cover the part to be rubbed with woollen cloth or flannel. The patient may be seated in an insulated chair, and rubbed with the ball of a director that is in contact with the conductor; or he may be connected with the conductor, and rubbed with a brafs ball which communicates with the ground. The friction thus produced is evidently more penetrating, more active, and more powerful, than that which is communicated by the flesh brush; and there is very little fear of being thought too faguing. This, when used but for a few minutes, will be found more efficacious than the other after several hours application.—Electricity applies here with peculiar propriety to spasm, pleurisy, and some stages of the palsy; and in every case answers the end of blistering where the discharge is not wanted, being the most safe and powerful stimulant we know.

4. By taking strong sparks from the patient. Here, as in every other case, the operator may connect the ball of the director with the positive or negative conductor, or he may connect the patient with either of these and the ball with the ground. Now it is clear from what has been already laid down, that if the director be connected with the positive conductor, the fluid is thrown upon the patient, if with the cushion the fluid is extracted from him. Let the patient be insulated, and the action is in some measure reversed; if he is joined to the negative conductor or cushion, he will receive a spark from a person standing on the floor; but if he communicates with the positive conductor, he will give the spark to the person on the ground.

5. By causing a current of the electric fluid to pass from one part of the body, and thus confining and concentrating its operation without communicating the shock. Place the patient in an insulated chair, and touch one part of the body with a director, joined to a positive conductor; then with a brafs-ball communicating with the ground touch another part; and when the machine is in action the fluid will pass through the required part from the conductor to the ball; the force of the stream will be different according to the strength of the machine, &c. Or connect one director with the cushion and the other with the positive conductor, and apply these to the part through which the fluid is to pass, and when the machine is in action the electricity will pass from one ball to the other. It is not necessary to insulate the patient in this case.

6. By the shock. Which may be given to any part of the human body, by introducing that part of the body into the circuit which is made between the outside and inside of the bottle. This is conveniently effected, by connecting one director by a piece of wire with the electrometer and the other with the outside of the bottle; then hold the directors by their glass-handles, and apply the balls of them to the extremity of the parts through which the shocks are to be passed. The force of the shock, as we have already observed, is augmented or diminished by increasing or lessening the distance between between the two balls, which must be regulated by the operator to the strength and sensibility of the patient.

Instead of the common bottle, we may have a small one with a glass tube proceeding from it, through which proceeds a wire and hook to hang it upon the machine, with a longer one from the outside coating, and which is to be carried by means of a director to the patient. When this is used as a common bottle, both wires are to be left there, and the shock is communicated by two directors, one connected with the bottom, the other with the top. The operator will often find himself embarrassed in giving small shocks, the fluid passing from the conductor to the ball of the electrometer, instead of going through the circuits he desires: when this happens, which may be known by the chattering noise of the spark, the resistance formed to the discharge is so great, that the fluid cannot force its way through the circuit: to remedy this, pass two metallic pins through the clothing, so that they may be in contact with the skin, which will lessen the resistance and conduct the fluid.

7. By a sensation between a shock and the spark, which does not communicate that disagreeable feeling attending the common shock. This is effected by taking out the long wire from the small medical bottle, and leaving the shorter one which is connected with the tube in its place, the directors to be connected and used as before. The effect of this species of shock, if it may be called one, is to produce a great vibration in the muscular fibres, without inducing that pungent sensation which the shock effects. It is therefore applicable to some stages of palsy and rheumatism; it may also serve as an artificial means of exercise.

8. By the bottle-director. Inflate the patient, and place one of the balls in contact with him; by which means this director is charged. Now if a wire is conveyed from the bottom of this to the top of another director, the bottle-director will be discharged whenever the other ball b is brought in contact with the patient; so that by bringing it down with rapidity, any number of small shocks may be procured in a minute; or connect the inflated patient with the top or inside of a large charged jar, and then this apparatus used in the foregoing manner will discharge from the large jar at each spark its own contents, and by repetition discharge the whole jar; thus a number of shocks may be given without continually turning the machine or employing an assistant.

9. By passing the whole fluid contained in the Leyden phial through a diseased part without giving the shock. Connect a director, by means of a wire, with the ball of a Leyden jar; charge the jar either completely or partially, and then apply the ball or point of the conductor to the part intended to be electrified, and the fluid which was condensed in the phial will be thrown on the part in a dense flow stream, attended with a pungent sensation which produces a considerable degree of warmth. If a wire that communicates with the ground is placed opposite to the end of the director, the passage of the fluid will be rendered more rapid, and the sensation stronger. Or inflate the patient, connect him with the top of a jar, charge this, and then apply a metal wire or piece of wood to the part through which you mean to make the fluid pass. It is obvious, that in this case the circuit between the inside and the outside of the jar is not completed, therefore electricity, the shock will not be felt. The condensed fluid passes in a dense flow stream through the required part, while the outside acquires a sufficient quantity from substances near it to restore the equilibrium.

It is in all cases most advisable to begin with the more gentle operations, and proceed gradually to increase the force as the strength and constitution of the patient or the nature of the disorder requires. The stream from a wooden point, a wooden ball, or brass point, may be first used; sparks, if necessary, may then be taken, or small shocks given.

In rheumatic cases the electric friction is generally used. If the pains are local, small shocks may be given. To relieve the toothache, very small shocks may be passed through the tooth; or, cover the part affected with flannel, and rub it with a director communicating with the machine.

In inflammations and other disorders of the eyes, the fluid should be thrown from a wooden point; the sensation here produced is that of a gentle cooling wind; but, at the same time, it generates a genial warmth in the part affected.

In palsies, the electric friction and small shocks are administered. Streams of the fluid should always be made to pass through the affected part.

The only treatise we have yet had from the faculty on the subject of medical electricity is a pamphlet, intitled, Considerations on the Efficacy of Electricity in removing Female Obstructions, by Mr Birch; and if its merits were to be confined to this disease alone (in which it may be reckoned a specific), it would be intitled to the attention of practitioners; but we have reason to expect much more from it, since the prejudices of the faculty seem removed, and the practice is becoming more general every day.

Sect. XIII. Of the Uses of the Electric Fluid in the System of Nature at large.

These are so many and so various, that it may be said without much exaggeration, that whether we look to the heaven above or to the earth beneath, we can scarcely perceive anything that is not acted upon, and in a manner perfectly subjected to the operations of this wonderful fluid. If we attend to the common phenomena of our atmosphere, experiments show that electricity is connected with every one of them. If we evaporate water by means of heat, it appears from clouds, rain, hail, snow, that a strong electricity is produced. If vapour is condensed into rain, a quantity of electricity is also produced; and if water is frozen into ice, if it descends in hail or snow, electricity appears to be equally concerned. When clouds emit their electricity in great quantities, they instantly dissolve in rain; which is more or less heavy according to the quantity of electricity discharged, as in thunder storms; and when this quantity is excessive, a vast many discharges are frequently made before the rain can descend. Hence it is reasonable to conclude, that though heat may be the cause of the first rise of vapour, it is the electric fluid which unites it with the air in such a manner as to be be perfectly dissolved and become transparent in it (p). This is confirmed by an observation related under the article Cloud; namely, that small clouds floating in the atmosphere will frequently be seen to attract one another, and to meet together; after which, if they have been of nearly an equal size, both will almost instantly vanish. Transparency itself, as we have seen in many instances through the course of this treatise, depends on the vibratory motion of the electric fluid; and when we are assured that it depends on this in several cases, we may conclude from analogy that it does so in all. In the case of vapour dissolved in the atmosphere, therefore, as long as this particular motion continues through it, the vapour remains dissolved and transparent; but when the electricity comes to be disposed to assume the other motion, of which it is exceedingly susceptible, viz. that of running in a stream from one place to another, the vibratory motion ceases, the vapour formerly dissolved loses its transparency, and appears in the form in which it was originally raised by heat, viz. that of an opaque smoke or mist. As this mist must always be electrified (for it is in the disposition of the fluid to fly to a distant place that electricity suffuses), the fluid then begins to exert its power of attraction, and the mist collects in bodies larger or smaller according to the quantity of motion with which the electric matter is affected; and thus we see how by means of this disposition of the fluid, cloudy weather, rain, or the most violent thunderstorms, may be produced.

On looking farther into the operations of nature, we find the electric fluid acting in a still higher capacity, and regulating the temperature of the different climates throughout the world. Under the article Chemistry, n° 99, it has been shown, that what is heat in summer becomes electric fluid in winter; and under the article Cold, it has been shown that cold as well as heat is a positive substance. In the present treatise it has been proved at length, that the electric fluid and the light of the sun are the same; the former being in truth no other than the solar light absorbed by the earth, entangled among its particles, becoming subject to new laws, and acting in many cases as if it were a distinct fluid. Hence it becomes a proper antagonist to the light itself; for as the latter is only the fluid of electricity moving in a vibratory manner, and what we call electricity is the same fluid either in a comparatively flagrant situation, or disposed to run with violence from one place to another; it is plain that the motion of the light must be opposed by the fluid tho' flagrant, and much more if it be moving in any opposite manner. But the action of light when augmented is heat: the power which opposes it therefore, i.e., the electric fluid moving in an opposite direction, as explained under Chemistry, n° 102, is cold itself; and hence the strong electric appearances in the atmosphere in cold countries, or in cold weather even in our own country. Hence also the electricity of the serene sky is weaker in summer than in winter; and the combustion, which is a very strong vibratory action of the electric matter, produces no electricity, the one action being inconsistent with the other. The electric fluid therefore regulates the light and heat of the sun throughout the whole world, and is itself regulated by them; so that neither heat nor cold can ultimately predominate anywhere.

Descending from the atmosphere into the earth itself, we find the electric matter no less concerned there than in the atmosphere. It has been already observed, that its vibratory motion probably gives transparency to all bodies. Sometimes this motion is augmented to a great degree, as in the waters of the ocean, which become unusually clear before tempests and hurricanes. Its action in producing earthquakes is explained at large under the article Earthquake, as well as in setting fire to volcanoes under the article Volcano. Like other fluids, its action seems to gain a great increase of power when it runs for a considerable way along any conductor. This may be easily conceived from the consideration, that the substance along which it runs is everywhere pressed by a fluid of the same kind, which continually accelerates its motions, and at last gives them an intensity capable of acting as the most vehement fire. The fact has been long observed, and is confirmed by the experiments of Mr. Wilson in the Pantheon as well as by those of later electricians. In the former, the spark taken from a vast conductor of 155 feet in length, was so strong that it resembled the discharge of a large jar, or rather a small battery; and was so very pungent, that few who had tried it once would venture on a second experiment. The latest experiments were made with a number of tin conductors joined to each other's ends: in which situation it was found that the spark taken from them was much stronger than when they were laid at each other's sides, though the surface was in both cases exactly the same. Hence we see, that if by any means the electric fluid shall meet with an unusually good conductor for a considerable way through the earth, the extremity of that conducting part may be heated, set on fire, or violent explosions issue from it; and the same thing will take place in the atmosphere. Upon this principle then may we account for natural hot-baths; explosions suddenly issuing from the earth, by which people have sometimes been killed; clouds and whirlwinds charged with an enormous quantity of electricity, and far beyond what in the ordinary way they could contain, &c.

Thus, to the action of the electric fluid we are in an especial manner to ascribe the temperature of the air throughout the whole globe; all the phenomena of rain, snow, hail, lightning, tempests, and in all probability the currents of the air itself named winds. Certain it is at least, that every electrified substance has an atmosphere round it resembling a gentle blast of

(p) In this there appears some inaccuracy of expression; but as it is somewhat difficult to find terms at once sufficiently accurate and intelligible, we shall hear observe, that by the word heat we mean the electric or universal fluid moving in a certain manner, viz. from a centre to a circumference; by cold, the same fluid pressing from a circumference to a centre; by the electric fluid simply, the same either comparatively flagrant, or moving in any other way than those just mentioned. Uses in the of cool air; and it is also very remarkable, that the electric fluid itself cannot be blown away from any substance, even by the most violent blast of air we can imagine. An undoubted evidence of this is, that if you set up a small ball or pointed body upon the conductor of a strong machine, so that a stream of electric light may issue from it, it will not be in your power to turn this flame aside in the smallest degree by the most violent blast of a bellows. On the contrary, if any body is presented to it which has a tendency to attract, the flame will move across the blast of air directly contrary to it, or in the same direction with it, in the very same manner as if no such thing was present. As the electric fluid therefore acts independent of the air, and cannot have its motions controlled by it, it is highly probable that all the motions of the atmosphere are controlled by this fluid alone; and indeed if we allow it to be the proper antagonist to the light of the sun itself, we must readily allow it also to be the regulator of every other power on this earth.

Its effects on vegetation have been treated of in the last section, though we cannot certainly say that it is the original cause of this process. It seems, however, to be the true cause of Crystallization; which, as remarked under that article, probably is only an incipient or imperfect vegetation. The most convincing proof of this is from the experiments of Mr Lichtenberg with a large electrophorus; in which the knob of an electrified phial being drawn over the surface of the electric plate, finely powdered rosin afterwards sifted upon the place assumed the figure of stars and other beautiful ramifications, indicating not only an inclination to arrange itself in the same regular order with the crystals of salts, but to run out into branches like those of vegetables. These experiments have been repeated to great advantage by the Reverend Mr Bennett, according to whose method the figures represented in Plate CLXXIX were made. The apparatus used for making them consisted only of a common Leyden phial, and a plate of glass 15 inches square covered on one side with a varnish of gum lac dissolved in spirit of wine (q.), and several times laid over. The other side is covered with tin-foil laid on with common paste. When it is to be used, the glass-plate is put upon a metallic stand with the tin-foil laid undermost; the phial is to be charged, and the knob drawn over the varnished side. Thus any kind of figure may be drawn or letters made as represented in the plate; and from every figure beautiful ramifications will proceed, longer or shorter according to the strength of the charge. On some occasions, however, the charge may be too strong, particularly where we wish to represent letters, so that the whole will be blended into one confused mass. The round figures are formed by placing metallic rings or plates upon the electrical plate; and then giving them a spark from the electrified bottle, or sending a shock through them. The figures may be rendered permanent by blowing off the loose chalk, and clapping on a piece of black-sized paper upon them; or if they are wanted of another colour, they may easily be obtained by means of lake, vermilion, rose-pink, or any of the ordinary colours ground very fine. The easiest way of applying them seems to be by a barber's puff-bellows.

This tendency of the electrical fluid to produce ramifications in its passage through other substances, is likewise evident from the figure of the positive flashes described by Mr Nicholson, and represented Plate CLXXXVIII. It may indeed be objected, that in both cases the fluid has to make its way through non-conducting substances, where it meets with considerable resistance; so that the case cannot be applicable to vegetation, where a ready conductor is always found in the moisture with which the earth abounds. But if we consider that the earth, and everything contained in it, is already saturated with electric matter, it must readily appear that no new quantity can be forced into it without meeting with a considerable resistance; and therefore it will branch out and diversify in the very same manner when passing through the earth, that it does when artificially sent through the air, or made to diffuse itself on the surface of an electric substance. If in the earth it meets with such particles as serve to facilitate its passage, these will be arranged according to the direction of the fluid itself; and thus these particles being consolidated by other powers, or by electricity itself acting in a different manner, may be supposed to assume the figures of branched roots; while the continual accumulation of new matter augment them in bulk, and is what we call the growth of the plant, or its drawing nourishment from the ground. It is not indeed pretended that we can explain the manner in which plants grow; the utmost we can do is to attain some slight and general idea of the cause, and how by the action of that cause, directing itself according to the laws given it by the Author of nature, the effects may be produced. This is sufficient to satisfy the curiosity natural to the human mind; a farther knowledge would not only be entirely useless, but in all probability is inconsistent with the limited state of our faculties at present. What is here said concerning vegetation, may be applied equally to the formation and growth of animal bodies; but this subject is still more obscure and difficult: it has been supposed by many, however, that the nervous fluid is the same with that of electricity; for which many probable reasons might be assigned, though the futility and invisibility of both must forever prevent us from obtaining any direct proof on this subject.

When we consider the rest of the terrestrial phenomena, we find the same fluid concerned in every one of them, or rather acting as their only cause. There is, in fact, nothing in nature a more surprising phenomenon than that of the magnet; and this, by repeated experiments, has of every been proved to depend on electricity. Magnetic needles have often been endowed with their virtue by means of artificial electricity, and iron has been known to receive it from lightning; whence we may reasonably conclude, that the power of the magnet at all times depends upon the secret operation of the electric fluid. By extending its power to the production of attractive and repulsive forces in all cases, and which from many causes...

Uses in the natural phenomena is extremely probable, we shall still give it a higher rank in the system of nature. We shall now find it guiding the planets in their courses through the heavens, giving stability and cohesion not only to terrestrial substances, but to the globe of earth itself, and to all other bodies in the universe.

A system of natural philosophy on this principle was begun in the year 1747, and lately published by the Count de Treflan. In this the electric fluid is considered as the first principle of motion in the universe, and the immediate agent by which the system of nature is governed. According to him, the fixed stars themselves are no other than as many foci of action communicating electricity to their surrounding planets, which have electric atmospheres of different extents. He shows the operation of the fluid in all the different phenomena of earth, air, water, fire, &c., descending even to the most minute, as well as considering the most grand and sublime exhibitions of nature. That the electric fluid is capable of imitating many of these phenomena, is certain; as for example, those of earthquakes, water-sprouts, tides, &c., of which an account is given under their proper articles. By means of the same fluid also we may imitate the planetary motions; and for this several contrivances have been fallen upon: the principal are as follow:

1. From the prime conductor of an electric machine suspend six concentric hoops of metal at different distances from one another, in such a manner as to represent in some measure the proportional distances of the planets. Under these, and at the distance of about half an inch, place a metallic plate, and upon this plate, within each of the hoops, a glass-bubble blown very thin and light. On electrifying the hoops, the bubbles will be immediately attracted by them, and will continue to move round the hoops as long as the electrification continues. If the electricity is very strong, the bubbles will frequently be driven off, run hither and thither on the plate, making a variety of surprising motions round their axis; after which they will return to the hoop, and circulate as before; and if the room is darkened, they will all appear beautifully illuminated with electric light.

2. Provide a ball of cork about three quarters of an inch in diameter, hollowed out in the internal part by cutting it in two hemispheres, scooping out the inside, and then joining them together with paste. Having attached this to a silk thread between three and four feet in length, suspend it in such a manner that it may just touch the knob of an electric jar, the outside of which communicates with the ground. On the first contact it will be repelled to a considerable distance, and after making several vibrations will remain stationary; but if a candle is placed at some distance behind it, so that the ball may be between it and the bottle, the ball will instantly begin to move, and will turn round the knob of the jar, moving in a kind of ellipse as long as there is any electricity in the bottle. This experiment is very striking, though the motions are far from being regular; but it is remarkable that they always affect the elliptical rather than the circular form.

3. Cut a piece of India paper in the shape of an isosceles triangle, whose sides are about two inches long, and two-tenths of an inch in breadth; then erect a brass ball of two or three inches diameter on a brass wire one-sixth of an inch in thickness, and two feet six inches long, on the prime conductor: electrify the conductor, and then bring the obtuse end of the piece of paper within the atmosphere of the ball; let it go, and it will revolve round the ball, turning often round its own axis at the same time.

We shall not here enter into any speculations concerning the way in which it might be supposed possible from an experiment to produce the planetary motions by means of the efflux of the sun's light, and the return of the electric fluid towards him. Before we can make excursions into these celestial spaces, it is absolutely necessary to remove an objection derived from Mr Morgan's experiment, that the electric fluid cannot pervade a perfect vacuum; and from which he concludes, that the electric fluid cannot pass beyond the limits of our atmosphere. On this experiment, however, we must observe, that though it were really proved in a much more decisive manner than is done by this experiment, that the fluid cannot be artificially driven through a vacuum, this would not prove that it cannot naturally pass through it, unless we should suppose the powers of nature and of art to be equal to one another. But that even the powers of art, in Mr Morgan's experiment, have not a fair chance of success, is evident from an inspection of fig. 80. Here he endeavours to force the electric fluid through a long course of perfect vacuum, and finds the power of his machine insufficient for the purpose. Yet one of Mr Morgan's own experiments might have led him to vary this one in such a manner as would perhaps have shown the possibility of transmitting the fluid through the most perfect vacuum that can be made. He informs us, that a spark, which in the open air cannot exceed one quarter of an inch diameter, will appear to fill the whole of an exhausted receiver four inches wide and eight inches long; though in the latter case it will be excessively faint in comparison with what it would have been in the atmosphere: yet, in order to prove that the faintness of the electric light in vacuo depends on the enlarged space through which it is diffused, we have only to introduce two pointed wires into the vacuum, so that the fluid may pass from the point of the one to the point of the other; and when the distance between them is not more than the tenth of an inch, in this case we shall find the spark as bright as in the open air.

The inference to be derived from this experiment is obvious. Had Mr Morgan, instead of attempting to cause the fluid pass through the whole length of the vacuum, put two wires in the inside at a small distance from each other, as described in the experiment just now mentioned, it is very probable that the fluid would have made its way through that small distance. It must be acknowledged indeed, that, considering the very great difficulty of making this experiment at any rate, we could scarce expect that this additional trouble could be taken; but without this, or something equivalent, his conclusion cannot by any means be allowed to be just; nor, even if it had been tried, would it have determined the question in his favour.

The great difficulty in this experiment is to give a reason why in a certain degree of exhaustion the vacuum should be so easily penetrated by the fluid, and in another should make such resistance; but the following considerations will probably throw some light on In all cases where the fluid is obliged to pervade the substance of any medium whatever, it moves with difficulty. Thus, if a vast quantity of electricity is sent through a small wire, the resistance it meets with is so great that the wire is dispersed with violence; and if the battery is large, it cannot be totally discharged, as was the case with Dr Van Marum's battery, mentioned no. 150. Again, if the spark be taken in water, a most violent explosion takes place; and yet both metals and water are good conductors of electricity.

2. In all cases where we set the electric fluid in motion, the utmost we can do is to give it a tendency to circulate; and unless we allow it to do so, no electricity will be produced. Thus, if we extricate the fluid from the earth by means of an electrical machine, discharge it upon a conductor, and form a communication between that and another part of the earth, the circulation will go on very readily, and the fluid will easily return to the place from whence it came. If the communication betwixt the earth and conductor be cut off by an electric, the circulation will nevertheless go on; the fluid will evaporate in the air, and from thence reach the earth by channels invisible to us. The effect will be the same in all cases where its motion in a certain direction is stopped: but what we call stopping it, is only rendering its passage more difficult in one particular place than in another; for as to any absolute stop or impediment, such as could resist the whole force of the fluid, as Mr Morgan supposes, there is not the least probability that it exists in nature. The whole that can be inferred from Mr Morgan's experiment therefore is, that the electric fluid will more readily evaporate and pass silently thro' the air than through a complete vacuum. The question, however, still recurs: Since this fluid passes very readily through rarefied air, why does it hesitate after a certain degree of rarefaction, and at last stop altogether when the air is totally exhausted? To this it may be replied, that when air is heated it becomes less electric than when cold, and by an increase of heat becomes at last an excellent conductor. On the other hand, by an increase of cold its electric properties become proportionally greater, and consequently the difficulty with which the fluid gets through it increases in proportion.

Under the article Elastic Vapours, it is shown that the true principle of elasticity is heat; and under the article Chemistry, no. 99, it is shown, that heat and electricity are convertible into one another. In proportion as the air is rarefied, therefore, it absorbs heat, and consequently becomes a better conductor; but when it is totally exhausted, nothing remains but the fluid of electricity itself; the same indeed with that of heat, but deprived of motion, and consequently capable of making a much greater resistance. Now the strongest spark that can be drawn from any of our machines perhaps does not equal 1/60th of an inch in diameter, as appears from the holes made by them in paper or cards when pierced, as directed in Sect. VIII. But when a perfect vacuum is made, this small spark is obliged to act upon a cylinder of electric matter perhaps 6000 or 7000 times greater in diameter than itself, each point of which resists with the whole force the explosion itself has; and what is worse, the whole of this must be put in motion before any discharge can be made. The resistance therefore is so violent, that the fluid rather passes through the air as already explained: nevertheless, if it were possible to make a perfect vacuum of no greater diameter than that of the electric spark, there is no reason to suppose that it would not be penetrated by it; and of this Mr Morgan's experiments with the two wires above mentioned seems to be a confirmation.

On the whole, it is evident, that we cannot from this or indeed any other experiment argue against the possibility of the passage of the electric fluid from any part of the creation to another. We cannot force it, it is true, because it is disposed by its own natural laws to resist our efforts; but where it is disposed by these laws to yield in one place, there will undoubtedly be a current of it thither from some other, which we would find ourselves equally unable to stop by all the machines that ever have been or will be invented. There is as yet therefore not the least proof that the electric fluid does not pervade the most distant regions of space, and there perform all those great operations which have been ascribed to unknown and inexplicable powers. For a further account of the operations of this fluid in producing the phenomena of nature, see the articles Atmosphere, Aurora Borealis, Earthquake, Hail, Hurricane, Lightning, Meteor, Rain, Snow, &c.

A

Archard's observations on the division of the scale of an electrometer, no. 183.

Epinus's experiments on the electricity of melted sulphur poured into metal cups, 53.

Agate, when discovered to be an electric substance, 1.

Air of a room, how electrified, 94.

How to charge a plate of it, 95.

Penetrated by the electric fluid, ib. Electricity only shows itself in the air, 106.

Changes the colour of the electric light by admission into a vacuum, 132.

Experiments on various kinds of it with the great machine at Haarlem, 179. Is always positively electrified, 200. Its electricity may be determined by a large electrometer, 241. See Atmosphere.

Amalgam for electrical purposes, best made of mercury and zinc, 21. Mr Nicholson's directions for preparing it, 169.

Amber, its electric properties discovered by Thales, 1.

Gives the name to electric substances from its Latin name Elektron, 2. Account of its electric properties, 53.

Animal fluids favour the passage of the electric flash over their surface, 92.

Animals, some naturally electrified, 254.

Apparatus, electrical, described, p. 424. Directions for using and making experiments with it, p. 465.

Atmosphere: visible electric one, p. 469, no. 13. The electric fluid supposed not to reach beyond the atmosphere of the earth, 137. How to collect a great quantity of electricity from it, 189. How high it is necessary to raise conductors in order to produce signs of electricity, 191. How to observe the electricity of the atmosphere, 192. Observations on the electricity of it, 195. A periodical flux and reflux observed in the electricity of the atmosphere, 196. M. Saussure's observations upon it in an extraordinary degree.

Blazing of vegetables supposed to be an effect of lightning, 246.

Bolognian stone, artificial, illuminated by electric light, p. 468, n° 9.

Bofe's account of the electric properties of jet and amber being discovered, 1. Glafs globes introduced by him, 8. His enthusiasm concerning the electric shock, 10. His imaginary process of beatification, 11. Obliged by Dr Watson to own the fallacy of his experiment, ib.

Bottles, how cemented so as to be useful after having been broken by a discharge, 41.

Boxwood, how rendered luminous by the electric shock, p. n° 477, 41. Split by Van Marum's large machine 145.

Boyle's discoveries in electricity, 2.

Brookes's method of constructing batteries, 40. His cement for mending bottles broken by a discharge, 41. His method of making mercurial gages perfectly free from air, p. 484, n.E. His experiments on the force of batteries, 159. His experiments on the Leyden phial, p. 497, n° 157. His electrometer, 183.

Candle, lighted, increases the sensibility of Bennet's electrometer, 213.

Cantons experiments on the durability of the electric virtue in glafs, 52.

Card, or other substance, how pierced by the electric explosion, p. 470, n° 16. Effect of a shock sent over the surface of it, p. 471, 17.

Catalogue of electric substances, 42.

Caterpillars do vast damage to vegetables, 248. Easily destroyed by an electric shock, 249. How destroyed in the root of a tree, 251.

Cavallo's experiments with glafs tubes, 51. His solution of a difficulty concerning the repulsion of bodies negatively electrified, 68. This solution insufficient, 69. His observations on the continuance of the virtue of the electrophorus, 113. Various experiments with it, 114.

INDEX.

Mistakes in his observations, 115. His experiments on colours, p. 478, col. 1. His experiments with an improved air-pump on the passage of the electric matter through a vacuum, 138. Conclusions from them, 139. His instrument for observing the electricity of the atmosphere, 217. His dissertation on measuring small quantities of electricity, 220. His opinion of the method by which these might be measured, 223. Cement proper to be used for electrical purposes, 17. Mr Brookes's cement for jars broken by an electric discharge, 41.

Chain shortened by the electric shock, 81.

Charcoal, in what manner the electric fluid passes through it, 97.

Cigna's experiments on ribbons, 44.

Clay swelled, and tubes broken by the electric explosion, p. 471, n° 18.

Coating for globes, most proper composition for that purpose, 18. Directions for coating jars, 25.

Goffier, a joiner in France, the first who took a spark from a rod electrified by thunder, 13.

Cold makes water electric, 128. M. Saussure's observations on the atmospherical electricity in a very great degree of cold, 197.

Colours: Mr Cavallo's experiments on them, p. 478 col. 1.

Combination produces no signs of electricity, 210.

Condenser: M. Volta's described, 221. Its defects, ib.

Conducting power of various substances ascertained by means of an electrometer, 187.

Conductors distinguished from electrics by M. du Fay, 5. Used for preserving houses from lightning, 16. Whether the electric fluid pervades their substance, 80. Of the discharge of electricity by sparks on blunt conductors and filletly by pointed ones, 105. The luminous conductor; p. 468, n° 10. Different metals compared as conductors, 148. How to produce both. both electricities in the same conductor, p. 501, n° 165.

Contact: Difficulty of bringing bodies into that state, 80.

Cotton electrified, p. 472, n° 26.

Cuneus, one of the first who exhibited the Leyden phial, and from whom it took its name, 9.

Cylinders of glass, &c. used for electric purposes, 17. Why an exhausted cylinder cannot be excited, 101. Nor one filled with condensed air, 102. State of the inside of one during excitation, 162. Effects of different cylinders excited after Mr Nicholson's improved method, 170. Why the cylinder of an electric machine always retains some electricity, 226.

D.

Dalibard, M. the first in Europe who erected an apparatus for atmospherical electricity, 13.

Dancing Balls, p. 473, n° 29.

Delor, M. erects an apparatus for atmospherical electricity, 13.

Dephlogisticated Air: how to fire a piece of iron wire in it, p. 476, n° 38. Effects of the great Haarlem machine on metals confined in this kind of air, 156.

Diamonds: their electric light observed by Mr Boyle, 2.

Discharger of electricity described, 36. Mr Henley's universal discharger, ib.

Discharging Rod described, 27.

Doubler of electricity, Mr Bennett's objected to by Cavallo, 222. Improved by Mr Nicholson, 229.

Du Fay discovers the vitreous and resinous electricities, 8. His hypothesis of two electric fluids, 56.

Duff driven off from a brass chain by a strong electric shock, 80.

E

Earthquakes: their phenomena imitated with the great Haarlem machine, n° 158.

Effluvia, unctuous, supposed to be the cause of the phenomena of electricity, 55.

Eggs, how rendered luminous by the electric spark, p. 476, n° 40.

Electric Substances described, p. 418, col. 1. Several of them discovered by Mr Gilbert, n° 2. Difference between them and conductors discovered by Mr Gray, 5. Perpetual attractive power discovered in them by him, 6. Identity of the electric fluid and lightning supported by Dr Franklin, 12. His suspicions verified, 13. Catalogue of electric substances, with their different powers, 42. Objections to the assigning any electric power to metals, 432, col. 2. Electric substances and conductors approximate each other in their properties, ib. Electric substances, how divided, ib. Durability of the electric virtue of glass in some cases, 52. Two electric fluids supposed by M. du Fay, 56. The electric matter supposed to come from the earth, 57. Difficulty in determining its course, 59. Different opinions concerning its nature, 61. Is found to act according to the quantity of surface, 64. Cannot be proved repulsive of itself, 78. Whether it pervades the substance of conductors, 80. A chain shortened by the electric shock, 81. An inquiry into its nature, p. 450. Proved to be the same with elementary fire or the light of the sun, ib. col. 2. Gunpowder fired by the electric blast, ib. n° 83. Its action compared with that of light, 84. Identity of electric matter and light farther considered, ib. Electric substances proved to be penetrated by the electric fluid, 85. Light proved to be a vibration of it, 88. Of the passage of the electric fluid over the surface and through the substance of different bodies, 89, et seq. Circular spots produced by its explosions, 93. The fluid pervades the substance of electrics, but generally moves over the surface of conductors, ib. Globes burst by the fluid, 96. Proofs of its passing over the surface of conductors, 97. Is resisted by the vacuum of an ordinary pump, ib. The vast strength and velocity of the electric fluid occasioned by the mutual action of the air and fluid upon themselves and one another, ib. The fluid is not repulsive of itself, 98. In what manner an electric substance becomes excited, or diffuses its electric virtue, 99. Proofs of the vibratory motion of the electric fluid, 100. Electric substances of the same kind will not produce any electricity by being rubbed upon each other, 102. How to determine the direction of the fluid, 104. Electric attraction and repulsion accounted for, ib. Why electric appearances continue so long, 105. Why a motion of the electric fluid on one side is suddenly propagated round any body, 107. Star and pencil of electric light exhibited, p. 467, n° 5. Electric light flashing between two metallic plates, p. 468, n° 7. Artificial Bolognian stone illuminated by electric light, ib. n° 9. The visible electric atmosphere, p. 469, n° 13. To pierce a card by the electric explosion, p. 470, n° 16. To swell clay and break small tubes by its means, p. 471, n° 18. To make the electric spark visible in water, ib. n° 19. Metals calcined and revived by the electric shock, n° 123. The fluid throws light conducting substances before it, 124. Dr Priestley's experiments on this subject, 125. Experiments concerning the velocity of the fluid, 126. Sometimes it seems to move more slowly, 127. Water becomes electric by cold, 128. Electric substances become conductors by heat, 129. Changes of colour in the electric light by the admission of air into a vacuum, 132. Tubes perforated by the electric spark, 134. Why the fluid assumes the form of a spark, 135. Supposed by Mr Morgan not to reach beyond the limits of our atmosphere, 137. Electric light always visible in the most perfectly exhausted receiver, 140. Diminishes in a great degree of rarefaction, 142. The fluid supposed by Van Marum to act in a different manner from fire, 147. Dr Priestley's experiments on the effects of electric fluid on different kinds of air, 181. Saussure's conjecture concerning the nature of the fluid, 207.

Electrical Apparatus for the pocket, 265.

Electricity defined, p. 418, col. 1. Definitions of terms used in the science, ib. History of it, p. 419. When first mentioned, n° 1. Discoveries in it by Mr Gilbert and Mr Boyle, 2. By Otto Guericke and Sir Isaac Newton, 3. By Mr Hauksbee, 4. By Mr Gray, 5, 6, 7. By M. du Fay, 8. By M. Van Kleitt, 9. By Cuneus, ib. Supposed discoveries by Bose and others, 11. By Dr Franklin, Dalibard, &c., 12, 13. By Signior Volta, 16. Apparatus for exciting it, p. 424, et seq. Different theories of it, p. 440. Theory of unctuous effluvia by the first electricians, n° 55. Of two electric fluids by M. du Fay, 56. Of afflux and efflux by Dr Watson, 58. Of two sets of pores along with the afflux and efflux by the Abbé Nollet, 60. Of the identity of the electric fluid with the ether of Sir Isaac Newton by Mr Wilson, 62. Great power of electricity accumulated by some philosophers, 63. Dr Franklin's theory of plus and minus, or positive and negative electricity, 65. Proofs of atmospherical electricity, 73. Great quantity of electricity drawn down from a cloud by M. Cavallo, 75. Attempt to explain the phenomena of electricity from the known laws by which fluids act upon one another, p. 450. Reason of the prodigious power of electricity, n° 99. Positive and negative elec- Electricity explained, 102. Methods of changing them into one another, 103. Its discharge by sparks on blunt conductors, and silently on pointed ones, 102. Shows itself only in the air, 106. Why positive electricity tends to introduce the negative kind, 106. Zones of positive and negative electricity accounted for, 108. Rules for using the apparatus and performing experiments in electricity, p. 465. Entertaining experiments, p. 467.

To draw off the electricity silently from the prime conductor by a point, p. 472. n° 25. The spider seemingly animated by electricity, p. 473, n° 28. How to produce the two electricities in the same conductor, 165. How to increase the intensity of electricity to a great degree, 169. Great quantity brought down by a kite during a thunder-storm, 171. Of the disposition of balls to receive electricity as their items are long or short, 176. Of the methods of measuring, condensing, and doubling, electricity, and of distinguishing the two kinds from one another, p. 508. Electricity of different substances distinguished by means of Saufure's improved electrometer, 188. How to collect a great quantity of atmospherical electricity, 189. Or to ascertain the kind of it, 190. How high it is necessary to raise conductors in order to produce signs of it, 191. How to discover the intensity at various heights, ib. How to observe the electricity of the atmosphere, 192. To render the signs of it permanent in the electrometer, 193. To distinguish the two electricities in the atmosphere, 194. Observations on atmospherical electricity, 195. A periodical flux and reflux of electricity observable in the atmosphere, 196. M. Saufure's observations on it in a great degree of cold, 197. Of the maximum and minimum of it in the atmosphere, 198. Is weaker in summer than in winter, 199. Always positive in the atmosphere, 200. Saufure's experiments to determine the cause of atmospherical electricity, 201. Immense quantity of it extricated from volcanoes, 202. Positive electricity always the effect of burning iron, 203. No signs of electricity to be obtained by combustion, properly so called, 210. Cavallo's dissertation on measuring small quantities of electricity, 220. His opinion concerning the methods by which they may be measured, 223. His apparatus for that purpose, 224. Universal diffusion of electricity, 225. Cavallo's instrument for measuring small quantities of electricity, 227. Bonnet's doubler of electricity improved by Nicholson, 229. Nicholson's instrument for distinguishing the two electricities, 230. How to apply artificial electricity to the purposes of vegetation, 242. Easy method of applying electricity in this manner, 243. Various ways of applying electricity medicinally, 268. Is concerned in the production of clouds, rain, hail, snow, &c., 268, p. 536. Regulates the heat of the various climates, 269. Acts in many various ways in the substance of the earth, 270. Almost all the terrestrial phenomena to be ascribed to it, 271. Is probably the cause of vegetation, 272. Curious figures made by its means on a plate of varnished glass, 273. Is the cause of magnetism, and probably of every other kind of attraction, 274. Objections from an experiment of Mr Morgan's answered, 277.

Electrometers: various kinds of them, 27, 182. Electrified corkball electrometer, p. 467. n° 1. Description of different electrometers, 182. Brooke's electrometer, 183. Mr Cavallo's atmospherical electrometer, 184. Improved by M. Saufure, 185. Mr Bennet's electrometer, 211. Mr Cavallo's electrometer for rain, 218. Imperfection of all these instruments, 219. A convenient pocket electrometer, 232.

Electrophorus invented by M. Volta, 16. Its phenomena accounted for, 111. Construction of the machine, 112. Mr Cavallo's observations, 113. Experiments with it, 114. Mistakes in Mr Cavallo's observations, 115. Singular appearance of an excited electrophorus, 116. General reason of all the phenomena, 117.

Electro-vegetometer described, 235.

Enthuafism, ridiculous, of some electricians, 10.

Ether: identity of it with the electric fluid supposed by Mr Wilson, 62.

Evaporation: electricity produced by it, 201. How to make experiments on the evaporation of water, 216.

Excitation: Beccaria's hypothesis concerning it, 79. The subject particularly considered, p. 457. col. 2. Nicholson's experiments upon it, p. 499. n° 160. The silk flap joined to the rubber the principal cause of it, p. 500, n° 161. Improved method of exciting electric substances, 166. How excitation takes place by a simple rubber without a silk flap, 167. Gradual improvements in it, 174.

Excited substances defined, p. 418. col. 2.

Experiments, entertaining, p. 467. The electrified corkball electrometer, ib. Attraction and repulsion of light bodies, ib. The flying feather or shuttle-cock, ib. Electric well, ib. Star and pencil of electric light, ib. Drawing sparks, p. 468. Electric light flashing between two plates, ib. To fire inflammable spirits, ib. Artificial Bolognian stone illuminated by electric light, ib. The luminous conductor, ib. The conducting glass tube, p. 469. Visible electric atmosphere, ib. Of charging and discharging a phial in general, ib. The Leyden vacuum, p. 470. To pierce a card or other substance by the electric explosion, ib. Effect of the shock felt over the surface of a card or other substance, p. 471. To swell clay and break small tubes by the electric explosion, ib. To make the electric spark visible in water, ib. To fire gunpowder, ib. To strike metals into glass, ib. To stain paper or glass, p. 472. The lateral explosion, ib. To discharge a jar silently, ib. Drawing electricity from the prime conductor by a point, ib. The electrified cotton, ib. The electrified bladder, ib. The spider seemingly animated by electricity, p. 473. The dancing balls, ib. The electrical jack, ib. The self-moving wheel, ib. The magic picture, p. 474. The thunder-houfe, ib. The electric fly, p. 475. The electrified bells, ib. To fire a pistol or cannon by inflammable air, p. 476. The spiral tube, ib. To fire a piece of iron wire in dephlogisticated air, ib. To illuminate eggs, ib. To render ivory or boxwood luminous, p. 477. To illuminate water, ib. To make a beautiful appearance in vacuo, ib. To render goldleaf or Dutch metal luminous, ib. To perforate a glass tube, ib. The inflammable air-lamp, ib.

Experiments, miscellaneous, p. 478. On colours, ib. On the calcination and revivification of metals, n° 123. M. Beccaria's experiments to show that the electric fluid throws light conducting substances before it to facilitate its passage through the air, 124. Dr Priestley's experiments on this subject, 125. On the velocity of the electric fluid, 126, 127. On the change of water into an electric substance by cold, 128. On the change of electricity into conductors by heat, 129. On the non-conducting power of a perfect vacuum, 130. Mr Morgan's experiments on this subject, 131. Changes of colour in the electric light by the admission of air into

A perfect vacuum, 132. Surprising case with which an exhausted tube is charged with electricity, 133. Tubes perforated by the electric spark, 134. Why the fluid assumes the form of a spark, 135. Cavallo's experiments on this subject with an improved air-pump, 138. Experiments with the great machine at Teyler's museum in Haarlem, 143. Force of the explosion of its battery calculated, 145. Lengths of wire of different kinds melted by it, 146. Comparative efficacy of the different metals as conductors, 148. Wires shortened by the discharge, 149. The battery cannot be entirely discharged by very small wires, 150. Curious phenomena with tin wire, 151. Accounted for, 152. With wire composed of lead and tin, 153. Stain on paper by the calcination of metals, 154. Effects of the battery on metals confined in phlogisticated air, 155. In dephlogisticated air, 156. Phenomena on calcining metals in water, 157. Phenomena of earthquakes imitated, 158. Brookes's experiments on the force of batteries, 159. Violent explosion from his battery, 161. Milner's account of the Leyden phial, and experiments upon it, 162—165. How to charge a phial without friction, 165. Brookes's experiments on the Leyden phial, p. 497, n° 157. Nicholson's experiments on excitation, p. 499, n° 160. How to produce both electricities in the same conductor, p. 501, n° 165. To augment the intensity of electricity to a great degree, 169. Effects of different cylinders excited after his manner, 170. Surprising appearance on raising an electrical kite during a thunder-storm, 171. On the different appearance of positive and negative sparks, 175. On the disposition of balls to receive the electric matter according to the length or shortness of their stems, 176. On the action of points, 177. On different kinds of air with Dr Van Marum's great machine, 179. Curious experiment with balloons filled with inflammable air, 180. Dr Priestley's experiments on different kinds of air, 181. Explosion, lateral, p. 472, n° 23. Violent from Mr Brookes's battery, n° 161.

F.

Feather, electrified, always keeps the same side towards the body which electrifies it, n° 3. Electrified feather or shuttle-cock, p. 467, n° 2. Figures, curious ones made by electricity on a varnished plate of glass, 274. Finger rendered transparent by electricity, 86. Fire supposed by Van Marum to act on metals in a different manner from electrical fluid, 147. Florence flasks incapable of receiving a charge of electricity, 25. Fly, electric, p. 475, n° 34. Franklin, Dr, suspects the identity of electric fluid and lightning, 12. His suspicions verified, 13. Account of his theory, 65. His explanation of the phenomena of the Leyden phial, 70. Insufficiency of his hypothesis concerning positive and negative electricity, 82. Friction, how to charge a phial without employing any, 165.

G.

Gages, mercurial, Mr Brookes's method of preparing them so as to be entirely free from air, 135, note E. Geometrical figures beautifully shown by means of the electric light, p. 468, n° 9. Gilbert's discoveries in electricity, n° 2. Glass, attraction and repulsion through it discovered by Sir Isaac Newton, 3. Most commonly used for producing electricity, 17. Composition for coating glass globes or cylinders, 18. Glass tubes a necessary part of the electrical apparatus, 24. Directions for coating glass jars, 25. A substance capable of being substituted for it, 26. Phenomena of excited glass, 41. All kinds of it not equally proper for electrical purposes, 45. Mr Symmer's experiments on glass plates, 47. Beccaria's experiments on them, 48. Mr Henley's experiments on the same, 49. Conducting power of glass tubes, 50. Cavallo's experiments with them, 51. Durability of the electric virtue of glass, 52. Arguments for the impermeability of glass by the electric fluid refuted, 77. Objection on that subject answered, 85. Conducting glass tube, p. 469, n° 11. To strike metals into glass, p. 471, n° 21. To stain it by an electrical explosion, p. 472, n° 22. To perforate a glass tube, p. 477, n° 45. Easily broke by electricity, when covered with cement, p. 499, n° 159. Globes introduced into the electrical apparatus by Mr Boe, 8. The proper size of them for this purpose, 17. How to adapt several globes to one machine, 29. Accounts of globes burst by electrical explosions, 96. Gold-leaf rendered luminous by the electric fluid, p. 477, n° 44. Gold-leaf electrometer, experiments with one, 226. See Bennet, Gray, Mr, discovers the difference between conductors and non-conductors, 5. Discovers a perpetual attractive power in electric substances, 6. Imagines he can imitate the planetary motions, 7. Probable reason for his opinions in this respect, 53. Guericke, Otto, his discoveries in electricity, 3. Gum-lac, its electrical phenomena, 52. Gunpowder fired by the electric blast from Mr Wilton's large conductor, 83. How to fire it with the ordinary machines, p. 471, n° 20. Gymnotus electricus, account of its electric properties, 256. Some very large ones found in Surinam river, 257. Remarkable difference of the metals in conducting its shock, 258.

H.

Hansen said by some to have introduced the use of glass globes into the electrical science, n° 8. Haukloer, curious experiment of his to render pitch and sealing-wax transparent, 4. Henley's experiments on glass plates, 49. Gives a remarkable account of the continuance of the electric virtue in a small bottle, 52. Heat changes electric into conducting substances, 129.

I.

Jack, electrical, p. 473, n° 30. Jars: directions how to coat them, n° 25. Remarkably loud report of a jar belonging to Mr Rackstraw, 92. To discharge a jar silently, p. 472, n° 24. Possibility of touching both sides of a jar so quickly that it has not time to discharge itself entirely, n° 127. Mr Brookes's method of preventing jars from being broken by a discharge, p. 499, n° 158. They are easily broken when covered with cement, ib. n° 159. Construction of jars to show the course of the electric fluid, n° 178. Ice, effect of sending the shock of a battery over its surface, 89. Jet, its electrical properties, when discovered, 1. Inflammable air: how to fire a pistol or cannon with it, p. 476, n° 36. Inflammable air-lamp, p. 477, n° 46. Ingenbouz, Dr, his machine, n° 30. Insects destroyed by electricity, 247. How to do this in a great number of trees at once, 250. How prevented from generating in plants, 252. Infusion defined, p. 418. Infusing flask described, n° 41. Jones, Mr William, his improvement on the spring of the rubber, 23. Iron; how to burn it in dephlogisticated air, p. 476, n° 38. Its combustion always... ways produces positive electricity, n° 203.

Ivory; how rendered luminous, p. 477, n° 41.

Kite, electrical, Mr Cavallo's directions concerning its construction, n° 74. Great quantity of electricity brought down by him with one, 75. And by Mr Baldwin in America, 171. Application of Mr Bennet's electrometer to the electrical kite, 214. Observations on the use of them, 231. Experiments with it sometimes dangerous, 233.

Kleiss, Mr Van, discovers the electric shock, 9.

Lamp, inflammable air one, p. 477, n° 46.

Lateral explosion, p. 472, n° 23.

Lead more easily destructible by electricity than other metals, n° 160.

Leyden vacuum, p. 470, n° 15.

Leyden phial discovered, n° 9. Described, 37. Explained, 46. Dr Franklin's method of accounting for the phenomena, 70. Another explanation, 108. To charge and discharge it, p. 409, n° 14. Milner's account of it, and experiments upon it, n° 162.

Light proved to be the same with the electric fluid, 82. Its action compared with that of electricity, 84. Further proofs of their identity, ib. Proved to be a vibration of the electric fluid, 88. How to exhibit the electric light flashing between two metallic plates, p. 468, n° 7. The star and pencil of electric light, p. 467, n° 5. Changes of colour in this light by the admission of air into a vacuum, n° 132. Always visible in the most perfectly exhausted receiver, 140. Diminishes in a great degree of rarefaction, 142.

Lightning suspected by Dr Franklin to be the same with the electric fluid, 12. Their identity proved, 13. Danger of making experiments with lightning, 14. Conductors used for preserving houses from it, 16. Passes over such parts of masts as have been covered with lamp-black and tar, p. 478, n° 1. Blasting and mildew supposed to be owing to it, 246.

Lycium, or tourmalin, its electricity discovered, 1. See Tourmalin.

Machine, electrical, described, 16, et seq. Different kinds of them, 28, et seq.

Magnetism given to large needles by the great machine at Haarlem, 143. Probably caused by electricity, 274.

Marum, Dr Van, his great electrical machine described, 35. Experiments with it, 143. On various kinds of airs, 179.

Medical apparatus, 33.

Medical electricity, 264, et seq.

Medicated tubes, 11.

Medicine, surprising powers in it ascribed to electricity, 11.

Mercurial gages, Mr Brookes's method of preparing them, p. 484, note E.

Metals; how to produce circular spots upon them by means of electric explosions, 93. Proofs of the electric fluid passing over the surface of them, 97. To strike metals into glass, p. 471, n° 21. Experiments on the calcination and revivification of them by the electric shock, 123. Comparative efficacy of them as conductors of electricity, 148. Remarkable difference among them in conducting the shock of the gymnus electricus, 258.

Mildea, supposed to be owing to lightning, 246.

Milner's account of the Leyden phial, and experiments upon it, 162, et seq.

Morgan's experiments on the non-conducting power of a vacuum, 131.

Negative electricity defined, p. 419, col. 1. See Electricity.

Newton, Sir Isaac, discovers that electric attraction and repulsion penetrates glass, n° 3.

Nicholson's experiments on excitation, &c., 160. See Excitation. His machines superior to those of Van Marum, 173.

Non-conductors defined, p. 418. See Electricity.

Nollet, Abbe, discovers the fallacy of what electricians had said concerning medicated tubes, 11. Cautioned them against making experiments with lightning, 14. His theory of electricity, 60.

Paper stained by the electric shock, p. 472, n° 22. By the calcination of metals upon it, n° 154.

Pencil. Stroke of a black lead one conducts a strong electric shock, 97. Pencil of electric light exhibited, p. 467, n° 5.

Phenomena of electricity accounted for by various theories, p. 440, et seq. See Electricity.

Philosophical purposes: electrical machine proper for them, n° 32.

Phlogisticated air: effects of a strong electric shock on metals confined in it, 155.

Picture, magic, p. 474, n° 32.

Pistol fired by inflammable air, p. 476, n° 36.

Planetary motions imitated by Mr Gray, 7. The electric fluid supposed to be concerned in producing them, 274. Various imitations of them, 276.

Plate machine inferior to a cylinder, 163.

Plates, metallic: electric light flash between them, p. 468, n° 7.

Points, discharge electricity violently, p. 472, n° 24, 25. Their action accounted for, n° 106. By Mr Nicholson, 177.

Positive Electricity defined, p. 418. See Electricity.

Priestley's electrical machine defined, n° 28. His method of observing the electricity of the tourmalin, 54. His experiments showing that the electric fluid throws light conducting substances before it, in order to facilitate its passage through the air, 125. His method of showing that it prefers a short passage through the air to a long one through a good conductor, 127. His experiments on different kinds of air, 181.

Prime conductor first added to electrical machines by Mr Bose, 8. Described, 24.

Rackfrow, Mr, remarkably loud report of a jar belonging to him, 92.

Rain: Mr Cavallo's instrument for observing its electricity, 217.

Rawfibb, experiments on sending the electric shock over its surface, 90.

Reid, Mr, his electrical machine, 31.

Report, remarkably loud one of a jar, 92. Of a battery, 161.

Repulsion, the only proper method of determining the power of electricity, 183. See Attraction.

Refined and vitreous electricity defined, p. 419, col. 1. Discovered by M. du Fay, n° 8.

Rielman, professor, killed by lightning, 15.

Ribbons: experiments on them by Mr Cigna, 44.

Room: how to electrify the air of one, 94.

Rosin, its electric phenomena considered, 52. Thrown off from a chain by an electric shock, 81.

Rubber of the electrical machine described, 20. How to insulate and support it, 22. Mr Jones's improvement on the spring of it, 23. How excitation takes place with a simple rubber without a silk flap, 167.

S.

Sauflure, M. his improvement on Mr Cavallo's electrometer, with experiments, 185, et seq.

Sealing-wax rendered transparent by Mr Haukbee, 4. Best rubber for exciting it, 24. Its electric phenomena, 53. Another experiment by Beccaria, in which it is rendered transparent, 86.

Shock, electrical, discovered, 9. How to give one to any part of the body, 266.

Shuttlecock, electrical, p. 467, n° 3.

Silk,

Silk, electric phenomena of it, p. 433, col. 1. The silk flap applied to the cylinder the principal cause of excitation, p. 500, n° 161.

Silurus Electricus, described, 262.

Sparks, electric, drawn from different substances, p. 468, n° 6. Electric spark made visible in water, p. 471, n° 19. Why the electric fluid assumes this appearance, n° 135. Difference between the appearance of positive and negative sparks, 175.

Spider, seemingly animated by electricity, p. 473, n° 28.

Spirits fired by electricity, p. 468, n° 8.

Spots, circular, made on metals by the electric explosion, n° 93.

Stockings, silk, strong attraction between them when electrified, 43.

Sulphur, the most proper rubber for exciting it, 24. Its electrical phenomena, 52.

Summer: electricity weaker in that season than in winter, 199.

Surface of bodies: passage of the electric fluid over it considered, 89.

Surinam river: the gymnopus electricus sometimes found here of a vast size, 257.

Symmer's experiments on electrified silk-stockings, p. 433, col. 1. On glass plates, 47.

Syphon, electrified capillary, p. 476, n° 39.

Terms in electricity defined, p. 418.

Teyler's museum, great machine there described, p. 35. Its vast power, p. 465, col. 2. Of the battery applied to it, 143, 144. See Battery. Experiments on different kinds of airs with it, 179.

Thales, the first who mentions electricity, 1.

Theophrastus discovers the electricity of the tourmalin, 1.

Theories, various, of electricity, n° 440, et seq.

Thermometer, electrical air one, n° 228.

Tinged glass readily transmits a shock of electricity over it, 25.

Tin-wire, curious phenomenon by exploding it, 151. Explained, 152.

Torpedo, its electrical properties, 259. Artificial one made by Mr Walib, 260. Why no spark is discovered in giving a shock with it, 261.

Tourmalin, its electricity discovered by Theophrastus, 1. Dr Priestley's experiments upon it, 53.

Transparency, surprising experiment of Hauksbee upon it, 4. Of Beccaria and Dr Priestley, 86.

Trefian, Count de, his system of natural philosophy, 275.

Tubes, glass, a necessary part of the electrical apparatus, 24. Broken by electrical explosions, p. 471, n° 18.

How perforated, p. 477, n° 45. Surprising case with which an exhausted tube is charged with electricity, 133. Tubes perforated in this manner, 134.

Vacuum of an ordinary air-pump, a good conductor, 97. The Leyden vacuum, p. 470, n° 15. To make a beautiful appearance in vacuo, p. 477, n° 43. Non-conducting power of a perfect vacuum, 130. Mr Morgan's experiments on this subject, 131. Conclusions from them against the ascent of the electric fluid above our atmosphere, 137. Shown to be insufficient, 277.

Vegetable injured by an electric shock, 245.

Vegetation, effects of electricity on it, 233 et seq.

Velocity of the electric fluid: experiments on it, 126. Of the velocity of a cylinder necessary to produce the utmost degree of excitation, p. 501, n° 164.

Vial. See Leyden Vial.

Vitreous electricity defined, p. 419.

Volcanoes, vast quantity of electricity emitted by them, 202.

Volta, Signior, discovers the electrophorus, 15. His condenser described, 221. Cavallo's electrometer, improved by Saussure, serves instead of this instrument, 186.

Walib, Mr, explains the electrical properties of the torpedo, 259. Makes an artificial one, 260.

Water, experiments by sending the electric fluid over its surface, 91. How illuminated, p. 477, n° 42. Becomes electric by cold, 128. Phenomena of the calcination of metals in it, 157. Evaporates more slowly on a red-hot metal than on one heated to a lesser degree, 204. How to electrify it in reservoirs, 244.

Watson, Dr, discovers the electric fluid to come from the earth, 57. His theory of afflux and efflux, 58. His experiments on the electric light in vacuo, 98. On the velocity of the electric fluid, 126.

Well, electric, p. 467, n° 9.

Wheel, self-moving, p. 473, n° 31.

Wilcke's experiments on sulphur, gum-lac, &c, 52.

Wilson's theory of electricity, 62.

Winter, electricity stronger in it than in summer, 199.

Wires, how melted by an electric battery, 81. Lengths of wires of different kinds melted by the great battery of the Haarlem machine, 146. Shortened by the discharge, 149. Very small wires cannot discharge this battery entirely, 150.

ELE