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. History of Electricity.

Tho' 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 lycium (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, of Doctor 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 eletrum), were called electrics. 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 further 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 he 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. See Astronomy, No. 101.

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 will 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.

Mr 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 that 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 stained a line across the place. The middle of this line was silk, the rest pack-thread. Over the filken 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 80 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 till their experiments; still adding more conducting line, 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 actually 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; tho' in this science, 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 imagined that he had discovered in all electric substances a perpetual attractive power, which required no kind of excitation either by heating, rubbing, or any kind of attrition. He took 19 different substances, 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 surfaces hardened, he says they 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 flockings 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 abovementioned. But the most remarkable experiments mentioned by Mr Grey's imagines are his imitations of the planetary motions. "I have he can immediately make, (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 of rosin 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 anything 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 day-light. 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 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, the vitreous and resinous 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 Bofe, professor of philosophy at Wittenburgh; 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 Cuneus, a native of Leyden, who exhibited it as he was repeating some experiments made by Messrs Mutenbroek 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 Lieberkühn 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 stuns my arms and shoulders.

"A tin tube, or a man placed upon electrics, is electrified much stronger by this means than in the common way. When I present this 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. Mutenbroek 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. Glass 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 Mutenbroek, who tried the experiment with a very thin glass bowl, told Mr Reaumur in a letter written 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, shewed their heroism and magnanimity, by receiving a number of electric shocks as strong as they could possibly make them; Mr Bofe abovementioned, 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 shewing 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 asserted by Signor Pivati at Venice, and after him by Verati at Bologna, Mr Bianchi 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; 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 doubts been found to transpire through the pores of excited glas, 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 (the Index subjoined to) MEDICINE.

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, tho' the experiment was several times made in the dark, and with some continuance. At last the doctor 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 familiarity 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 Abbe 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 suspicions 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 suspicion 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 centry-box, where the rain could not come; while on the outside it was fastened to three wooden poles by long silk strings defended from the rain. This machine happened to be the first that was favoured with a visit of the etherial fire. Mr Dalibard himself was not at home; but, in his absence, he had entrusted the care of his apparatus to one Coiffer a joiner, who had served 14 years among the dragoons, and on whose courage and understanding he could depend. This artian 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, Coiffer heard a pretty loud clap of thunder. Immediately he ran to the machine, taking with him a vial furnished with a brafs wire; and prefenting 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 vial, or from the bar. He did not attend to it at the time; but the pain continuing, he uncovered covered his arm when he went home in the presence of Coiffier. 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 high 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 cautions 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 received such a shock, that the ball fell out of his hands, and he was struck backwards four or five paces.

The greatest influence 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 gunpowder), 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 spirited 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 stocking was entire; as was the coat everywhere, the waistcoat only being fingered on the forefinger 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 fingered 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 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 Sect. II. Of the Phenomena of Electricity.

These are so many, and so various, that, in order to avoid confusion, it is necessary to divide them into distinct classes. It is, however, necessary, before entering upon any particular discussion of the phenomena, to say something concerning the general method by which the electrical phenomena are made to appear, and the distinction between electrics and non-electrics.

The most common method by which any substance is made to exhibit signs of electricity, is by rubbing it. Warming without rubbing, or blowing air violently upon it, will also in many cases produce signs of electricity; and thus the discharge of cannon, blowing up of powder-magazines, &c., has been found to electrify glass-windows. But these appearances are comparatively slight; and the only effectual method by which any considerable effects can be produced, is by friction.

Every substance which, by any of the above-mentioned methods, is made to exhibit the signs of electricity, such as attracting and repelling light bodies, emitting light, &c., will communicate the same properties to any other that touches it; and the latter is said, during the time that these appearances continue, to be electrified.

Every substance which, by rubbing, warming, or blowing upon it, can be made to exhibit signs of electricity, is called an electric per se; and those substances which cannot be made to exhibit any appearances of this kind, without touching another substance which already shows them, are called non-electrics, or conductors.

At first the catalogue of electric substances was very small; but the industry of philosophers hath now enlarged it to such a degree, that, according to some, there is not a perfectly non-electric substance in nature. This, however, seems carrying the matter too far; for it is certain, that by rubbing a piece of metal as much as we please, it will never be made to exhibit the least sign of electricity while we hold it in our hands. If we fix it upon one of the substances generally called electrics, such as a stick of sealing-wax, a glass tube, &c., and then rub it, we shall in that case indeed produce signs of electricity; but here we certainly have a right to conclude, that it derives its electrical properties from its particular situation, and consequently is not an electric per se.

The catalogue of electric substances is, as we have already said, prodigiously extensive. We are not, however, to imagine, that all of them are equally fit for electrical experiments. There is, in this case, a very great diversity; and some are found to be more proper for one purpose, and some for another. It is therefore very difficult to distinguish absolutely between the strength of one electric and another in all cases; for a substance that cannot be made to emit sparks but with great difficulty, will perhaps attract very strongly; and another which attracts but weakly, will emit sparks very vigorously.

This distinction, though hitherto not taken notice of, seems to be the most natural foundation for the classing of electric substances; and thus we may divide them in the following manner:

1. For exhibiting a permanent and very strong attractive and repulsive power, silk is preferable to all other substances yet discovered.

2. For exhibiting the electric light, attraction and repulsion in quick succession, and in general all the phenomena of electricity, in a very vigorous, though not a durable, manner, glass is preferable to every other body, and is the most generally made use of.

3. Those substances commonly called negative electrics, such as amber, gum-lac, sulphur, rosin, and all the resinous gums, exhibit electric appearances for the greatest length of time; a single friction being sufficient to make them do so for months together, in favourable circumstances. They are also very remarkable for the strong electric power they communicate to conducting bodies which come into contact with them, and which they will continue to do for a great length of time, as if they contained an inexhaustible supply of the fluid.

In this order, therefore, we shall treat of the electric powers of different substances. It is, however, still necessary to premise an explanation of some terms made use of by electricians, without the frequent repetition of which, it is impossible to speak intelligibly on the subject.

1. If any substance shall, by friction, or any other means, be made to exhibit signs of electricity, the idea of electric virtue of that substance is said to be excited; or, terms, to avoid a circumlocution, the substance itself is said to be excited. This phrase differs from the other already mentioned, of being electrified; because the latter implies that the electricity is communicated by some external body; whereas the being excited implies, that the electric power is originally inherent in the body itself.

2. Any non-electric, or conducting body, being placed upon an electric per se, and thus having its communication with other non-electrics cut off, is said to be insulated.—Here it must be observed, that the common air we breathe is an electric substance, so that a body is perfectly insulated though it should remain in contact with the air all round. The great use of insulation, is to prevent any substance from losing its electric virtue in such a short time as otherwise it would do; and because this is found to be the case, it has been supposed that the current of electric matter is flopped by the electric or insulating substance; whence electrics have also obtained the name of non-conductors.

3. There is observed a very strange difference between the electricity produced by some bodies, and that exhibited by others. If two bodies electrified by glass are presented to each other, they will mutually repel, or separate to a greater distance than before. The same thing will happen to two bodies electrified by sulphur, sealing-wax, rosin, &c. But if a body electrified by glass is presented to one electrified by sulphur, or rosin, they will be mutually attracted; and when they meet, there will be no more signs of electricity in either of them, supposing both to have been equally electrified at first. That kind produced by the glass is called the positive, and that produced by the sulphur or rosin the negative, electricity.—Formerly it was thought, that these two kinds of electricity were essentially distinct, and belonged to the glass and sulphur without a possibility of alteration; but now it is found, that glass may be made to electrify ### Electricity

Phenomena trify negatively, and sulphur positively, by very slight alterations in the surface, or the substances with which they are rubbed.—We shall now present the reader with an ample catalogue of electric substances, and the different kinds of electricity produced by them.

| 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. | | Rough glass | Positive | Dry oiled silk, sulphur, or metals. | | | Negative | Woollen-cloth, quills, wood, paper, sealing-wax, white-wax, the human hand. | | Tourmalin | Positive | Amber, or air blown upon it. | | | Negative | Diamond, the human hand. | | Hare's skin | Positive | Metals, silk, loadstone, leather, hand, paper, baked wood. | | | Negative | Other finer furs. | | Black silk | Positive | Sealing-wax. | | | Negative | Hare's, weasel's, and ferret's skin, loadstone, braids, silver, iron, hand. | | White silk | Positive | Black silk, metals, black cloth. | | | Negative | Paper, hand, hare's, weasel's skin. | | Sealing-wax | Positive | Metals. | | | Negative | Hare's, weasel's, and ferret's skin, hand, leather, woollen-cloth, paper. | | Baked wood | Positive | Silk. | | | Negative | Flannel. |

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.

### Wool, Silk

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 brais 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 a turkey, s; scale of a carp; the chrysalis of a moth, recent from the earth, cleansed; cranium 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; quinquina, or Peruvian bark, s; tamarind-flone; coffee-berry roasted, s; nutmeg, s; ginger, s; white pepper, freed from the husk, s; cinnamon, s; cloves, s; mace, s; all-spice, s; capsicum, both sides of the pod, s; hemlock, s; a clove of garlic; ditto of echalot, freed from the husk, 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; sage, s; thyme, s; carrot; turnip; potatoe; 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 maff, w; an apple, s; down of the cotton-ruhb, 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-christi nut; horseradish.

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

### Coral

Wool. Silk. Phenomena

Neg. Pos.

Pos. Neg. Sea-fan; the horny part, w; rough coral, w. Spange, w; coral polished, w.

S A L T S. Allum, w. Neg. Neg. Borax, Nitre purified, smooth surfaces; Pos. Pos.

F O S S I L A N D M I N E R A L S U B S T A N C E S. Common pebble-stones of all colours, s; marble, s; pit-coal, s; black-lead, w; jet, s; amber; mineralized sulphur; thunder-bolt stone; cornammonium; shark's-tooth; coat of petrifaction. Several smooth native crystals; brown Iceland ditto; talc, s; Ceylon pebble, smooth and transparent; agate, s; cornehan; amethyst, s. A specimen of gypsum. Neg. Pos.

A R T I F I C I A L S U B S T A N C E S. 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, burnt into glass, unburnished; pearl-barley, w; Indian ink, w; blue vitriol, s. Dr Lewis's Glass porcelain. Neg. Pos.

Here it must be observed, that a great number of the substances in Mr Henly'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 tho' 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.

§ 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 tracted and highly excited; the white flocking positively, and the black negatively. While they were kept at a distance from each other, both of them appeared inflated to stockings such a degree, that they exhibited the entire shape of the leg. When two black, or two white flockings, 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 flocking 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 flockings catch at each other at greater distances than otherwise they would have done, and afforded a very curious spectacle.

When the flockings 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 phenomena 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 flocking 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 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 forever they were removed from the plain, the upper one was negatively, and the lower one positively, electrified.—If both ribbons were black, all these experiments succeeded in the same manner as with the white. If, instead of the ivory ruler, he made use of any skin, or a piece of smooth glass, the event was the same; but if he made use of a stick of sulphur, the electricities were in all cases the reverse of what they had been before the ribbon was 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 abovementioned, the ribbons acquired both of them the negative electricity. If the ribbons were one black, and the other white, whichever of them was laid uppermost, and in whatever manner the friction was made, the black generally acquired the negative, and the white the positive, electricity.

He also observed, that, when the texture of the upper piece of silk was loose, yielding, and retiform, like that of a flocking, so that it could move, and be rubbed against the lower one, and the rubber was of such a nature as could communicate but little electricity to glass, the electricity which the upper piece of silk acquired, did not depend upon the rubber, but upon the body on which it was laid. In this case, the black was always negative, and the white positive. But, when the silk was hard, rigid, and of a close texture, and the rubber of such a nature as would have imparted a great degree of electricity to glass, the electricity of the upper piece depended on the rubber. Thus, a white silk 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 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 electric 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 electric 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, 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 spark 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 tinfoil, 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 Phenomena of electrified silk, was induced to try the cohesive power of electrified glass. For this purpose, he got 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 glass, he took off the coating from the negative side, and applied another uncoated and uncharged (or unelectrified) 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 too often, after remaining 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 or 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. By Mr when he made use of plates of looking-glass, or window Healey 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 Æpinus 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 insulated 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.

This diffusive power of glass he thought proper to try in a different manner. A tube was procured of about Phenomena 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 streamed 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 sensibly 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; 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 diffused electricity was discharged; and his whole arm was violently shocked. The old tube, after being heated as abovementioned, shewed 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 successive charge, and at last became exceedingly small; but after the tube had stood a few hours uncharged, it was as vigorous as ever.

Mr Cavalli 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 Plate XCIX. fig. 13. 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 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 thicknesses, which he found to be between seven and eight parts of a thousand of an inch. The balls retained their virtue for six years, but in a less degree. Mr. Lollini 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. Henry in the 6th 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 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, Rosin, 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 distinguished it by the name of spontaneous electricity. He melted sulphur in an earthen vessel, which he placed upon conductors; then, Mr. Wilcke, 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 conductors 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.

Pursuing 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. Epinus. 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 shewing signs of electricity on the removal of the glass. Gum lac, rosin, &c. agree in the same general properties with sulphur, but do not become so strongly electrified spontaneously, nor are they so easily excited.

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* See no. 6. 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, the moving engine, and the prime conductor, i.e., an inflated conductor, which immediately receives the electricity from the excited electric.

Formerly, different kinds of electrics were used, as glass, rosin, sulphur, sealing-wax, &c. Their forms were also 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 insulates 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 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 yet 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 workman to let them pass gradually from the heat of the glass-house, 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 apparatus, 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 must be stirred very often: afterwards it is left to cool, and referred 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 as to 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 strap is put over the wheel and the pulley; and by these means, when the winch is turned, the globe or cylinder makes three or four revolutions, for one revolution of the wheel. There is an inconvenience generally attending this construction, which is, that the string is sometimes so very slack, that the machine cannot work. To remedy this inconvenience, the wheel should be made movable 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 answers perhaps as well as any other; 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, is nothing more than a silk-cushion stuffed with hair; and over this cushion is put a piece of leather, on which Apparatus. Some amalgam has been rubbed so as to stick as fast as possible to the leather. This amalgam has been found to excite smooth glass the most powerfully of anything yet tried. That generally used, is made with two parts of quicksilver and one of tin-foil, with a small quantity of powdered chalk, mixed together until they become a mass like paste. But an amalgam of quicksilver and bismuth is now found to be much more powerful. Some time ago it was generally used, and it is now customary also, to make the rubber of red silk skin stuffed with hair; but the silk one, as above described (which is an improvement of Dr Nooth) is much preferable. If this silk cushion, on account of adapting it to the surface of the glass, is to be fixed upon a metal plate, then care should be taken to make the plate free from sharp points, edges, or corners; and it should be as much as possible concealed, or covered with silk. In short, to construct the rubber properly, it must be made in such a manner, that the side of it, which the surface of the glass enters in whirling, may be as perfect a conductor as it can be made, in order to supply electricity as quickly as possible; and the opposite part should be as much 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 by experiment to be the case when the rubber is not made in a proper manner.

The rubber should be supported by a spring, by which means it may easily lute any inequalities that may be found on the surface of the glass; and by a screw, it may be made to press harder or softer as occasion requires. It should also be insulated, in whatever manner is most convenient; for, whenever insulation is not required, a chain or wire, &c. may be occasionally hung upon it, and thus communicate with the earth, or with any other body, at pleasure; whereas, when there is not a contrivance for insulating the rubber, many of the most curious experiments in electricity will never be performed with the machine.

We come now to consider the prime conductor, or conductor, &c. 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 plate-lead 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 dissipation 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 incumbrance.

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 directions electricity, are coated electrics; among which, glass for coating, coated with conductors obtains the principal place: on jars, &c. 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 it will contain; its thickness only is to be considered: for the thinner it is, the higher charge it is capable of receiving; 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 15 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 with gum-water, bees-wax, &c. but not with varnish, for this is apt to be set on fire by the Apparatus. 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 this purpose, at least be cheaper; but, except Father Beccaria's method, which may be used very well, no remarkable discovery has been made relating to this point. He took equal quantities of very pure colophonium, and powder of marble fitted 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 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 broke, can never 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 Apparatus 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-balls electrometer, was invented by Mr Canton; the discharging electrometer was invented by Mr Lane, and hath been improved by Mr Henly; another on a different principle by Mr Kinnersley; and the quadrant electrometer, which is of latest invention, is a contrivance of Mr Henly.

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, which are kept in a situation parallel to one another, about four inches asunder, by two small pieces of board properly adapted to that purpose. These boards, when set horizontally upon a table, and there fixed by fastening the lower of them with iron cranks, 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, which reaches almost the whole length of the upper board; and, by means of screws, may be placed at any required distance from the other pillar, which is fixed, being let through a mortice in the upper board, and strongly 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 also cylinders, or spheroids of different sizes. In this machine, says Dr Priestley, more than one globe or cylinder may be used at once, by fixing them one above the other in the different holes of the pillars; and, by adapting to each a proper pulley, they may be whirled all at once, and their power united in order to increase the electricity (a): but in this construction different rubbers cannot be conveniently applied to them all; which is a capital imperfection.

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 baked wood, the opposite part of which is inserted into the socket of a bent steel spring. These parts are easily separated, so that

(a) When several globes are used at once, and their power united, it has been found by experiment, that the electricity does not increase in proportion to their number, although it is more than what may be produced by a single globe. Plate CII. fig. 1. shews a machine of this kind formerly used by Dr Watson. Apparatus that the rubber, or the piece of wood that serves to insulate it, may be changed at pleasure. The spring admits of a twofold alteration of position. It may be either slipped along the groove, or moved in the contrary direction (the groove being wider than the screw which fastens the spring), so as to give it every desirable position with respect to the globe or cylinder; and it is, besides, furnished with a screw, which makes it press harder or lighter, as the operator chooses.

The wheel of this machine is fixed to the table; it 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 thus suited to the variable length of the string.

The prime conductor is hollow copper, made in the shape of a pear, fitted with its neck upwards, and with its bottom or rounder part upon a stand of baked wood; and an arched wire proceeds from its neck, having an open ring at its end, in which some small pointed wires are hung, that, by playing lightly upon the electric, collect the electric fluid from it.

Next to Dr Priestley's machine is one invented by Dr Ingenhouz, 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 flender and smaller ones are raised, which lie parallel 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 brass; 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.

The last machine we shall describe, is that represented in fig. 1. (Plate XCIX.) which has all the improvements hitherto made, except that it is not capable of admitting different kinds, or more than one electric; but which, indeed, it seems not to stand in need of. The electric power of such a machine is equal to what may be obtained by any other construction; and at the same time its size, being neither remarkably large, nor at all inconvenient, renders it the completest hitherto contrived.—These machines are made and sold by Mr George Adams, in Fleet-street, London, philosophical instrument-maker to his majesty.

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

The rubber G of this machine is on each end two inches shorter than the cylinder, (i.e. the cylinder exclusive of the necks), and it is made to rub about one fourth part of the cylinder's circumference. It consists of a thin quilted cushion of silk, stuffed with hair, and fastened by silk strings upon a piece of wood, which is properly adapted to the surface of the cylinder. From the upper extremity of the cushion proceeds a piece of oiled silk, that covers almost all the upper part of the cylinder; and to the lower extremity of the cushion, or rather of the piece of wood to which the cushion is tied, a piece of leather is fastened, which is turned over the cushion, i.e. stands between it and the surface of the cylinder. Upon this leather, which reaches from the lower to almost the upper extremity of the cushion, some of the above-described amalgam is to be worked, so as to be forced as much as possible into its substance. This rubber is supported by two springs, screwed to its back, and from which it may be easily unsecured when occasion requires. The two springs proceed from the wooden cap of a strong glass pillar (b), perpendicular to the bottom board of the machine. This pillar has a square wooden basis, that slides in two grooves in the bottom board ABC, upon which it is fastened by a screw. In this manner the glass pillar may be fastened at any required distance, and in consequence the rubber may be made to press harder or lighter upon the cylinder. The rubber in this manner is perfectly insulated; and when insulation is not required, a chain with a small hook may be hung to it, so as to have a regular communication with the piece of leather; the chain then falling upon the table, renders the rubber uninsulated.

Fig. 2. represents the prime conductor AB belonging to this machine. This is of hollow brass; and is supported by two glass pillars varnished, which by two brass sockets are fixed in the board CC. This conductor receives the electric fluid through the points of the collector L, which are set at about half an inch distance from the surface of the cylinder of the machine.

(b) This glass pillar, as well as the glass feet of insulating stools in general, should be covered with varnish, or rather with sealing-wax; otherwise they will insulate very imperfectly, on account of the moisture that they attract from the air in damp weather. If the handle E, fig. 1, of the wheel, be turned, (and, on account of the rubber, it should be turned always in the direction of the letters a b c) this machine, standing in the situation that is represented in the figure, will give positive electricity, i.e., the prime conductor will be electrified positively, or overcharged with 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 conductor. But if a negative electricity is required, then the chain must be removed from the rubber and hung to the prime conductor; for in this case the electricity of the prime conductor will be communicated to the ground, and the rubber remaining insulated will appear strongly negative. Another conductor equal to the conductor A B, fig. 2, may be connected with the insulated rubber, and then the operator may obtain as strong negative electricity from this, as he can positive from the conductor A B, fig. 2.

Fig. 4 represents a stand supporting the electrometers D D C C. B is the basis of it, made of common wood. A is a pillar of wax, glats, or baked wood. To the top of the pillar, if it be of wax or glats, 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 glats, or baked wood, suspending at their ends four electrometers, two of which D D are silk threads about eight inches long, suspending each a small downy feather at its end. The other two electrometers C C are those with very small balls of cork, or of the pith of alder; and they are constructed in the following manner. A b is a stick of glats 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 (c) c about five inches long, each suspending a cork or pith-ball d about one-eighth of an inch in diameter. When this electrometer is not electrified, the threads c c hang parallel to each other, and the cork-balls are in contact; but when electrified, they repel one another, as represented in the figure. The glats stick a b serves for an insulating handle, by which the electrometer may be supported when it is used without the stand A B.

Another species of the above electrometer is represented in fig. 3, 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. 7 represents Mr Henly'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.

The properest wood, for the purpose of making the pillar and index of this electrometer, is box; and this pillar and index should be well rounded, and made as smooth as possible. When this electrometer is not electrified, the index hangs parallel to the pillar, as in fig. 7; but, when it is electrified, the index recedes more or less, according to the quantity of electricity, from the item; as represented on the prime conductor E, fig. 2.

The main of M. Lane's discharging electrometer consists in a brass ball, about one inch and a half in diameter, screwed to a brass graduated rod, and adapted to a proper frame, so that it may be set at any required distance from the prime conductor, or the knob of an electric jar. The principal use of this electrometer is to let a jar discharge by itself through any proper circuit, without using any discharging rod, or removing any part of the apparatus; and to give shocks nearly of the same strength. Suppose, for instance, that the above-mentioned brass ball be set at half an inch distance from the prime conductor, and that a coated jar be situated so as to touch the prime conductor with its knob, and to have its outside coating communicating with the abovementioned brass ball. Now, it is evident, that the circuit, from the outside to the inside of the jar, is interrupted only between the prime conductor and the brass ball, which lie half an inch asunder; therefore, when the jar is charging, and the charge is become so high as to strike through half an inch of air, the jar will discharge itself; and by keeping the brass ball at the same distance from the prime conductor, and charging the jar successively, the shocks will be of the same strength.

This electrometer is, however, subject to a great inconvenience; which is, that the surface of the brass ball is often deprived of its smoothness by the force of the explosion, in which case it becomes unfit for use. The principal use for which this electrometer is intended, i.e., to give shocks of the same strength, may be more elegantly obtained by the above-described quadrant electrometer, which suffers no damage by the discharges; hence a delineation and a more particular description of the discharging electrometer is unnecessary.

Fig. 5 represents Mr Henly's universal discharger, Mr Henly's universal discharger, which is of a very extensive use, and is composed of the following parts. A is a flat board 15 inches long, four inches broad, and one thick, or thereabouts, which forms the basis of the instrument. B B are two brass pillars cemented in two holes upon the board A, and furnished at their top with brass caps, each of which has a turning joint, and supports a spring tube, through which the wire D C slides; each of these caps is composed of three pieces of brass, connected so that the wire D C, besides its sliding through the socket, has two other motions, viz. an horizontal and a vertical one. Each of the wires D C, D C, is furnished with an open ring at one end, and at the other end has a brass hall D, which, by a short spring socket, is slipt upon its pointed extremity, and it may be removed from it at pleasure. E is a strong circular piece

(c) These threads should be wetted in a weak solution of salt. piece of wood five inches in diameter, having, on its surface, a lip of ivory inlaid, and furnished with a strong cylindrical foot, which fits the cavity of the socket F, which is fastened in the middle of the bottom board, and has a screw G, which serves to fasten the foot of the circular board E at any required height. H is a small press belonging to this instrument; it consists of two oblong pieces of board, which may be pressed against each other by means of two screws a a: the lower of these boards has a cylindrical foot equal to the foot of the circular board E. When this press is to be used, it is fixed into the socket F, in the place of the circular board E, which must, in that case, be removed.

Fig. 11. is an electric jar coated with tinfoil on the inside and outside, within three inches of the top of the cylindrical part of the glass, having a wire with a round brass knob A at its extremity. This wire passes through the cork D, that stops the mouth of the jar, and at its lower end is bent so as to touch the inside coating in several places. When corks are used to stop electric jars, they should be made very dry, and dipped in melted bees-wax or varnished.

Fig. 10. represents a battery composed of 16 jars coated in the inside and outside with tinfoil, which all together contain about 12 feet of coated glass. About the middle of each of these jars is a cork that sustains a wire, which at the top is fastened round or soldered to the wire E knobbled at each end, which connects the inside coatings of four jars; and by the wires F F F F the inside coatings of all the 16 jars are connected together. Each of the wires F has a ring at one end, through which one of the wires E passes, and the other end has a brass knob. If the whole force of the battery is not required, one, two, or three rows of jars may be used at pleasure: for as each of the wires F F F F is moveable round the wire E, which passes thro' its ring, and rests upon the next wire E, it may be easily removed from that, and turned upon the contrary wire E; and thus the communication between one row of jars and another may be discontinued at pleasure. See the figure.

The square box that contains these jars is of wood lined at the bottom with sheet-lead or tin, and has two handles on two opposite sides, by which it may be easily removed. In one side of the box is a hole, thro' which an iron hook B passes, which communicates with the metallic lining of the box, and consequently with the outside coating of all the jars. To this hook is fastened a wire, the other-end of which is connected with the discharging rod.

The discharging rod consists of a glass handle A, and two curved wires B B, which move by a joint C, fixed to the brass cap of the glass handle A. The wires B B are pointed, and the points enter the knobs D D, to which they are screwed, and may be unscrewed from them at pleasure. By this construction we have the opportunity of using the balls or the points, as occasion requires; and as the wires are moveable by the joint C, they may be adapted to smaller or larger jars at pleasure.

The battery, represented in the plate, is a small one in comparison to those now frequently used, and much too weak for the purpose of some experiments, hereafter to be described. But when a large battery is to be constructed, it is better to make two, three, or more small ones, as represented in the plate, than a single large battery, which is heavy, and, on several accounts, inconvenient. The force of several small batteries may be easily united by a wire or a chain, and thus they may be made to act in every respect like a large one.

F in fig. 2. is a circular brass plate hung on the prime conductor by a chain, and resting in an horizontal position. Underneath this, there is another plate P parallel and equal to the former (but it would be better if it was a little larger), which is supported by a stand H of brass, having also a socket to receive the foot of the plate, and a screw G to fix it at different distances.

D in fig. 2. is a fly made of small brass wires fixed in a cap of brass also, which is to be put upon the pointed wire K, that is screwed to the prime conductor, upon which it must stand in equilibrium, like the needle of a compass. The other ends a, b, c, d, of the wires are pointed and bent all one way.

It is highly requisite for an electrician to have by him several insulating stools, or stands, they being very necessary for several experiments. The best materials to construct these are glass covered with sealing-wax, and baked wood (a). A large stool, proper to insulate a chair upon, or two or three persons standing, may be made with a strong board, about two feet and a half square; and may be supported by four feet of glass, or baked wood, about eight inches long. But small stands are better made with one foot or pillar, and all of baked wood or glass, without any conducting substance in their construction. Drinking-glasses, either varnished, or in part covered with sealing wax, answer this purpose very well.

§ 3. Practical Rules concerning the Use of the Electrical Apparatus, and the performing of 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

(A) The wood should be baked very well, even till it becomes quite brown, it being then in the best state for insulation; and to make it still better, i.e., to defend it from moisture, it may be slightly varnished as soon as it comes out of the oven, or else boiled in linseed oil; but in this case, after boiling, it should be made hot again, and then it is fit for use. 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 fuet, 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 conducting state. 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 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 burnt 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 proportionably; 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, 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. IV. Entertaining Experiments.

I. The electrified cork-ball Electrometer.

Fix at the end of the prime conductor the knobbed rod I.B., fig. 2, and hang on it the electrometer with the cork-balls, fig. 3. 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 E, 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 shewn by the quadrant electrometer, with a large downy feather, or the like; for if these be connected with the prime conductor, and the winch be turned, the electrometer will raise its index, and the feather, by the divergency of its down, will appear swelled in a beautiful manner.

II. Attraction and Repulsion of light Bodies.

Connect with the prime conductor the two parallel brass plates F, P, as represented in fig. 2, at about three inches distance from one another; and upon the lower plate put any kind of light bodies, as bran, bits of 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.

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

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.

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-cork 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, of the kind represented fig. 3; 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, 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 end of the knobbed rod IB, fixed at its end B, fig. 2; for the electric fluid seems to acquire an impetus by going through a long conductor, when electrified by a powerful machine.—This spark (which has the same appearance whether drawn from a prime conductor positively, or negatively electrified) 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 brushes of light sidewise in every direction.

VI. 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.

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 affording light when exposed to it, and afterwards to appear lucid when brought into the dark*.—Take some of this powder, and, by means of spirits of wine or ether, stick it all over the inside of a clear glass phial, and stop it with a glass stopper, or a cork and sealing-wax. If this phial be kept in a darkened room (which for this experiment must be very dark), it will give no light; but let two or three strong sparks be drawn from the prime conductor, when the phial is kept at about two inches 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 these 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 so 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. 6. Plate XCIX. represents a prime conductor invented by Mr Henry, 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 it receives the electric fluid, when set near the excited cylinder of the electrical machine, and the other has a knobbled wire G, from which a strong spark may be drawn; and from each of the pieces F.D., B.E., a knobbled 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 supporters of this instrument are two glass pillars fastened in the bottom-board H, like the prime conductor represented fig. 2. When the glass tube of this conductor is exhausted of air by means of an air-pump, and the brass ball is screwed on, as represented in the figure, then it is fit for use, and may serve for a prime conductor to an electrical machine. If the point C of this conductor is set near the excited cylinder of the machine, it will appear illuminated with a star; at the same time the glass tube will appear all illuminated with a weak light; but from the knobbled 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 C, instead of being presented to the cylinder, be connected with the rubber of the machine, the appearance of light within the tube will be reversed; the knob which communicates with the piece 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. 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 glass is rubbed in the common manner used to excite electricities, it will appear luminous within, being full of a flashing light, which plainly resembles the aurora borealis or northern light. This phial may also be made luminous, by holding it by either end, and bringing the other end to the prime conductor; in this case, all the cavity of the glass will instantly appear full of flashing light, which remains in it for a considerable time after it has been removed from the prime conductor.—Instead of the above-described glass vessel, a glass tube, exhausted of air and hermetically sealed, may be used, and perhaps with 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. 1. Plate C represents the receiver with the plate of an air-pump. In the middle of the plate E, 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, as D E, fig. 2. Plate XCIX, 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 E, fig. 2, 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 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. 8. and 9. of Plate XCIX, 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. 9.; and if it is discharged, a star will appear in the place of the pencil, as represented in fig. 8. 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.

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 thickness of that card, or quire of paper, only is interposed; lastly, by bringing the other knob of the discharging rod near the knob of the jar, make the discharge, and the electric 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 burs on both surfaces of each card. The hole, or holes, are larger or smaller, according as the card, &c., is more damp or more dry. It is remarkable, that if the nostrils are presented to it, they will be affected with a sulphurous, or rather a phosphorescent, 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 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. Henly 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 six 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. 5. of Plate XCIX.

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 C D, fig. 2. Plate C, and insert in it two wires A, B, 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 A or B 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 as represented fig. 3. 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 five 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. C in fig. 4. Plate C, repre- 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-slings, 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. 5. Plate XCIX., 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 of that 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 a terrible shock, put one of your hands in contact with its outside coating; with the other hold a sharp pointed needle, and keeping the point directed towards the knob of the jar, proceed gradually near it, until the point of the needle touches the knob. This operation discharges the jar entirely; and you will either receive no shock at all, or so small a one as can hardly be perceived. The point of the needle, therefore, has silently and gradually drawn all the superfluous fluid from the inside surface of the electric jar.

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 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 electric Fly.

Fix the fly formerly described upon the prime conductor, as represented by D, fig. 2. of Plate XCIX. then turn the winch of the machine, and the fly will immediately begin to move round, in an horizontal position, and in the direction of the letters adeb, i.e. contrary to the direction of the points of the wires. If the experiment is repeated with a conductor negatively electrified, the fly will turn the same way as before, viz. in the direction of the letters adeb. The above fly does not move in vacuo; and even if placed under a close receiver, it will turn but for a little while, and then stop; for the quantity of air contained in the receiver may become readily and equally electrified. If, when the fly under the close receiver is stopped, you put the end of your finger on the outside of the glass, opposite to one of the points of the fly, this will move again briskly; and by altering the position of your finger occasionally round the glass, you may continue its action a considerable time, viz. till most of that part of the glass is charged.

XXVII. 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.

XXVIII. 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 knoked 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 knoked 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.

XXXI. The electrified Capillary Syphon.

Let a small bucket of metal, full of water, be suspended from the prime conductor; and put in it a glass syphon of narrow extremity, so that the water will just drop from it. If, in this disposition of the apparatus, the winch of the machine be turned, the water, which, when not electrified, only dropt from the extremity of the syphon, will now run in a full stream, which will even be subdivided into smaller streams; and if the experiment be made in the dark, it will appear beautifully illuminated.

XXX. The electrified Bells.

Figure 5. of Plate C. represents an instrument having three bells, which are caused to ring by the power of electric attraction and repulsion. B is a brass piece furnished with a hook, by which it may be suspended from the rod proceeding from the extremity of the prime conductor A. The brass 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 part of the bell D a brass chain proceeds, which falls upon the table, and has a silk thread F at its extremity. The apparatus being disposed as in the figure, if the cylinder of the machine be turned, the clappers will fly from bell to bell with a very quick 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 then run again to the bells C, E, from which they acquire more electricity, &c. 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 bell D, remaining insulated, will soon become as strongly electrified as either of the two other bells; in which case the clappers, having no opportunity to deposit the electricity that they acquire from the bells C, E, must consequently stop.

If this experiment be made in the dark, sparks will be seen between the clappers and the bells.

XXXI. The Spider seemingly animated by Electricity.

Fig. 6. of Plate C. represents an electric jar, having a wire C D E 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 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.

XXXII. The Spiral Tube.

Fig. 7. of Plate C represents an instrument composed of two glass tubes CD, one within another, and closed with two knobbled brass caps A and B. The innermost of these tubes 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.

The small round pieces of tin-foil are sometimes stuck upon a flat of glass ABCD, fig. 8. So as to represent curve lines, flowers, letters, &c.; and they are illuminated after the same manner as the spiral tube, i.e., by holding the extremity C or B in the hand, and presenting the other extremity to the prime conductor, when the machine is in motion.

XXXIII. 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. 9. Plate C, and will continue their motion for a considerable time.

XXXIV. 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 purposes 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 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 thro' 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.

XXXV. 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-celler. The upper end of its axis passes through a hole in a thin brass plate, cemented to a long and strong piece of glass; which keeps it fix or eight inches distant from any non-electric, and has a small ball of wax or metal on its top 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.

XXXVI. The Magic Picture.

This is a contrivance of Mr Kinnersley; 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 brass well with leaf-gold or bras. 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 foreside exactly over the back gilding; and when it is dry, cover it, by pasting on the panel of the picture that has been cut out; observing to bring the correspondent parts of the board and picture together, by which the picture will appear as 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.

XXXVII. Imitations of the Planetary Motions.

From the prime conductor suspend six concentric hoops of metal, at different distances from each other; and under them, on a stand, place a metal plate at the distance of about half an inch. Then place upon the plate within each hoop, and near to it, a round glass bubble blown very light: these bubbles and the distances between the hoops should correspond to the different diameters of the planets and those of their orbits; but as that cannot be on account of the vast disproportion between them, it must suffice here to make a difference that bears some relation to them. Now, the hoops being electrified, the bubbles placed upon the plate, near the hoops, will be immediately attracted by them, and they 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 from the hoops, and make a variety of surprising motions round their axis, and running hither and thither on the plate, after which they will come back to the hoops and run round them as before.

If the room is darkened, all the glass balls will appear beautifully illuminated.

Another method of imitating the planetary motions is, by means of a hollow cork or pith ball, suspended by a silk thread, as mentioned under the article Astronomy, No. 102. The same experiment will succeed with a metallic ball strongly electrified either way. It is similar to that by Mr Grey formerly mentioned. As it will not succeed without the candle, (for a vial charged with an electricity opposite to the former will not do), it seems most likely that Mr Grey had succeeded in his experiments by the unheeded circumstance of sometimes having a candle near him when he made them. Other imitations of these motions have been contrived, and an ingenious person may contrive to vary these and other electrical experiments almost infinitely. Small orreries, planetariums, clocks, &c. have been constructed to go by the blast of electric matter issuing from a point: but as these are in no way connected with electricity, and would move as well by means of the draught of air through a chimney, or a current of water, we apprehend it is needless to give any particular description of them.

XXXVIII. The Thunder-house.

Fig. 10. of Plate C. 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 CD is also fixed in a hole about eight inches distant from the basis of the board A. A square hole LMK, about a quarter of an inch deep, and nearly one inch wide, is made in the board A, and is filled with a square piece of wood nearly of the same dimensions. It is mentioned nearly of the same dimensions, because it must go so easily into the hole, that it may drop off by the least shaking of the instrument. A wire LK is fastened diagonally to this square piece of wood. Another wire IH of the same thickness, having a brass ball H, screwed on its pointed extremity, is fastened upon the board A; so also is the wire MN, which is shaped in a ring at O. From the upper extremity of the glass pillar CD, a crooked wire proceeds, having a spring socket F, through which a double knobbled wire slips perpendicularly, the lower knob G of which falls just above the knob H. The glass pillar DC must not be made very fast into the bottom board; but it must be fixed so as 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 EFG. Now when the square piece of wood LMIK (which may represent the shutter of a window or the like), is fixed into the hole so, that the wire LK stands in the dotted representation IM, then the metallic communication from H to O is complete, and the instrument represents a house furnished with a proper metallic conductor; but if the square piece of wood LMIK is fixed so, that the wire LK stands in the direction LK, as represented in the figure, then the metallic conductor HO, from the top of of the house to its bottom, is interrupted at IM, in which case the house is not properly secured.

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

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

Further. Unscrew the brass ball H from the wire HI, 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 IM 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. See Thunder.

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 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 first electricians, all the phenomena were derived from unctuous 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 fastened 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 unctuous 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 frequently enough 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 all experiments, the positive electricity doth manifest a superiority in strength over the negative, something like that superior degree of vigour which is observed in the north pole of a lodestone over the south pole. 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, however, it was imagined, that the electric matter, whether it consisted of one or more fluids, was produced from the electric body by friction; but by a discovery of Dr Watson's, it became the earth 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... ping 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, seeing the rubber, or the man who had a communication with it, gave no sparks but when the conductor was connected with the floor, he as naturally concluded, that the globe was supplied from the conductor, as he had before concluded that it was supplied from the rubber.—From all this he was at last led to form a new theory of electricity, namely, that, in all electric operations, there was both an afflux of electric matter to the globe and the conductor, and likewise an efflux of the same electric matter from them.—Finding that a piece of leaf-silver was suspended between a plate electrified by the conductor, and another communicating with the floor, he reasons from it in the following manner. "No body can be suspended in equilibrium but by the joint action of two different directions of power: so here the blast of electric ether from the floor setting through it, drives the silver towards the plate electrified. We find from hence, likewise, that the draught of electric ether from the floor is always in proportion to the quantity thrown by the globe over the gun-barrel (the prime conductor at that time made use of), or the equilibrium by which the silver is suspended could not be maintained."—Some time after, however, the Doctor retracted this opinion concerning the afflux and efflux, and supposed that all the electric phenomena might be accounted for from the excess or diminution of the quantity of electric matter contained in different bodies. This 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 supposed 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 every body to which electricity is communicated, has two orders of pores, one for the emission of the effluvia, and another for the reception of them.—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, concerning that fire was not a distinct element, but arose from some violent repulsions, rarefactions, &c. among the particles of ignited bodies. The great resemblance of the electric fluid to elementary fire, however, seemed strongly to militate against this opinion. The hypothesis therefore 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 grosser parts had been driven away by the friction of the rubber.

This last opinion, however, soon received a full refutation. futation from the experiments of Dr Watson above- mentioned; by which it was proved, that the elec- tric 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 pre- judice 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 ac- quiesced in, rather than the simple position above- mentioned.—It would be tedious, and indeed impos- sible, to give an account of all the theories which were now invented. One of the most remarkable, and most confident, was that of Mr Wilson.—According to this gentleman, the chief agent in all the operations of elec- tricity, 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 ful- phureous and unctuous bodies. To this ether are ascribed the principal phenomena of attraction and re- pulsion: the light, the sulphureous or rather phos- phoreal smell with which violent electricity is always attended, and other sensible qualities, are ascribed to the grossest 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 sub- tle medium at the surface of all bodies; which is the cause of the refraction and reflection of the rays of light, and also reflects 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 denser and resists it.—The same medium is rarefied by heat, which thus changes conductors into non-conduc- tors.—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 phe- nomena were discovered by the affluence of a number of different electricians in different countries.—Mr Winck- ler observed, that if glass was rubbed on the inside, 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 proportionate; though this has since been denied. 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 ac- counts, 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 o- pened 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. Gor- don, a Scots Benedictine monk, and professor of philo- sophy 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 fall- ing down with giddiness, and small birds were killed by them. This was effected by conveying the elec- tricity 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, ano- ther question of great importance was likewise decided, namely, Whether electricity acted according to the laws of the surface of bodies. This was found to be in proportion to the surface, and not the solid con- tents. The magnetic effluvia also were found not to in- terfere in the least with the electrical ones. An elec- trified 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 attrac- tive virtue of electricity was also found to pervade glass so powerfully, that a thread was attracted thro’ 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 concern- ing 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 subtle and elastic. Be- tween the particles of this fluid there subsists a very strong repulsion with regard to each other, and as strong an attraction with regard to other matter. Thus, according to Dr Franklin’s hypothesis, one quantity of electric matter will repel another quantity of the same, but will attract and be attracted by any terrestrial matter that happens to be near it. The pores of all bodies are supposed to be full of this subtle fluid; and when its equilibrium is not disturbed, that is, when there is in any body neither more nor less than its na- tural 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 di- turbed, 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 elec- trified 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 ei- ther in the atmosphere contiguous to them, or in other neighbouring bodies; which occasions them still to re- cede 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 densest 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 resistance of the air would be taken off, and the electric fluid could come from all other quarters by the attraction of the bodies.

Mr Cavalli, who seems to have undertaken the defense 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 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 any 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 assertion 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 Cavalli 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 inflated 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, 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 Cavalli's supposition.

What gave the greatest reputation to Dr Franklin's theory, however, is the easy solution which it affords of all the phenomena of the Leyden phial. The fluid is supposed to move with the greatest ease in bodies which are conductors, but with extreme difficulty in electrics per se; insomuch 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 part of any electric substance, except the same quantity go out at another, and the gain be exactly equal to the loss. These things being previously supposed, the phenomena of charging and discharging a plate of glass admit of an easy solution. In the usual manner of electrifying by a smooth glass globe, all the electric matter is supplied by the rubber from all the bodies which communicate with it. If it be made to communicate with nothing but one of the coatings of a plate of glass, while the conductor communicates with the other, that side of the glass which communicates with the rubber must necessarily be exhausted in order to supply the conductor, which must convey the whole of it to the side with which it communicates. By this operation, therefore, the electric fluid becomes almost entirely exhausted on one side of the plate, while it is as much accumulated on the other; and the discharge is made by the electric fluid rushing, as soon as an opportunity is given it by means of proper conductors, from the side which was overloaded, to that which is exhausted.

It is not, however, necessary to this theory, that the very same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the space of an equal portion of the natural quantity of fluid belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue till the exhausted side of the glass is supplied by the fluid naturally 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 easily 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 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 repulsion 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... According to the opinions of all electricians before Dr Franklin.

This hypothesis has been in some measure improved by Mr Ampinus, in a treatise entitled, "Tentamen theorice Electricitatis & Magnetismi." He extends the property of impermeability to air, and all electric substances, 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 fluid, 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 facts, which seem in the strongest manner to confirm them.

1. "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 schoolboy'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 A B (Plate CI. fig. 2.) 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 thunder-storm, there is no great danger for the operator to receive any shock. Although I have raised my electrical kite hundreds of times without any caution whatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunder-storm, if the kite has not been raised before, I would not advise a person to raise it while the stormy clouds are just overhead; the danger at such 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 corkscrew electrometer held in the hand in an open place, or, if it rains, by the electroscope for rain, to be described hereafter.

"When the kite has been raised, I generally introduce the string thro' a window into a room of the house, and fasten it to a strong silk lace, the extremity of which is generally tied to a heavy chair in the room. In fig. 14. of Plate XCIX. A B represents part of the string of the kite which comes within the room; C represents the silk lace; D E a small prime conductor, which, by means of a small wire, is connected with the string of the kite; and F represents the quadrant electrometer fixed upon a stand of glass covered with sealing wax, which I used to put near the prime conductor rather than to fix it in a hole upon the conductor, because the string A B sometimes shakes so as to pull the prime conductor down, in which case the quadrant electrometer would be broken. G represents a glass tube about 18 inches long, with a knotted wire cemented to its extremity; which instrument I use to observe the quality of the electricity, when the electricity of the kite is so strong, that I think it not safe to come very near the string. The method is as follows. I hold the instrument by that extremity of the glass tube which is farthest from the wire, and touch the string of the kite with the knob of its wire; which being insulated... Sect. V. ELECTRICITY.

Theory.

fulated, acquires a small quantity of electricity from it, which is sufficient to ascertain its quality when the knob of the instrument is brought near an electrified electroscope. Sometimes when I raise the kite in the night-time, out of the house, where I have not the convenience of observing the quality by the attraction and repulsion, or even by the appearance of the electric light, I make use of a coated phial, which I can charge at the string; and, when charged, put into my pocket, where it will keep charged even for several hours. The construction of this phial is as follows. Besides the coating on the inside and outside, which this phial has in common with others of the same kind, a glass tube open at both ends is cemented into its neck, and proceeds within the phial, having a small wire fastened to its lower extremity, which touches the inside non-electric coating. The wire, with the knob of this phial, is cemented into another glass tube, which is nearly twice as long, and smaller than the tube cemented into the neck of the phial. The wire is cemented so, that only its knob projects out of one end, and a small length of it out of the other end of the tube. If this piece with the wire be held by the middle of the glass tube, it may be put in or out of the tube which is in the neck of the phial, so as to touch the small wire at the lower extremity of it, and that without discharging the phial if it is charged. I have kept such a phial charged for six weeks together, and probably it would keep much longer if it was to be tried.

"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 Cavallio could not always avoid danger, even when there was no thunder; as appears from the following account.—October 18th, 1775. After having rained a great deal in the morning and night before, the weather became a little clear in the afternoon, the clouds appearing separated, and pretty well defined. The wind was weak, 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 insulated, 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 electroscope. 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 electroscope 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 electroscope 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 electroscope 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 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 electroscope 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 isochronous with the shocks which I had received; but, in about a minute's time, oftener; so that the people of the house compared their sound to the rattling noise of a jack going when the fly is off. The cloud now was just over the kite; it was black, and well defined, 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 wind- ing 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 farther observation; being then ten minutes after four o'clock.—N.B. There was neither thunder nor lightning perceived that day, nor indeed for some days before or after."

Besides the kite, Mr Cavallo has given us the following description of some other instruments he uses for discovering the electricity of the atmosphere. "Fig. 11. of Plate C. 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. A B is a common jointed fishing-rod, without the last or smallest joint. From the extremity of this rod proceeds a slender glass tube C, covered with sealing-wax, and having a cork D at its end, from which a pith-ball electrometer is suspended. H G I is a piece of twine fastened to the other extremity of the rod, and supported at G by a small ring F G. 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 K L; 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; and from them I have deduced the following general observations, which seem to coincide with those made with the kites.

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 day time.

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 instance 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 vial to it, rises surprisingly quick to its usual place; but in dry and warm weather it rises exceedingly slow.

From these observations, little doubt can be entertained of the atmosphere's being always full of electric matter. From Mr Cavallo's observations, however, it appears also, that the rain which descends from the clouds is full of electric matter. The method of proving this, is by an instrument called by Mr Cavallo an electrometer for rain, and of which he gives the following description. "A B C I, Plate XCIX. fig. 12. 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. F D is a piece of cane, round which braids wires are twisted in different directions, so as to catch the rain easily, and at the same time to make no resistance to the wind. This piece of cane is fixed into the tube; and a slender wire proceeding from it goes through the bore of the tube, and communicates with the strong wire A G, which is thrust into a piece of cork fastened to the end A of the tube. The end G of the wire A G is formed in a ring, from which I suspend a more or less sensible pith-ball electrometer. This instrument is fastened to the side of the window-frame, where it is supported by strong brafs hooks at C B; which part of the tube is covered with a silk lace, in order to adapt it better to the hooks. The part F C 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 C B. 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, 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 A G. 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.

Plate C, fig. 12, represents a pocket electrometer, which on several accounts seems preferable to those generally in use. The case or handle of this electrometer is formed by a glass tube about three inches long, and three tenths of an inch in diameter, half of which is covered with sealing-wax. From one extremity of this tube, 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. 13. 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. 12. 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. 14 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 has a piece of ivory fastened upon a piece of amber B C. This amber B C serves only to insulate the ivory, which, when inflated, and rubbed against woollen clothes, acquires a positive electricity, and is therefore useful to electrify the electrometer positively."

From this very full explanation of the methods by which the electric fluid can be procured from the atmosphere itself, from rain and vapour, at all times, it is impossible to doubt of the truth of the first position on which Dr Franklin's theory depends, viz. that "all terrestrial substances, as well as the atmosphere which surrounds the earth, are filled with electric fluid."

2. The second position requisite for establishing Dr Franklin's theory is, "That glass and other electric substances, tho' 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 greater charge it can receive.

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 of glass is in that case an expulsion of fire from the outside at 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 insulating stand, and the knob of another phial be brought near the coating of the first. As soon 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 insulated, 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, 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, say 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 repells 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 loses, 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 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 shews, 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 hypotheses 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 most commonly adduced in favour of this position, is that of the following experiment. Take a wire of any kind of conductor, 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 appear 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 twenty 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 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. 4. Plate CI. 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 fluid 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 surface. 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, 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, are all filled with 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.

§ 1. 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 no 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 knobbled 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 15 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 gun-powder in the most unfavourable circumstances that can be imagined, namely, when it was drawn off 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 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 scarcely 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.—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 himself. 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 insufficient quantity, as was observed in the experiment with the large conductor abovementioned. 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 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 darker 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 or 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 further 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 surprising case with which the latter penetrates glass, and the seeming stop which is put to the motions of 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, (for all others we know are 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 shewn that this is not the case. Sealing-wax and pitch are as opaque bodies as we are acquainted with; yet in Mr Hawksbee's experiments mentioned, n° 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. One was made by S. Beccaria. He discharged an electric shock through some brafs 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 at it 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 pursued by any electrician, seem to be of more worthy of notice than almost all others. One them, 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 Hawksbee's experiments. The other is, by sending the flash 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 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 through 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 scarcely 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 be full of electric matter; 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 fluid. They are conducted through 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 consists 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 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 shewn 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 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 flesh piece of raw flesh, he took a leg of mutton, and laying the chain that communicated with the outside of the battery over the flank, he took the explosion on the outward membrane, about seven inches from the chain; but was greatly surprized to observe the electric fire not 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 flank, 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 ter. 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 fluid 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, though 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 fluid over With many the surfaces of a great number of different bodies, but other bodies found it impossible with a great number 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 fluid passed over the surface of a touch-stone, and likewise over a piece 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 through 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 Rackstraw'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 pieces of metal, the effects are very different from any 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 center 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 centre. The appearance of the whole, exclusive of the black dust, is represented Plate CI, fig. 1, n° 1.

"June 14. 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 CI, fig. 1, 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..." 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 thro' 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 abovementioned, and some others. The electric most universally present is air. That the fluid pervades its substance is evident to our eye-light; 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 abovementioned: One or more needles are fixed on the prime conductor, which is kept strongly electrified for about ten 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 inflate 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 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 sent 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 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 Beraud, at Lyons; Mr Boze, at Wittenberg; Mr Le Cat, at Rouen; and Mr Robineau, 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... 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 the glass vibrate in a manner he could not conceive.

When Mr Berard'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 showered 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 Abbé 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 too 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 substance of electric bodies, but that it sometimes moves with extreme violence; so that its repulsive power separates even the minutest particles from each other; and this could not happen without a thorough penetration of the electric body.—It seems, however, more difficult to shew, that the electric matter does not generally pass directly through the substance of metals, but over their surface. A little consideration, however, will shew, 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 been supposed to be dissipated as 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 subtlety, 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 reluctance; 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 n° 1, 2, 3. fig. 1. Plate CI. 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 scarce 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 shewn 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 derable 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 proportionate 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 thro' 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 speckles, it is probable the fluid may run along the surface of the speckle, 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, as 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 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. 15. Plate C. 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 thro' 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, 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 can 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 lamplight 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 stream 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. 16. 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, may most of the phenomena of electricity be explained. The first thing to be considered is, From what source it originally derives the astonishing agility and strength displayed in its electric motions. If it is granted that the electric fluid is the only 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 through 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 silverizing 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 betwixt 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 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, tho' 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 set 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 Wilcke 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 minuteest 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, ended 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 they 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 insulated 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 negative insulated phial will instantly be electrified negatively. Now, though we 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 insulated 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... charge of positive electricity as it can contain, and then inflate 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. The only method, therefore, 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 refilled by it; and this is evidently 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 up on 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 shewn, 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 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 with it 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 to electrify. 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 for ever, 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 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 doth appear is the air. The prime conductor of an electrical 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 shewn, 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 electricity. 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. 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 thro' the atmosphere. 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 Thunder.

§ 9. 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 infallible, 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 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 fluidity 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 be found on the surface of glass tubes*; and especially in electrified air. When the prime conductor of a machine is strongly electrified positively, it is throwing out the fluid from it in all directions. The air cannot receive this fluid without throwing out that which it also contains; and this shews, 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 conductor, 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 resistance 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.

§ 10. 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. surface. Nevertheless, it will soon shew 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 resistance it met with, and runs along the conductor into the earth. But no sooner is this done, than the power which resisted the vibration outward from the glass, having got the better in the manner just now explained, a new vibration is produced by that resisting 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 resistance 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 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.

§ 11. The Phenomena of the Electrophorus accounted for.

The electrophorus is a machine represented Plate CI. fig. 3. 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 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 near 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 pasteboard covered with tinfoil 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 (I), 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 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 (I), 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, this 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, 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 six 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 plate with the wax upon the table, and the glass uppermost, i.e., contrary to the common method; then, on making the usual experiment of putting the metal plate on it, and taking the spark, &c., I observe it to be attended with the contrary electricity: that is, if I lay the metal plate upon the electric one, and, while in that situation, touch it with an insulated body, that body acquires the positive electricity; and the metallic, removed from the electric plate, appears to be negative; whereas it would become positive, if laid upon the excited wax. This experiment, I find, answers in the same manner if an electric plate is used which has the sealing-wax coating on both sides, or one 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, (fig. 3. Plate CI.) 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 have been mistaken. He tells us, that "if instead of laying the opposite 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, that 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 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 restored 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 reliance 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 reflected by the air; a vibration 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. Miscellaneous Experiments.**

In this section are comprehended some of those effects of the electric matter which may properly be reckoned anomalous, and for which it is impossible to assign any reason. Some very remarkable ones of this kind are those on colours, of which Mr Cavallo gives the following account. "Having accidentally observed, that an electric shock sent over the surface of a card, marked a black stroke upon a red spot of the card, I was from this induced to try what would be the effect of sending shocks over cards painted with different water-colours. Accordingly, I painted several cards with almost every colour I had, and sent shocks (a) over them, when they were very dry; making use of the universal discharger, fig. 5. Plate XCIX. The effects were as follow.

Vermilion was marked with a strong black track, about one tenth of an inch wide. This stroke is generally single, as represented by A B, fig. 17c. of Plate XCIX. Sometimes it is divided in two towards the middle, like E F; and sometimes, particularly when the wires are set very distant from one another, the stroke is not continued, but interrupted in the middle, like G H. 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

(A) The force generally employed was the full charge of one foot and a half of coated glass. wire D, is neither so strongly marked, nor surrounds the wire so much, as the other extremity E.

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

"Verdigris was shook 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.

"Desirous 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 (a), I procured some pieces of paper painted on both sides with oil colours; and sending 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 propositions.

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

"It is remarkable, that vermilion receives the black impression, when painted with linseed oil, nearly as well as when painted with water. The paper painted with white lead and oil, receives also a black mark; but its nature is very singular. The track, when first made, is almost as dark as that marked on white-lead, painted with water; but it 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, block cal-metals. The calcination of them appears from Dr Cines, vitrification from Mr Priestley's experiments with the brass chain, mentioned n° 75, 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 phosphorus 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 throws 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-filter 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 filter, at the place where it was discontinued; the filter was found melted, and part of it dispersed all along the track, that the electric matter took between the plates of wax, from the filter 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.

(a) "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." For a particular account of such facts, see the article Thunder. 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 brass dust, laid on a floor of baked wood, making interruptions in various parts of the train; and always found the brass 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 insulated a jar of three square feet, and upon an adjoining glass-stand laid a heap of brass dust; and at the distance of seven or eight inches a brass 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 thro' 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 brass rod communicating with the inside of my battery, in order to observe what variety it might occasion in the circular spots abovementioned, I was greatly surprised to find the explosion made all at once, at the distance of two inches.

"I afterwards put some brass dust upon a plate of metal communicating with the inside of the battery; and making the discharge thro' 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 fluid 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 conductors, though the length of it be ever so great.

The first attempt these gentlemen made, was to convey the electric shock across the river Thames, making use of the water of the river for one part of the chain of communication. This they accomplished on the 14th and 18th of July 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 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 snapings, being numerous in the whole length of a chain, very sensibly lessened the great discharge at the gun-barrel.

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 shewed, 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, everything 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 smartly struck, though they were distant from each other above 500 feet.

The same gentlemen, pleased with the success of their former experiments, undertook another, the object of which was, to determine whether the electric 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 Islington, 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... A method of ascertaining this hath been contrived by Dr Priestley, thus: Bend a wire, about five feet long, in the form represented Plate C. fig. 17, so that the parts A B may come within half an inch of one another; 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 A and B; which shews that the fluid chooseth a short passage through the air, rather than the long one through the wire. The charge, however, does not pass entirely between A and B, but part of it goes also through the wire. This may be proved by putting a slender wire between A and B; 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 A D B be cut in D, so as to discontinue the circuit A D B, 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 shew that the electric fluid always meets with resistance, it is by no means easy to shew 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 glass 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

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, becomes 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 C E F D, fig. 16. Plate XCIX; and tie a silk string G C D to it, which serves to hold it by when it is to be let near the fire; fill the middle part of this tube with rosin, sealing-wax, &c., then introduce two wires A E, B F, through its ends, so that they may touch the rosin, or penetrate a little way into 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. 7. of Plate XCIX, 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.

Besides these, there are a number of other anomalous appearances exhibited by the electric fluid. Some of the principal of them are the phenomena of the Tourmaline, the Gymnotus Electricus, Torpedo, &c., for a particular account of which, see these articles. See also Magnetism, Lightning, Thunder, &c. The effects of medical electricity are considered under the article Medicine. On this last subject we shall just mention the construction of an instrument which, Mr Cavallo says, is very useful for curing the tooth-ach. It is represented Plate XCIX, fig. 15, and consists of two wires A E, B E, fixed in two holes in the piece of baked wood H. These wires, from C to D, and G to F, are bended in a plane inclined to the rest of the wires; their extremities D E, F E, being again bended towards one another, and in the plane C A G B. The extremities A B are bended in a ring. When this instrument is to be used, it must be applied in such a manner that the affected tooth may be pretty closely embraced by the two wires at E; which being flexible, may be adjusted so that they will receive teeth of different sizes: then the end A, or B, of one of the wires, must be connected with the outside of a charged jar, and the end of the other wire with the knob of the jar, so as to make the shock pass through the wires of the instrument, and, of consequence, through the tooth. "A single shock," (says Mr Cavallo), "sent through an affected tooth in this manner, will often cure it instantaneously; it is, however, always proper to send two or three shocks through it."