An account of this discovery of Mr. Grey, is thus abridged from the Philosophical Transactions, by Dr. Priestley. “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 a tube would attract equally well with the ends shut, as with them open. In this respect there was no difference; but he found that the corks attracted and repelled light substances, as well, and rather better than the tube itself. He then fixed an ivory ball upon a stalk of fir about four inches long, and thrusting the end of the stalk into one of the corks, he found the ball endowed with a strong attractive and repulsive virtue. This experiment he repeated in many different ways; fixing the ball upon long sticks, and upon pieces of brass and iron wire, always with the same success; but he constantly observed, that the ball at the end attracted more vigorously, than that part of the wire nearest the tube.
“The inconvenience of using long wires in this manner, put Mr. Grey upon trying whether the ball might be suspended by a pack-thread, with a loop on the tube, with equal success; and the event fully answered his expectation. Having thus suspended bodies of the greatest length he conveniently could, to his tube, he ascended a balcony 26 feet high, and fastening a string to his tube, found that the ball would attract light bodies on the ground below. This experiment succeeded in the greatest heights to which he could ascend; after which, he attempted to carry the electricity horizontally. His first attempt miscarried, because he suspended his line, which was intended to carry the electricity horizontally, by a pack-thread; and thus the fluid got off from it; but though Mr. Grey knew this was the case, he could not at any time think of any method to prevent it.
“On the 30th June 1729, Mr. Grey paid a visit to Mr. Wheeler, in order to give him a specimen of his experiments; but told him of the unsuccessful attempt he had made to carry the electric fluid horizontally; Mr. Wheeler proposed to suspend the conducting line by silk instead of pack-thread. For this advice he could give no reason, but that the silk thread was smaller than the other; however, with it they succeeded perfectly well. Their first experiment was in a matted gallery at Mr. Wheeler’s house, on the 2d of July 1729. About four feet from the end of the gallery they fastened a line across the place. The middle of this line was silk, the rest pack-thread. Over the silken 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 about 80 1–2 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 lines had been doubled. Thus they proceeded with their experiments; still adding more conducting line, till at last their silk string broke with the weight. This they endeavoured to supply, first with a small iron wire, and then with a brass one. The result of these experiments, however, soon convinced them, that the silk refused to conduct the electric fluid, not on account of its smallness, as they had supposed, but on account of some difference in the matter. The wires were smaller than the silk threads, yet the electricity was effectually carried off by them. They had recourse, therefore, to thicker lines of silk; and thus conveyed the electric matter to the distance of 765 feet: nor did they perceive the virtue to be at all diminished by the distance to which it was carried.” In the manner in which silk was found to be a non-conductor, the same quality was also discovered in many other substances, such as hair, rosin, &c.
Mr. Grey also made many electrical experiments on fluids and animal bodies. As he knew no other method of trying whether bodies were electrified or not, but by making them raise light bodies placed under them, to put a fluid in this situation, he dissolved soap in Thames water, and suspending a tobacco pipe, he blew a bubble at the head of it; and bringing the excited tube near the small end, he found the bubble to attract leaf brass to the height of two and of four inches.[[6]] He contrived afterwards, by a curious experiment to shew the effects of electricity upon water, in a more satisfactory manner. He filled a small cup with water higher than the brim, and when he had held an excited tube over it, at the distance of about an inch or two, he says, that if it were a large tube there would first arise a little mountain of water from the top of it, of a conical form; from the vertex of which there proceeded a light, very visible when the experiment was performed in a dark room, and a snapping noise almost like that which was made when the finger was held near the tube, but not quite so loud, and of a more flat sound. Upon this, says he, immediately the mountain, if I may so call it, falls into the rest of the water, and puts it into a tremulous and waving motion. This experiment he repeated in the sun-shine, when he perceived small particles of water thrown from the top of the mountain; and sometimes a fine stream of water would arise from the vertex of the cone, in the manner of a fountain, from which issued a fine stream or vapour, whose particles were so small as not to be seen. This last circumstance he inferred, from the under side of the tube being wet. And by after experiments, he found that though the cylinder of water does not always rise, yet that there is always a stream of particles thrown on the tube, and sometimes to such a degree as to become visible.
In April 1730, Mr. Grey suspended a boy on hair lines, in a horizontal position, just as all electricians had before been used to suspend their hempen lines of communication, and their wooden rods; then bringing the excited tube near his feet, he found that leaf brass was attracted by his head, with a vigour sufficient to raise it to the height of eight, and sometimes of ten inches. When the leaf brass was put under his feet, and the tube brought near his head, the attraction was small; and when the leaf brass was brought under his head, there was no attraction at all. While the boy was thus suspended, Mr. Grey amused himself with making the electricity operate on several parts of his body at the same time, and at the ends of long rods, which he made him hold in his hands, and in diversifying the experiments several other ways.
Mr. Grey continued to study electricity as long as he lived; and besides giving a set of fanciful experiments, by which he supposed he had discovered a perpetual attractive power in electrics, he, a little while before his death, entered on another course by which he hoped he should be able to astonish the world with a new sort of planetarium. “I have lately made (says he) several new experiments upon the projectile and pendulous motions of small bodies by electricity; by which small bodies may be made to move about large ones, either in circles or ellipses, and those either concentric or excentric 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 round 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 apogean, than in the perigean part of their orbits; which is directly contrary to the motion of the planets round the sun.” The manner in which these experiments were made, as delivered by him on his death-bed 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, greatly 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 excentricity 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 excentricity 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 circumstance 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 substance to support it: but he acknowledged that he had not found the experiment succeed if the thread was supported by any thing but the 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 the witness. 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 electric fluid, but what is attended with much irregularity.
Not long after the discovery of Mr. Grey of the difference between conductors and non-conductors, Mr. Du Fay, a French philosopher, (for the “spirit of electricity” had passed from England to France,) discovered, what was afterwards called positive and negative electricity; or as he denominated them the vitreous and resinous electricities. “Chance (says he) has thrown in my way a principle, which casts a new light on the subject of electricity. The principle is, that there are two distinct kinds of electricity, very different from one another, one of which I call vitreous, and the other resinous electricity. The first is that of glass, rock crystal, precious stones, hair of animals, wool and many other bodies. The second is that of amber, copal, gum lac, silk thread, paper, and a vast number of other substances. The characteristics of these two electricities is, that they repel themselves and attract each other. Thus a body of the vitreous electricity repels the vitreous, and on the contrary attracts all those of the resinous. The resinous also repels the resinous and attracts the vitreous. This discovery of Mr. Du Fay 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 chace about the room with a piece of excited gum copal, instead of being repelled by it, as it was by the glass tube, was eagerly attracted.
This doctrine of two different electricities, produced by exciting different substances, was dropped after Mr. Du Fay; and even this philosopher himself adopted at last the opinion of Dr. Franklin that the two electricities differ only in degree, and that the stronger attracts the weaker. Although many of the experiments of Mr. Grey led directly to it, yet to the French philosopher just mentioned, belongs the merit of first drawing the electrical spark from the human body.—And we cannot forbear remarking, in this place on the regular and progressive advances which the human mind makes in the investigation of science. Electrical attraction was, for a long period, the single phenomenon known to philosophers.—Repulsion was then observed to be also a property of electrics.—In the investigation of these we read of the accidental discovery of the electric light.—To this naturally succeeded, Mr. Grey’s distinction between conductors and non-conductors; and then the difference between vitreous and resinous electricities by Mr. Du Fay. We shall have to remark in the sequel of this history, how each succeeding fact and invention grew out of that which immediately preceded it.
The knowledge of electricity did not stop in France. The Germans began to labour in the same field; and with laudable success. Their success arose chiefly from the improvements they made in the electrical apparatus. The simple experiments of Gilbert, and the early electricians, were made by exciting a piece of amber or sulphur. Mr. Boyle found the electric power increased by smoothing the surface of bodies. Otto Guericke made his experiment with a globe of sulphur, formed by melting that substance in a hollow globe of glass, and afterwards breaking the glass from off it, little supposing that the glass itself would better have answered his intention. In 1709 Mr. Hawksbee first observed the great electric power of glass. He used a glass globe, which he mounted upon an axis, whirling it round, and at the same time applying his hand to it. He also, to increase the power, inclosed an exhausted cylinder within another, exciting the outermost. After Mr. Hawksbee’s death, the glass globe was laid aside, and his successors confined themselves to the use of tubes. Mr. Boze, professor of philosophy at Wittemburgh, in 1742 returned to the use of the globe. He also added a prime-conductor of tin or iron, supported, at first, by a man standing on cakes of rosin, but afterwards by silken lines extended horizontally, under the conductor. Mr. Winckler, of Leipsic, to excite the globe, substituted a cushion, instead of the hand. The electrical star and the electrical bells were also the invention of the German philosophers. Dr. Desagulier, likewise, assisted electricians by some electrical terms. He first gave to bodies conveying electricity the name of conductors; and those in which electricity may be excited by heating and rubbing he calls electrics per se.