Mr. Boyle, who so eminently distinguished himself in the latter part of the seventeenth century, was led by the study of chemistry, to give some attention to electricity. He enlarged the catalogue of electrics; and noticed some circumstances relating to electrical attraction, which had escaped former philosophers. The electrical properties of bodies he found were increased by wiping and warming them, before they were rubbed. Bodies of all kinds, he observed, were indiscriminately attracted; and this attraction he supposed took place in vacuo as well as in the open air.
Hitherto the attraction of electrics was the single phenomenon noticed by philosophers. Gilbert, even when remarking on the characteristic differences between magnetism and electricity, observes, that in magnetism there is both attraction and repulsion, but in electricity only the latter, and not the former.[[5]] Boyle made an approach to the discovery of this fact of electrical repulsion, by remarking that light bodies, as feathers &c. would cling to his fingers and other substances, after they had been attracted by electrics.
Otto Guericke, the celebrated inventor of the air pump, 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. He could thus excite the electricity with greater power, and try all the experiments of his predecessors to greater advantage. His was the full discovery of electric repulsion. “A body once attracted, he remarks, by an excited electric, is repelled by it, and not attracted again till it has been touched by some other body.” In this manner he kept a feather a long time suspended in the air, above his sulphur globe. He also made another remarkable discovery, which has since been very generally overlooked; namely, that a feather, when repelled by an excited electric, always keeps the same face towards the body which repels it, as the moon does to the earth. The electric light was probably observed by Mr. Boyle in the diamond; but Otto Guericke saw it more clearly in the excitation of his glass globe, and also heard the hissing sound which attends it. As this light, however, was exhibited to Dr. Wall, about the same time, in a much finer manner, we shall rather give his account of it.
“I found, says he, upon swiftly drawing a well polished piece of amber in the dark, through a piece of woollen cloth, and squeezing it pretty hard with my hand, a prodigious number of little cracklings were heard, and every one of them produced a flash of light; but when the amber was drawn gently and slightly through the cloth, it produced only a light, but no crackling; but by holding one’s finger at a little distance from the amber, a large crackling is produced, with a great flash of light succeeding it. And, what to me is very surprising, upon its eruption, it strikes the finger very sensibly, wheresoever applied, with a push or puff, like wind. This light and crackling seems, in some respects, to represent thunder and lightning.
Sir Isaac Newton is the next in chronological order, who made any discovery of importance. He first observed that the electrical attraction and repulsion, penetrated through glass. It cannot but be lamented, that this great philosopher, among the vast variety of important subjects which he cultivated and improved, had not applied himself to electricity, with greater assiduity.
Mr. Hawksbee, in 1709, wrote a treatise on electricity, and distinguished himself by discoveries which far surpassed those of his predecessors. Besides a variety of new facts in regard to attraction and repulsion, he observed the electric light distinctly, and made some delicate and curious experiments on its nature.
The electric light was considered by Mr. Hawksbee, as well as by all those who first observed it, as a species of phosphorus, and all the experiments made, were conducted under this impression.
Holding an exhausted globe within the effluvia of an excited one, he observed a light in the former, which presently died away, if it was kept at rest; but was revived, and continued very strong, if the exhausted globe was kept in motion. The greatest electrical light he produced, was when he enclosed an exhausted cylinder within one not exhausted, and excited the outermost of them, putting them both in motion. He observed no difference, whether the globes were turned in the same direction, or otherwise.
He made many experiments to shew the extreme subtlety of the electric light, and found out a method of rendering opaque bodies transparent. He lined with sealing wax more than half the inside of a glass globe, and having exhausted it, put it in motion. On 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 superficies of the wax within. It was as if there had been pure glass, and no wax interposed between the glass and his hand. This lining was in many places the eighth of an inch thick; and in some places where it did not adhere so closely to the glass as in others, yet the light on these appeared just as on the rest. He repeated these experiments with pitch instead of sealing wax, and with equal success. It is to be regretted that these facts have not engaged more of the attention of philosophers.
After the death of Mr. Hawksbee, twenty years elapsed before any farther improvements were made. The great discoveries which were then making in other branches of philosophy, by Sir Isaac Newton, so absorbed the public attention, that electricity was entirely overlooked. Mr. Grey, after this long interval, took up the subject, and by his discovery of the distinction between electrics and non-electrics, formed an important epoch in the history of electricity.