had 'stuffed the booksellers' shops by copying from one another extravagant stories concerning the attraction of magnets and amber, without giving any reason from experiment.' He himself makes some important steps in the subject. He distinguishes magnetic from electric forces,3 and is the inventor of the latter name, derived from λEKTρov, electron, amber. He observes rightly, that the electric force attracts all light bodies, while the magnetic force attracts iron only; and he devises a satisfactory apparatus by which this is shown. He gives a considerable list of bodies which possess the electric property; 'Not only amber and agate attract small bodies, as some think, but diamond, sapphire, carbuncle, opal, amethyst, Bristol gem, beryl, crystal, glass, glass of antimony, spar of various kinds, sulphur, mastic, sealing-wax,' and other substances which he mentions. Even his speculations on the general laws of these phenomena, though vague and erroneous, as at that period was unavoidable, do him no discredit when compared with the doctrines of his successors a century and a half afterwards. But such speculations belong to a succeeding part of this history..

In treating of these Sciences, I will speak of Electricity in the first place; although it is thus separated by the interposition of Magnetism from the succeeding subjects (Galvanism, &c.) with which its alliance seems, at first sight, the closest, and although some general notions of the laws of magnets were obtained at an earlier period than a knowledge of the corresponding relations of electric phenomena: for the theory of electric attraction and repulsion is somewhat more simple than of magnetic; was, in fact, the first obtained; and was of use in suggesting and confirming the generalization of magnetic laws.

3 De Magnete, p. 52.

4 Ib. p. 48.

8

CHAPTER I.

DISCOVERY OF LAWS OF ELECTRIC PHENOMENA.

W

E have already seen what was the state of this branch of knowledge at the beginning of the seventeenth century, and the advances made by Gilbert. We must now notice the additions which it subsequently received, and especially those which led to the discovery of general laws, and the establishment of the theory; events of this kind being those of which we have more peculiarly to trace the conditions and causes. Among the facts which we have thus especially to attend to, are the electric attractions of small bodies by amber and other substances when rubbed. Boyle, who repeated and extended the experiments of Gilbert, does not appear to have arrived at any new general notions; but Otto Guericke of Magdeburg, about the same time, made a very material step, by discovering that there was an electric force of repulsion as well as of attraction. He found that when a globe of sulphur had attracted a feather, it afterwards repelled it, till the feather had been in contact with some other body. This, when verified under a due generality of circumstances, forms a capital fact in our present subject. Hawkesbee, who wrote in 1709 (Physico-Mechanical Experiments,) also observed various of the effects of attraction and repulsion upon threads hanging loosely. But the person who appears to have first fully seized the general law of these facts, is Dufay, whose experiments appear in the Memoirs of the French Academy, in 1733, 1734, and 1737.1 I discovered,' he says, very simple principle, which accounts for a great part of the irregularities, and, if I may use the term, the caprices that seem to accompany most of the experiments in electricity. This principle is, that electric bodies attract all those that are not so, and repel them

1 Priestley's History of Electricity, p. 45, and the Memoirs quoted.

a

as soon as they are become electric by the vicinity or contact of the electric body. . . . Upon applying this principle to various experiments of electricity, any one will be surprized at the number of obscure and puzzling facts which it clears up.' By the help of this principle, he endeavours to explain several of Hawkesbee's experiments.

A little anterior to Dufay's experiments were those of Grey, who, in 1729, discovered the properties of conductors. He found that the attraction and repulsion which appear in electric bodies are exhibited also by other bodies in contact with the electric. In this manner he found that an ivory ball, connected with a glass tube by a stick, a wire, or a packthread, attracted and repelled a feather, as the glass itself would have done. He was then led to try to extend this communication to considerable distances, first by ascending to an upper window and hanging down his ball, and, afterwards, by carrying the string horizontally supported on loops. As his success was complete in the former case, he was perplexed by failure in the latter; but when he supported the string by loops of silk instead of hempen cords, he found it again become a conductor of electricity. This he ascribed at first to the smaller thickness of the silk, which did not carry off so much of the electric virtue; but from this explanation he was again driven, by finding that wires. of brass still thinner than the silk destroyed the effect. Thus Grey perceived that the efficacy of the support depended on its being silk, and he soon found other substances which answered the same purpose. The difference, in fact, depended on the supporting substance being electric, and therefore not itself a conductor; for it soon appeared from such experiments, and especially from those made by Dufay, that substances might be divided into electrics per se, and nonelectrics, or conductors. These terms were introduced by Desaguliers,3 and gave a permanent currency to the results of the labours of Grey and others.

2 Mém. Acad. Par. 1734.

3 Priestley, p. 66.

Another very important discovery belonging to this period is, that of the two kinds of electricity. This also was made by Dufay. 'Chance,' says he, "has thrown in my way another principle more universal and remarkable than the preceding one, and which casts a new light upon 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, the other resinous, electricity. The first is that of glass, gems, hair, wool, &c.; the second is that of amber, gum-lac, silk, &c. The characteristic of these two electricities is, that they repel themselves and attract each other.' This discovery does not, however, appear to have drawn so much attention as it deserved. It was published in 1735; (in the Memoirs of the Academy for 1733;) and yet in 1747, Franklin and his friends at Philadelphia, who had been supplied with electrical apparatus and information by persons in England well acquainted with the then present state of the subject, imagined that they were making observations unknown to European science, when they were led to assert two conditions of bodies, which were in fact the opposite electricities of Dufay, though the American experimenters referred them to a single element, of which electrized bodies might have either excess or defect. 'Hence,' Franklin says, 'have arisen some new terms among us: we say B,' who receives a spark from glass, and bodies in like circumstances, is electrized positively; A,' who communicates his electricity to glass, 'negatively; or rather B is electrized plus, A minus.' Dr. (afterwards Sir William) Watson had, about the same time, arrived at the same conclusions, which he expresses by saying that the electricity of A was more rare, and that of B more dense, than it naturally would have been. But that which gave the main importance to this doctrine was its application to some remarkable experiments, of which we must now speak.

Electric action is accompanied, in many cases, by light and a crackling sound. Otto Guericke observes

4 Prestley, p. 115. 5 Experimenta Magdeburgica, 1672, lib. iv. cap. 15.

that his sulphur-globe, when rubbed in a dark place, gave faint flashes, such as take place when sugar is crushed. And shortly after, a light was observed at the surface of the mercury in the barometer, when shaken, which was explained at first by Bernoulli, on the then prevalent Cartesian principles; but, afterwards, more truly by Hawkesbee, as an electrical phenomenon. Wall, in 1708, found sparks produced by rubbing amber, and Hawkesbee observed the light and the snapping, as he calls it, under various modifications. But the electric spark from a living body, which, as Priestley says, 'makes a principal part of the diversion of gentlemen and ladies who come to see experiments in electricity,' was first observed by Dufay and the Abbé Nollet. Nollet says he 'shall never forget the surprize which the first electric spark ever drawn from the human body excited, both in M. Dufay and in himself.' The drawing of a spark from the human body was practised in various forms, one of which was familiarly known as the 'electrical kiss.' Other exhibitions of electrical light were the electrical star, electrical rain, and the like.

As electricians determined more exactly the conditions of electrical action, they succeeded in rendering more intense those sudden actions which the spark accompanies, and thus produced the electric shock. This was especially done in the Leyden phial. This apparatus received its name, while the discovery of its property was attributed to Cunæus, a native of Leyden, who, in 1746, handling a vessel containing water in communication with the electrical machine, and happening thus to bring the inside and the outside into connexion, received a sudden shock in his arms and breast. It appears, however, that a shock had been received under nearly the same circumstances in 1745, by Von Kleist, a German prelate, at Camin, in Pomerania. The strangeness of this occurrence, and the

6 P. p. 47.

7 Priestley, p. 47. Nollet, Leçons de Physique, vol. vi. p. 408. 8 Fischer, v. 490.