Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/18

Rh 8 ELECTRICITY [_HISTOKY. Theoplirastus, attracted light bodies, was the tourmaline, a Ceylon mineral, in which the Dutch had early recog nized the same attractive property, whence it got the name of AschentrikTcer, or attractor of ashes. In 1717 M. Lemery exhibited to the Academy of Sciences a stone from Ceylon which attracted light bodies; and LinnaBus, in mentioning the experiments of Lemery, gives the stone the name of Lapis Electricus. The Duke de ISToya was led in 1758 to purchase some of the stones called tourma line in Holland, and, assisted by Daubenton and Adanson, he made a series of experiments with them, a description of which was published. The subject, however, had en- -Epinus g a g e( i the attention of ^pinus, a celebrated German philo- (1724- sopher, who published an account of them in 1756. 1806). Hitherto nothing had been said respecting the necessity of heat to excite the tourmaline ; but it was shown by vEpinus that a temperature between 99 1 and 212 Fahr. was requisite for the development of its attractive powers. Benjamin Wilson (Phil. Trans., 1763, &amp;lt;fec.), Priestley, and Canton continued the investigation; but it was reserved for the Abbe Haiiy to throw a clear light on this curious branch of the science (Traite de Mineralogie}. He found that the electricity of the tourmaline decreased rapidly from the summits or poles towards the middle of the crystal, where it was imperceptible ; and he discovered that if a tourmaline is broken into any number of frag ments, each fragment, when excited, has two opposite poles. Haiiy discovered the same property in the Siberian and Brazilian topaz, borate of magnesia, mesotype, preh- nite, sphene, and calamine. He also found that the polarity which minerals receive from heat has a relation to the secondary forms of their crystals, the tourma line, for example, having its resinous pole at the summit of the crystal which has three faces, and its vitreous pole at the summit which has six faces. In the other pyro- electrical crystals above mentioned, Haiiy detected the same deviation from the rules of symmetry in their second ary crystals which occurs in tourmaline. Brard discovered that pyro-electricity was a property of the axinite; and it was afterwards detected in other minerals. In repeating and extending the experiments of Haiiy, Sir David Brewster discovered that various artificial salts were pyro-electrical ; and he mentions tartrate of potash and soda, and tartaric acid, as exhibiting this property in a very strong degree. He also made many experiments with the tourmaline when cut into thin slices, and reduced to the finest powder, in which state each particle preserved its pyro-electricity ; and he showed that scolezite and mesolite, even when de prived of their water of crystallization and reduced to powder, preserve their property of becoming electrical by heat. When this white powder is heated and stirred about by any substance whatever, it collects in masses like new fallen snow, and adheres to the body with which it is stirred. (For Sir David Brewster s work on pyro-electricity see Trans. R.S.E., 1845; Phil. Mag., Dec. 1847; Edin burgh Journal of Science, Oct. 1824 and 1825). In addition to his experiments on the tourmaline, ^Epinus made several on the electricity of melted sulphur ; and in conjunction with Wilcke, he investigated the subject of electric atmospheres, and discovered a beautiful method of charging a plate of air by suspending large wooden boards coated with tin, and having their surfaces near each other and parallel. vEpinus, however, has been principally distinguished by his ingenious theory of electricity, which he has explained and illustrated in a separate work (Ten- tamen Theories Electricitatis et Magnetismi) which ap peared at St Petersburg in 1759. This theory is founded on the following principles. 1. The particles of the elec tric fluid repel each other with a force decreasing as the distance increases. 2. The particles of the electric fluid

attract the particles of all bodies, and are attracted by them, with a force obeying. the same law. 3. The electric fluid exists in the pores of bodies ; and while it moves without any obstruction in non-electrics, such as metals, water, &c., it moves with extreme difficulty in electrics, such as glass, rosin, &c. 4. Electrical phenomena are pro duced either by the transference of the fluid from a body containing more to one containing less of it, or from its attraction and repulsion when no transference takes place. The electricity of fishes, like that of minerals, now be- gan to excite very general attention. The ancients, as we tricit j have seen, were acquainted with the benumbing power of the torpedo, but it was not till- 1676 that modern naturalists attended to this remarkable property. The Arabians had long before given this fish the name of raad or lightning ; but Redi was the first who communicated the fact that the shock was conveyed to the fisherman by means of the line and rod which connected him with the fish. Lorenzini published engravings of its electrical organs; Reaumur described the electrical properties- of the fish ; Kampfer compared the effects which it produced to lightning ; but Bancroft was the first person who distinctly suspected that the effects of the torpedo were electrical. In 1773 Walsh (Phil. Trans., 1773-5) and Ingenhousz proved, by many curious experiments, that the shock of the torpedo was an electrical one; and Hunter (Phil. Trans., 1773-5) examined and described the anatomical structure of its electrical organs. Humboldt (Ann. de Chim. et de Phys., i. 15), Gay-Lussac, and Geoffroy pursued the subject with success; and Cavendish (Phil. Trans., 1776) constructed an artificial torpedo, by which he imitated the actions of the living animal. The subject was also investigated by Todd, Sir Humphrey Davy (Phil. Trans., 1829), John Davy, and Faraday (Exp. Res., vol. ii.). The power of giving electric shocks has been discovered also in the Gymnotus electricus, 1 the Malapterurus electricus, 2 the Trichiurus electricus,&quot; 1 and the Tetraodon electricus. 2 The most interesting and the best known of these singular fishes is the Gymnotus or Surinam eel. Humboldt gives a very graphic account of the combats which are carried on in South America between the gymuoti and the wild horses in the vicinity of Culabozo. Among the cultivators of electricity Henry Cavendish is entitled to a distinguished place. Before he had any knowledge of the theory of ^Epinus, he had communicated to the Royal Society a similar theory of electrical pheno mena. As, however, he had carried the theory much further, and considered it under a more accurate point of view, he did not hesitate to give his paper to the world (Phil. Trans., 1771). Cavendish made some accurate experi ments on the relative conducting power of different sub stances. He found that electricity experiences as much resistance in passing through a column of water one inch long as it does in passing through an iron wire of the same diameter 400,000,000 inches long, whence he con cluded that iron wire conducts 400,000,000 times as well as rain or distilled water. He found that a solution of one part of salt in one of water conducts a hundred times better than fresh water, and that a saturated solution of sea-salt conducts seven hundred and twenty times better than fresh water. Cavendish likewise determined by nice experiments that the quantity of electricity on coated glass of a certain area increased with the thinness of the glass, and that on different coated plates the quantity was as the area of the coated surface directly, and as the thick ness of the glass inversely. Although electricity Lad been employed as a chemical agent in the oxidation and fusion of metals, yet it is to Cavendish that we owe the first of those brilliant inquiries which have done so much for the Powerful 2 Weak,