Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/19

Rh HISTORY.] ELECTRICITY advancement of modern chemistry. By using different proportions of oxygen and hydrogen, and examining the products which they formed after explosion with the elec tric spark, he obtained a proportion of which the product was pure water (Phil. Trans., 1784-5). The decom position of water by the electric spark was first effected by Pacts Van Troostwijk and Deiman ; improved methods of effecting it were discovered and used by Pearson, Cuth- bertson, and Wollaston (Phil. Trans., 1801). The great discovery made by Galvani in 1790, that the contact of metals produced muscular contraction in the frog, and the invention of the voltaic pile, in 1800, by Volta led to the recognition of a new kind of electricity called Gal vanic or Voltaic Electricity, which is now proved to be identical with frictional electricity. The chemical effects of the voltaic pile far transcend those of ordinary electricity. In 1800 Nicolson and Carlisle discovered the power of the pile to decompose water; and in 1807 (Bakerian Lecture) Sir Humphry Davy decomposed the earths and the alkalies, and thus created a new epoch in the history of cheTnistry. Contemporaneous with Cavendish was Coulomb, one of the most eminent experimental philosophers of the last century. In order to determine the law of electrical action, he invented an instrument called a torsion balance, which has since his time been universally used in all delicate researches, and which is particularly applicable to the measurement of electrical and rnagnetical actions. ^Epinus and Cavendish had considered the action of elec tricity as diminishing with the distance ; but Coulomb proved, by a series of elaborate experiments, that it varied, like gravity, in the inverse ratio of the square of the distance. Dr Robison had previously deter mined, without, however, having published his experi ments, that in the mutual repulsion of two similarly electrified spheres, the law was slightly in excess of the inverse duplicate ratio of the distance, while in the attraction of oppositely electrified spheres the deviation from that ratio was in defect ; and hence he con cluded that the law of electrical action was similar to that of gravity. Adopting the hypothesis of two fluids, Coulomb investigated experimentally and theoretically the distribution of electricity on the surface of bodies. He determined the law of its distribution between two con ducting bodies in contact ; he measured the density of the electricity at different points of two spheres in contact ; he ascertained the distribution of electricity among several spheres (whether equal or unequal) placed in contact in a straight line ; he measured the distribution of electricity on the surface of a cylinder, and its distribution between a sphere and cylinder of different lengths but of the same diameter. His experiments on the dissipation of electri city possess also a high value. He found that the momen tary dissipation was proportional to the degree of electrifi cation at the time, and that, when the charge was mode rate, its dissipation was not altered in bodies of different kinds or shapes. The temperature and pressure of the atmosphere did not produce any sensible change ; but he concluded that the dissipation was nearly proportional to the cube of the quantity of moisture in the air. In examining the dissipation which takes place along imperfectly insu lating substances, he found that a thread of gum-lac was the most perfect of all insulators ; that it insulated ten times as well as a dry silk thread ; and that a silk thread covered with fine sealing-wax insulated as powerfully as gum-lac when it had four times its length. He found also that the dissipation of electricity along insulators was chiefly owing to adhering moisture, but in some measure also to a slight conducting power. For the memoirs of Coulomb see Mem. de Math, et Phys. de I Acad. de Sc., 1785, &c. Towards the end of the last century a series of experi- ments was made by Laplace, Lavoisier, and Volta (Phil. Trans., 1782, or Collezione dell Op.}, from which it ap- peared that electricity is developed when solid or fluids bodies pass into the gaseous state. The bodies which were to be evaporated or dissolved were placed upon an insu lating stand, and made to communicate by a chain or wire with a Cavallo s electrometer, or with Volta s condenser, when it was suspected that the electricity increased gra dually. When sulphuric acid diluted with three parts of water was poured upon iron filings, hydrogen was disen gaged with a brisk effervescence ; and at the end of a few minutes the condenser was so highly charged as to yield a strong spark of negative electricity Similar results were obtained when charcoal was burnt on a chafing dish. Volta, who happened to be at Paris when these experi ments were made, and who took an active part in them, subsequently observed that the electricity produced by evaporation was always negative. He found that burning charcoal gives out negative electri city ; and in other kinds of combustion he obtained dis tinct electrical indications. In this state of the subject Saussure (Voyage dans les Alpes, t. ii. p. 808, et seqq.) undertook a series of elaborate experiments on the electri- city of evaporation and combustion. In his first trials he found that the electricity was sometimes positive and sometimes negative when water was evaporated from a heated .crucible of iron ; but he afterwards found it to be always positive both in an iron and a copper crucible. In a silver and a porcelain crucible the electricity was nega tive. The evaporation of alcohol and of ether in a silver crucible also gave negative electricity. Saussure made many fruitless trials to obtain electricity from combustion, and he likewise failed in his attempt to procure it from evaporation without ebullition. Many valuable additions were about this time made to electrical apparatus, as well as to the science itself, by Van Marum, Cavallo, Nicholson, Cuthbertson, Brooke, Bennet, Read, Morgan, Henley, and Lane ; but these cannot here be noticed in detail. The application of analysis to electrical phenomena may be dated from the commencement of the present century. Coulomb had considered only the distribution of electri- city on the surface of spheres ; but Laplace undertook to investigate its distribution on the surface of ellipsoids of revolution, and he showed that the thickness of the coat ing of fluid at the pole was to its thickness at the equator as the polar is to the equatorial diameter. Biot (Traite de Physique Exp. et Math.) has extended this investigation to all spheroids differing little from a sphere, whatever may be the irregularity of their figure. He likewise determined analytically that the losses of electricity form a geometrical progression when the two surfaces of ajar or plate of coated glass are discharged by successive contacts ; and he found that the same law regulates the discharge when a series of jars or plates are placed in communication with each other. It is to Poisson (Mem. de I Inst. Math, et Phys., 12, 1811, etc.) however, that we are mainly indebted for having brought the phenomena of electricity under the dominion of analy sis, and placed it on the same level as the more exact sciences. Assuming the hypothesis of two fluids, he deduced theorems for determining the distribution of the electric fluid on the surface of two conducting spheres when they are either placed in contact or at any given distance. The truth of these theorems had been estab lished by experiments performed by Coulomb long before the theorems themselves had been investigated. Voltaic electricity had now absorbed the attention of experimental philosophers. The splendour of its phe nomena, as well as its association with chemical discovery, contributed to give it popularity and importance ; but the VIII. 2 Lapla&amp;lt; Volta, g aus. sure. Appli tion Biot. Poissc