136 ELECTRICITY. 



In the year 1770, Dr. Priestley published his works on electricity. This 

 philosopher did not contribute materially to the advancement of the science by 

 the development of any new facts ; but in his History of Electricity he collected 

 and arranged much useful information respecting the progress of the science. 

 At this period the Honorable Henry Cavendish, whose name has been distin- 

 guished in other departments of physics, engaged in some original investiga- 

 tions respecting electricity. The discovery of the composition of water, by 

 transmitting an electric spark through a mixture of oxygen and hydrogen gases, 

 has been generally ascribed to him.* Cavendish conceived the notion of re- 

 ducing the phenomena of electricity to mathematical analysis, and had pro- 

 ceeded with a memoir on that subject, which was completed before he learned 

 that ^Epinus had produced a work with the same object. On comparing his 

 own paper with the Tentamen of ^Epinus, he found that they were nearly simi- 

 lar. Nevertheless, Cavendish published his memoir. 



The year 1785 formed an important epoch in the history of electrical sci- 

 ence, marking, as it did, the commencement of those labors by which Coulomb 

 i;iid the foundations of ELECTRO-STATICS. This great experimental philoso- 

 pher was the first who really brought the phenomena of electricity within 

 the reach of numerical calculation, and thereby prepared the way for his fol- 

 lowers in the same field to reduce this most subtle of all physical agents to the 

 rigorous sway of mathematics. It is to Coulomb we owe it that statical elec- 

 tricity is now a branch of mathematical physics. 



The immediate instrument by which this vast object was attained was the 

 balance of torsion, which he had already used with signal success in other deli- 

 cate physical inquiries. This apparatus, which will be fully explained in the 

 following pages, consisted of a needle suspended in a horizontal position by an 

 exceedingly fine wire or filament of silk attached to its centre of gravity. The 

 attraction, or other force of which the intensity is to be measured, is made to 

 act on one end of this needle, so as to twist the filament by which it is sus- 

 pended ; and it is resisted in its effort to effect this by the reaction proceeding 

 from the torsion so produced. This reaction, and therefore the force which 

 produces it, and is in equilibrium with it, was proved by Coulomb to be pro- 

 portionate to the angle described by the needle round its centre of motion. 

 Such was the sensibility of this exquisite instrument, that it was found to be 

 perceptibly affected by a force not exceeding the twenty-millionth part of a 

 grain. 



"With this instrument Coulomb measured the force with which electrified 

 bodies attract and repel each other ; and the first result of this investigation 

 was the discovery, that the law of this attraction and repulsion was the same 

 which Newton showed to prevail among the great bodies of the universe. In 

 fact, he showed that two bodies, oppositely electrified, attract each other with 

 a force which, c&teris paribus, is the same at equal distances, and which aug- 

 ments in the same proportion as that in which the square of the distance is di- 

 minished. Also if two bodies be similarly electrified, they will repel each 

 other by a force which increases according to the same proportion when the 

 distance between them is diminished. 



By attaching a very small circular disk of paper coated with metallic foil to 

 an insulating handle, Coulomb found that by touching with the face of the disk 

 an electrified surface, and then submitting the disk itself thus electrified by 

 contact to the test of the balance of torsion, he could determine the depth of 

 the electric fluid on the surl'ace'touched by the disk. In this manner was he 

 enabled to gauge or sound the electricity on the surface of bodies, so as to com- 



This claim has been recently called in question. See Larduer on the Steam-Engine. Seventh 

 Edition, p. 303. 



