ELECTRICITY FROM THALES TO FARADAY. 253 



then a most intense and pure evolution of light followed, which on 

 separating the points extended to a gorgeous arc." It was at first 

 supposed that the galvanic or voltaic electricity was distinct from 

 the so-called " frictional " or " ordinary " electricity. 



A distinguished contemporary of Cavendish was Coulomb, the 

 value of whose work in developing certain exceedingly important 

 mathematical laws with regard to action at a distance, surface den- 

 sities, and rates of charge dissipation can hardly be overestimated. 

 His name was given to the torsion balance which, since his day, has 

 been the standard instrument for measuring electric and magnetic 

 attractions and repulsions. The importance of his work has since 

 been recognized by the perpetuation of his name in connection with 

 the unit of quantity of electricity, as that of Yolta has been hon- 

 ored by its use, abbreviated (volt), to designate the unit of elec- 

 trical tension or pressure. 



Certain highly instructive and interesting data were accumulated 

 about this time by Yolta, Laplace, Saussure, and the renowned 

 chemist Lavoisier, in connection with the subject of electrification 

 produced when evaporation, and the liberation of gases and vapors in 

 general from any cause, occurs. The liquid, solid, or mixture liber- 

 ating the gas was contained in a metallic dish and the resultant elec- 

 trification of the latter examined qualitatively. Volta's observations 

 led him to conclude that the electrification was always negative, but 

 Saussure demonstrated finally that its sign was dependent on the ma- 

 terial of the dish. These experimenters covered, between them all, 

 a somewhat extensive field, examining, among other things, the 

 electrification resulting from the ebullition of various liquids, from 

 the ordinary combustion of fuel, and from the decomposition of acids 

 by metals to liberate hydrogen. 



About the end of the first decade of the century Poisson attacked 

 the phenomena of electricity analytically, and succeeded in demon- 

 strating the right of electrical investigation to rank among the exact 

 sciences. Of his most important mathematical propositions is one 

 in which, assuming as a working hypothesis the existence of two 

 mutually attracting fluids, he deduced formulae covering the distri- 

 bution of these fluids on the surfaces of two conducting spheres, in or 

 out of contact. 



A great deal of work was done during the end of the last cen- 

 tury and the beginning of the present one on what is now known 

 as pyro-electrification. The Abbe Haiiy discovered that fragments 

 of tourmaline crystal exhibited opposite electrifications on opposite 

 extremities of their lines of cleavage. It is this crystal also which has 

 unusually remarkable powers of polarizing light, and which, under 

 electro-magnetic stress, suffers modifications of the latter j^roperty. 



