PROGRESS IN NINETEENTH CENTURY 53 



he was followed by F. Neumann (1841), Briot (1864), Rayleigh 

 (1871), and others, treating an ether variously loaded with material 

 particles. Among theories beginning with the phenomena observed, 

 that of Boussinesq (1867, et seq.) has received the most extensive 

 development. 



The difficult surface conditions met with when light passes from 

 one medium to another, including such subjects as ellipticity, total 

 reflection, etc., have been critically discussed, among others, by Neu- 

 mann (1835) and Rayleigh (1888); but the discrimination between 

 the Fresnel and the Neumann vector was not accomplished without 

 misgiving before the advent of the work of Hertz. 



It appears, therefore, that the elastic theories of light, if Kelvin's 

 gyrostatic adynamic ether be admitted, have not been wholly routed. 

 Nevertheless, the great electromagnetic theory of light propounded 

 by Maxwell (1864, Treatise, 1873) has been singularly apt not only 

 in explaining all the phenomena reached by the older theories and 

 in predicting entirely novel results, but in harmoniously uniting, as 

 parts of a unique doctrine, both the electric or photographic light 

 vector of Fresnel and Cauchy and the magnetic vector of Neumann 

 and MacCullagh. Its predictions have, moreover, been astonishingly 

 verified by the work of Hertz (1890), and it is to-day acquiring added 

 power in the convection theories of Lorentz (1895) and others. 



Electrostatics 



Coulomb's (1785) law antedates the century; indeed, it was known 

 to Cavendish (1771, 1781). Problems of electric distribution were 

 not seriously approached, however, until Poisson (1811) solved the 

 case for spheres in contact. Afterwards Clausius (1852), Helmholtz 

 (1868), and Kirchhoff (1877) examined the conditions for discs, the 

 last giving the first rigorous theory of the experimentally important 

 plate-condenser. In 1845-48 the investigation of electric distribu- 

 tion received new incentive as an application of Kelvin's beautiful 

 method of images. Maxwell (Treatise, 1873) systematized the treat- 

 ment of capacity and induction coefficients. 



Riess (1837), in a classic series of experiments on the' heat produced 

 by electrostatic discharge, virtually deduced the potential energy 

 of a conductor and in a measure anticipated Joule's law (1841). In 

 1860 appeared Kelvin's great paper on the electromotive force needed 

 to produce a spark. As early as 1855, however, he had shown that 

 the spark discharge is liable to be of the character of a damped vibra- 

 tion and the theory of electric oscillation was subsequently extended 

 by Kirchhoff (1867). The first adequate experimental verification 

 was due to Feddersen (1858, 1861). 



The specific inductive capacity of a medium with its fundamental 



