Page:Aether and Matter, 1900.djvu/64

28 the other in a plane mirror, will present nothing strange to those physicists who regard with equanimity even the hypothesis of the possible existence of both positive and negative matter.

On this view of the constitution of atoms the transit of a material body through the aether does not involve any disturbance in bulk or pushing aside of that medium, unless the body carries an electric charge or is electrically or magnetically polarized.

19. In Maxwell's final presentation of electric theory, in his "Treatise," he deals with displacement but not with anything called electricity : so that a diagram of molecular polarization is foreign to it. When electric current (recognized electrodynamically) flows from A to B along a wire, the circuit is completed by displacement from B to A through the dielectric: and the notion of charges at A and B is (but only to this limited extent) irrelevant. At the same time there is little doubt that this scheme was the outcome of consideration of the theory of Kelvin and Mossotti, who were the first (in 1845) to extend Poisson's theory of magnetic polarization to dielectrics, of which the electric activity had then just been rediscovered by Faraday: and it seems possible that this notion of electrically polar molecules was dropped by Maxwell because his model of the electrodynamic field did not suggest to him any means of representing the structure of a permanently existing electric pole.

This agnostic attitude as to the nature of electric displacement and electric charge does not however limit the application of his theory on the electromotive side, so far as regards bodies at rest; for on any view the most that can be made of conduction in bodies at rest amounts to the direct application of Ohm's law, while the electrodynamics of stationary circuital currents had been already made out by Ampere,