110 ON DIELECTRICS. 



contact. If the tube traverses the surface of separation of the two 

 electricities, the displacement is the same in the two media, but the 

 polarization is not the same, and the surface would have an apparent 

 electricity equal to the difference of the electrical layers on the 

 surfaces of the two media in contact. 



It is evident that, since the electrification of the conductor is 

 only apparent, all the energy due to the electrification must reside 

 in the medium. It is equal to the work expended in effecting 

 the displacement in a direction opposite that of the elastic forces. 

 From what we have seen (120), the value of this work for unit 



volume is - or . . F : it is therefore equal to half the pro- 

 STT 2 477 



duct of the electrical force with the displacement. 



Maxwell's theory of displacement accounts thus for the properties 

 of the medium in a satisfactory manner. It furnishes a physical 

 interpretation of Faraday's specific inductive capacity; when multi- 

 plied by a factor , it is the inverse of the coefficient of electrical 



47T 



elasticity of the medium. 



It explains Faraday's view that it is not possible to impart an 

 absolute charge of electricity to matter: on this theory, in short, 

 electricity behaves like an incompressible fluid ; the quantity which 

 can be contained in a closed surface is invariable, and the production 

 of two quantities of electricity of equal and opposite signs appears to 

 be a consequence of one and the same phenomenon. 



In conclusion, it is natural to suppose that if the explanation 

 of electrical phenomena postulates the existence of an incompressible 

 medium, diffused in space, this medium can be none other than the 

 ether to which luminous and thermal phenomena are attributed ; this 

 theory enables us to discern a dependence between the two orders of 

 phenomena, the confirmation of which would be one of the most 

 important conquests of physical science. 



