548 JAMES CLERK MAXWELL. 



associated with a complex chemical constitution, glass con- 

 sisting essentially of metallic silicates, including silicates of 

 the alkaline and alkaline-earthy metals. 



The measurement of the specific inductive capacity of 

 glass is attended with great difficulty on account of the 

 phenomenon generally known as residual charge or electric 

 absorption, that is the apparent soaking of the electricity 

 into the substance of the glass. This is a subject in which 

 Maxwell took very great interest, and in his work on electricity 

 and magnetism he has given a mechanical illustration of 

 the action on the supposition that it is due to a want of 

 homogeneity in the glass, some parts of which he supposed 

 to conduct electricity better than others, though badly at the 

 best. A form of experiment, very beautiful in its design, 

 was devised by Maxwell for measuring specific inductive 

 capacities, and was carried out by Mr. J. E. H. Gordon, who 

 was able to reverse the electric stress in the glass 12,000 

 times per second ; but this is of course no approximation to 

 the rapid alternations of the " waves " of light. With the 

 apparatus employed, however, the reduction of the observa- 

 tions involves great mathematical difficulties, and the results 

 must therefore be received with caution whether we regard 

 them as supporting the theory or as opposed thereto. 



In applying the hypothesis of molecular vortices to the 

 action of a magnetic field on polarised light, Maxwell " found 

 that the only effect which the rotation of the vortices will 

 have on the light will be to make the plane of polarisation 

 rotate in the same direction as the vortices, through an 

 angle proportional 



(A) to the thickness of the substance. 



(B) to the resolved part of the magnetic force parallel 

 to the ray. 



(C) to the index of refraction of the ray. 



(D) inversely to the square of the wave length in air. 



(E) to the mean radius of the vortices. 



(F) to the capacity for magnetic induction." 



The relation (E) between the amount of rotation and the size 

 of the vortices, shows that different substances may differ in 



