﻿Theory of Dispersion. 467 



4. Maxwell's remarks [Art. 1 (4) note] had reference to 

 the negative results of the experiments, whereby he proposed 

 to detect in a direct manner the inertia of an electric charge. 

 Experimental determination of the mass of an electron as 

 well as their velocities under certain conditions has supplied 

 the data sought by Maxwell on which to build up the further 

 development of the electromagnetic theory. 



5. We have thus justification for regarding the pheno- 

 menon of electric conduction in gases as due to " ionization " 

 or generation of charged particles or " ions " which are 

 carriers of electricity. 



6. An electric current would then, on this view, consist of 

 two parts — one due to electric displacement which we may 

 still regard as sethereal, and the other due to motion of 

 -electrons; or if u is the total current, in the direction of x, 

 we shall have 



u=j+px, (2) 



where p is the volume density of (free) electrification whose 

 transfer or procession gives rise to convection or conduction 



Tsf 



current, and a?, its velocity in the direction of x, while /'= ~ 



7. Now, when there is free electrification, we must have 



3/ , <>£ , d 7t _„ /0\ 



5i + dy + sr'- ;; ■ ; ■ (3) 



This must therefore be the further condition satisfied by 

 J\ g, h, while in the case of conduction there must, in addi- 

 tion, be a viscous decay which has to be taken account of in 

 a suitable dissipation function. 



8. When we proceed to interpret these equations, coupled 

 with the observed phenomena of metallic conduction and 

 dielectric polarization, in terms of the electron theory, we 

 are naturally led to the conclusion that : — 



9. A dielectric medium must be conceived to have electrons 

 interspersed in it, giving rise to a constrained (sethereal) 

 motion in the medium defined by the above relation ; this 

 also necessarily imposes a constraint on the motion of electrons. 

 On the other hand, motion of electrons in a conductor must be 

 free (subject however to certain dissipation of energy) so that 

 the static charge in them always resides on the surface. 



10. A conception of this kind naturally suggests that the 

 dielectric property of a medium may be explained as arising 

 from the constrained motion imposed on it. We proceed to 

 .show that it does, and that in this way we can explain 

 dispersion and allied phenomena. 



2 H2 



