( 819 ) 



Hence, if' we alter tlie relative position of tlie ceni res of the electrons 



in -^ by applying the delbrnialion [kl, I, I), and if, in the points 



thus obtained, we place the centres of electrons that remain at rest, 



we shall get a system, identical to the imaginary system 2£', of 



which we have spoken in § 6. The forces in this system ami those 



in 2i^ will bear to each other the relation expressed by (21). 



In the second place I shall snpj)Ose that the forces between unchar- 



(jed partich's, <is irell us tliose between such [jart'icles and electrons, are 



injiuencecl by a translation, in, quite tlie same way as tlce electric forces 



in an electrostatic system. In other terms, wliatever be the nature of 



the particles composing a ponderable body, so long as they do not 



move relatively to each other, we shall have between the forces 



acting in a system (2£') without, and the same system (2) with a 



translation, the relation specified in (21), if, as regards the relative 



position of the particles, -2" is got from ^ by the deformation (/./,/, /), 



/I 1 ' n 

 or ^ from 2S' by the detormation I —-, -—, — I. 



\n'V t ^ J 



We see by this that, as soon as the resulting force is for a 

 particle in 3', the same must be true for the corresponding particle 

 in 2^. Conse({uently, if, neglecting the effects of molecular motion, 

 we suppose each particle of a solid body to be in equilibrium under 

 the action of the attractions and repulsions exerted by its neighbours, 

 and if we rake for granted that there is but one configuration of 

 equilibrium, we may draw the conclusion that the system ^\ if the 

 velocity //; is imparted to it, will of itself change into the system 

 ^. In other terms, the translation will produce the deformation 



1 1 1 



kV T 1 



The case of molecular motion will be considered in § 12. 



It will easily be seen that the hypothesis that has formerly been 

 made in connexion with Michelson's experiment, is implied in what 

 has now been said. However, the present hypothesis is more general 

 because the only limitation imposed on the motion is that its velocity' 

 be smaller than that of light. 



§ 9. We are now in a [tosition to calculate the electromagnetic 

 momentum of a single electron. For simplicity's sake I shall suppose 

 the charge e to be uniforndy ilistributed over the surface, so long 

 as the electron remains at rest. Then, a distribution of the same 

 kin<l will exist in the system ^' with which we are concerned in 

 the last integral of (22). Hence 



54* 



