flie Theory of Electrons. 271 



4. And a mobility of this nature, even if it be not 

 necessary in order to surmount difficulties in the theory of 

 aberration of light, is absolutely required for the existence 

 of a force between two electrons or other conductors. In 

 the development of the theory, electric force appears in the 

 variation of the energy as the coefficient multiplying the 

 virtual displacement of the centre of the electron nucleus *, 

 and the mechanical traction on the surface of a material 

 conductor as the coefficient multiplying the virtual displace- 

 ment of a portion of the surface of the conductor. But this 

 variation of the energy is made on what amounts to the 

 supposition that in the displacement of the surface of the 

 nucleus or conductor any region of sether which is thereby 

 encroached on loses its elasticity, and any region which 

 is thereby vacated regains it. (The fact that this is probably 

 what occurs effectively in the case of a finite material 

 conductor, as the charge consists of a distribution of free 

 electrons over its surface which, when in equilibrium, keep 

 the interior free from electric stress, in no way vitiates the 

 argument here presented. If this hypothesis be correct, the 

 consideration of what happens when a portion of the surface 

 of a material conductor is moved involves, and should be 

 preceded by, the discussion of the motion of an electron.) 

 If, on the other hand, the variation be made on the supposition 

 that there goes on no such process of electric straining on 

 one side and release on the other, but that the displace- 

 ment of the surface involves the displacement of the sur- 

 rounding sether in the same way as the displacement of a 

 portion of the surface of a bubble of air involves that of 

 the surrounding medium, then the corresponding coefficient 

 may be proved to be zero. Such a supposition would therefore 

 afford no room for electrostatic force between electrons or 

 conductors, any more than ordinary hydrostatic theory shows 

 an attraction or repulsion between two balls immersed in a 

 material fluid. 



And the difficulty could not be surmounted by supposing 

 that the nucleus is not devoid of elasticity, but is a region of 

 sether in which the concentration of the rotation-vector is not 

 zero, corresponding to a volume-density of electricity therein, 

 or that the nucleus is so small that the energy of the irro- 

 tational motion which would be caused by its displacement 

 if the surrounding sether did not resist rotation, is negligible; 

 or that the resistance of the sether to compression, though 

 enormously great, is not infinite. On any of these hypotheses, 



* Phil. Trans. 1897, A. p. 213 ; ' .Ether and Matter,' p. 97. 



U2 



