Closing Years of the Nineteenth Century. 419 



theory of Lorentz was, like those of Weber, Kiemann, and 

 Clausius,* a theory of electrons ; that is to say, all electro- 

 dynamical phenomena were ascribed to the agency of moving 

 electric charges, which were supposed in a magnetic field to 

 experience forces proportional to their velocities, and to com- 

 municate these forces to the ponderable matter with which 

 they might be associated.t 



In spite of the fact that the earlier theories of electrons 

 had failed to fulfil the expectations of their authors, the 

 assumption that all electric and magnetic phenomena are due 

 to the presence or motion of individual electric charges was 

 one to which physicists were at this time disposed to give a 

 favourable consideration ; for, as we have seen,* evidence of 

 the atomic nature of electricity was now contributed by the 

 study of the conduction of electricity through liquids and gases. 

 Moreover, the discoveries of Hertz had shown that a molecule 

 which is emitting light must contain some system resembling 

 a Hertzian vibrator; and the essential process in a Hertzian 

 vibrator is the oscillation of electricity to and fro. Lorentz 

 himself from the outset of his career! | had supposed the inter- 

 action of ponderable matter with the electric field to be effected 

 by the agency of electric charges associated with the material 

 atoms. 



The principal difference by which the theory now advanced 

 by Lorentz is distinguished from the theories of Weber, 



* Cf. pp. 226, 231, 262. 



+ Some writers have inclined to use the term ' electron-theory ' as if it were 

 specially connected with Sir Joseph Thomson's justly celebrated discovery (cf . p. 407, 

 supra) that all negative electrons have equal charges. But Thomson's discovery, 

 though undoubtedly of the greatest importance as a guide to the structure of the 

 universe, has hitherto exercised hut little influence on general electromagnetic 

 theory. The reason for this is that in theoretical investigations it is customary 

 to denote the changes of electrons by symbols, e\, e-z, . . . ; and the equality or 

 non-equality of these makes no difference to the equations. To take an illustration 

 from Celestial Mechanics, it would clearly make no difference in the general 

 equations of the planetary theory if the masses of the planets happened to be 

 all equal. 



* Cf. chapter xi. 



Cf. pp. 357-363. 



|| Verb. d. Ak. v. Wetenschappen, Amsterdam, Deel xviii (1878). 



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