ELECTRONS 79 



there is much evidence too much in fact for exposition 

 within the limits of a single chapter. 



Energy, the second unknown of modern science, is 

 accepted by the physicist, not only because it is a 

 necessary assumption in the eternal sequence of changes 

 which occur within us and about us, but also because 

 there is an unknown something which correlates exactly 

 with all these changes and seems to be conserved 

 throughout them. Whenever changes occur in the 

 form,' chemical composition, or location of bodies of mat- 

 ter the magnitude of the change is always definitely to be 

 predetermined upon the assumption that this mysteri- 

 ous energy will be constant and unchanged in amount. 



Its existence is an inference from the motions in- 

 volved in the change. Only in this kinetic form is it to 

 be detected and measured; and then, of course, only by 

 the motions of ponderable matter. Between such occa- 

 sions it masks its potentialities and appears as harmless 

 as the explosive shell, the high-tension electric wires, 

 or the reservoir of still water in the hills above the hydro- 

 electric plant. But when released, the magnitude of 

 the changes which occur shows that it has not been 

 altered by quiescence. 



In a science where the ether is a convenient postulate, 

 and energy a formless unknown, the electron stands 

 out in stark reality as a definite ponderable particle, 

 the tiny material element of the universe. 



Its identification in 1897 by Professor J. J. Thomson 

 followed close upon the discoveries of radium and 

 X-rays to which it now furnishes the mechanism for a 

 valid explanation. The quarter of a century since his 



