468 The Theory of Aether and Electrons in the 



devised and successfully carried out* for determining experi- 

 mentally the kinetic energy possessed by the ions after 

 emission. The mean kinetic energy of both negative and 

 positive ions was found to be the same for various metals 

 (platinum, gold, silver, etc.), and to be directly proportional to 

 the absolute temperature; and the distribution of velocities 

 among the ions proved to be that expressed by Maxwell's law. 

 The ions may therefore be regarded as kinetically equivalent 

 to the molecules of a gas whose temperature is the same as that 

 of the metal. 



By the investigations which have been recorded, the hypo- 

 thesis of atomic electric charges has been, to all appearances, 

 decisively established. But all the parts of the theory of 

 electrons do not enjoy an equal degree of security; and in 

 particular, it is possible that the future may bring important 

 changes in the conception of the aether. The hope was 

 formerly entertained of discovering an aether by reference to 

 which motion might be estimated .absolutely ; but such a hope 

 has been destroyed by the researches which have sprung from 

 Fitz Gerald's hypothesis of contraction; and in some recent 

 writings it is possible to recognize a tendency to replace the 

 classical aether by other conceptions, which, however, have 

 been as yet but indistinctly outlined. 



In any event, the close of the nineteenth century brought to 

 an end a well-marked era in the history of natural philosophy ; 

 and this is true not only with respect to the discoveries them- 

 selves, but also in regard to the conditions of scientific organiza- 

 tion and endeavour, which in the last decades of that period 

 became profoundly changed. The investigators who advanced 

 the theories of aether and electricity, from the time of Descartes 

 to that of Lord Kelvin, were, with very few exceptions, 

 congregated within a narrow territory : from Dublin to the 

 western provinces of Russia, and from Stockholm to the north 

 of Italy, may be circumscribed by a circle of no more than six 



* 0. W. Richardson and F. C. Brown, Phil. Mag. xvi (1908), pp. 353, 890 ; 

 F. C. Brown, Phil. Mag. xvii (1909), p. 355 ; xviii (1909), p. 649 



