191 1.] AGGREGATES OF ELECTRONS. 363 



plicit evidence of the existence of these minute charged particles, 

 whose mass is nearly 2,000 times smaller than that of the lightest 

 known chemical atom. 



The emission of ordinary heat radiation such as is given out 

 by all substances, and in increasing amount the higher the tempera- 

 ture, is very intimately connected with the theory of electrons. As 

 is well known this radiation is electromagnetic in character and in- 

 cludes visible light as a particular case. We also know that when 

 an electron is accelerated it emits electromagnetic radiation. It 

 is natural therefore to attribute the origin of this radiation to the 

 motions of the electrons of which material bodies are made up. 

 By making use of the principles of thermodynamics we can prove 

 that the nature of the radiation of this character which is to be 

 found in any enclosure maintained at a given temperature is inde- 

 pendent of the nature of the walls of the enclosure. The amount 

 and character of this radiation is thus independent of the material 

 from which it originates. If therefore we can calculate the amount 

 of this radiation of each wave-length for any particular substance, 

 for a series of temperatures, we shall know what it is for all sub- 

 stances at the same temperatures. Unfortunately when such a 

 calculation is carried out in the most logical and natural way it 

 leads to results which are not in agreement with those given by 

 experimental measurements. Another mode of calculation given by 

 Planck leads to a formula which agrees with the experimental values. 

 It has been shown however that Planck's calculation involves the 

 implicit assumption that energy is an atomic or discontinuous quan- 

 tity. This idea is distasteful to many physicists and it is so revolu- 

 tionary that it is not desirable to adopt it without very convincing 

 evidence. One of the authorities on this very intricate subject, 

 Jeans, maintains that the reason for the discrepancy between the 

 less revolutionary theory and experiment is due to the fact, as he 

 asserts, that the experiments do not measure the true equilibrium 

 radiation. However this may be, the difficulties which lie in the 

 explanation of the connection between radiation and temperature 

 do not belong to the electron theory proper but are outside of it. 

 No matter how it may be decided the outcome of this question is 

 not likely to shake the foundations of the electron theory of matter. 



