191 1.] AGGREGATES OF ELECTRONS. 353 



is higher the higher the temperature. Clearly, at a sufficiently high 

 temperature some of the particles will have enough energy from their 

 heat motion to be able to break through the surface. Aloreover, the 

 number which are able to escape will be greater the higher the 

 temperature. 



A theory following these lines has succeeded in predicting the 

 way in which the emission of the electrons depends upon the tem- 

 perature as well as a number of other interesting relations between 

 the thermal and electrical behavior of substances. It will be re- 

 marked that the view which has been outlined is very similar to the 

 view of the phenomenon of evaporation which is afforded by the 

 kinetic theory of matter. According to that theory the particles of 

 the liquid escape into the vapor when their kinetic energy (to be 

 accurate we ought to say that part of it which depends on the com- 

 ponent of velocity normal to the surface) exceeds the work they 

 have to do in order to pass through the surface. Thermionic emis- 

 sion may be looked upon then simply as the evaporation of electrons 

 which may be regarded as dissolved in the metal. Just as water is 

 cooled when it evaporates and heated when steam condenses into it; 

 so we should expect a conductor to be cooled when it emits electrons 

 and heated when it absorbs them. Both these effects have recently 

 been discovered, the former by W'ehnelt and Jentzsch and the latter 

 by Richardson and Cook. 



There is one point in this connection which is worthy of further 

 consideration. We have seen that it is necessary to suppose that 

 the electrons in a metal behave like the molecules of a gas. The 

 same will be at least as true of them after they have been emitted. 

 Thus when a metal at a high temperature lies in an air-tight en- 

 closure there will be two atmospheres of electrons, one at a high 

 pressure inside the metal and the other at a low pressure in the 

 enclosure outside of the metal. If the principles of the kinetic 

 theory of matter are well grounded it can be shown that in both of 

 these atmospheres the electrons are moving about with all possible 

 speeds but that the proportion of them which have a given speed is 

 the same for each atmosphere. Moreover, the proportion is the same 

 known function of the temperature in each case and in each case also 



PROG. AMER. PHIL. SOC, L. 200 \V, PRINTED AUG. 4, I9II. 



