September 30, 1921] 



SCIENCE 



285 



will not. If they can be both intensified and 

 controlled then we shall have at our disposal 

 an almost illimitable supply of power which 

 will entirely transcend anything hitherto 

 known. It is too early yet to say whether the 

 necessary conditions are capable of being 

 realized in practise, but I see no elements 

 in the problem which would justify us in 

 denying the possibility of this. It may be 

 that we are at the beginning of a new age, 

 which will be referred to as the age of sub- 

 atomic power. We can not say; time alone 

 will tell. 



THERMIONIC EMISSION 



With your permission, I will now descend 

 a little way from the summit of Mount Olym- 

 pus, and devote the rest of my address to a 

 sober review of the present state of some of 

 the questions with which my own thoughts 

 have been more particularly occupied. At 

 the Manchester meeting of the Association 

 in 1915 I had the privilege of opening a dis- 

 cussion on thermionic emission — that is to 

 say, the emission of electrons and ions by in- 

 candescent bodies. I recall that the opinion 

 was expressed by some of the speakers that 

 these phenomena had a chemical origin. That 

 view, I venture to think, is one which would 

 find very few supporters now. It is not that 

 any new body of fact has arisen in the mean- 

 time. The important facts were all estab- 

 lished before that time, but they were insuffi- 

 ciently appreciated, and their decisiveness was 

 inadequately realized. 



It may be worth while to revert for a mo- 

 ment to the issues in the controversy, already 

 moribund in 1915, because it has been closely 

 paralleled by similar controversies relating to 

 two other groups of phenomena — namely, 

 photoelectric emission and contact electromo- 

 tive force — which, as we shall see, are inti- 

 mately connected with thermionic emission. 

 The issue was not as to whether thermionic 

 emission may be looked upon simply as a type 

 of chemical reaction. Such an issue would 

 have been largely a matter of nomenclature. 



matter for a very long time without anything very 

 serious happening so far as we know. 



Thermionic electron emission has many feat- 

 ures in common with a typical reversible 

 chemical reaction such as the dissociation of 

 calcium carbonate into lime and carbon dioxide 

 There is a good deal to be said for the point 

 of view which regards thermionic emission as 

 an example of the simplest kind of reversible 

 chemical action, namely, that kind which con- 

 sists in the dissociation of a neutral atom 

 into a positive residue and a negative elec- 

 tron, inasmuch as we know that the negative 

 electron is one of the really fundamental ele- 

 ments out of which matter is built up. The 

 issue in debate was, however, of a different 

 character. It was suggested that the phenom- 

 enon was not primarily an emission of elec- 

 trons from the metallic or other source, but 

 was a secondary phenomenon, a kind of by- 

 product of an action which was primarily a 

 chemical reaction between the source of elec- 

 trons and some other material substance such 

 as the highly attenuated gaseous atmosphere 

 which surrounded it. This suggestion carried 

 with it either implicitly or explicitly the view 

 that the source of power behind the emission 

 was not the thermal energy of the source, but 

 was the chemical energy of the postulated 

 reaction. 



This type of view has never had any suc- 

 cess in elucidating the phenomena, and I do 

 not feel it necessary at this date to weary you 

 with a recital of the facts which run entirely 

 counter to it, and, in fact, definitely exclude 

 it as a possibility. They have been set forth 

 at length elsewhere on more than one occa- 

 sion. I shall take it to be established that the 

 phenomenon is physical in its origin and re- 

 versible in its operation. 



Establishing the primary character of the 

 phenomenon does not, however, determine its 

 nature or its immediate cause. Originally I 

 regarded it as simply kinetic, a manifesta- 

 tion of the fact that as the temperature rose 

 the kinetic energy of some of the electrons 

 would begin to exceed the work of the forces 

 by which they are attracted to the parent sub- 

 stance. With this statement there is, I think, 

 no room for anyone to quarrel, but it is per- 

 missible to inquire how the escaping electrons 



