November 17, 192 1] 



NATURE 



173 



know from the amount ot heat liberated in radio- 

 active disintegration that the amount of energy- 

 stored in the nuclei is of a higher order of magni- 

 tude altogether, some millions of times greater, 

 in fact, than that generated by any chemical 

 reaction such as the combustion of coal. In this 

 comparison, of course, it is the amount of energy 

 per unit mass of reacting or disintegrating matter 

 which is under consideration. The amounts of 

 energy which have thus far been released by arti- 

 ficial disintegration of the nuclei are in them- 

 selves small, but they are enormous in comparison 

 with the minute amounts of matter affected. If 

 these effects can be sufficiently intensified there 

 appear to be two possibilities. Either they will 

 prove uncontrollable, which would presumably 

 spell the end of all things,- or they 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 any- 

 thing hitherto known. It is too earlv yet to say 

 whether the necessary conditions are capable of 

 being realised in practice, 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 cannot 

 say ; time alone will tell. 



Thermionic Emission. 



At the Manchester meeting of the Association in 

 191 5 I had the privilege of opening a discussion on 

 thermionic emission — that is to say, the emission 

 of electrons and ions by incandescent 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 

 meantime. The important facts were all estab- 

 lished before that time, but they were insufficiently 

 appreciated, and their decisiveness was inade- 

 quately realised. 



It may be worth while to revert for a moment 

 to the issues in that controversy, already moribund 

 in 1915, because it has been closely paralleled by 

 similar controversies relating to two other groups 

 of phenomena — namely, photo-electric emission 

 and contact electro-motive force — which, as we 

 shall see, are intimately connected with thermionic 

 emission. The issue was not as to whether therm- 

 ionic emission may be looked upon simply as a 

 type of chemical reaction. Such an issue' would 

 have been largely a matter of nomenclature. 

 Thermionic electron emission has many features 

 in common with a typical reversible' chemical 

 reaction such as the dissociation of calcium car- 

 bonate 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, 



« To reassure the nervous I would, however, interpolate the comfortiDE 

 thought that this planet has held considerable qi^ties of rSS^t^l 

 ^"wfl^^ow ""^ ^°^ '""' *"''°"' ^"yihing very serious happening so far 



XO. 2716, VOL. 108] 



namely, that kind which consists in the dissocia- 

 tion of a neutral atom into a positive residue and 

 a negative electron, inasmuch as we know that 

 the negative electron is one of the really funda- 

 mental elements out of which matter is built up. 

 The issue in debate was. however, of a different 

 character. It was suggested that the phenomenon 

 was not primarily an emission of electrons 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 electrons and some other material 

 substance such as the highly attenuated gaseous 

 atmosphere which surrounded it. This sugges- 

 tion carried with it either implicitly or explicitly 

 the view that the source of power behind the 

 ; emission was not the thermal energv of the 

 source, but was the chemical energy of the postu- 

 lated reactions. 



This type of view has never had any success 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 mor'e 

 than one occasion. I shall take it to be estab- 

 lished that the phenomenon is physical in its origin 

 and reversible in its operation. 



Establishing the primary character of the phe- 

 nomenon does not, however, determine its nature 

 or its immediate cause. Originally I regarded it 

 as simply kinetic, a manifestation of the fact that 

 as the temperature rose the kinetic energv of some 

 of the electrons would begin to exceed the work 

 of the forces by which they are attracted to the 

 parent substance. With this statement there is, 

 I think, no room for anyone to quarrel, but it is 

 permissible to inquire how the escaping electrons 

 obtain the necessary energ\-. One answer is that 

 the electrons have it already in the interior of the 

 substance by virtue of their energy of thermal 

 agitation. But thermal agitations now appear 

 less simple than they used to be regarded, and in 

 any event they do not exhaust the possibilities. 



We know that w hen light of short enough wave- 

 length falls on matter it causes the ejection of 

 electrons from it — the so-called photo-electric 

 effect. Since the formula for the radiation 

 emitted by a body at any given temperature con- 

 tains every wave-length without limitation, there 

 must be some emission of electrons from an incan- 

 descent body as the result of the photo-electric 

 effect of its own luminosity. Two questions 

 obviously put themselves. Will this photo-electric 

 emission caused by the whole spectrum of the hot 

 body vary as the temperature of the incandescent 

 body is raised in the way which is known to 

 characterise thermionic emission? A straight- 

 forward thermodynamic calculation shows that 

 this is to be expected from the theoretical point of 

 view, and the anticipation has been confirmed by 

 the experiments of Prof. W. Wilson. Thus 

 the autophoto-electric emission has the correct 

 behaviour to account for the thermionic emission. 

 The other question is: Is it large enough? This 



