A.—MATHEMATICS AND PHYSICS. 27 
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 anything hitherto known. It is too early 
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. 
With your permission, I will now descend a little way from the 
summit of Mount Olympus, 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 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 pheno- 
mera 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 established before that time, but they were insufficiently appreciated, 
_and their decisiveness was inadequately 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, photoelectric emission and contact electro- 
motive force—which, as we shall see, are intimately 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. 
Thermionic electron emission has many features 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 consists in the dissociation 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 fundamental 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 elec- 
trons and some other material substance such as the highly attenuated 
1 To reassuré the nervous I would. however, interpolate the comforting 
thought that this planet has held considerable quantities of radioactive matter 
for a very long tinie Without anything very seriotis Hidppenihg $0 far as we 
know: 
£2 
