20 THE PRESIDENTIAL ADDRESS. 
that, if we were able to hasten the radioactive processes in uranium and 
thorium so that the whole cycle of their disintegration could be confined 
to a few days instead of being spread over thousands of millions of 
years, these elements would provide very convenient sources of energy 
on a sufficient scale to be of considerable practical importance. Un- 
fortunately, although many experiments have been tried, there is no 
evidence that the rate of disintegration of these elements can be altered 
in the slightest degree by the most powerful laboratory agencies. With 
increase in our knowledge of atomic structure there has been a gradual 
change of our point of view on this important question, and there is ~ 
by no means the same certainty to-day as a decade ago that the atoms 
of an element contain hidden stores of energy. It may be worth while 
to spend a few minutes in discussing the reason for this change in out- 
look. This can best be illustrated by considering an interesting analogy 
between the transformation of a radioactive nucleus and the changes in 
the electron arrangement of an ordinary atom. It is now well known 
that it is possible by means of electron bombardment or by appropriate 
radiation to excite an atom in such a way that one of its superficial 
electrons is displaced from its ordinary stable position to another tem- 
porarily stable position further removed from the nucleus. This 
electron in course of time falls back into its old position, and its potential 
energy is converted into radiation in the process. There is some reason 
for believing that the electron has a definite average life in the displaced 
position, and that the chance of its return to its original position is 
governed by the laws of probability. In some respects an ‘ excited’ 
atom of this kind is thus analogous to a radioactive atom, but of course 
the energy released in the disintegration of a nucleus is of an entirely 
different order of magnitude from the energy released by return of the 
electron in the excited atom. It may be that the elements, uranium 
and thorium, represent the sole survivals in the earth to-day of types 
of elements that were common in the long distant ages, when the 
atoms now composing the earth were in course of formation. A frac- 
tion of the atoms of uranium and thorium formed at that time has 
survived over the long interval on account of their very slow rate of 
transformation. It is thus possible to regard these atoms as haying 
not yet completed the cycle of changes which the ordinary atoms have 
long since passed through, and that the atoms are still in the ‘ excited’ 
state where the nuclear units have not yet arranged themselves in posi- 
tions of ultimate equilibrium, but still have a surplus of energy which 
can only be released in the form of the characteristic radiation from 
active matter. On such a view, the presence of a store of energy ready 
for release is not a property of all atoms, but only of a special class 
of atoms like the radioactive atoms which have not yet reached the 
final state for equilibrium. 
