THE PRESIDENTIAL ADDRESS. 19 
pected forces may come into importance at the very small distances 
separating the protons and electrons in the nucleus. Until we gain more 
information on the nature and law of variation of the forces inside the 
nucleus, further progress on the detailed structure of the nucleus may 
be difficult. At the same time, there are still a number of hopeful 
directions in which an attack may be made on this most difficult of 
problems. A detailed study of the y rays from radioactive bodies may 
be expected to yield information as to the motion of the electrons inside 
the nucleus, and it may be, as Ellis has suggested, that quantum 
laws are operative inside as well as outside the nucleus. From a 
study of the relative proportions of the elements in the earth’s crust, 
Harkins has shown that elements of even atomic number are much 
more abundant than elements of odd number, suggesting a marked 
difference of stability in these two classes of elements. It seems 
probable that any process of stellar evolution must be intimately 
connected with the building up of complex nuclei from simpler ones, 
and its study may thus be expected to throw much light on the evolution 
of the elements. 
The nucleus of a heavy atom is undoubtedly a very complicated 
system, and in a sense a world of its own, little, if at all, influenced by 
the ordinary physical and chemical agencies at our command. When 
we consider the mass of a nucleus compared with its volume it seems 
certain that its density is many billions of times that of our heaviest 
_ element. Yet, if we could form a magnified picture of the nucleus, 
we should expect that it would show a discontinuous structure, occupied 
but not filled by the minute building units, the protons and electrons, 
in ceaseless rapid motion controlled by their mutual forces. 
Before leaving this subject it is desirable to say a few words on 
the important question of the energy relations involved in the formation 
and disintegration of atomic nuclei, first opened up by the study of 
radioactivity. For example, it is well known that the total evolution 
of energy during the complete disintegration of one gramme of radium 
is many millions of times greater than in the complete combustion of 
an equal weight of coal. It is known that this energy is initially 
mostly emitted in the kinetic form of swift « and @ particles, and the 
energy of motion of these bodies is ultimately converted into heat 
when they are stopped by matter. Since it is believed that the radio- 
active elements were analogous in structure to the ordinary inactive 
elements the idea naturally arose that the atoms of all the elements 
contained a similar concentration of energy, which would be available 
for use if only some simple method could be discovered of promoting 
and controlling their disintegration. This possibility of obtaining new 
_ and cheap sources of energy for practical purposes was naturally an 
alluring prospect to the lay and scientific man alike. It is quite true 
