16 THE PRESIDENTIAL ADDRESS. 
of all more complicated structures, including matter in bulk, but we 
may not, therefore, be justified in expecting that its processes can be 
explained in terms of concepts derived entirely from a study of molar 
properties. The atomic processes involved may be so fundamental that 
a complete understanding may be denied us. It is early yet to be 
pessimistic on this question, for we may hope that our difficulties may 
any day be resolved by further discoveries. 
We must now turn our attention to that new and comparatively 
unexplored territory, the nucleus of the atom. In a discussion on the 
structure of the atom ten years ago, in answer to a question on the 
structure of the nucleus, I was rash enough to say that it was a problem 
that might well be left to the next generation, for at that time there 
seemed to be few obvious methods of attack to throw light on its con- 
stitution. While much more progress has been made than appeared 
possible at that time, the problem of the structure of the nucleus is 
inherently more difficult than the allied problem already considered ot 
the structure of the outer atom, where we have a wealth of information 
obtained from the study of light and X-ray spectra and from the chemical 
properties to test the accuracy of our theories. 
In the case of the nucleus, we know its resultant charge, fixed by 
Moseley’s law, and its mass, which is very nearly equal to the mass of 
the whole atom, since the mass of the planetary electrons is relatively 
very small and may for most purposes be neglected. We know that 
the nucleus is of size minute compared with that of the whole atom, 
and ¢an with some confidence set a maximum limit to its size. The 
study of radioactive bodies has provided us with very valuable informa- 
tion on the structure of the nucleus, for we know that the « and 6 
particles must be expelled from it, and there is strong evidence that the 
very penetrating y rays represent modes of vibration of the electrons 
contained in its structure. In the long series of transformations which 
occur in the uranium atom, eight « particles are emitted and six elec- 
trons, and it seems clear that the nucleus of a heavy atom is built up, 
in part at least, of helium nuclei and electrons. It is natural to sup- 
pose that many of the ordinary stable atoms are constituted in a similar 
way. It is a matter of remark that no indication has been obtained 
that the lightest nucleus, viz. that of hydrogen, is liberated in these 
transformations, where the processes occurring are of so fundamental a 
character. At the same time, it is evident that the hydrogen nucleus 
must be a unit in the structure of some atoms, and this has been 
confirmed by direct experiment. Dr. Chadwick and I have observed 
that swift hydrogen nuclei are released from the elements boron, 
nitrogen, fluorine, sodium, aluminium, and phosphorus when they are 
bombarded by swift « particles, and there is little room for doubt that 
these hydrogen nuclei form an essential part of the nuclear structure. 
