18 THE PRESIDENTIAL ADDRESS. 
different radioactive properties, for it is to be anticipated that the stability 
of a nucleus will be much influenced by its mass and arrangement. 
Our knowledge of isotopes has been widely extended in the last few 
years by Aston, who has devised an accurate direct method for showing 
the presence of isotopes in the ordinary elements. He has found that 
some of the elements are ‘ pure ’—i.e. consist of atoms of identical mass— 
while others contain a mixture of two or more isotopes. In the case of 
the isotopic elements, the atomic mass, as ordinarily measured by the 
chemist, is a mean value depending on the atomic masses of the indi- 
vidual isotopes and their relative abundance. These investigations have 
not only shown clearly that the number of distinct species of atoms is 
much greater than was supposed, but have brought out a relation between 
the elements of great interest and importance. The atomic masses of 
the isotopes of most of the,elements examined have been found, to an 
accuracy of about one in a thousand, to be whole numbers in terms of 
oxygen, 16. This indicates that the nuclei are ultimately built up of 
protons of mass very nearly one and of electrons. It is natural to 
suppose that this building unit is the hydrogen nucleus, but that its 
average mass in the complex nucleus is somewhat less than its mass in 
the free state owing to the close packing of the charged units in the 
nuclear structure. We have already seen that the helium nucleus of 
mass 4 is probably a secondary unit of great importance in the building 
up of many atoms, and it may be that other simple combinations of 
protons and electrons of mass 2 and 3 occur in the nucleus, but these 
have not been observed in the free state. 
While the mass of the majority of the isotopes are nearly whole 
numbers, certain cases have been observed by Aston where this rule is 
slightly departed from. Such variations in mass may ultimately prove 
of great importance in throwing light on the arrangement and closeness 
of packing of the protons and electrons, and for this reason it is to be 
hoped that it may soon prove possible to compare atomic masses of 
the elements with much greater precision even than at present. 
While we may be confident that the proton and the electron are the 
ultimate units which take part in the building up of all nuclei, and can 
deduce with some certainty the number of protons and electrons in the 
nuclei of all atoms, we have little, if any, information on the distribution 
of these units in the atom or on the nature of the forces that hold them 
in equilibrium. While it is known that the law of the inverse square 
holds for the electrical forces some distance from the nucleus, it seems 
certain that this law breaks down inside the nucleus. A detailed study 
of the collisions between « particles and hydrogen. atoms, where the 
nuclei approach very close to each other, shows that the forces between 
nuclei increase ultimately much more rapidly than is to be expected 
from the law of the inverse square, and it may be that new and unex- 
