§ 119. In previous chapters we have discussed some of the properties of radioactive nuclei and the types of radiation which accompany their transformations. The instability of these nuclei has given us important information on their structure, but unfortunately such information is not available in the case of the ordinary non-radioactive elements. Apart from their instability, there is no reason to believe that the nuclei of the radioactive elements differ in any marked way in their general type of structure from ordinary elements of high atomic weight. It is thus important to examine the properties of atomic nuclei in general to see whether we can obtain evidence to throw light on their structure and their connection with one another. In particular, it is of great interest to see whether any definite evidence can be obtained of the reasons why the property of radioactivity only manifests itself in any marked degree in the two elements of highest atomic weight, thorium and uranium, and their products of transformation.

In chapter vn an account has been given of the genesis of the nuclear theory of the atom and the evidence in its support. On this theory, the ordinary physical and chemical properties of the atom, excluding its mass, depend on the magnitude of the nuclear charge, for on this depends the number and distribution of the outer electrons.