§ 75. The investigation of radioactivity during the last twenty-five years has led to the accumulation of a wealth of data concerning the emission of energy in the form of α, β, and γ rays from the radioactive nuclei and in nearly all cases the rate of disintegration of the element has been determined. This information must have an intimate bearing on the structure of the radioactive nuclei and it provides a variety of quantitative tests which can be applied to any hypothesis of this structure. It is, however, only within very recent times that a picture, even of the most general type, has been given which will explain satisfactorily the spontaneous disintegration of a nucleus with the emission of an α particle, and no application of this has yet been made to the emission of the β, and γ rays.

The difficulty in the way of an adequate theory of the structure of the radioactive nuclei has been twofold. In the first place, early in the study of radioactivity it became clear that the time of disintegration of an atom was independent of its previous history and depended only on chance. Since a nuclear particle, say an α particle, must be held in the nucleus by an attractive field, it seemed necessary, in order to explain its ejection, to invent some mechanism which would provide a spontaneous revulsion to a repulsive field.