§ 79. Introduction. The β rays from radium and other radioactive bodies were early shown to consist of a stream of electrons projected with a wide range of speed, the swifter ones with a velocity close to that of light. Apart from a study of the apparent absorption and reflection of the β rays from different radioactive substances in traversing matter, further progress was comparatively slow, and it was not until recent years that a clear idea was obtained of the origin of the β rays and of their connection with radioactive transformations.

Compared with the complex emission of β rays, a study of the homogeneous emission of a particles and their absorption by matter presented a relatively simple and direct problem, and in a surprisingly short time the main facts of the material nature of the α rays and their origin were established and the results interpreted in terms of the transformation theory.

Radioactivity was then at once recognised to be an actual disintegration of the nucleus, which in the a ray case consisted in the emission of a helium nucleus and in the β ray case of an electron from the nucleus. These disintegration particles had to be distinguished from any subsidiary electrons that might be detached from the outer electronic levels during the disintegration. It will be seen later that this separation has been rendered possible by the recognition of the important part played by γ rays in the disintegration.