Page:Radio-activity.djvu/161

 PART III.

[Greek: alpha].

87. The [Greek: alpha] rays. The magnetic deviation of the [Greek: beta] rays was discovered towards the end of 1899, at a comparatively early stage in the history of radio-activity, but three years elapsed before the true character of the [Greek: alpha] rays was disclosed. It was natural that great prominence should have been given in the early stages of the subject to the [Greek: beta] rays, on account of their great penetrating power and marked action in causing phosphorescence in many substances. The [Greek: alpha] rays were, in comparison, very little studied, and their importance was not generally recognized. It will, however, be shown that the [Greek: alpha] rays play a far more important part in radio-active processes than the [Greek: beta] rays, and that the greater portion of the energy emitted in the form of ionizing radiations is due to them.

88. The nature of the [Greek: alpha] rays. The nature of the [Greek: alpha] rays was difficult to determine, for a magnetic field sufficient to cause considerable deviation of the [Greek: beta] rays produced no appreciable effect on the [Greek: alpha] rays. It was suggested by several observers that they were, in reality, secondary rays set up by the [Greek: beta] or cathode rays in the active matter from which they were produced. Such a view, however, failed to explain the radio-activity of polonium, which gave out [Greek: alpha] rays only. Later work also showed that the matter, which gave rise to the [Greek: beta] rays from uranium, could be chemically separated from the uranium, while the intensity of the [Greek: alpha] rays was unaffected. These and other results show that the [Greek: alpha] and [Greek: beta] rays are produced quite independently of one another. The view that they are an easily absorbed type of Röntgen rays fails to explain a characteristic property of the [Greek: alpha] rays, viz. that the absorption of the rays in a given thickness of matter, determined by the electrical method, increases with the thickness of matter previously traversed. It does not seem probable that such an effect could be produced by a radiation like X rays, but the result is to be expected if the rays consist of projected bodies, which fail to