PART III.
The [Greek: alpha] Rays.
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
