g
from thorium C2, which reaches a distance of 8.6 cms. In the
uranium family the fastest ray is

215

that of radium C. It attains 6.94 cms. There is thus an
appreciable difference between the ultimate distances traversed
by the most energetic rays of the two families. The shortest
ranges are those of uranium 1 and 2.

The ionisation effected by these rays is by no means uniform
along the path of the ray. By examining the conductivity of the
gas at different points along the path of the ray, the ionisation
at these points may be determined. At the limits of the range the
ionisation

{Fig. 13}

ceases. In this manner the range is, in fact, determined. The
dotted curve (Fig. 13) depicts the recent investigation of the
ionisation effected by a sheaf of parallel rays of radium C in
air, as determined by Geiger. The range is laid out horizontally
in centimetres. The numbers of ions are laid out vertically. The
remarkable nature of the results will be at once apparent. We
should have expected that the ray at the beginning of its path,
when its velocity and kinetic energy were greatest, would have
been more effective than towards the end of its range

216

when its energy had almost run out. But the curve shows that it
is just the other way. The lagging ray, about to resign its
ionising properties, becomes a much more efficient ioniser than
it was at first. The maximum efficiency is, however, in the case
of a bundle of parallel rays, not quite at the end of the range,
but about half a centimetre from it. The increase to the maximum
is rapid, the fall from the maximum to nothing is much more
rapid.

It can be shown that the ionisation effected anywhere along the
path of the ray is inversely proportional to the velocity of the
ray at that point. But this evidently does not apply to the last
5 or 10 mms. of the range where the rate of ionisation and of the
speed of the ray change most rapidly. To what are the changing
properties of the rays near the end of their path to be ascribed?
It is only recently that this matter has been elucidated.

When the alpha ray has sufficiently slowed down, its power of
passing right through atoms, without appreciably experiencing any
effects from them, diminishes. The opposing atoms begin to exert
an influence on the path of the ray, deflecting it a little. The
heavier atoms will deflect it most. This effect has been very
successfully investigated by Geiger. It is known as "scatte