ring."
The angle of scattering increases rapidly with the decrease of
velocity. Now the effect of the scattering will be to cause some
of the rays to complete their ranges

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or, more accurately, to leave their direct line of advance a
little sooner than others. In the beautiful experiments of C. T.
R. Wilson we are enabled to obtain ocular demonstration of the
scattering. The photograph (Fig. 14.), which I owe to the
kindness of Mr. Wilson, shows the deflection of the ray towards
the end of its path. In

{Fig. 14}

this case the path of the ray has been rendered visible by the
condensation of water particles under the influence of the
ionisation; the atmosphere in which the ray travels being in a
state of supersaturation with water vapour at the instant of the
passage of the ray. It is evident that if we were observing the
ionisation along a sheaf of parallel rays, all starting with
equal velocity,

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the effect of the bending of some of the rays near the end of
their range must be to cause a decrease in the aggregate
ionisation near the very end of the ultimate range. For, in fact,
some of the rays complete their work of ionising at points in the
gas before the end is reached. This is the cause, or at least an
important contributory cause, of the decline in the ionisation
near the end of the range, when the effects of a bundle of rays
are being observed. The explanation does not suggest that the
ionising power of any one ray is actually diminished before it
finally ceases to be an alpha ray.

The full line in Fig. 13 gives the ionisation curve which it may
be expected would be struck out by a single alpha ray. In it the
ionisation goes on increasing till it abruptly ceases altogether,
with the entire loss of the initial kinetic energy of the
particle.

A highly remarkable fact was found out by Bragg. The effect of
the atom traversed by the ray in checking the velocity of the ray
is independent of the physical and chemical condition of the
atom. He measured the "stopping power" of a medium by the
distance the ray can penetrate into it compared with the distance
to which it can penetrate in air. The less the ratio the greater
is the stopping power. The stopping power of a substance is
proportional to the square root of its atomic weight. The
stopping power of an atom is not altered if it is in chemical
union with another atom. The atomic weight is the one quality of
importance. The physical

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