in the solid. In the former, initial recombination may obscure
the intense ionisation near the end of the range. We can only
detect the true end-effects by artificially separating the ions
by a strong electric force. If this recombination happened in the
mineral we should not have the concentric spheres so well defined
as we see them to be. What, then, hinders the initial
recombination in the solid? The answer probably is that the newly
formed

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ion is instantly used up in a fresh chemical combination. Nor is
it free to change its place as in the gas. There is simply a new
equilibrium brought about by its sudden production. In this
manner the conditions in the complex molecule of biotite,
tourmaline, etc., may be quite as effective in preventing initial
recombination as the most effective electric force we could
apply. The final result is that we find the Bragg curve
reproduced most accurately in the delicate shading of the rings
making up the perfectly exposed halo.

That the shading of the rings reproduces the form of the Bragg
curve, projected, as it were, upon the line of advance of the ray
and reproduced in depth of shading, shows that in yet another
particular the alpha ray behaves much the same in the solid as in
the gas. A careful examination of the outer edge of the circles
always reveals a steep but not abrupt cessation of the action of
the ray. Now Geiger has investigated and proved the existence of
scattering of the alpha ray by solids. We may, therefore, suppose
with much probability that there is the same scattering within
the mineral near the end of the range. The heavy iron atom of the
biotite is, doubtless, chiefly responsible for this in biotite
haloes. I may observe that this shading of the outer bounding
surface of the sphere of action is found however minute the
central nucleus. In the case of a nucleus of considerable size
another effect comes in which tends to produce an enhanced
shading. This will

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result if rays proceed from different depths in the nucleus. If
the nucleus were of the same density and atomic weight as the
surrounding mica, there would be little effect. But its density
and molecular weight are generally greater, hence the retardation
is greater, and rays proceeding from deep in the nucleus
experience more retardation than those which proceed from points
near to the surface. The distances reached by the rays in the
mica will vary accordingly, and so there will