tube. The dark central part is the capillary. The tubular halo
surrounds it. This experiment has, however, been anticipated by
some scores of millions of years, for here is the same effect in
a biotite crystal (Pl. XXV). Along what are apparently tubular
passages or cracks in the mica, a solution, rich in radioactive
substances, has moved; probably during the final consolidation of
the granite in which the mica occurs. A continuous and very
regular halo has developed along these conduits. A string of
halo-spheres may lie along such passages. We must infer that
solutions or gases able to establish the radioactive nuclei moved
along these conduits, and we are entitled to ask if all the
haloes in this biotite are not, in this sense, of secondary
origin. There is, I may add, much to support such a conclusion.
The widespread distribution of radioactive substances is most
readily appreciated by examination of sections of rocks cut thin
enough for microscopic investigation. It is, indeed, difficult to
find, in the older rocks of granitic type, mica which does not
show haloes, or traces of haloes. Often we find that every one of
the inclusions in the mica—that is, every one of the earlier
formed substances—contain radioactive elements, as indicated by
the presence of darkened borders. As will be seen presently the
quantities involved are generally vanishingly small. For example
it was found by direct determination that in one gram of the
halo-rich mica of Co. Carlow there was rather less than twelve
billionths of a gram of radium, We are
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entitled to infer that other rare elements are similarly widely
distributed but remain undetectable because of their more stable
properties.
It must not be thought that the under-exposed halo is a recent
creation. By no means. All are old, appallingly old; and in the
same rock all are, probably, of the same, or neatly the same,
age. The under-exposure is simply due to a lesser quantity of the
radioactive elements in the nucleus. They are under-exposed, in
short, not because of lesser duration of exposure, but because of
insufficient action; as when in taking a photograph the stop is
not open enough for the time of the exposure.
The halo has, so far, told us that the additive law is obeyed in
solid media, and that the increased ionisation attending the
slowing down of the ray obtaining in gases, also obtains in
solids; for, otherwise, the halo would not commence its
development as a spherical shell or envelope. But here we learn
that there is probably a certain difference in the course of
events attending the immediate passage of the ray in the gas and
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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