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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 be a gradual cessation of the effects of the rays.

The result of our study of the halo may be summed up in the statement that in nearly every particular we have the phenomena, which have been measured and observed in the gas, reproduced on a minute scale in the halo.  Initial recombination seems, however, to be absent or diminished in effectiveness; probably because of the new stability instantly assumed by the ionised atoms.

One of the most interesting points about the halo remains to be referred to.  The halo is always uniformly darkened all round its circumference and is perfectly spherical.  Sections, whether taken in the plane of cleavage of the mica or across it, show the same exactly circular form, and the same radius.  Of course, if there was any appreciable increase of range along or across the cleavage the form of the halo on the section across the cleavage should be elliptical.  The fact that there is no measurable ellipticity is, I think, one which on first consideration would not be expected.