322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
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 differ- 
ence 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 ionization 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 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 con- 
ditions 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 result 
if rays proceed from different depths in the nucleus. If the nucleus 
was 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 retarda- 
tion 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 state- 
ment that in nearly every particular we have the phenomena which 
have been measured and observed in the gas reproduced on a minute 
