PLEOCHROIC HALOES—JOLY. Sok 
In most of the cases referred to above the structure evidently shows 
the existence of concentric spherical shells of darkened biotite. This 
is a very interesting fact, for it proves that in the mineral the alpha 
ray gives rise to the same increased ionization toward the end of its 
range as Bragg determined in the case of gases; and we must con- 
clude that the halo in every case grows in this manner. A spherical 
shell of darkened biotite is first produced, and the inner coloration is 
only effected_as the more feeble ionization along the track of the ray 
in course of ages gives rise to sufficient alteration of the mineral. 
This more feeble ionization is, near the nucleus, enhanced in its 
effects by the fact that there all the rays combine to increase the 
ionization, and, moreover, the several tracks are there crowded by 
the convergency to the center. Hence the most elementary haloes 
seldom show definite rings due to uranium, etc., but appear as 
embryonic disk-hke markings. The photographs on the screen! illus- 
trate many of the phases of halo development. Rutherford suc- 
ceeded in making a halo artificially by compressing into a capillary 
glass tube a quantity of the emanation of radium. As the emanation 
decayed the various derived products came into existence and all the 
several alpha rays penetrated the glass, darkening the walls of the 
capillary out to the limit of the range of radium Cin glass. Figure 1 on 
plate 3 is a magnified view of the tube. The dark central part is the 
capillary. The tubular halo surrounds it. This experiment has, how- 
ever, been anticipated by some scores of millions of years, for here is 
the same effect in a biotite crystal (pl. 3, fig.2). Along what are appar- 
ently tubular passages or cracks in the mica a solution rich in radio- 
active 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. 
It must not be thought that the underexposed halo isa recent 
creation. By no means. All are old, appallingly old; and in the 
same rock all are probably of the same, or nearly the same, age. 
The underexposure is simply due to a lesser quantity of the radio- 
active elements in the nucleus. They are underexposed, 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 ionization attending the slowing down 
1 Shown by lecturer but not here reproduced. 
73176°—sMm 1914——21 
