are due to uranium, although their inner darkening
is doubtless aided by the passage of rays which were too few to
extend the darkening beyond the vigorous effects of the two
uranium rays. Then we find haloes carried out to the radii 0.016,
0.018 and 0.019 mm. The last sometimes show very beautiful outer
rings having radial dimensions such as would be produced by
radium A and radium C. Finally we may have haloes in which
interior detail is lost so far out as the radius due to emanation
or radium A, while outside this floats the ring due to radium C.
Certain variations of these effects may occur, marking,
apparently, different stages of exposure. Plates XXIII and XXIV
(upper figure) illustrate some of these stages;

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the latter photograph being greatly enlarged to show clearly the
halo-sphere of radium A.

In most of the cases mentioned 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
ionisation towards the end of its range, as Bragg determined in
the case of gases. And we must conclude that the halo in every
case grows in this manner. A spherical shell of darkened biotite
is first produced and the inner colouration is only effected as
the more feeble ionisation along the track of the ray in course
of ages gives rise to sufficient alteration of the mineral. This
more feeble ionisation is, near the nucleus, enhanced in its
effects by the fact that there all the rays combine to increase
the ionisation and, moreover, the several tracks are there
crowded by the convergency to the centre. Hence the most
elementary haloes seldom show definite rings due to uranium,
etc., but appear as embryonic disc-like markings. The photographs
illustrate many of the phases of halo development.

Rutherford succeeded 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 C in glass. Plate XXV shows a magnified
section of the

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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 sam