contain radioactive elements. It is now well known how these
haloes are formed. The particle in the centre of the halo
contains uranium or thorium, and, necessarily, along with the
parent substance, the various elements derived from it. In the
process of transformation giving rise to these several derived
substances, atoms of helium--the alpha rays--projected with great
velocity into the surrounding mineral, occasion the colour
changes referred to. These changes are limited to the distance to
which the alpha rays penetrate; hence the halo is a spherical
volume surrounding the central substance.[1]

The time required to form a halo could be found if on the one
hand we could ascertain the number of alpha rays ejected from the
nucleus of the halo in, say, one year, and, on the other, if we
determined by experiment just how many alpha rays were required
to produce the same

[1] _Phil. Mag._, March, 1907 and February, 1910; also _Bedrock_,
January, 1913. See _Pleochroic Haloes_ in this volume.

21

amount of colour alteration as we perceive to extend around the
nucleus.

The latter estimate is fairly easily and surely made. But to know
the number of rays leaving the central particle in unit time we
require to know the quantity of radioactive material in the
nucleus. This cannot be directly determined. We can only, from
known results obtained with larger specimens of just such a