haped halo. The halo is a spherical volume containing the
radioactive nucleus at its centre. The true radius of the halo
may, therefore, only be measured on sections passing through the
nucleus.

226

In order to understand the mode of formation of a halo we may
profitably study on a diagram the events which go on within the
halo-sphere. Such a diagram is seen in Fig. 15. It shows to
relatively correct scale the limiting range of all the alpha-ray
producing members of the uranium and thorium families. We know
that each member of a family will exist in equilibrium amount
within the nucleus possessing the parent element. Each alpha ray
leaving the nucleus will just attain its range and then cease to
affect the mica. Within the halosphere, there must be, therefore,
the accumulated effects of the influences of all the rays. Each
has its own sphere of influence, and the spheres are all
concentric.

The radii in biotite of the several spheres are given in the
following table

URANIUM FAMILY.
Radium C - 0.0330 mm.
Radium A - 0.0224 mm.
Ra Emanation - 0.0196 mm.
Radium F - 0.0177 mm.
Radium - 0.0156 mm.
Ionium - 0.0141 mm.
Uranium 1 - 0.0137 mm.
Uranium 2 - 0.0118 mm.

THORIUM FAMILY.
Thorium CE - 0.040 mm.
Thorium A - 0.026 mm.
Th Emanation - 0.023 mm.
Thorium Ci - 0.022 mm.
Thorium X - 0.020 mm.
Radiothorium - 0.119 mm.
Thorium - 0.013 mm.

In the photograph (Pl. XXIV, lower figure), we see a uranium and
a thorium halo in the same crystal of mica. The mica is contained
in a rock-section and is cut across the cleavage. The effects of
thorium Ca are clearly shown

227

as a lighter border surrounding the accumulated inner darkening
due to the other thorium rays. The uranium halo (to the right)
similarly shows the effects of radium C, but less distinctly.

Haloes which are uniformly dark all over as described above are,
in point of fact, "over-exposed"; to borrow a familiar
photographic term. Haloes are found which show much beautiful
internal detail. Too vigorous action obscures this detail just as
detail is lost in an over-exposed photograph. We may again have
"under-exposed" haloes in which the action of the several rays is
incomplete or in which the action of certain of the rays has left
little if any trace. Beginning at the most under-exposed haloes
we find circular dark marks having the radius 0.012 or 0.013 mm.
These haloes