luence of the radioactive
substances contained in the zircon. The

224

extraordinary uniformity of the radial measurements of perfectly
formed haloes (to use the name by which they have long been
known) suggests that they may be the result of alpha radiation.
For in that case, as we have seen, we can at once account for the
definite radius as simply representing the range of the ray in
biotite. The furthest-reaching ray will define the radius of the
halo. In the case of the uranium family this will be radium C,
and in the case of thorium it will be thorium C. Now here we
possess a means of at once confirming or rejecting the view that
the halo is a radioactive phenomenon and occasioned by alpha
radiation; for we can calculate what the range of these rays will
be in biotite, availing ourselves of Bragg's additive law,
already referred to. When we make this calculation we find that
radium C just penetrates 0.033 mm. and thorium C 0.040 mm. The
proof is complete that we are dealing with the effects of alpha
rays. Observe now that not only is the coincidence of measurement
and calculation a proof of the view that alpha radiation has
occasioned the halo, but it is a very complete verification of
the important fact stated by Bragg, that the stopping power
depends solely on the atomic weight of the atoms traversed by the
ray.

We have seen that our examination of the rocks reveals only the
two sorts of halo: the radium halo and the thorium halo. This is
not without teaching. For why not find an actinium halo? Now
Rutherford long ago suggested that this element and its
derivatives were

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probably an offspring of the uranium family; a side branch, as it
were, in the formation of which relatively few transforming atoms
took part. On Rutherford's theory then, actinium should always
accompany uranium and radium, but in very subordinate amount. The
absence of actinium haloes clearly supports this view. For if
actinium was an independent element we would be sure to find
actinium haloes. The difference in radius should be noticeable.
If, on the other hand, actinium

was always associated with uranium and radium, then its effects
would be submerged in those of the much more potent effects of
the uranium series of elements.

It will have occurred to you already that if the radioactive
origin of the halo is assured the shape of a halo is not really
circular, but spherical. This is so. There is no such thing as a
disc-s