rised attractions,
there is equal retardation and equal ionisation in the molecule
in whatever

234

direction it is approached. Or, again, if the encounters indeed
differ in number, then some compensating effect must exist
whereby a direction of lesser linear density involves greater
stopping power in the molecule encountered, and vice versa.

The nature of the change produced by the alpha rays is unknown.
But the formation of the halo is not, at least in its earlier
stages, attended by destruction of the crystallographic and
optical properties of the medium. The optical properties are
unaltered in nature but are increased in intensity. This applies
till the halo has become so darkened that light is no longer
transmitted under the conditions of thickness obtaining in rock
sections. It is well known that there is in biotite a maximum
absorption of a plane-polarised light ray, when the plane of
vibration coincides with the plane of cleavage. A section across
the cleavage then shows a maximum amount of absorption. A halo
seen on this section simply produces this effect in a more
intense degree. This is well shown in Plate XXIII (lower figure),
on a portion of the halo-sphere. The descriptive name "Pleochroic
Halo" has originated from this fact. We must conclude that the
effect of the ionisation due to the alpha ray has not been to
alter fundamentally the conditions which give rise to the optical
properties of the medium. The increased absorption is probably
associated with some change in the chemical state of the iron
present. Haloes are, I believe, not found in minerals from which
this

235

element is absent. One thing is quite certain. The colouration is
not due to an accumulation of helium atoms, _i.e._ of spent alpha
rays. The evidence for this is conclusive. If helium was
responsible we should have haloes produced in all sorts of
colourless minerals. Now we sometimes see zircons in felspars and
in quartz, etc., but in no such case is a halo produced. And
halo-spheres formed within and sufficiently close to the edge of
a crystal of mica are abruptly truncated by neighbouring areas of
fclspar or quartz, although we know that the rays must pass
freely across the boundary. Again it is easy to show that even in
the oldest haloes the quantity of helium involved is so small
that one might say the halo-sphere was a tolerably good vacuum as
regards helium. There is, finally, no reason to suppose that the
imprisone