hey are
formed. We see here a peculiarity of the ionisation effected by
alpha rays. It is linear in distribution and very local. Much of
the ionisation in gases is again undone by recombination before
diffusion leads to the separation of the ions. This "initial
recombination" is greatest towards the end of the path of the ray
where the ionisation is a maximum. Here it may be so effective
that the form of the curve is completely lost unless a very large
electromotive force is used to separate the ions when the
ionisation is being investigated.

221

We have now reviewed recent work at sufficient length to
understand something of the nature of the most important advance
ever made in our knowledge of the atom. Let us glance briefly at
what we have learned. The radioactive atom in sinking to a lower
atomic weight casts out with enormous velocity an atom of helium.
It thus loses a definite portion of its mass and of its energy.
Helium which is chemically one of the most inert of the elements,
is, when possessed of such great kinetic energy, able to
penetrate and ionise the atoms which it meets in its path. It
spends its energy in the act of ionising them, coming to rest,
when it moves in air, in a few centimetres. Its initial velocity
depends upon the particular radioactive element which has given
rise to it. The length of its path is therefore different
according to the radioactive element from which it proceeds. The
retardation which it experiences in its path depends entirely
upon the atomic weight of the atoms which it traverses. As it
advances in its path its effectiveness in ionising the atom
rapidly increases and attains a very marked maximum. In a gas the
ions produced being much crowded together recombine rapidly; so
rapidly that the actual ionisation may be quite concealed unless
a sufficiently strong electric force is applied to separate them.
Such is a brief summary of the climax of radioactive
discovery:--the birth, life and death of the alpha ray. Its advent
into Science has altered fundamentally our conception of

222

matter. It is fraught with momentous bearings upon Geological
Science. How the work of the alpha ray is sometimes recorded
visibly in the rocks and what we may learn from that record, I
propose now to bring before you.

In certain minerals, notably the brown variety of mica known as
biotite, the microscope reveals minute circular marks occurring
here and there, quite irregularly. The m