the slow breakdown of the commoner elements may be dismissed. The
halo shows that the mica of the rocks is radioactively sensitive.
The fundamental criterion of radioactive change is the expulsion
of the alpha ray. The molecular system of the mica and of many
other minerals is unstable in presence of these rays, just as a
photographic plate is unstable in presence of light. Moreover,
the mineral integrates the radioactive effects in the same way as
a photographic salt integrates the effects of light. In both
cases the feeblest activities become ultimately apparent to our
inspection. We have seen that one ray in each year since the
Devonian period will build the fully formed halo: an object
unlike any other appearance in the rocks. And we have been able
to allocate all the haloes so far investigated to one or the
other of the known radioactive families. We are evidently
justified in the belief that had other elements been radioactive
we must either find characteristic haloes produced by them, or
else find a complete darkening of the mica. The feeblest alpha
rays emitted by the relatively enormous quantities of the
prevailing elements, acting over the whole duration of geological
time—and it must be remembered that the haloes we have been
studying are comparatively young—must have registered their
effects on the sensitive minerals. And thus we are safe in
concluding that the common elements, and, indeed, many which
would be called rare, are possessed of a degree of stability
which has preserved them un
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changed since the beginning of geological time. Each unaffected
flake of mica is, thus, unassailable proof of a fact which but
for the halo would, probably, have been for ever beyond our
cognisance.
THE USE OF RADIUM IN MEDICINE [1]
IT has been unfortunate for the progress of the radioactive
treatment of disease that its methods and claims involve much of
the marvellous. Up till recently, indeed, a large part of
radioactive therapeutics could only be described as bordering on
the occult. It is not surprising that when, in addition to its
occult and marvellous characters, claims were made on its behalf
which in many cases could not be supported, many medical men came
to regard it with a certain amount of suspicion.
Today, I believe, we are in a better position. I think it is
possible to ascribe a rational scientific basis to its legitimate
claims, and to show, in fact, that in radioactive treatment we
are pursuing methods which have been already tried extensively
and found to be of definite value; and that new methods differ
from the old mainly in their power and availability, and little,
or not at all, in kind.
Let us briefly review the basis of the science. Radium is a
metallic element chemically resembling barium. It
[1] A Lecture to Postgraduate Students of Medicine in connection
with the founding of the Dublin Radium Institute, delivered in
the School of Physic in Ireland, Trinity College, on October 2nd,
1914
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possesses, however, a remarkable property which barium does not.
Its atoms are not equally stable. In a given quantity of radium a
certain very small percentage of the total number of atoms
present break up per second. By "breaking up" we mean their
transmutation to another element. Radium, which is a solid
element under ordinary conditions, gives rise by transmutation to
a gaseous element—the emanation of radium. The new element is a
heavy gas at ordinary temperatures and, like other gases, can be
liquified by extreme cold. The extraordinary property of
transmutation is entirely automatic. No influence which chemist
or physicist can apply can affect the rate of transformation.