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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