239

and regularity which undoubtedly prevail in the periods of the members of the radioactive families, it appears to me that it is allowable to ask if the change rate of uranium has been always what we now believe it to be.  This comes to much the same thing as supposing that atoms possessing a faster change rate once were associated with it which were capable of yielding both helium and lead to the rocks.  Such atoms might have been collateral in origin with uranium from some antecedent element.  Like helium, lead may be a derivative from more than one sequence of radioactive changes.  In the present state of our knowledge the possibilities are many.  The rate of change is known to be connected with the range of the alpha ray expelled by the transforming element; and the conformity of the halo with our existing knowledge of the ranges is reason for assuming that, whatever the origin of the more active associate of uranium, this passed through similar elemental changes in the progress of its disintegration.  There may, however, have been differences in the ranges which the halo would not reveal.  It is remarkable that uranium at the present time is apparently responsible for two alpha rays of very different ranges.  If these proceed from different elements, one should be faster in its change rate than the other.  Some guidance may yet be forthcoming from the study of the more obscure problems of radioactivity.

Now it is not improbable that the halo may contribute directly to this discussion.  We can evidently attack

240

the biotite with a known number of alpha rays and determine how many are required to produce a certain intensity of darkening, corresponding to that of a halo with a nucleus of measurable dimensions.  On certain assumptions, which are correct within defined limits, we can calculate, as I have done above, the number of rays concerned in forming the halo.  In doing so we assume some value for the age of the halo.  Let us take the maximum radioactive value.  A halo originating in Devonian times may attain a certain central blackening from the effects of, say, rob rays.  But now suppose we find that we cannot produce the same degree of blackening with this number of rays applied in the laboratory.  What are we to conclude?  I think there is only the one conclusion open to us; that some other source of alpha rays, or a faster rate of supply, existed in the past.  And this conclusion would explain the absence of haloes from the younger rocks; which, in view of the vast range of effects possible in the development of haloes, is, otherwise, not easy to account for.  It is apparent that the experiment on the biotite has a direct bearing on the validity of the radioactive method of estimating the age of the rocks.  It is now being carried out by Professor Rutherford under reliable conditions.

Finally, there is one very certain and valuable fact to be learned from the halo.  The halo has established the extreme rarity of radioactivity as an atomic phenomenon.  One and all of the speculations as to