er something as to
the past history of our Globe. Nothing is, on the other hand,
known respecting the origin of uranium or thorium--the parent
radioactive bodies. And while not questioning the law

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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

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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 c