7 8 PROF. J. JOLY ON THE GENESiS OF RLEOCHROIC HALOES. 
may have differed from those known to-day. There must certainly have been 
a convergence towards the properties of the existing shorter-lived members of the 
series, for the outer parts of the halo are in fair agreement with the recognised 
ranges of the final products of transformation. 
According to the relation discovered by Carruthers, # the range is numerically 
proportional to a certain power of the atomic weight. If, then, the radioactive losses 
sufficed to lead the former line of radioactive descent through existing atomic 
weights, it might possibly be the case that abnormalities in the ranges of the earlier 
members of the series might not appear in the later products of change. 
It may be urged that, if there had been a gradual change progressing in the 
average range of certain of the a-rays, this should be revealed in a want of definition 
onic haloes. The answer to this is that such an effect would be very 
difficult to detect. It may be that the inward widening of the embryonic band has 
been influenced in this manner. We possess no standard for comparison. We have 
no means of deciding between what might have been due to a convergence in the 
ranges of the rays and what might be due to development in accordance with the 
form of the curve. Nor can we determine in how far the observed amount of 
imperfection in the definition is due to the one cause or to the other. Evidence in 
this direction—that is, from the actual appearance of the haloes presented to us 
from rocks of any age—seems, unfortunately, to fail us. 
Rutherford, in discussing the origin of actinium,! has remarked on the 
possibility that radioactive change may give rise to simultaneously formed products 
of different atomic weight, periods, etc. If we supposed uranium to be derived from 
some antecedent element, the original uranium may well have possessed different 
radioactive properties for similar reasons. The shorter-lived atoms would get 
scarcer during geological time, and there would be a convergence in the value of A, 
which may be still going on. Such may have been the history or evolution of many 
of the really or apparently stable elements. Indeed, in discussing the law of Geiger 
and Nuttall, Rutherford has advanced a theoretical explanation for the relation 
between range and longevity, which would seem to have bearings on the views I am 
stating. The short range is due to the gradual waste of energy by radiation during 
long periods of time. 
The high lead ratio of uranium-bearing minerals would find explanation on the 
view that there was a more rapid decay of early uranium atoms, or, rather, that a 
large number of uranium atoms formerly transformed at a more rapid rate than the 
value revealed by present-day observation. Similarly, if thorium gives rise 
ultimately to lead—-as seems probable—-the scarcity of lead in thorium-bearing 
ores is in perfect harmony with the close agreement between the past and present 
ranges of the a-rays emitted by members of this series. It is almost unnecessary to 
* ‘ Nature,’ January 20, 1916. 
t ‘Radioactive Substances and their Transformations, 1 p. 522. 
of the embry 
