292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
mentation it still remains incredible. But, if possible, still more 
incredible is the conclusion respecting solvent denudation to which 
radioactive time drives us. Jf the sodium in the ocean has taken 
1,400,000,000 years to accumulate, the rivers are now bearing to the sea 
about fourteen times the average percentage of the past—not less than 
nine times. It seems quite impossible to find any explanation of such 
an vncrease. 
With these difficulties in view it is excusable to direct attention to 
the foundations of the radioactive method and ask how far they are 
secure. The fundamental assumption is that the parent radioactive 
substance, uranium, has always in the past disintegrated at the 
present rate. Is this assured? I am not now suggesting that the 
rate of change has been effected by external physical conditions, such 
as heat or pressure, but I assume that there may have been a different, 
and from the evidence as well as from probability, a greater rate of 
decay in the past, arising intrinsically, and ultimately due possibly 
to conditions of origin. 
I venture to suggest—I do so with diffidence—that our assumption 
of a constant rate of change for the parent substances—uranium or 
thorium—is really without any very strong basis. It rests upon 
analogy with the behavior of the substances which have been derived 
from them. But there may be a very profound distinction. The 
latter are of radioactive origin. That particular distribution of 
stability or of intrinsic energy among the atoms of these bodies 
obtaining at the moment of their formation, upon which the subse- 
quent constant change rate depends, may be conditioned by the 
events of radioactive transformation, or by their past history, or by 
both. In a word, a radioactive origin may be essential. 
Now we know nothing as to the origin of the primary radioactive 
elements. No substances of greater atomic weight are known from 
which they may be derived. Nor is it unphilosophic to assume that 
they have had some other mode of origin, seeing that the radioactive 
ascent must terminate somewhere. Uranium can not be regarded, 
therefore, as in all senses one of a series any more ae we should regard 
lead as such. 
The matter seems to turn upon the oes of the assumption 
that the mere existence of radioactive change progressing in the sub- 
stance involves such a particular distribution of instability among 
its atoms as will insure that a constant fraction of these disintegrate 
each unit of time from their first origination—however this was 
brought about—till all are transformed. Ifsuch an hypothesis is not 
sufficiently secure to overbear the opposing evidence we must agree 
to judge the former by the latter. In this case the accumulation of 
1 See Sir J. J. Thomson’s Presidential Address to the British Association, 1909. 
