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. If the sodium in the ocean has taken 

 1,4-00,000,000 years to accumulate, the rivers are now hearing 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 increase. 



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, 1 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 anymore than we should regard 

 lead as such. 



The matter seems to turn upon the legitimacy 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. If such 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, 



