380 On the Age of tie Earth. 



have been a different and, from the evidence as well as from 

 probability, a greater rate of decay in the past, arising intrin- 

 sically, 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 sub- 

 stances— uranium or thorium — is really without any very 

 strong basis. It rests upon analogy with the behaviour of the 

 substances which have been derived from them. But there 

 may be a very profound distinction. The latter are of radio- 

 active 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 subsequent 

 constant change-rate depends *, may be conditioned by the 

 events of radioactive transformation, or by their past history, 

 o<r 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 cannot be regarded, therefore, as in 

 all senses one of a series any more than we should regard 

 lead as such. 



The matter seems to turn upon the legitimacy of the assump- 

 tion that the mere existence of radioactive change progressing 

 in a substance involves such a particular distribution of insta- 

 bility among its atoms as will ensure 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 transformation pro- 

 ducts in minerals, in place of being a measure of geological 

 time, serves to shed light upon the rate of transformation of 

 the primary radioactive bodies in the past. Apart from its 

 interest in other respects, the importance of such a conclusion 

 to geological science would be great. If we supposed the 

 curve, found by plotting the time results derived from lead- 

 ratios against the sedimentary thicknesses^ represented an 

 approximation to the facts, the rate of change of uranium 

 150 million years ago may have been many times what it 

 now is. The radiothermal effects of the whole series must 

 have been proportionately increased, and the convergence of 

 the radioactive activity must have had an influence upon the 

 secular cooling of the earth. 

 July 18, 191 L 



* See Sir J. J. Thomson's Presidential Address to the British Asso- 

 ciation, 190,:). 



