THE BIRTH-TIME OF THE WORLD 51 



ments or the dissolved sodium. We can give reasons why these 

 measurements might afford too great an age, but we can find 

 absolutely no good reason why they should give one much too low. 



By the storage of radioactive products ages have been found 

 which, while they vary widely among themselves, yet claim to 

 possess accuracy in their superior limits, and exceed those 

 derived from denudation from nine to fourteen times. 



In this difficulty let us consider the claims of the radioactive 

 method in any of its forms. In order to be trustworthy it must 

 be true : (1) that the rate of transformation now shown by the 

 parent substance has obtained throughout the entire past, and 

 (2) that there were no other radioactive substances, either now 

 or formerly existing, except uranium, which gave rise to lead. 

 As regards methods based on the production of helium, what 

 we have to say will largely apply to it also. If some unknown 

 source of these elements exists we, of course, on our assumption 

 over-estimate the age. 



As regards the first point : In ascribing a constant rate of 

 change to the parent substance — which Becker (loc. cit.) describes 

 as " a simple though tremendous extrapolation " — we reason 

 upon analogy with the constant rate of decay observed in the 

 derived radioactive bodies. If uranium and thorium are really 

 primary elements, however, the analogy relied on may be mis- 

 leading ; at least, it is obviously incomplete. It is incomplete 

 in a particular which may be very important : the mode of 

 origin of these parent bodies — whatever it may have been — is 

 different to that of the secondary elements with which we com- 

 pare them. A convergence in their rate of transformation is 

 not impossible, or even improbable, so far as we know. 



As regards the second point : It is assumed that uranium 

 alone of the elements in radioactive minerals is ultimately 

 transformed to lead by radioactive changes. We must consider 

 this assumption. 



Recent advances in the chemistry of the radioactive elements 

 has brought out evidence that all three lines of radioactive descent 

 known to us — i.e. those beginning with uranium, with thorium, 

 and with actinium — alike converge to lead. 1 There are difficulties 

 in the way of believing that all the lead-like atoms so produced 

 (" isotopes " of lead, as Mr. Soddy proposes to call them) actually 

 remain as stable lead in the minerals. For one thins: there is 



1 See Soddy's Chemistry of the Radioactive Elements (Longmans, Green & Co.). 



