Ratio between Uranium and Radium in Minerals. 349 



a direct change of uranium into lead, this quantity should be 

 formed in a million years*. The results of Mile. Gleditsch 

 and ourselves with autunites therefore can be taken as con- 

 firmatory evidence of the existence of at least one inter- 

 mediate body of verv long period between uranium and 

 radium. Incidentally they indicate that Portuguese autunite 

 is considerably more recent than the French mineral, and 

 indeed it would appear not improbable that the Portuguese 

 mineral has been laid down within a period not very many 

 times longer than that covered by historical records. 



In the present state of the subject the possibility has to be 

 taken into account that the two a-particles, known from the 

 work of Boltwood (Am. J. Sci. 1908, xxv. p. 269), to be 

 expelled from uranium may be due to two successive changes. 

 The similar low range of these two a-particles is an argu- 

 ment, according to Rutherford's rule (Phil. Mag. 1907, xiii. 

 p. 110), that the second change cannot be a very rapid one ; 

 and we are therefore faced with the possibility that uranium 

 may be a mixture of two elements of atomic weights 238*5 

 and 234*5, which, like ionium, thorium, and radio-thorium, 

 are chemically so alike that they cannot be separated. Now, 

 if the element of atomic weight 234*5 had a period at all 

 comparable to uranium, the ratio of uranium to radium must 

 vary with the age of the mineral, both for very recent and 

 for very old minerals, as Mile. Gleditscrr's results indicate. 

 In this case the estimate of the minimum period of ionium 

 given in the last paper, which depends on there being only 

 one long-lived intermediate body, would not necessarily be 

 fallacious, for by hypothesis uranium and its first product 

 are so alike chemically that they cannot be separated. 



To obtain evidence on this point the specific a-activities 

 of specimens of uranium oxide separated from the three 

 minerals were compared. On the view suggested, the specific 

 a-activity of the uranium from autunite should be lower than 

 that from pitchblende, and the latter should be lower than that 

 from thorianite. Within 5 per cent, the specific a-activities 

 of all three substances proved to be the same. The small 

 differences can be well explained by experimental error and 

 by the fact that in some of the preparations the /3-radiation 

 has not yet reached equilibrium. This, therefore, is fairly 

 conclusive evidence that the variations in the uranium- 

 radium ratio cannot be due to two successive slow a-ray 

 changes in uranium itself. 



Physical Chemistry Laboratory, 

 University of Glasgow. 



* The correction to 10,000 years on p. 937 of the same volume must 

 be supplemented by a second correction bringing back the value to that 



first stated. 



