238 SCIENCE PROGRESS 



Boltwood points out that since the uranium-radium ratio is 

 constant within the limit of experimental error, the inevitable 

 and only possible conclusion is that uranium is the parent of 

 radium. He adds also that, since the uranium-radium ratio in 

 thorite, which contains about 50 per cent, of thorium, is the 

 same as in minerals, such as uranoplane and carnotite, containing 

 a mere trace of thorium, thorium cannot possibly be regarded as 

 the parent of radium. 



He tested thirteen such minerals for lead, and found an 

 appreciable quantity in all, except in a specimen of uranoplane, 

 which contains a mere trace, and which is the youngest geo- 

 logically. Lead minerals do not exist side by side with most 

 of the minerals tested, so that this possible source of lead is 

 excluded. 



In a later paper, Boltwood examined previously published 

 analyses of uranium minerals ; the ratio of lead to uranium in 

 forty-three minerals varies from 0*04 to 0*25. Minerals from the 

 same locality show very good agreement. He quotes Barrell's 

 opinion that the variations of the ratio for minerals from different 

 localities " are not contradictory to the order of the ages attri- 

 buted by geologists to the formations in which the different 

 minerals occur." Assuming that the helium in minerals is 

 formed by disintegration (this statement will be discussed 

 immediately), and according to the equation — 



Uranium (238'5) = lead (2o6 - o) -I- helium (31*6) 

 (this assumes that helium is identical with the a-particle, and 

 that eight a-particles are liberated in the uranium disintegration) 

 — he finds that the actual amount of helium in minerals never 

 exceeds the theoretical quantity, and varies from 6—100 per cent, 

 of that quantity. The denser minerals usually retain the larger 

 proportion. 



Strutt has analysed twenty minerals for radium and uranium, 

 and his figures, although showing greater variation than those 

 of Boltwood, yet point conclusively to the same result. 



McCoy and Ross found that the total radioactivity of minerals 

 free from thorium, but containing actinium, bears a constant 

 ratio to the amount of uranium present. This led them to the 

 conclusion that actinium is also a product of uranium. As it has 

 been shown (see Part I. vol. ii. p. 544) that actinium is not in 

 the uranium-radium series, this would point to the existence of 

 an entirely different uranium-actinium series of disintegrations. 



