Products of the Radio-active Elements. 257 



of its parent and long as compared with the oldest of the known 

 minerals. This hypothesis is supported by the circumstance 

 noted by Strutt, that in general the minerals containing high 

 proportions of thorium also contain a comparatively high pro- 

 portion of helium, a point which will be referred to later in 

 the course of this paper. Since the present knowledge of 

 radio-active phenomena leads to the assumption that the aver- 

 age life of uranium is of the order of 2 X 10 9 years ? while the 

 average life of thorium is apparently in excess of that number, 

 it seems scarcely reasonable to expect that minerals will be 

 found which are sufficiently old for a state of equilibrium to 

 have been reached between thorium and uranium. The pro- 

 duction of a slowly changing disintegration product from a 

 more rapidly changing parent is in no way contradictory to 

 the disintegration theory, since a number of examples of this 

 are at present recognized.* The common association of the 

 other rare earths with thorium may indicate, as suggested by 

 Strutt, that these are possible final products of the latter ele- 

 ment. 



Bismutfi. 



The occurrence of bismuth as a constituent of the more 

 highly radio-active minerals is another significant indication 

 of a possible end product. The proportion of bismuth which 

 is present in the older radio-active minerals is, however, very 

 small, so small indeed that its occurrence is but seldom detected 

 in the ordinary course of analysis. It is only in treating 

 considerable quantities of material for the extraction of 

 polonium that the presence of bismuth becomes evident. 

 This occurrence of bismuth in small quantities is suggestive 

 of its formation from the disintegration, either of a parent 

 having a relatively long life, or of one which is itself produced 

 in only relatively small quantities. The former requirement 

 would seem to be fairly well filled by thorium, in which case 

 it is to be expected that in two minerals of equal age, the one 

 containing the greater proportion of thorium w T ould also con- 

 tain the greater relative amount of bismuth. An opportunity 

 has not yet been found for the experimental investigation of 

 this question. The fact that the atomic weight of bismuth 

 differs from the atomic weight of thorium by exactly 24 units, 

 an even multiple of 4, is possibly significant. 



Barium. 

 Another element which persistently appears as a minor consti- 

 tuent of uranium minerals is barium. Its production, if it is 



* One example is the production of the active deposit from the thorium 

 emanation, the parent with a half -value period of less than one minute, the 

 product with a half-value period of eleven hours. 



