262 B. B. Boltwood — Ultimate Disintegration 



so much altered that its original condition is unknown." 

 This generalization can apparently be extended to include lead 

 also. 



2. When the analyses of samples from the same actual 

 locality are compared it will be evident that, in general, 



a) The content of rare earths increases with the amount of 

 lead present. This is most strikingly shown in the groups 

 i-y, vi-yin, xm-xiv and xvii-xvm. The simultaneous 

 variation of thorium is also indicated somewhat imperfectly 

 in those instances where this constituent was separately deter- 

 mined. 



h) That in those specimens having the highest specific grav- 

 ity (v and viii) the proportion of helium compared with the 

 lead present is greatest. It is in general to be expected that 

 the denser and therefore less porous material would retain a 

 greater proportion of the helium formed within it. The low 

 proportion of gas compared with lead in x and xix might 

 well be due to the high emanating power of the former* #nd 

 the greater porosity of the latter indicated by its low density. 

 It is moreover interesting to note that those specimens (x, 

 xix, xxi) containing disproportionately large amounts of 

 water contain a relatively low amount of helium compared 

 with the lead present. It is possible that these minerals were 

 sufficiently porous to permit the entrance of water from with- 

 out while at the same time a part of the helium formed has 

 escaped from within them. 



It is evident that, in Table I, a lack of agreement exists 

 between the proportion of lead and rare earths and the pro- 

 portion of helium in the Connecticut material and the propor- 

 tions of the corresponding constituents in the Norwegian sam- 

 ples. In the latter the amounts of lead and rare earths as 

 compared with the gas present are much greater than in the 

 former. This can be explained by assuming that the Nor- 

 wegian minerals are considerably older than the American varie- 

 ties, and that the Norwegian specimens examined by Hillebrand 

 have in some manner lost a large part of their helium. 

 The geological data available on the relative ages of the 

 American and Norwegian occurrences, while not entirely in 

 accord with the assumption of such a great difference in age, 

 would not appear to be sufficiently definite to preclude such a 

 possibility. 



In considering the bearing of the results of the analyses 

 of the two secondary uraninites, ix and xxn, on the general 

 theories proposed in this paper, it is evident that the presence 

 * Phil. Mag. (6), ix, 609. 



