652 Profs. J. Joly and E. Rutherford on the 



He also found * 0*34 millionths of one per cent, of radium 

 in zircon ; and in another specimen 0*52 millionths of one 

 per cent. The latter quantity involves about 1*5 per cent, 

 of uranium. The closely allied, and probably derived, 

 mineral cyrtolite was found by Strutt f to contain 3' 6 7 per 

 cent, of U 3 8 and 5'05 per cent, of Th0 2 . Two analyses of 

 cyrtolite are cited by Dana ; one showing 1'59, the other 

 1*4 per cent of U 3 8 . 



J. W. Waters showed that in gneiss of the Inner Hebrides 

 and granite of Mourne the radioactivity was concentrated 

 mainly in the zircons ; in Cornish granite chiefly in anatase 

 or rutile ; and in Dalbeattie granite chiefly in orthite. 

 Quantitative results are not given J. 



It will be seen that the highest of the above results is that 

 for cyrtolite, which rises to 3*67 per cent, of U 3 8 . This 

 figure does not, presumably, limit the quantity of uranium 

 which might be present. But there is no reason to believe 

 that radioactive constituents ever constitute a large part of 

 zircon. On the contrary, it has been shown by Miigge § and 

 by Gockel || that the radioactive matter constitutes an im- 

 purity or foreign substance in zircon. 



Uranium is not recorded by Dana as a constituent of 

 orthite or brookite. Strutt H found in one specimen of 

 orthite O073 per cent, of U 3 8 , and in another radium 

 equivalent to 3 per cent, of uranium. 



From the foregoing results it would seem safe to assume 

 10 per cent, of uranium as an upper limit to the amount now 

 contained in the nuclei of the haloes in this mica. 



The periods of time which emerge from the measurements 

 to be presently cited are such that the wasting of the uranium 

 should be taken into account. We must, accordingly, calcu- 

 late back, deriving the mass of transformed uranium from the 

 number of alpha rays required to form the halo according to 

 the observations and the experimental data. The estimated 

 existing mass of uranium in the nucleus thus becomes the 

 amount remaining after the time t ; so that if M is the mass 



transformed and W the existing mass, W/(M-fW) is the 



t_ 

 fraction remaining, or <?~l. Exponential tables give the 



t 

 value of £/L corresponding to e~Z, where L is the average 



* Proc. R. S., A, lxxvi. p. 312. 



t Proc. R. S., A, lxxvi. p. 88. 



X Phil. Mag. 1909 and 1910. 



§ Centralblf. Min. &c, 1909, p. 148. 



| Chemiker Zeitung, 1909, No. 12G. 



II Proc. R. S., A, lxxvi. p. 312. 



