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LXXI. Lead and the End Product of Thorium. (Part II.)* 

 By Arthur Holmes, A.R.C.S., B.Sc, F.G.S., Imperial 

 College, London, and Robert W. Lawson, M.Sc, Radium 

 Institute, Vienna |. 



Contents. 

 § 9. Constant of Disintegration of Thorium E. 

 § 10. Bismuth as a Possible End Product of the Thorium Series. 

 § 11. Further Evidence from Atomic Weight Estimations. 

 § 12. Kemarks on the Selection of Material for Atomic Weight 



Determinations. 

 § 13. The Possible End Product of the Actinium Series. 

 § 14. Conclusions. 



§ 9. Constant of Disintegration of Thorium E. 



THE results we have so far obtained show in direct! y 

 that thorium E is an unstable element, even though, 

 the radiation from it has not hitherto been detected. In 

 order to obtain information as to its half-life period we must, 

 then, resort to some method other than those usually used. 

 The present section gives a more complete treatment of a 

 method outlined recently by one of the authors %. Before 

 passing on to this, however, the suggestion made recently by 

 Fajans in this connexion must be considered §. Using the 

 results of an analysis by Bolt wood || of a thorium mineral 

 poor in uranium (thorite), and assuming that all the lead 

 estimated had its origin in thorium, Fajans gave 2.10 7 years 

 as the maximum possible value of the half-period of thorium 

 E. Absence of knowledge regarding the age of the mineral 

 referred to, renders it difficult if not impossible to determine 

 by how much this value is too great; and in the case of 

 another thorite analysis (by Holmes, loc. cit. 1911), also used 

 by Fajans, where the age is known, it is certain that not 

 more than 12 per cent, of the total lead found can be due to 

 thorium. Using this fact, it can be readily shown that 

 the ThE/Th ratio is 8.10" 5 , from which the half-period of 

 thorium E is found to be about 1'2.10 6 years. The thorium 

 content of this mineral has been given in Table I. No. 3 

 (Part I.). 



Since thorium E is unstable, it is clear that definite and 

 reliable results for the half-period of this element can only 

 be obtained from analyses of minerals rich in thorium. A 



* Part I. of this paper was published in this Magazine, vol. xxviii. 

 pp. 823-840 (Dec. 1914). 



t Communicated by the Authors. 



X Lawson, < Nature,' July 9, 1914, p. 479. 



§ Fajans, Heidelberger Akad. Bcr., Abh. xi. p. 12 (1914). 



)1 Boltwood, Am. Journ. Sci. xxiii. p. 88 (1907). 



Phil. Mag. S. 6. Vol. 29. No. 173. May 1915. 2 X 



