£24 Messrs. A. Holmes and R. W. Lawson en 



Table II. (b) Minerals from Moss district, Norway. 

 Table III. (c) „ „ Arendal „ „ 



Table IV. (d) „ „ Ceylon. 



(e) Conclusion. 

 § 7. Evidence from Atomic Weight Determinations. 

 § 8. Conclusion. 



§ 1. Introduction. 



f I^HE suggestion that lead is the end product of disinte- 

 JL gration of the uranium family, first put forward by 

 Boltwood in 1905, and afterwards developed by him and by 

 one of the present writers*, may now be regarded as practi- 

 cally established. Although the actual production of lead 

 has not yet been demonstrated beyond doubt, a considerable 

 amount of indirect evidence may be cited in support of that 

 view. In unaltered primary minerals o£ the same antiquity 

 the lead-uranium ratio is remarkably constant ; whereas in 

 minerals of varying geological age it is found that the ratio 

 increases progressively with the antiquity. Moreover, since 

 within the wide range of geological time this ratio does not 

 appear to reach a maximum constant value, the stability of 

 radium G must be regarded as of at least a high order. The 

 end product or products o£ the thorium series are, however, 

 still unrecognized. Recent generalizations based on the 

 distribution of the radio-elements in the Periodic Classifi- 

 cation f have tentatively suggested that the end products of 

 uranium, thorium, and actinium should be isotopic with 

 ordinary lead. As far as our knowledge goes these end 

 products emit no detectable radiations, and therefore until 

 independent evidence of instability is forthcoming, they 

 would naturally be considered as stable, isotopic elements. 

 From a consideration of the analytical data available, it has 

 been considered unlikely that the end product of thorium is 

 isotopic with bismuth J. As will be pointed out later, 

 however, a systematic examination of this question is very 

 desirable, as well as of the occurrence of thallium in 

 radioactive minerals §. In any case it is of the greatest 



* Boltwood, Phil. Mag. [6] ix. p. 613 (1905) : Am. Jour. Sc. [4] xx. 

 p. 253(1905); Am. Jour. Sc. [4] xxiii. p. 77 (1907). Holmes, Pruc. 

 Roy. Soc. A. lxxxv. p. 248 (1911) ; < The Age of the Earth/ Chapter X. 

 London, 1913. 



f Soddy, < The Chemistry of the Radioelements,' Part II. (1914). 

 Fnjans, Die Naturivissenschaften, xviii. pp. 429-434 (1914) ; xix. pp. 463- 

 408 (1911). Held. Akad. Sitz.-Ber. A. 11 Abh. (1914). 



X Rutherford, ' Radioactive Substances,' p. 599 (1913). 



§ Exner and Haschek, Wien. Sitz.-Ber. cxxi. p. 1077 (1912). Fajans, 

 Phi/s. Zeit. xiv. p. 140 (1913) ; Le Radium, x. pp. 57-65 (1913). Abegg, 

 llandbuch d. anorg. Chemie, iii. 1. p. 409. 



