MEASUREMENTS BASED ON RADIOACTIVITY 849 



trutli than the smaller one. Whether it is too high or too ^ low can not 

 as yet be determined by the method of attack pursued in this investigation. 



ACCUMULATION OF LEAD^^ 



Discussion of the evidence. — The suggestion that lead is the final prod- 

 uct of the uranium family was made by Boltwood in 1905. All the evi- 

 dence which has become available since that date convincingly upholds tlie 

 correctness of his view, which from the first was much more than a mere 

 guess or assumption. Up to the present the generation of lead from a 

 radioactive preparation has not been directly demonstrated, but the reason 

 is not far to seek. Such a preparation, originally free from spectroscopic 

 traces of lead, would require many years to generate within itself a per- 

 ceptible accumulation of lead. Two or three experiments were begun a 

 few years ago with this end in view, but more time must elapse before 

 there can be any hope of detecting the final product. The following lines 

 of deductive evidence, however, leave no room for doubt tliat the final 

 product really is chemically identical with lead : 



(a) When the radio-elements are arranged in their proper positions 

 in the periodic classification, the final product of uranium naturally falls 

 into the division already occupied by lead. 



(h) The atomic weight of ordinary lead is 207.1 (International, 1915). 

 or 207.19 according to Baxter, Thorvaldsen, and Grover (1915). How- 

 ever, according to the data previously given, the atomic weight of the 

 final product should be 206.2, or 206 according to Honigschmid's meas- 

 urement of the atomic weight of radium. This was for some years a 

 source of difficulty, but it has now led to the discovery that lead which 

 has accumulated in radioactive minerals, and which is recognized as lead 

 by its chemical and spectroscopic behavior, has actually a lower_ atomic 

 weight than that of ordinary lead. Eichards and Lembert in America, 

 Maurice Curie in France, and Honigschmid in Austria have investi- 

 gated lead prepared from pitchblende and other uranium minerals, and 

 the values of the atomic weight found by them range from 206 to 206.5.^^ 

 The higher values are all from pitchblendes of secondary origin, which 

 are liable to contamination with ordinary lead, owing to their association 

 with galena. The lowest values are from pnre uraninite (German East 

 Africa) and from a similar mineral, Broggerite (Norway), both of which 

 were fresh primary minerals quite free from traces of galena. Atomic 

 weight evidence, therefore, strongly supports the view that the final prod- 

 uct of uranium is an isotopic variety of lead. 



8* B}^ Arthur Holmes. 



05 For references and a table of results see Holmes and Lawson : Phil. Mag., vol. xxix, 

 1915, p. 682. 



