838 Messrs. A. Holmes and R. W. Lawson on 



weight for radium is 226*4, while Honigschmid's value 

 is 226. These give 206*4 and 206 respectively as the atomic 

 weight of RaG. RaG being isotopic with lead (atomic 

 weight 207*1), it would be expected that the atomic weight 

 of lead prepared from primary uranium-bearing minerals 

 should lie between 226 and 226*5. 



The atomic weight of thorium is 232*4 (International 1914) 

 or 232*2 (Honigschmid, 1914). Subtracting the atomic 

 weights of the six helium atoms which are expelled before 

 Th E is reached, we have for the atomic weight of the latter 

 208*4 or 208*2. Thus if ThE is stable and isotopic with 

 lead, the atomic weight of lead extracted from primary 

 thorium minerals ought to be about 208*4 or 208'2. The 

 most probable values for ordinary lead, Ra G, and Th E are,, 

 then, as follows: — 



Lead Atomic weight = 207*1 



Uranium Lead or Ra G „ =206*0-206*5 



Thorium Lead or Th E „ =208-2-208*4 



These theoretical values have been tested by a direct 

 appeal to experiment, and the results so far obtained are 

 summarized in Table Y. It will at once be noticed that in 

 the case of Ra G the agreement is very satisfactory, for 

 pitchblende and carnotite, being secondary minerals, are 

 liable to contain original lead, which would have the effect 

 of raising the atomic weight above the theoretical value. 

 These results strongly support the view that the end product 

 of uranium is a stable isotope of lead, if indeed they do not 

 amount to definite proof. 



Unfortunately in the case of ThE the determinations 

 are fewer and widely at variance. The value of Soddy and 

 Hymau, however, is admittedly not final, and since it is even 

 higher than would have been expected on theoretical grounds 

 (<: 208*2-208*4), it would perhaps be unwise to lay too 

 much stress on it at present. On the other hand, the results 

 of Richards and Lembert are of extreme accuracy, a deter- 

 mination of the atomic weight of ordinary lead by the 

 methods which they adopted in the other cases giving a 

 value of 207*15 + 0*01. Their result for lead from thorianite 

 seems to show that at least the greater part of whatever lead 

 was not born of uranium was probably original ordinary 

 lead. Certainly there are no grounds for believing that any 

 appreciable quantity of their lead preparation had an atomic 

 weight higher than 208. A similar conclusion may be 

 drawn from the still more recent results obtained by 

 M. Maurice Curie. No analysis is given of the monazite 



