RADIOACTIVE LEAD RICHARDS. 213 



Hypotlietical calculation of atomic weight of uranium-lead. 



Atomic weight of uranium = 23S.1S 



S X atomic weight of helium = 32.00 



Residue (lead?) 206.18 = 206.18 



Atomic weight of radium = 225.96 



5 X atomic weight of helium = 20.00 



Residue (lead?) 205.96 = 205.96 



Average hypothetical value for lead = 206.07 



Observed value for uranium-lead 1 = 206.08 



Difference 0.01 



The agreement is remarkably good. Each of the individual calcu- 

 lated values shows less than 0.05 per cent, deviation from the 

 average, and the average itself shows essential identity with fact — 

 a striking confirmation of the theory. This is perhaps the most suc- 

 cessful attempt on record to compute an atomic weight from hypo- 

 thetical assumptions. Usually we are wholly at a loss as to the 

 theory underlying the precise relationships, and must determine our 

 values by careful experiment alone. 



The value 206.08 for the atomic w-eight of lead has further sup- 

 port in the fact that it is more nearly half way between thallium, 

 204, and bismuth, 208, the two neighboring elements in the periodic 

 system, than is the atomic weight 207.2 possessed by ordinary lead. 

 It appears, then, that 206, the value pertaining to uranium-lead, is 

 a very reasonable value. 



But, as has been repeatedly pointed out, ordinary lead, consti- 

 tuting the vast bulk of the lead in the world, has without doubt a 

 much higher atomic weight, 207.2, not to be expected from either of 

 the lines of reasoning just given. In order to test the uniformity 

 of this circumstance, Professor Baxter, with the help of one of his 

 assistants, investigated ordinary lead from the nonuraniferous ores 

 from many parts of the world, and discovered that the constancy 

 of its quantitative behavior is as striking as that of copper or silver. 

 His figures agreed very closely, within the limit of error of experi- 

 mentation, with those obtained as a part of the present comparison 

 of the two kinds of lead, so that there could be no question as to lack 

 of identity of methods or precautions. 



Before leaving the subject of the relative atomic weights of these 

 two types of lead, it is not without interest to note the exact absolute 

 weights of the atoms. If, as we have excellent reason for believing 

 on the basis of the brilliant work of Professor Millikan, a so-called 



1 This is the TTarvard result. If ITonigschmid's value is given equal weight, the average 

 observed value would be 20G.07, exactly idenical with the hypothetical valne. 



136650°— 20 15 



