6 



SCIENCE 



[N. S. Vol. XLIX. No. 1253 



from radioactive sources in widely separated 

 parts of the world. Messrs. E. R. Bubb and 

 S. RadcliflF, of the Eadium Hill Company, of 

 New South Wales, kindly sent a large quantity 

 of lead from their radium mines, and a par- 

 ticularly valuable specimen prepared from se- 

 lected crystals of pure mineral was put at our 

 disposal by Professor Gleditsch — ^not to men- 

 tion otlier important contributions from others, 

 including Professor Boltwood and Sir William 

 Ramsay. Each of these samples gave a 

 different atomic weight for the lead obtained 

 from them, and the conclusion was highly 

 probable that they contained varying admix- 

 tures of ordinary lead in the uranium-radium- 

 lead. This was verified by the knowledge that 

 in at least some cases the uranium ore actually 

 had been contaminated with lead ore. The 

 purest N^orwegian specimen thus acquired 

 especial importance and significance, because 

 it was only very slightly, if at all, vitiated in 

 this way. As a matter of fact, it gave 206.08 

 for the atomic weight in question — the lowest 

 of all. Here are typical results, showing the 

 outcome; many more of similar tenor were 

 obtained. 



ATOMIC WEIGHTS 



r207 201 

 Common lead J 207*19 }■■■■■ 207.19 



Australian Eadioaetive Lead oofiafi 



containing probably 25 per-j QQg'gg r. .. .206.34 

 cent, ordinary lead | 20636 J 



Purest XJranio-lead {206'o9}- • ' -206.08 



Honigschmid, from similar pure material, 

 had found figures (206.05) agTceing almost 

 exactly with the last value. One can not help 

 believing that this last specimen of lead is a 

 definite substance, probably in a state almost 

 pure, because of the unmixed quality of the 

 carefully selected mineral from which it was 

 obtained. 



A further question now arises: is it a per- 

 manent substance — really an end-product of 

 the disintegration? Soddy's hypothesis as- 

 sumes that it is. The only important fact 

 militating against this view is the observation 

 that uranium-lead is always radioactive, and 

 hence might be suspected of being unstable. 



In various impure specimens, however, the 

 radioactivity is not proportional to the change 

 in the atomic weight; hence the radioactivity 

 is probably, at least in part, to be referred not 

 to the lead itself, but rather to contamination 

 with minute, unweighable amounts of intensely 

 radioactive impurities — other more transitory 

 products of disintegration.- If weighable, 

 such impurities would almost certainly in- 

 crease, not diminish, the atomic weight; hence 

 their presence could not account for the low 

 value. 



Let us compare the actual result for the 

 atomic weight of this kind of lead with the 

 theory of Soddy and Eajans. If this theory 

 is sound, the simple subtraction of eight times 

 the atomic weight of helium from that of 

 uranium, or -RYe times the atomic weight of 

 helium from that of radium, should give the 

 atomic weight of the lead resulting from the 

 disintegration, as follows: 



HYPOTHETICAL CALCULATION OP ATOMIC WEIGHT OP 

 URANIUM-LEAD 



Atomic weight of uranium. . =238.18 



8 X atomic weight of helium = 32.00 



Eesidue (lead!) 206.18 • = 206.18 



Atomic weight of radium. . . = 225.96 



5 X atomic weight of helium = 20.00 



Eesidue (lead?) 205.96 = 205.96 



Average hypothetical value for lead = 206.07 

 Observed value for uranium-leads. . — 206.08 

 Difference byol 



The agreement is remarkably good. Each 

 of the individual calculated 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 successful 



2 For this reason the term ' ' radio-active lead ' ' al- 

 though it describes the fact, is perhaps not from a 

 theoretical point of view the best designation of 

 either uranium or thorium lead; but the term is 

 convenient because it distinguishes between these 

 two forms and common lead. 



s This is the Harvard result. If Honigschmid 's 

 value is given equal weight, the average observed 

 value would be 206.07, exactly identical with the 

 hypothetical value. 



