RADIOACTIVE LEAD RICHARDS. 



217 



Let us bring all these results together into one tulde, so that we 

 may better grasp their combined significance. 



Comparison of properties of different kinds of lead. 1 



Common 

 lead. 



A 



Mixture 

 (Austra- 

 lian). 



B 



Uranio- 

 lead. 



Percentage 

 difference. 



A-B A-C 



Atomic weight 



Density 



Atomic volume 



Melting point (absolute) 



Solubility (metal as nitrate). 

 Refractive index of nitrate. . . 



Thermoelectric effect 



Spectrum wave length 



207. 19 

 11.337 

 18.277 



600. 53 

 37. 281 

 1.7815 



206.34 

 11. 289 

 18.278 



600. 59 

 37. 130 

 1.7814 



206. OS 

 11.273 

 18. 2S1 



0.42 

 0.42 

 0.01 

 0.01 

 0.41 

 0.01 

 0.00 

 0.00 



0.54 

 0.56 

 0.02 



0. 00 



Summed up in a few words, the situation appears to be this : At 

 least two kinds of lead exist — one, the ordinary metal disseminated 

 throughout the world, in nonuraniferous ores; another, a form of 

 lead apparently produced by the decomposition of uranium, radium 

 being one of the intermediate products. If we leave out of con- 

 sideration the probably inessential difference in radioactivity, the 

 two kinds are very closely, if not exactly, alike in every respect, 

 excepting atomic weight, density, and immediately related prop- 

 erties involving weight, such as solubility. Thorium- lead may 

 be a third variety, with similar relations. Shall we call these 

 substances different elements, or the same? The best answer is 

 that proposed by Professor Soddy, who invented a new name, and 

 called them " isotopes " of the same element. 



Since every new fact concerning the behavior of the elements gives 

 a new possible means of discovering something about their nature, 

 and since these facts are of especially significant kind, the anomaly 

 is of more than passing interest, and may be said to constitute one 

 of the most interesting and puzzling situations now presented to the 

 chemist who looks for the deeper meanings of things. 



Many new queries arise in one's mind from a study of the data. 

 Among them is a question as to the nature of ordinary lead, which 

 possesses a less reasonable atomic weight than the radioactive variety. 

 Why should this state of things exist ? 



Ordinary lead may be either a pure substance, or else a mixture of 

 uranium-lead with lead of yet higher atomic weight, perhaps 208. 

 The latter substance might be formed, as Soddy points out, if thorium 

 (over 232) lost six atoms of helium, and he and Honigschmid have 

 found quantitative evidence of its existence in thorium minerals. 



After reviewing all the data, Prof. F. W. Clarke has brought 

 forward an interesting and reasonable hypothesis explaining the 



1 For the sake of better comparison, all the results given are those obtained at Harvard. No results of 

 experiments elsewhere are inconsistent with these. 



