Januaet 3, 1919] 



SCIENCE 



9 



two kinds of lead from each other by frac- 

 tional crystallization. This was predicted by 

 Soddy, and tested by him and by others. Va- 

 rious vain attempts have been made to sep- 

 arate the different kinds of lead from one 

 anotlier, but apparently when once they are 

 mixed, no chemical method can separate them, 

 since the properties of the different kinds are 

 so nearly alike. The latest attempt at the 

 Gibbs Memorial Laboratory involved one thou- 

 sand fractional crystallizations of the Austra- 

 lian lead nitrate, which is believed to contain 

 both ordinary and uranium-radium-lead. The 

 extreme fraction of the crystals (representing 

 the least soluble portion, if any difference in 

 solubility might exist) gave within the limit 

 of error the same atomic weight as the ex- 

 treme fraction of the mother liquor (repre- 

 senting the most soluble portion), thus con- 

 firming the work of others in this direction. 



When wires constructed of two different 

 metals are joined, and the junction heated, 

 an electrical potential or electromotive force 

 is produced at the junction. This property 

 seemed, then, to be a highl.y interesting one to 

 test, in order to find out how great may be the 

 similarity of the two kinds of lead. In fact, 

 wires made of radioactive lead and ordinary 

 lead tested in the Gibbs Laboratory gave no 

 measurable thermoelectric effect, the wires act- 

 ing as if the.v were made of the same identical 

 substance, although the atomic weights and 

 densities were different. No other known 

 case of this sort is known, so far as I am 

 aware. The melting points of the two kinds 

 of lead were likewise found, with the assist- 

 ance of N. F. Hall, to be identified within 

 the probable accuracy of the experiment. 



Let us bring all these results together into 

 one table, so that we may better grasp their 

 combined significance. 



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 non-uraniferous ores; 

 another, a form of lead apparently produced 

 by the decomposition of uranium, radium be- 

 ing one of the intermediate products. If we 

 leave out of consideration the probably in- 

 essential difference in radioactivity, the two 



kinds are very closely if not exactly alike in 

 ever.v respect, excepting atomic weight, density 

 and immediatelj' related properties involving 

 weight, such as solubility. Thorium lead ap- 

 pears to be a third variety, with similar rela- 

 tions. Shall we call these substances different 

 elements, or the same? The best answer is 

 that proposed by Soddy who invented a new 

 name, and called them " isotopes " of the 

 same element. 



COMPAKISOX OF PROPERTIES OF DIFFERENT KINDS OF 

 LEAD5 



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 in- 

 terest, 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 ques- 

 tion 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 sub- 

 stance, or else a mixture of uranium-lead with 

 lead of yet higher atomic weight, perhaps 208. 

 The latter substance niight be formed, as 



■'• For the sake of better comparison, all the re- 

 sults given are those obtained at Harvard. Xo re- 

 sults of experiments elsewhere are inconsistent with 

 these. 



