158 CHEMICAL DISCOVERY AND INVENTION 



same series. The consequence of such changes is that the radio- 

 active elements and their ultimately inactive products are found 

 in one or other of the last twelve places of the periodic table 

 from thallium to uranium, and though not identical in atomic 

 weight with the long known elements are indistinguishable 

 from them. Thus it is believed that the final product of the 

 decay of thorium and the final product of radium is lead, but 

 the lead derived from thorium has a calculated atomic weight 

 2084, while the lead from radium has an atomic weight 206. 

 These figures are derived from the atomic weights 2324 for 

 thorium and 226 from radium, in consequence of the loss of six 

 and five atoms of helium respectively. 



Estimations of the atomic weight of lead from uraniferous 

 and thoriferous minerals lend some support to this view. Thus 

 the following values for the atomic weight of lead, derived 

 from the radio-active minerals named in the list, were published 

 in 1914 by Professor Richards and Mr. Lembert, both experienced 

 in the work of atomic weight determinations. 



Lead from N. Carolina uraninite . . 20640 



,, ,, Joachimsthal pitchblende . 206-57 



Colorado carnotite . . 206-59 



Ceylon thorianite . . . 206-82 



Cornish pitchblende . . 206-86 



Common Lead 207-15 



It deserves to be mentioned that earlier experiments made at 

 Harvard with the object of testing this question, namely, the 

 possible variation of atomic weights in the cases of copper, 

 calcium, sodium, and iron, all gave negative results. 



The evidence so far, therefore, tends to sustain the hypo- 

 thesis, but the conclusion cannot yet be considered satisfactorily 

 established in view of the fundamental importance of the issues 

 involved. For it is obvious that these conclusions, if accepted, 

 involve an entirely new view of the nature of the elements and 

 the constitution of matter. If as is suggested there may be two 

 or more different atoms with atomic weights divergent from 

 each other to the extent of several units, but which are indis- 

 tinguishable from each other by chemical or physical properties, 

 and which exhibit, it is said, the same spectrum, the properties of 

 the elements are not wholly dependent on their atomic weights 

 as the periodic law prescribes. It may be that hereafter the 



