191 1-] BOLTWOOD— RADIOACTIVITY. 345 



nucleus finally attains a permanently stable form which undergoes 

 no further alterations. If such is indeed the case, then we might 

 expect that these ultimate end products of radioactive decay would 

 accumulate in old radioactive minerals where the process of trans- 

 formation has been proceeding for long geological periods. This 

 line of reasoning has enabled us to identify at least one of these 

 products and that is, in all probability, the one following radium F. 

 The residual atom in this case appears to be no other than the atom 

 of ordinary lead. There are, moreover, certain theoretical argu- 

 ments which point to the same conclusion. The accepted atomic 

 weight of uranium is 238.5. It has been found that two alpha par- 

 ticles are emitted during its transformation and one by the succeed- 

 ing product, ionium. This would correspond to the loss of three 

 alpha particles or helium atoms with an atomic weight of four or a 

 total of twelve units. Two hundred and thirty-eight and one half 

 less twelve give two hundred and twenty-six and one half for the 

 atomic weight of radium, which corresponds to the value obtained 

 in the actual determination of the atomic weight of this element 

 by ]\Ime. Curie. The transformations of the atoms of radium, the 

 emanation, radium A, radium C and radium F are each accompanied 

 by the expulsion of another alpha particle, making five in all. Five 

 times four is twenty and two hundred and twenty-six and one half 

 less twenty is two hundred and six and one half. The latter number 

 is sufficiently near to two hundred and seven and one tenth, the most 

 recently determined atomic weight of lead, to support the conclusion 

 that lead is the ultimate disintegration product of radium. It has 

 not yet been possible to determine the end products of the actinium 

 or the thorium series but they will undoubtedly be identified among 

 the various elements occurring in small proportions in the older ura- 

 nium and thorium minerals. 



Before completing this necessarily brief resume of the present 

 status of the study of radioactive phenomena it is necessary to make 

 some reference to the series of papers published by Sir William 

 Ramsay associated with A. T. Cameron and F. L. Usher. These 

 papers, which deal with the action of radium emanation on various 

 other substances, suggest the occurrence of certain changes, which 

 if they really took place would be of fundamental importance to the 



