l'.U2] The Origin of Radium. 399 



WEEKLY EVENING MEETING, 



Friday, March 15, 1912. 



His Grace the Duke of Northumberland, K.G. P.C. D.C.L. 

 LL.D. F.R.S., President, in the Chair. 



Frederick Soddy, Esq., M.A. F.R.S. 



The Origin of R((dium. 



The philosophical explanation of radioactivity arose out of some very 

 simple chemical observations on the nature of the radioactive process. 

 The apparently permanent and constant radioactivity of radioactive 

 substances in general consists of two parts. One part cannot, as a 

 rule, be separated from the substance, whilst another part is readily 

 separated by many ordinary chemical and physical operations. The 

 latter part is often the larger part of the whole radioactivity, but it 

 is associated frequently with a practically infinitesimal proportion of 

 the material. Naturally, the first explanation to present itself was 

 the ordinary one that the radioactive substance is not homogeneous, 

 but consists of a mixture of more than one radio-element. In this 

 way Mme. Curie accounted for her analysis of the radioactivity of 

 pitch-blende, and the separation from the preponderating radioactive 

 constituent, uranium, of new radio-elements in minute amount, to 

 which the names radium, polonium, and actinium were given. Simi- 

 larly, Sir William Crookes, who separated by chemical methods the 

 /3-activity of uranium compounds, gave the name uranium X to the 

 constituent responsible for this activity. 



In a chemical examination of the radioactivity of thorium com- 

 pounds ten years ago, Professor Rutherford and I found that one 

 portion of the activity was separable from the thorium by chemical 

 methods, and to the constituent responsible for this separable activity 

 the name thorium X was given. Then an astonishing observation was 

 made. Unlike any other chemical separation which had hitherto 

 been carried out, it was found that thorium, after having been deprived 

 of thorium A^ by simple chemical processes, spontaneously regenerated 

 this constituent with lapse of time. After a month's interval from 

 the separation a new quantity of thorium X could be obtained from 

 the thorium, the activity of which was as great as tliat obtained in 

 the first separation. In the same interval the activity of the thorium A' 

 first separated completely decayed, according to an exponential law. 

 This continuous regeneration of the constituents responsible for tlie 



