Boltwood — Radio-Activity of Thorium Salts. 99 



It is obvious, therefore, that the chemical process first 

 described by Rutherford and Socldy for preparing thorium-X 

 from thorium, consisting in the precipitation of the thorium as 

 hydroxide from a solution of the nitrate, can be employed also 

 for the separation of mesothorium from thorium. It has the 

 advantage that the mesothorium is obtained in a relatively con- 

 centrated form, but it can be applied with advantage only 

 where thorium nitrate of some age is available, since fresh 

 thorium nitrate will contain little if any mesothorium. 



The fact that the two oxides numbered 3 and 4, which were 

 obtained through the intermediate formation of the normal 

 and " basic " sulphates, respectively, also show a decrease in 

 their activities is not in itself very suggestive, since in the pre- 

 paration of the sulphates the nitrate was first converted into 

 hydroxide. Also in the cases of the other oxides, No. 5 to 

 No. 11 inclusive, the chemical operations through which the 

 thorium passed from the first decomposition of the mineral to 

 the final separation of the pure thorium salt were too compli- 

 cated to make it possible to determine at what particular step 

 the mesothorium was removed. From various data it appears 

 probable, however, that the precipitation of thorium by sodium 

 thiosulphate is effective in separating thorium from mesotho- 

 rium. 



A further point which appears to be worthy of notice in 

 passing is the similarity in chemical behavior shown by thorium 

 and radiothorinm on the one hand, and by thorium-X and 

 mesothorium on the other. We have no good evidence as yet 

 of the chemical separation of thorium and radiothorium. 

 These two elements appear to remain together most persist- 

 ently through elaborate chemical operations which result in 

 the separation of the thorium from every other known element. 

 In contrast to this is the facility with which thorium-X and 

 mesothorium can be separated from thorium and radiothorium. 

 The chemical similaritj 7 of mesothorium and thorium-X is fur- 

 ther indicated by what follows. Nearly two years ago an 

 attempt was made to separate radiothorium from thorium by 

 precipitating barium sulphate in a dilute solution of a thorium 

 salt. This experiment was performed because it was thought 

 that the entrainment of radiothorium by barium sulphate 

 might explain the presence of radiothorium in the radium- 

 barium sulphate residue for thorianite where Halm had first 

 obtained it. The precipitated barium sulphate was highly 

 active when first prepared, but its activity fell regularly at a 

 rate corresponding to half- value in 4 days, until at the end of 

 about 40 days it had reached a negligible value. At the start 

 it therefore contained thorium-X but no appreciable amount 

 of radiothorium. This precipitate of barium sulphate has 



