100 JBoltwood — Radio- Activity of Thorium Salts. 



recently been examined and is now quite active. It gives 

 off thorium emanation but contains no thorium. Its present 

 activity is therefore due to radiothorium formed from the 

 mesothorium which was precipitated with it. From this it 

 seems probable that the radiothorium which Halm separated 

 from the residue of insoluble sulphates, obtained some time 

 before in working up a considerable quantity of thorianite, 

 had not been present in the residues when first prepared, but 

 had been formed in them later through the disintegration of 

 mesothorium. This supposition is further strengthened by 

 Halm's statement* that his radiothorium probably contained 

 some mesothorium. 



It appears quite likely, therefore, that the entraining action 

 of barium sulphate on mesothorium was directly responsible 

 for the presence of radiothorium in the thorianite residues. 



Summary. 



Measurements of the changes which have taken place in the 

 activity of certain thorium preparations have given results 

 which are strongly in support of Halm's assertion that an inter- 

 mediate product having a slow rate of change and not emit- 

 ting a-rays exists in the thorium series between thorium and 

 radiothorium. 



Certain methods for the separation of this intermediate 



product from thorium have been described. 



Sloane Laboratory, Yale University, 



New Haven, Conn., June 16, 1907. 



* Loe. cit. 



