436 



McCoy and Hoss — Thorium Compounds. 



close agreement of the results of the three methods shows 

 clearly the purity of the sample. The activity of sample A 

 was determined more than a month after preparation; when it 

 had attained a constant (maximum) value. 



Table I gives the results obtained with very thin films of 

 sample A of thorium dioxide ; w is the weight of the film ; 

 a is the observed activity of the film, the unit of activity being 

 that due to l sq cm of a thick film of pure U s 8 . The area of 

 each film was 39-8 sq cm . 



Table I. Thorium Dioxide — A. 







a 



2798 



41-36 



■00676 



2020 



30-80 



•00599 



1026 



20-70 



•00495 



0546 



12-27 



•00444 



= -00387 



.cojo 



.Cc6o 



, Oos~o 



oofo 



oo So 



Weight, w. 

 Curve I, which is nearly a straight line, represents Table I. 



( — j = -00387. Accordingly k„ the specific activity of this 



• 2 

 sample of thorium dioxide, is equal to rz: = 517. 



1 ' I -00387 



The observed activity of thicker films of thorium com- 

 pounds must be corrected for the effect of the emanation. To 

 do this the activity of the evolved emanation must be known. 

 This was found as follows : the film was allowed to remain 



