438 • JleCoy and J2oss — Thorium Compounds. 



of this sample of thorium dioxide, since there would be no 

 absorption of the emanation by such a film. The fraction 

 emanated, of the whole amount of emanation produced by the 

 sample, can now be calculated. About one-fourth of the 

 activity of Th0 2 is due the emanation.* Therefore the total 

 activity of the whole of the emanation of l g of the sample 



= 5 - = 129. -— = -013. Therefore 1-3 per cent of the 

 4 129 x 



whole emanation is emanated. The last column of Table II 



gives the fraction, E, of the emanated emanation which is 



evolved by the film. 



The above results may now be used to correct the observed 



activity of a thick film. The observed activity, a t1 of a film of 



~E '7 



Weight, w. 



a thorium compound is too great by the amount E, the activity 

 of the evolved emanation. But the value a^—E, which repre- 

 sents the activity of the solid film alone, is too low, because the 

 film has lost a fraction of its emanation and ThB. The lost 

 activity is equal to %E since the activity of the ThB is equal 

 to that of the equilibrium amount of the emanation. The 

 total activity of the film is \v). Therefore the true activity 

 of the film is 



a^—E 

 a = 1 — 2E 



k x w 



Table III gives in column 2 the observed values of the 

 activity, a lt and in column 3 the corrected values, a. The 



* Rutherford, loc. cit. 



