Radioactivity of Radium and Thorium. 



455 



In the second column the final activity is taken as 100* 

 In the third column the proportion of the lost activity 

 regained is tabulated. These results are shown graphically 

 (curve A). The curve B shows the decay of activity of 



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the emanation drawn from the table in § 2. The two 

 curves are quite analogous to those obtained for the decay* 

 and recovery of UrX and ThX respectively. The proportion 

 of the activity regained after an interval t is given by 



I. 



Io 



= l-<r 



where A, is the coefficient of decay of activity of the radium 

 emanation. The curve of recovery of the activit} r of radium 

 can thus be deduced if the rate of decay of the emanation is 

 known. In these experiments the non-separated activity was 

 25 per cent, of the final activity. The activity of the emana- 

 tion, together with the excited activity it produced, made up 

 the other 75 per cent. 



The above equation is only approximate owing to the 

 existence of a period of retardation of a few hours, due to the 

 time taken for the excited activity to reach a steady value 

 for any given quantity o£ emanation present. It is, how- 

 ever, too small to affect the results appreciably and may be 

 neglected. 



Somewhat similar experiments were performed tor thorium. 

 A sample of thorium hydroxide of high emanating-power 

 was ignited over a blast-lamp and converted into the de- 

 emanated oxide. Its radioactivity was found to rise 



2 H 2 



ID 



