8 Johnstone and Boltwood — Relative Activity 



thorium oxide (and other thorium products in equilib- 

 rium with this) per gram of mineral. The above value 

 for the specific activity of the mineral includes the incre- 

 ment due to the thorium products which must be elimi- 

 nated. To accomplish this measurements were made of 

 a series of films prepared from a specimen of thorite 

 containing 52 per cent of thorium oxide and 0-37 per cent 

 of uranium. The results are tabulated in table III. 





Table III. 





Film 



Weight 



Activity per gram in 



Number 



gram 



divisions per minute 



39 



0-0056 



511 



40 



0-0076 



515 



42 



0-00776 



514 



Av. 513 



The activity of the uranium and its radioactive prod- 

 ucts contained in the thorite will closely equal 



— — X 0-003? = 13*3 divisions per min. 



Correcting the specific activity of the thorite by this 

 number and dividing the remainder by 0-52 (the weight 

 of thorium oxide per gram) we obtain 960 divisions per 

 minute as the activity of one gram of thorium in equi- 

 librium with its products. We may now correct the 

 value found for the activity of the uraninite by an 

 amount equal to the activity of the thorium components 

 (960 X 0-019) and obtain the value 2606 divisions per 

 minute per gram of mineral for the uranium series of 

 products which it contains and a value of 3570 divisions 

 per minute per gram of uranium present (2606/-73). 



The uraninite, however, contained less than the full 

 equilibrium amounts of the uranium-radium products 

 because of the fact that in the finely divided form in 

 which it was used it spontaneously lost a small propor- 

 tion of its radium emanation. The loss of this and the 

 absence of the proportionate amounts of radium A and 

 radium C would cause a deficiency which must be cor- 

 rected for. 



The relative proportion of radium emanation lost by 

 the films of uraninite was determined by the method de- 



