Bolt-wood — Radio-activity of Uranium Minerals. 275 



no tendency to fall off even when the plate was completely 

 inverted. 



In measurements of a similar character to those here 

 described McCoy* has used films of such appreciable thickness 

 that a correction had to be made for the absorption of the 

 radiations in the material itself. By using very thin films the 

 absorption becomes inappreciable and the activity is directly 

 proportional to the weight of the material present. This 

 relation is indicated in the following table (Table I) giving the 

 results of the measurements of a series of films prepared from 

 an oxide of uranium containing 82 - l per cent of uranium: 





Table 



I. 







Weight of oxide 



Activity : Divisions 



Activity 



Film No. 



grams 



per minute 



Weight 



1 



0-0324 



3-16 



98 



2 



0-0131 



1-36 



104 



3 



0-0087 



0-885 



102 



4 



0-0088 



0-889 



101 



5 



0-00435 



0-446 



102 





Table 



II. 











Activity : Divisions 



Activity 



Weight grams 







per minute 



Weight 



0-0364 







13-76 



378 



0-0250 







9-56 



382 



0-0229 







9-11 



397 



0-0049 







2-07 



422 



In the case of the uranium minerals the absorption with 

 increasing thickness of the film is more apparent, as is in- 

 dicated in the following table (Table II) : 



Film No. 



1 



2 

 3 

 4 



The results given in Table II are shown graphically in fig. 

 2, in which the ordinates are proportional to the activity and 

 the abscissas to the weights. The dotted straight line is drawn 

 through the first point and the origin and indicates the locus 

 of the points if no absorption took place. It is evident that 

 for films weighing about 5 milligrams (and approximately 

 60 cm in area) the absorption of the radiations in the material of 

 the film is very slight. The films of minerals used in these 

 experiments weighed in no case more than 11 milligrams and 

 no corrections were therefore made in the activities for any 

 absorption of the radiations in the films themselves. 



Determination of the Uranium in the Minerals. 



An accurate determination of the proportion of uranium 

 contained in the minerals was highly essential as it was desired 

 to express the activity of all the other substances in terms of 

 *Jour. Am. Chem. Soc, xxvii, 391, 1905. 



