•±20 Boltwood — Thorium Minerals and Salts. 



was determined for each of the electroscopes,* and after deduct- 

 ing that part of the activity of the thorium minerals tine to 

 uranium, the remainder was divided by the weight of thorium 

 oxide per gram of the mineral. The number thus obtained 

 represented the activity of one gram of thorium in the min- 

 erals. The general results obtained are given in the table 

 which follows (Table I). The activities are given in terms of 

 the fall of the gold-leaf in the scale divisions per minute. The 

 numbers printed in ordinary type refer to the measurements 

 made in the smaller electroscope, those in italics to the meas- 

 urements made in the larger electroscope. 



Table I. 



Activity per gram of mineral. 











» 





^ Activity 





Per cent 



Per cent 





Due to 



Due to 



per gram 



Mineral. 



uranium. 



Th0 2 



Total. 



uranium. 



thorium. 



of Th0 2 . 



Thorianite, 















Ceylon 



11-2 



78-8 



85 



35 



50 



63 









153 



59 



93 



118 



Thorite, 















Norway 



. 0-4 



520 



34 



1 



33 



63 









62 



2 



60 



115 



Orangite, 















Norway 



. 9-4 



51-1 



63 



30 



33 



64 









108 



50 



58 



114 



Monazite, 















N. Carolina 



035 



4-66 



4-1 



1-1 



3-0 



64 









7-1 



1-8 



5-3 



113 



It will be seen from the numbers given in the last column 

 that the activity of one gram of thorium oxide in the four dif- 

 ferent minerals varied less than one per cent when determined 

 in the smaller electroscope and not over two per cent from the 

 average when determined in the larger electroscope. This 

 variation is within the limits of error of the several measure- 

 ments. 



It is therefore evident that for the four minerals examined 

 the activity of thorium per gram, or as it will be called, the 

 specific activity of thorium, was a constant. The difference in 

 value between the specific activity as determined in the smaller 

 electroscope is due in part to a greater sensitiveness for the 

 larger instrument and in part to the fact that more complete 

 ionization was obtained in the larger ionization vessel than in 

 the smaller. In the smaller electroscope a portion of the a-par- 

 ticles of longer range are stopped by the walls and charged 



* The experimental determination of this factor will be described in a 

 later paper. 



