696 Dr. Geiger and Prof. Rutherford on the Number of 



to be proportional to the weight of the film for thin films, 

 but for the thicker films the activity was relatively smaller 

 on account of absorption. 



Eadioactive Substances. 



Number of et particles emitted 

 per second per gram of 

 Uranium or Thorium. 



Uranium film No. 1. 

 1043 mgrs. U 3 O tf on area 5*9 cm. 2 



233 XlO 4 



Average 

 237 X10 4 



Uranium film No. 2. 

 285 mgrs. U 3 8 on area 128 cm. 2 



236 X10 1 



Uranium film No. 3. 

 304 mgrs. U 3 8 on area 14*9 cm. 2 



2-43X10' 



Mineral film No. 1. 



1095 mgrs. Uraninite. 



(Joachimstahl) on area 5 - 9 cm. 2 



9-5 Xl0 4 



Average 

 9-6 XlO 1 



Mineral film No. 2. 



1273 mgrs. Uraninite. 



(Joachimstahl) on area 5*9 cm. 2 



9-7 XlO 1 



Thorium film No. 1. 

 4-43 mgrs. Th0 2 on 61 cm. 2 



2-55 X10 1 



Average 

 2-7 XlO 4 



Thorium film No. 2. 

 1-21 mgrs. Th0 2 on 64 cm. 2 



2-84 X 10 * 



Thorium film No. 3. oAtwiru 

 358 mgrs. Th0 2 on 615 cm . 2 - t>D X l u 



It w T ill be seen that there is a good agreement between the 

 experiments and the numbers calculated on the assumption 

 considered in the beginning of this paper. This is brought 

 out by the Table below. 



Number of a particles per gram 



of Uranium per second. 

 Calculated. Observed. 



Uranium 2'32 x 10 4 2-37 x 10 4 



Uranium mineral 9*67 x 10 4 9'6 X 10 4 



Thorium, number of a particles per gram : 2*7 x 10 4 



No doubt the agreement is closer than would be expected 

 under the conditions of the experiments. 



