vil] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 187 
cessive filtrates. In three successive precipitations the activities of 
the residues were proportional to 100, 8, 1°6 respectively. Thus two 
precipitations are nearly sufficient to free the thorium from Th X. 
The thorium freed from Th X was then allowed to stand for 
a definite time, and the amount of Th X formed during that time 
found by precipitating it, and measuring its radio-activity. Ac- 
cording to the theory, the activity J; of the thorium formed in the 
time ¢ is given by 
rf => Cm 
I, 
where J, is the total activity of Th X, when there is radio-active 
equilibrium. 
If At is small, 
I, 
2 ING 
Tie 
Since the activity of Th X falls to half value in 4 days, the 
value of X expressed in hours =-0072. After standing a period 
of 1 hour about 1/140, after 1 day 1/6, after 4 days 1/2 of the 
maximum should be obtained. The experimental results obtained 
showed an agreement as good as could be expected, with the equa- 
tion expressing the result that the Th X was being produced at 
a constant rate. 
The thorium-nitrate which had been freed from Th X was 
allowed to stand for one month, and then it was again subjected 
to the same process. The activity of the Th X was found to be 
the same as that obtained from an equal amount of the original 
thorium-nitrate. In one month, therefore, the Th X had been 
regenerated, and had reached a maximum value. By leaving the 
thorium time to fully recover its activity, this process can be re- 
peated indefinitely, and equal amounts of Th X are obtained at 
each precipitation. Ordinary commercial thorium-nitrate and the 
purest nitrate obtainable showed exactly the same action, and 
equal amounts of Th X could be obtained from equal weights. 
These processes thus appear to be independent of the chemical 
purity of the substance?. 
1 The general method of regarding the subject would be unchanged, even if it 
were proved that the radio-activity of thorium is not due to thorium at all but to a 
small constant amount of a radio-active impurity mixed with it. 
