in the salt and the reagents, a very small amount of active matter being mixed with them.
132. Rate of production of Th X. If the recovery of
the activity of uranium or thorium is due to the continuous
production of new active matter, it should be possible to obtain
experimental evidence of the process. As the case of thorium
has been most fully investigated, a brief account will be given of
some experiments made by Rutherford and Soddy[1] to show that
Th X is produced continuously at a constant rate. Preliminary
experiments showed that three successive precipitations were
sufficient to remove the Th X almost completely from the thorium.
The general method employed was to precipitate a solution of
5 grams of thorium-nitrate with ammonia. The precipitate was
then redissolved in nitric acid and the thorium again precipitated
as before, as rapidly as possible, so that the Th X produced in the
time between successive precipitations should not appreciably
affect the results. The removal of the Th X was followed by
measurements of the activity of the residues obtained from successive
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. According to the theory, the activity I_{t} of the thorium formed in the time t is given by
I_{t}/I_{0} = 1 - e^{-λt},
where I_{0} is the total activity of Th X, when there is radio-active equilibrium.
If λt is small,
I_{t}/I_{0} = λt.
Since the activity of Th X falls to half value in 4 days, the
- ↑ Rutherford and Soddy, Phil. Mag. Sept. 1902.
