the Rate of Decay of Thorium Emanation. 109 
as possible through a calcium-chloride drying-tube’ and the 
time noted. The cylinder was then closed, the earth con- 
nexion broken, and readings taken every 5 seconds over a 
period of 400 seconds; any residual emanation was imme- 
diately blown out, and the rate of charge due to the excited 
activity taken as quickly as possible. 1t was unnecessary to 
apply any correction for the lag of the needle, as it did not 
require more than 5 seconds to become steady. 
An example will make the method clear. | 
After introducing the emanation, an operation which lasted 
10 seconds, readings were begun 30 seconds later (see table, 
p- 108). . 
The emanation was then blown out, and the subsequent 
rate of charge, which was due solely to the induced activity 
developed during the experiment, was found to be ‘65 cm. 
in 200 seconds. 
Fig. 2 
560 400 
SECONDS 
In the annexed diagram (fig. 2) the values given above are 
plotted against time and a smooth curve A drawn through the 
points, from which it will be seen that they are nearly all on the 
