230 RADIO-ACTIVE EMANATIONS [CH. 
into the tube B. The current through the tubes A and B was 
measured at regular intervals by an electrometer, with a suitable 
capacity in parallel. Initially there is no current in B, but after 
the opening of the slide, the amount in A decreased and the 
amount in B steadily increased. After several hours the amount 
in each half is nearly the same, showing that the emanation is 
nearly uniformly diffused throughout the cylinder. 
It can readily be shown? that if 
K = coefficient of diffusion of the emanation into air, 
t = duration of diffusion experiments in secs., 
a = total length of cylinder, 
S, = partial pressure of emanation in tube A at end of diffusion, 
S, = partial pressure of emanation in tube B at end of diffusion, 
then 

Say Sie ies (ot iol teeny!) 
Se aS a = a eh 
eos 7 9 ) 
Now the values of S, and S, are proportional to the saturation 
ionization currents due to the emanations in the two halves of the 
cylinder. From this equation K can be determined, if the relative 
values of S, and S, are observed after diffusion has been in progress 
for a definite interval t. 
The determination of S,; and S, is complicated by the excited — 
activity produced on the walls of the vessel. The ionization due 
to this must be subtracted from the total ionization observed in 
each half of the cylinder, for the excited activity 1s produced from 
the material composing the emanation, and is removed to the 
electrodes in an electric field. The ratio of the current due to 
excited activity to the current due to the emanation depends on 
the time of exposure to the emanation, and is only proportional to 
it for exposures of several hours. 
The method generally adopted in the experiments was to open 
the slide for a definite interval, ranging in the experiments from 
15 to 120 minutes. The slide was then closed and the currents 
mm each half determined at once. The central rods, which had 
1 See Stefan, Sitzwngsber. d. Wien. Akad. 63, u. p. 82, 1871. 
