234 RADIO-ACTIVE EMANATIONS [CH. 
exponential law, p= p,e- where p, is the value of p when ¢=0 
and 2 is the radio-active constant of the emanation. 

dp _ 
Then Pra rp, 
ap _ 
and ee Ap. 
x 
Thus pe Me ax “+ B 
Since p= 0 whena=a, B=0. 
If p=p, when «=0, A= py. 
A 
Thus Dine ~ I, 
It was not found convenient in the experiments to determine 
the activity of the emanation along the cylinder, but an equivalent 
method was used which depends upon measuring the distribution 
of “excited activity,” produced along a central rod AB, which is 
charged negatively. 
It will be shown later (section 167) that the amount of excited 
activity at any point is always proportional to the amount of 
emanation at that point. The distribution of “excited activity ” 
along the central rod from the plate C upwards thus gives the 
variation of p for the emanation along the tube. 
In the experiments, the cylinder was filled with dry air at 
atmospheric pressure and was kept at a constant temperature. 
The central rod was charged negatively and exposed from one to 
two days in the presence of the emanation. The rod was then 
removed, and the distribution of the excited activity along it 
determined by the electric method. It was found that the amount 
of excited activity fell off with the distance « according to an 
exponential law, falling to half value in about 1°9 cms. This is in 
agreement with the above theory. 
Since the activity of the emanation falls to half value in 
1 minute, »\=°0115. The value K =:09 was deduced from the 
average of a number of experiments. This is a slightly greater 
value than K =-07, obtained for the radium emanation, but the 
results show that the two emanations do not differ much from 
one another in molecular weight. 
