Rate of Decay of Thorium Emanation. 107 
this conclusion still remains undisturbed, even though the 
shape be not one of revolution. 
Whether the conditions of the limit can be sufficiently 
attained in experiment is a question upon which I am not 
prepared to express a decided opinion. From the logarithmic 
character of the infinity upon which the argument is founded, 
one would suppose that there might be practical difficulty in 
reducing the section sufficiently. Even if an adequate re- 
duction were possible mechanically, the conductivity of 
actual materials might fail. We must remember that in the 
theory the conductivity is supposed to be perfect. 
Terling Place, Witham, 
June 12, 1904. 
XI. The Rate of Decay of Thorium Emanation. 
By ©. Le Rosstenou and C. T. Grurnenam*. 
URING the investigation of a substance, giving an 
emanation which was suspected to be that of thorium, 
we had occasion to determine the time taken for known 
thorium emanation to decay to half-value, and finding that 
the value obtained was rather less than Prof. Rutherford’s 
(Phil. Mag. 1900, xlix. p. 1), we carried out a series of 
experiments, which go to prove that the true value is 51 
seconds, and not 60 as found by him. 
Before going on to describe our method of measurement, 
it might be advantageous to give a short account of the 
apparatus employed (fig. 1). 
The electrometer was of the Thomson-White pattern, with 
the bifilar suspension removed and replaced by a quartz fibre; 
the electrodes were insulated by means of ebonite coated 
Fig. 1. 
Ey 
ry, : 
2 Caterer & 
HN eas 
ceil R ee 
ots LL 
Fg ene ii 
K 
II --------- ie re 
E 
with shellac, and the instrument was practically dead-beat. 
A transparent celluloid scale was used, divided in millimetres; 
* Communicated by Prof. F. T. Trouton, F.R.S. 
