vii] RADIO-ACTIVE EMANATIONS 207 
The activity falls off according to an exponential law with the 
time, and decays to half value in 3°71 days. With the usual 
notation 
I, —At 
Te 
the mean value of X deduced from the results is given by 
r= 2:16 x 10-* = 1/463000. 
P. Curie determined the rate of decay of activity of the emana- 
tion by another method. The active matter was placed at one end 
of a sealed tube. After sufficient time had elapsed, the portion of 
the tube containing the radium compound was removed. The loss 
of activity of the emanation, stored in the other part, was tested at 
regular intervals by observing the ionization current due to the 
rays which passed through the 
walls of the glass vessel. The 
testing apparatus and the con- 
nections are shown clearly in 
Fig. 39. The ionization current 
is observed between the vessels 
BB and CC. The glass tube 
A contains the emanation. 
Now it will be shown later 
that the emanation itself gives 
off only « rays, and these rays 
are completely absorbed by the 
glass envelope, unless it is made 
extremely thin. The rays pro- 
ducing ionization in the testing 
vessel were thus not due to the 
a rays from the emanation at 
all, but to the 8 and y rays due to the excited activity produced 
on the walls of the glass tube by the emanation inside it. What 
was actually measured was thus the decay of the excited activity 
derived from the emanation, and not the decay of activity of the 
emanation itself. Since, however, when a steady state is reached, 
the amount of excited activity is nearly proportional at any time 
to the activity of the emanation, the rate of decay of the excited 



Harth 
Electrometer 
Fig. 39. 
