
208 - RADIO-ACTIVE EMANATIONS [CH. 
activity on the walls of the vessel indirectly furnishes a measure 
of the rate of decay of the emanation itself. This is only true if 
the emanation is placed for four or five hours in the tube before 
observations begin, in order to allow the excited activity time to 
reach a maximum value. 
Using this method P. Curie obtained results similar to those 
obtained by Rutherford and Soddy by the direct method. The 
activity decayed according to an exponential law with the time 
falling to half value in 3:99 days. 
The experiments were performed under the most varied con- 
ditions but the rate of decay was found to remain unaltered. The 
rate of decay did not depend on the material of the vessel contain- 
ing the emanation or on the nature or pressure of the gas with 
which the emanation was mixed. It was unaffected by the amount 
of emanation present, or by the time of exposure to the radium, 
provided sufticient time had elapsed to allow the excited activity 
to reach a maximum value before the observations were begun. 
P. Curiet found that the rate of decay of activity was unaffected 
by exposing the vessel containmg the emanation to different 
temperatures ranging from + 450° to — 180°C. 
In this respect, the emanations of thorium and radium are 
quite analogous. The rate of decay seems to be unaffected by 
any physical or chemical agency, and the emanations behave in 
exactly the same way as the radio-active products Th X and Ur X, 
already referred to. The radio-active constant » is thus a fixed 
and unalterable quantity for both emanations, although in one. 
case its value is about 5000 times greater than in the other. 
Emanations from Actinium. 
137. Debierne? found that actinium gives out an emanation 
similar to the emanations of thorium and radium. The loss 
of activity of the emanation is even more rapid than for the 
thorium emanation, for its activity falls to half value in a few 
seconds. In consequence of the rapid decay of activity, the 
emanation is able to diffuse through the air only a short distance 
from the active matter before it loses the greater proportion of its 
1 ¢C. R. 136, p. 223, 1903. 2 CO. R. 136, p. 146, 1903. 
