VIIt] RADIO-ACTIVE EMANATIONS 205 
(1) A supply of fresh radio-active particles produced at a 
constant rate. 
(2) A loss of activity of the particles following an exponential 
law with the time. 
In the case of Ur X and Th X, the active matter produced 
manifests its activity in the position in which it is formed; in this 
new phenomenon, a proportion of the active matter in the form of 
the emanation escapes into the surrounding gas. The activity of 
the emanation, due to a thorium compound kept in a closed vessel, 
thus reaches a maximum when the rate of supply of fresh emana- 
tion particles from the compound is balanced by the rate of change 
of those already present. The time for recovery of half the final 
activity 1s about 1 minute, the same as the time taken for the 
emanation, when left to itself, to lose half its activity. 
If q is the number of emanation particles escaping into the 
gas per second, and J, the final number when radio-active equi- 
librium is reached, then (section 124), 
qo = XN. 
Since the activity of the emanation falls to half value in 1 minute 
esl Sie 
and V,=87q, or the number of emanation particles present when 
a steady state is reached is 87 times the number produced per 
second. 
Radium Emanation. 
135. Discovery of the emanation. Shortly after the 
discovery of the thorium emanation, Dorn! repeated the results 
and, in addition, showed that radium compounds also gave off 
radio-active emanations and that the amount given off was much 
increased by heating the compound. The radium emanation differs 
from the thorium emanation in the rate at which it loses its 
activity. It decays far more slowly, but in other respects, the 
emanations of thorium and radium have much the same properties. 
Both emanations ionize the gas with which they are mixed, and 
affect a photographic plate. Both diffuse readily through porous 
1 Abh. der naturforsch, Ges. fiir Halle-a-S., 1900. 
