

358 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
The results obtaimed by Elster and Geitel for the air removed 
from the earth at Wolfenbiittel were also obtained later by Ebert 
and Ewers! at Munich. They found a strongly active emanation 
in the soil, and, in addition, examined the variation with time of 
the activity due to the emanation in a sealed vessel. After the 
introduction of the active air into the testing vessel, the activity 
was observed to increase for several hours, and then to decay, 
according to an exponential law, with the time, falling to half 
value in about 3°2 days. This rate of decay is more rapid than 
that observed for the radium emanation, which decays to half 
° 











value in a little less than four days. The increase of activity with 
time is probably due to the production of excited activity on the 
walls of the vessel by the emanation. In this respect it is analogous 
to the increase of activity observed when the radium emanation 
is introduced into a closed vessel. No definite experiments were 
made by Ebert and Ewers on the rate of decay of this excited 
activity. In one experiment the active emanation, after standing 
in the vessel for 140 hours, was removed by sucking ordinary air 
1 Phys. Zeit. 4, p. 162, 1902. 
