Dadourian — Radio-activity of Underground Air. 1 9 



observations being taken at longer intervals. About n've hours 

 after the potential difference was removed from the negatively 

 charged wire, the ionization due to radium excited activity was 

 vanishingly small, all the activity, about five per cent of the 

 total initial activity, being of a very much more slowly decay- 

 ing type. The rate of decay of this slowly decaying activity 

 was calculated from the data of two experiments ; in one case 

 the half-value period was 10 hours and 12 minutes, and in the 

 other case it was 10 hours and 47 minutes, giving an average 

 value of 10J hours. This is very near the half-value period of 

 thorium excited activity, which is about 11 hours. None of 

 the excited activities of the known radio-active substances or a 

 combination of them can account for the slowly decaying ex- 

 cited activity of ground air, except thorium excited activity. 



It is very improbable that there is an unknown radio-active 

 substance whose excited activity falls to half-value in about the 

 same time as that of thorium. Hence there is no reason to 

 doubt that the slowly decaying activity obtained by exposing a 

 negatively charged wire to underground air is thorium excited 

 activity. 



In order to see if the presence in the underground air of 

 radium and thorium emanations was enough to account for the 



