16 Dadourian — Radio-activity of Underground Air, 



Art. II. — Radio-activity of Underground Air / by 

 H. M. Dadourian. 



Atmospheric and underground air have been shown to be 

 radio-active by Elster and Geitel,* and others. Experiments, 

 by several investigators, on the rate of decay and other prop- 

 erties of the atmospheric radio-activity have proved it to be due 

 to the presence of a radio-active gas similar to radium emana- 

 tion. The activity of this radio-active gas decays just about 

 as fast as radium emanation, that is, it falls to haif value in 

 about three days and a half. But the rates of decay of the 

 excited activities obtained by exposing a negatively charged 

 wire to the air and to radium emanation do not agree so well. 

 Rutherford and Allanf obtained 45 minutes for the half-value 

 period of air excited activity, whereas radium excited activity 

 falls to half value, after the first two hours, in 28 minutes. 



This is to be expected if we suppose that there is thorium 

 emanation in the air as well as radium emanation. Thorium 

 emanation decays very rapidly, having a half-value period equal 

 to about one minute ; so the experiments on the rate of decay 

 of the radio-active gas obtained from the air determine the rate 

 of decay of the radium emanation in the air only, the thorium 

 emanation having decayed during the few minutes which it 

 takes to begin to observe the ionization currents, after the 

 removal of the difference of potential from the negatively 

 charged wire. This accounts for the close agreement between 

 the rates of decay of the ionizations of radium emanation and 

 the radio-active gas obtained from the air. On the other hand, 

 the excited activity obtained by exposing a negatively charged 

 wire to the air, decays very much more slowly than radium 

 excited activity ; the half-value period of the former is about 

 11 hours while that of the latter is 28 minutes. Thus measure- 

 ments of the rate of decay of the air excited activity give the 

 rate of decay of a combination of the excited activities of 

 radium and thorium. In fact, BumsteadJ has recently shown 

 that the excited activity obtained by exposing a negatively 

 charged wire in the open air is fairly accounted for by the 

 assumption of the presence of radium and thorium emanations 

 in the air. 



The following experiments were undertaken in order to see 

 if this was the case with the excited activity obtained from 

 underground air, also. The method employed in taking the 

 ground-air excited activity is illustrated in figure 1. 



* Elster and Geitel, Phys. Zeit,, iii, p. 574, 1902. 

 | Rutherford and Allan, Phys. Mag., Dec, 1902. 

 \ Bumstead, this Journal, xviii, 1, 1904. 



