336 H. 31. Dadourian — Atmospheric Radio-activity. 



the above paper by Elster and Geitel, Allan* published a paper 

 in which he came to this conclusion : " The difference of the 

 rates of decay of excited activity obtained under different con- 

 ditions seems to point to the fact that the radio-activity of the 

 atmosphere is of a very complex nature." 



Bumsteadf was the first to show the true nature of the 

 atmospheric activity and to account for the agreements as well 

 as disagreements among the rates of decay of the active-deposits 

 obtained from the air, from radium emanation and from thorium 

 emanation. Like other investigators of atmospheric activity 

 he exposed to the air a negatively charged wire and observed 

 the rate of decay of its activity, after the electric field was 

 removed from the wire. But instead of confining his obser- 

 vations to the first few hours of the process of disintegration, 

 he continued them beyond the time necessary for the complete 

 disintegration of the rapidly changing radium products. Then 

 he observed that there remained a slowly decaying activity, 

 which proved to have a half-value period equal to that of the 

 active-deposit of thorium emanation. For exposures of three 

 hours duration the activity of the slowly decaying product was 

 found to be from three to five one-hundredths of the total 

 initial activity, while for an exposure of twelve hours it was 

 15 one-hundredths. When the activity of this product was 

 subtracted from the total activity at any instant, the rate of 

 decay of the residual activity was found to agree very closely 

 with that of an active-deposit obtained from radium emana- 

 tion. He therefore concluded that atmospheric activity was 

 due to the presence of radium and thorium emanations in the 

 air. 



The presence in the ground air of radium emanation was 

 demonstrated by several observers:}: long before the discovery 

 of the exact nature of the atmospheric radio-activity. These 

 investigators found that there was a close agreement between 

 the decay curves of radium emanation and of the " radio-active 

 gas" obtained from the ground. Later it was shown by the 

 writer§ that the radio-activity of the ground air was due to 

 the presence of not only radium emanation but also thorium 

 emanation. It was found that the close agreement between 

 the rates of decay of the radio-active gas and of radium ema- 

 nation was due not to the absence of thorium emanation in 

 the ground, but to its decay during the short time taken in 

 transferring the radio-active gas from the ground into the 



* Allan, Phil, Mag., vii, 140, 1904. 



fBumstead. this Journal, xviii, 1. 1904. 



% Elster and Geitel. Phys. Zeitschr., Hi, 574, 1902, and v, 11, 1904; 

 Ebert and Ewers, Phys. Zeitsehr., iv, 162, 1902; Gockel, Phys. Zeitschr., 

 iv, 604, 1903 ; Bumstead and Wheeler, this Journal, xvii, p. 97, 1904. 



§ Dadourian, this Journal, xix, 16, 1905. 



