Dadourian — Radio-activity of Underground Air. 21 



charged negatively for three hours and the rate of decay of 

 the activity was observed as before. The results of two such 

 experiments are shown by curves I and II, figure 4, where the 

 ordinates represent the ionization currents and the abscissae 

 the time. These curves have half-value periods of about 11 

 hours and agree very closely with those given by Rutherford. 



The component due to thorium of the total ionization of the 

 ground-air excited activity was calculated by a method to be 

 described below, and was subtracted from the latter in order to 

 compare the result and activity with radium excited activity. 

 It is obvious that the resultant activity will be nothing more 

 nor less than radium excited activity provided that the ground- 



Fig. 4. 



air excited activity is composed solely of radium and thorium 

 excited activities. It was shown above that the activity of the 

 negatively charged wire exposed to ground-air was entirely 

 thorium excited activity at the end of five hours after the 

 removal of the potential difference from the wire. Hence the 

 thorium element of the ground-air excited activity at any time 

 can be found by multiplying the ionization of the thorium ex- 

 cited activity, for the corresponding time, by the ratio of the 

 ionizations of the ground-air and thorium excited activities at 

 any time after five hours. The ratio of the ionizations, at the 

 end of nine hours of the ground-air and thorium excited activi- 

 ties, represented by the curves I of figures 2 and 4 respectively, 

 was found to be 0*0216. Each of the observed values of the 

 ordinates of curve I, figure 2, was multiplied by this ratio and 



