Soil on Local Atmospheric Radioactivity. 177 



Hence the concentration 

 nq 



(I.) 



N = 



XA;i 



PU- 



(i- 





where £, = 101*5 and h = 2750. 



To test the accuracy of the measurements involved in this 

 equation, a series of readings was made with the pump running 

 at different speeds ; i. e. for different values of q. By taking 

 the mean of several measurements, the activity when q = 34 # 5 

 (pump fully on) was found to be 2 - 06 scale divisions per minute. 



Fig. 2. 



Ordinate? 

 for 



Curve I 



3-2 

 2-4 

 1-6 

















An 





























-~£^2 



*£-. 







0-8 









Liters 



bcr vvTw 



\ute 



Ordinates 



for 

 Curve II 



168 



11-2 



5-6 



12 3 4 5 



Ordinates represent scale divisions per minute. 



The corresponding value given by the equation was 1*64 n. 

 This ratio was used to determine the ordinates of equation 

 I, which was plotted for values of q from zero to 5 liters per 

 minute. The other observed values of the activity correspond- 

 ing to different values of q were then plotted directly and it 

 will be observed that they fit the curve very closely. (Curve I, 

 figure 2.) Since 1 - 61 n gave a rate of 2*06 scale divisions per 

 minute, one scale division per minute must indicate a mean 

 concentration in the electroscope of 0*80 n per cubic centi- 

 meter. 



The Thorium Standard. 



In selecting a standard for comparison with the amount of 

 thorium emanation obtained with the underground air, a num- 

 ber of difficulties are apparent. ]STo one as yet has devised a 



