17+ J. C. Sanderson — Probable Influence of the 



integration of radium A and C, and finally, of thorium B and 

 C. Figure 1 shows graphically the growth and decay of the 

 activity due to the presence of radium and thorium emanations 

 in the underground air. The ordinates are activity in scale 

 divisions per minute and the ahscissas represent time ; for con- 

 venience, the scale of the ahscissas is different in different 

 parts of Hie curve, as indicated. 



From the accepted values of X for radium A and C. a curve 

 was plotted showing the increase with time of the ionization 

 due to the a-rays from radium A and C. The number of ions 

 produced by an a-particle of each kind was taken as propor- 

 tional to its range. The ordinates were percentages of the 

 total activity. Assuming that the curve passed through the 

 point obtained empirically at 105 minutes, it was successively 

 made to include five or six of the preceding points and in each 

 case the value of the total activity due to radium A and C 

 deduced. The mean of these values was taken, and from this 

 mean the value of the ordinate when t = o was obtained. This 

 represented the combined activities of radium and thorium 

 emanation. 



The part of the curve showing the disintegration of thorium 

 emanation is plotted from the accepted value of A, for that 

 substance. The observed values are in good agreement with 

 those calculated ; readings had to be taken at time intervals so 

 short that certain irregularities in the motion of the gold leaf 

 became very apparent and were an obvious source of error. 



In plotting the rise of the activity of radium A and C, the 

 active deposit of thorium was neglected, because the first dis- 

 integration product of thorium emanation is rayless. In the 

 last part of the curve, however, thorium A, B and C are in 

 equilibrium with the emanation, and in the curve for the decay 

 of the activity due to radium A and C, their contribution to 

 the total ionization had to be taken into consideration. The 

 initial values of the activity, in scale divisions per minute, for 

 the active deposits of thorium and radium, were deduced as 

 follows : 



At 35 minutes and 95 minutes after the emanations had been 

 swept out, the total activity was 1*65 and 0*90 scale divisions 

 per minute respectively. At 35 minutes the radium A and C 

 was 46 - 7 per cent of its initial value ; the thorium B and C, 

 96*4 per cent. At 95 minutes the values were 14--5 per cent 

 and 90*5 per cent, respectively. If x be the initial value of 

 the radium A and C, and y the initial value of the thorium B 

 and C, we have: -467 x + -964 y = 1-65 and -145 x + -905 

 y = 0-90 which give, x = - 64 and y = 2-22 scale divisions 

 per minute. During the disintegration of radium A and C, 

 the curve for thorium B and C is practically a straight line and 



