Weights of Radium and Thorium Emanations. 75 



A, through V into B, and out into the open air, as in the 

 previous experiment. It is important to notice that in this 

 case the ratio of the rates at which the central rods in A and B 



charge up is not equal to —• For, let the concentration in A 



at a point in a plane at right angles to the axis and at a dis- 

 tance x from the ehonite plug be ~ ; then, since the emana- 

 tion decays according to an exponential law in passing from 

 one end to the other of A, 



where z Q is the concentration on entering A. 



Xow since the central rod connected to the electrometer 

 reaches nearly the whole length of the testing-vessel, the 

 ionization current in A is proportional to 



sf X ^= %(!-«-**) 



where S is the cross-sectional area of A and X is its length. 

 Similarly for B the ionization currrent is proportional to 



Hen< 



The ionization current in A 

 The ionization current in B 



To rind c T it is necessary to measure the decrease in activity 

 of the emanation in passing through the cylinder A. This 

 was done by replacing the diffusion-vessel by a glass one of 

 nearly the same volume, and measuring the ionization current 

 in A and B as before. Knowing the volumes of A and of 

 the glass vessel connecting A and B. it was possible to calcu- 

 late the fall in concentration due to the natural decay of the 

 thorium emanation in passing through A. It was also possible 

 to calculate the fall in concentration in passing through the 



glass vessel, from which the value of log- 1 , could be found. 



In these experiments the rate of flow of air was somewhat less 

 than in the previous experiments, on which account an appro- 

 priate correction was added to the observed value of zj. 

 The results of the experiments with thorium emanation are 

 given in Table> VIII. and IX. 



