726 Dr. P. B. Perkins : Determination of Periods of 



that an error of 10 per cent, in reading a deflexion during 

 decay or in finding the capacity ratios would change the 

 value of the decay period less than 1*5 per cent. The amount 

 of emanation drawn into the chamber was so regulated that 

 deflexions varying from 5 to 10 divisions were observed. 

 Since the leaf was at rest when readings were made, it was 

 possible to read to tenths with an error of not more than 1 or 

 2 per cent. The small percentage variations of the individual 

 determinations of the capacity ratios given in Table L, show 

 that the error made in obtaining these ratios could not 

 appreciably affect the half-value periods. 



Corrections. 



In order to correct the observed readings for the activity 

 produced in the chamber by the formation of the active 

 deposit, the theoretical curves for the rise of activity of the 

 C products were plotted *. The larger part of the ionization 

 obtained by the production of the active deposit is due to the 

 a particles from the C products, as the A products practically 

 decay with the emanation. The /3 ray effect of the B 

 products was also neglected. 



Fig. 2. 



8 \Z 16 



T/me in Minutes. 



80 



Figs. 2 and 3 show these curves for actinium and thorium 

 respectively. In the case of actinium emanation the maxi- 

 mum activity was observed about 9 minutes from the 

 beginning of the experiment, a result in agreement with the 

 theoretical curves shown in figs. 2 and 3. The activity at 



* See Rutherford's ' Radioactive Substances/ p. 421 (1913). 



