457 



parison with the others. Thus some time, about three hours, after the em- 

 anation has been placed in a vessel we have Rad. Em. changing through the 

 intermediate products into Rad. D. giving off three a particles, one from Rad. 

 Em; one from Rad. A; and one from Rad. C. This complex radiation has 

 after the first few hours the half value period of the longest of the series, 

 which is that of Rad. Em., 3.85 days. Thus if a quantity of radium emanation 

 gas is placed in an electroscope the rate of "leak" of the electroscope in- 



C 



Fig. 2. 



creases for three hours and then slowly decreases, dropping to one-half value 

 of the maximum in 3.85 days from the time it reached the maximum. 



The rise of activity during the first few hours is shown by the curves 

 in Fig. I. These curves are plotted from data given in Rutherford's Radio- 

 active Substances and Their Radiations, and in Makower & Geiger's Practical 

 Measurements in Radioactivity. The final values (4 hours) are based on 

 the number of ions produced by a particles from the various products. Thus 



