Rate of Transformation of the Radium Emanation. 551 



Let two volumes V l5 V 2 communicating with each other 

 be filled with air containing Ra-emanation uniformly dis- 

 tributed. Disconnect them; then, if at the time £=0 the 

 smaller volume V x , when passed into an electroscope, produces 

 the current i u and at the time t later, the other volume V 2 , 

 when passed into the same electroscope, gives the effect i 2 , it 

 can readily be deduced, that the period T of Ra-emanation 

 calculated from these data is 



T _ Slog 2 



~~ V i 9 



h>£TT — loo' ■? 



the theory is based on the assumption that the radium 

 emanation decays exponentially with the time. 



In order to obtain accurate results it is advisable to make 



t and -— large, and arrange the time t between the two 



observations so as to make the tw T o currents approximately 

 equal. The electroscope is then used under exactly the same 

 conditions, and no comparison of currents of very different 

 magnitude is needed, 



To eliminate errors due to changes of pressure and tempe- 

 rature of the air, as well as to possible changes of sensitive- 

 ness of the gold-leaf system, immediately before the emanation 

 is introduced into the electroscope, the latter is standardized 

 by the 7-rays from a few milligrams of radium bromide, 

 enclosed in an airtight capsule and placed in a definite 

 position near the electroscope. 



The readings were not taken immediately after the intro- 

 duction of a quantity of emanation, but three hours later, in 

 order to allow the emanation to reach radioactive equilibrium, 

 when the rate of movement of the gold-leaf is sensibly constant 

 over the time required to make the observations. 



A quantity of air containing emanation, supplied by boiling 

 some RaBr 2 solution, was collected into a little gasometer 

 over water and thence passed into a partially exhausted glass 

 vessel of the shape shown in fig. 1 . 



Fig-. 1. 



The vessel was then sealed off at A and brought into a 

 constant-temperature room for about two days, to allow the 

 emanation to distribute itself by diffusion uniformly through 

 the air in the vessel, which was generally at a pressure of 

 about half an atmosphere. > By means of a small flame, the 



