Valency of the liadioelements. 



407 



§ 7. Ion mobility of the Radio-ions. 



The mobility of ions may be calculated, either from the 

 coefficient of migration or the conductivity as the case may 

 be, or may be directly determined. The first method, which 

 is that most generally used, is not applicable to the case of 

 most radio-ions, owing to their small concentration. The 

 conductivity of a solution of radium bromide has been deter- 

 mined, however, by Kohlrausch and Henning*, and from 

 their result the mobility of the radium ion was found to be 

 57-4. 



The mobilities of the radio-ions must therefore be deter- 

 mined directly. The methods in common use for determining 

 the mobility of an ion directly may be divided into two 

 classes. The first of these may be called the volume method* 

 and the second the surface method, hi the volume method 

 the apparatus (see fig. 2) is filled with an indifferent electro- 

 lyte as far as the tube /j. This tube itself is filled with a 



solution say of EaBr 2 of the same conductivity as the 

 indifferent electrolyte. After electrolysis has taken place 

 for a given time t seconds, the concentration c of the radium 



* Verh. d. Phys. Ges. vol. vi. p. 144 (1904), 



