394 Dr. Gr. von Hevesy on the 



radium chloride to diffuse in excess of! some chloride, e. g. 

 in hydrochloric acid. The electrical forces act on the ions 

 so that their velocity is proportional to their concentration. 

 Since, however, the concentration of the chlorine ions is 

 much greater than that of the radium ions, the rate of dif- 

 fusion of the RaCl 2 will depend only on the mobility of the 

 radium ion. It follows from this that dilute hydrochloric 

 acid in a concentrated solution of potassium chloride diffuses 

 much more rapidly than it does in pure water, because in 

 the second case the slow motion of the CI ions hinders that 

 of the H ions. Experiments on this particular point have 

 been made by Abegg and Bose *. 



The relation between the diffusion velocity of RaCl 2 in an 

 excess of hydrochloric acid, the velocity of the ions, and the 

 valency of the radium ion will be next derived. 



If c be the concentration of the RaCl 2 diffusing, <y that of 

 the hydrochloric acid, and U and V be constants proportional 

 to the mobilities of the radium ion and the chlorine ion 

 respectively, then under the action of the osmotic forces 



1 dc 



N'_= URT - 7 - cations 



k n ax 



and. -v^, J- TTT1 m^ 



N'x= VRT -j- anions 



n dx 



pass unit cross-section in unit time. 



Under the action of the compensating electric forces also, 



W f ~ = — Uce-^ cations 

 K ax 



and XT// , - Yr dyfr 



N _ = +YC6-—- anions 

 a' ax 



pass unit cross-section in unit time. 



The total number of cations migrating is 



Where N K = -U'e 7 ^ 



is the number of H cations diffusing. 



* Zeit.phys. Chem. vol. xxx. p. 545 (1895). 



