Valency of the liadioelements. 39$ 



be expressed in days, the right-hand side of the equation 

 must be multiplied by 8*64 . 10 4 . 

 We therefore obtain the expression 



~ 0-04485 uv /1N 



n(u + v) 



Equation (1) was tested by Nernst * for the special case 

 of monovalent electrolytes (n = l), and found to be in good 

 agreement with experimental results. The general equation 

 was not tested, however, owiug probably to the absence of 

 experimental data, and to lack of knowlege of the conditions 

 under which multivalent electrolytes dissociate. Nernst's 

 view is based on the assumption that dissociation is complete, 

 that is for instance, that NaCl dissociates into Na* and Cl r 

 ions only. In a solution of the bivalent BaCl 2 , however, not 

 only are Ba" and CI' ions present, but also Bad* ions, the 

 last named being monovalent and of unknown mobility. 

 Although the validity of the general equation cannot be 

 tested strictly on account of this difficulty, its truth is 

 rendered very probable by the results tabulated in Table I. 

 In this table are given the relative number of molecules of 

 various substances diffusing in equal times f . 



Table I. 



KCl 803] 



NH 4 C1 689 monovalent. 



NaCl 600 J 



Bad., 45(n 



CaCl.; 429 ,. , , 



SrCi; 432 1 divalent. 



MgCL 392j 



The mobilities of Ba, Ca, Sr, and Mg are approximately 

 the same as those of K and Na. The esentially slower dif- 

 fusion of these bivalent halogen compounds must therefore 

 be ascribed to their greater valency. 



§ 3. Connexion beticeen the Diffusion Constant of a Salt and 

 the Mobility of the Cation. Diffusion in excess of the Anion. 



In equation (1) a relation between the valency and the 

 mobility either of the cation or of the anion of the salt is 

 given. A direct relation between the diffusion velocity of a 

 salt, such as for instance RaCl 2 , and the mobility and valency 

 of the radium cation, may be obtained by allowing the 



* Nernst, Zeit. phys. Chem. vol. ii. p. 616 (1888). 

 t Long, Wied. Ann. vol. ix. p. 613 (1880). 



