RECENT ADVANCES IN SCIENCE 275 



A. n 



being 7 = ^-^, where v v denotes the viscosity of the solu- 



tion at dilution v and rj is the viscosity of the pure solvent. 

 In general the viscosity of a solution is greater than the vis- 

 cosity of the solvent, so that the correction term introduced 

 assumes that the mobilities of the ions are smaller at moderate 

 concentrations than they are at great dilution. By making 

 use of this expression, data have been found for the degree 

 of dissociation or ionisation of a considerable number of 

 acids, bases, and salts. Two strong electrolytes very care- 

 fully investigated are hydrochloric acid and potassium chloride. 

 Judging from conductivity measurements, the degree of dis- 

 sociation of HC1 is distinctly greater than that of KC1 at the 

 same concentration. Associated with this is the fact that the 

 mobility of H ion is much greater than that of any other 

 ion, and this very naturally raises the question whether the 

 larger ionisation of HC1 is not more apparent than real. 

 Recently G. N. Lewis has even suggested that the degree of 

 ionisation of HC1 is to be taken as identical with that of KC1 

 as given by the conductivity expression. The doubt which 

 exists regarding the accuracy of the conductivity expression 

 is accentuated by the lack of agreement between the 7 values 

 obtained by this method and those obtained by measurements 

 of the electromotive force of concentration cells. One would 

 have expected to obtain values from electromotive force mea- 

 surements for the degree of ionisation and the concentration 

 of an ion (the latter being simply the product of the degree 

 of dissociation into the total concentration of the electrolyte) 

 in agreement with those obtained from conductivity measure- 

 ments. The type of cell used in an inquiry of this kind may 

 be represented thus : 



silver electrode 



silver nitrate 

 concentration 



silver nitrate 



concentration 



c 2 



silver electrode 



It can be shown thermodynamically, that the E.M.F. of such 

 a cell depends upon the concentration, or, as we shall call it, 

 the " effective concentration " of the silver ions, the E.M.F. 

 being proportional to the logarithm of the ratio of the effective 

 concentration of the ion in c x to the effective concentration of 

 the ion in c 2 . If we denote by a the factor which connects 



