344 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



be due to a reduction in the mean carrying capacities of the ions at higher 

 concentrations. 



Thus far, the conductance of electrolytic solutions as a function of 

 their concentration has not been accounted for with equal success. Mil- 

 ner 27 has considered this problem, but without arriving at an expression 

 for the conductance as a function of concentration. He has concluded, 

 however, that the decrease of conductance at low concentration must be 

 mainly due to a decrease in the ionic mobilities and not to a decrease in 

 their number. The argument here does not appear to be altogether con- 

 vincing. Milner assumes, for example, that the undissociated molecules 

 are normal in their osmotic behavior. The justification for this assump- 

 tion is by no means obvious. Moreover, experimental facts weigh heavily 

 against the generality of the conclusion reached. Weak electrolytes in 

 water, and, apparently, all classes of electrolytes in non-aqueous solu- 

 tions, approach the mass-action law as a limiting form at low concentra- 

 tions. The difficulty is not alone to account for the failure of the mass- 

 action law in solutions of strong electrolytes in water but, also, to account 

 for the applicability of this law to solutions in other solvents where, judg- 

 ing by the lower value of the dielectric constant, the interionic forces are 

 much greater than in water. Furthermore, according to Milner 's theory, 

 different electrolytes in dilute solutions should exhibit practically identi- 

 cal properties both as regards their osmotic and their electrical properties. 

 This condition is approximately fulfilled in water, but not in solutions in 

 non-aqueous solvents. In these latter solvents, the electrolyte appears 

 to retain its individuality even at exceedingly low concentrations. Any 

 theory which cannot give an account of this fundamental property of 

 electrolytic solutions is obviously incomplete. 



It is not difficult to see in what manner the conductance would be 

 influenced by interionic action at higher concentrations. According to 

 Milner, the ions are not distributed haphazard throughout the medium, 

 but, on the average, as the result of interaction between the charges, ions 

 having like charges are somewhat farther apart and ions having unlike 

 charges somewhat nearer together than would otherwise be the case. 

 Ordinarily it is assumed that a charged particle moves in a uniform 

 electric field. If, however, the ions are combining and dissociating, or, 

 in any case, if charged particles approach each other sufficiently closely, 

 the surrounding field will be influenced and the speed of the ions will vary 

 for different individuals, depending upon the proximity of other ions. 



According to this view, the ratio y = -*- is a measure, not of the number 



- A 



"Milner, PUl. Mag. S5, 214 and 352 (1918). 



