ELECTROLYTES AND THEIR ACTION 181 



Of all liquids, with the exception of prussic acid, hydrogen peroxide, and 

 formamide, water has the highest dielectric constant, about 80 times that of air, 

 while the majority of other liquids have valves which vary between 40 for nitro- 

 methane, and 6 -46 for acetic acid. When a substance is soluble in more than one of 

 these various liquids, it is found that its conductivity, or, in other words, the degree 

 to which it is dissociated, is greater, the higher the dielectric constant of the solvent 

 (J. J. Thomson, 1893, and Nernst, 1894, independently). The following numbers 

 will serve as illustrations (Walden, 1906). The solute is tetra-ethyl-ammonium 

 iodide, on account of its solubility in a variety of organic solvents. 



Centnerszwer (1902, p. 223) gives the molecular conductivity of potassium iodide 

 in prussic acid as 262, compared with that in water as 80. The dielectric 

 constant of liquid prussic acid is 95. 



The significance of this fact in connection with the meaning of the dielectric 

 constant as allowing charged bodies to approach nearer to one another without 

 union of their charges is that, supposing we assume that the oppositely charged 

 ions have been separated, a solvent with a high dielectric constant will enable 

 them to come much nearer to one another without combination than in a solvent 

 with a low dielectric constant. The kinetic energy they possess enables them to 

 resist the attraction of the opposite ions when much nearer together, owing 

 to this attractive force being less the higher the dielectric constant of the solvent ; 

 so that, on an average, a larger number are free at any given moment. 



Although considerations of such a kind enable us to form some idea of the reasons why ions 

 do not all combine with their oppositely charged fellows, it is not obvious what causes their 

 original separation, when a solid salt is placed in water. If we admit Faraday's view of the 

 electrical nature of chemical affinity, it seems possible that the electronic forces of the dielectric 

 may be involved. When molecules are separated from one another, as in the process of dis- 

 solving a solid, it may be that they are more accessible to forces tending to break the 

 combination between their constituent ions, and as the separation is effected, the high 

 insulating power, or dielectric constant, of the solvent prevents, to a varying degree, their 

 recombination. 



Perhaps the most serious difficulty in the Arrhenius theory is the behaviour 

 of strong acids, strong bases and salts, as compared with that of weak acids and 

 weak bases. In the latter case, as Ostwald showed, the proportion of dis- 

 sociated to combined molecules, when the solution is diluted, obeys a law deduced 

 from mass action simply and known as Ostwald's "dilution law." In the former 

 case the law is quite different. In a paper by A. A. Noyes, Melcher, Cooper, 

 and Eastman (1910, p. 375), attention is called to the fact that the electrolytic 

 dissociation in the former case of salts, strong acids, and strong bases "is a 

 phenomenon primarily determined not by specific chemical affinities, but by 

 electrical forces arising from the charges on the ions ; that it is not effected 

 (except in a secondary degree) by chemical mass action, but is regulated by 

 certain general, comparatively simple, laws, fairly well established empirically, but 

 of unknown theoretical significance ; and that, therefore, it is a phenomenon quite 

 distinct in almost all its respects from the phenomenon of dissociation ordinarily 

 exhibited by chemical substances, including that of the ionisation of weak acids 

 and bases." 



The reasons for this view can be found in the original paper ; we must be content here 

 with reference to the similar dissociation values for salts of different chemical nature but of 



