182 PRINCIPLES OF GENERAL PHYSIOLOGY 



the same ionic type, the proportion of these values to valency, the small effect of temperature 

 on the dissociation of salts, strong acids and bases, and its parallelism with that on the 

 dielectric constant, the exponential relation between dissociation and concentration, which is 

 not the same as that required by the law of mass action, and the fact that the optical and 

 similar properties of dissociated salts (in equimolar concentration) is independent of this 

 actual concentration, and therefore of their dissociation, if the solution is even moderuti-lv 

 dilute. 



With respect to the influence of temperature, the actual effect on dissociation 

 must be distinguished from that on the rate of migration of the ions. The 

 temperature coefficient of conductivity of a salt is about 2 per cent, per degree, as 

 shown by Arrhenius (1901, p. 136), but this is almost entirely accounted for by the 

 increased velocity of the ions, due to diminution of internal friction of the solvent. 

 The actual increase in number of ions is very small indeed. In another class 

 of cases, which are regarded by Noyes and his co-workers (1910) as being of 

 a more strictly chemical nature (see below), the increase in number of ions is 

 considerable as the temperature is raised. Water itself is a striking example. 

 According to the data of Kohlrausch and Heydweiller (1894, p. 209), the 

 temperature coefficient of ionisation of water at 18 is 5'32 per cent. (Nernst, 

 1911, p. 670). This fact is in agreement with the great heat of electrolytic 

 dissociation of water. 



As remarked above, Noyes and his coadjutors (1910, p. 376) suggest that 

 ions may form two different kinds of molecules, electrical and chemical. In 

 the first case the union is not so strong, and the constituents still retain their 

 electrical charges and their characteristic optical effects. " Secondarily, the 

 ions may unite in a more intimate way to form ordinary uncharged molecules, 

 whose constituents have completely lost their identity and original characteristics." 

 " In the case of salts, inorganic acids and bases, the tendency to form chemical 

 molecules is comparatively slight, so that the neutral electrical molecules 

 predominate. In the organic acids, as a rule, chemical molecules predominate. 

 These latter are formed in accordance with the law of mass action, while 

 electrical molecules are formed in accordance with an entirely different principle, 

 whose theoretical basis is not understood." 



G. N. Lewis (1910, p. 218) also calls attention to the deviation of these salts, strong acids 

 and bases, from the mass action law, and points out that it is the moderately concentrated 

 solutions that are abnormal ; in highly dilute solutions the behaviour is in agreement. The 

 ions themselves seem to obey the laws of perfect solutions, so that we must turn to the 

 undissociated molecules for an explanation of the anomalies. The author refers to cases where, 

 assuming normal behaviour of ions, correct results are predicted, although the undissociated 

 part is neglected. 



A deduction from the electrolytic dissociation theory, which has been verified 

 by independent methods, is the constancy of the product of the concentrations 

 of H' and OH' ions in dilute aqueous solutions. Finally, the Nernst equation 

 for the electromotive force of concentration batteries gives good results when 

 the concentration of the ions alone is considered. Lewis (p. 219) also refers 

 to a calculation which he made involving the use of three principles all founded 

 on the Arrhenius theory, viz., the Nernst equation, the solubility product, and 

 the dissociation constant of water. The result was different from the value 

 accepted, but independent investigation by Haber and by Nernst immediately 

 afterwards showed perfect agreement with the calculated value. As the author 

 remarks : " The calculation would obviously have been vitiated if any one of the 

 principles used had been unreliable." On the whole, the evidence indicates 

 that later and better theories will be developments of the first simple one of 

 Arrhenius, not substitutes for it. It must not be forgotten that the propounder 

 of the theory has always been ready to admit the difficulties. Whether the views 

 of Noyes will be found to explain some of these remains to be seen ; there are no 

 doubt many objections to be made to their bare present form. Perhaps this 

 point of view may also supply an answer to the question why a concentrated 

 solution, say of potassium chloride, in which only 25 per cent, is dissociated, 

 exhibits only the properties of ions. Has the KC1 molecule no properties of 

 its own 1 



