50 



Mr. W. R. Bousfield and Dr. T. M. Lowry. [June 19, 



Nevertheless, when the influence of changing ionic-mobility is 

 eliminated, and the coefficient of ionisation is dealt with separately, 

 the decay of ionisation as temperature rises is in many cases very 

 clearly marked, even at 0° C, and in presence of a large excess of the 

 solvent. Thus Whetham* has shown that at a dilution of 1000 litres 

 per molecule of solute the coefficient of ionisation of potassium chloride 

 falls from 0*992 at 0° to 0*982 at 18°, and that of barium chloride 

 from 0*969 at 0° to 0*954 at 18°; in the case of copper sulphate no 

 decrease occurs at dilutions greater than 1000 litres, but at a dilution 

 of 100 litres the coefficient falls from 0*638 at 0° to 0*623 at 18°. 

 These and other measurements that have been made between 0° and 

 100° serve, however, mainly to show that within these limits the 

 coefficient of ionisation is influenced only to a relatively small extent 

 by temperature, and that the upper conductivity zero must lie very 

 much further from the boiling point than the lower zero is removed 

 from the freezing point. 



The different behaviour of acids and salts, which is unimportant in 

 the case of the lower conductivity zero, becomes a vital factor in 

 discussing the possible existence of an upper conductivity zero. 

 Armstrong! has already called attention to the importance of the 

 fact that whilst the majority of salts are conductors per se, the acids 

 are, in the pure state, dielectrics, and only become electrolytes by 

 interaction with an " ionising solvent." In virtue of their inherent 

 power of " self-ionisation " the salts may therefore exhibit some con- 

 ducting power independently of that due to the ionising properties of 

 the solvent, whilst it is to be anticipated that the acids would cease to 

 be electrolytes if the solvent, by reason of increasing temperature, 

 should lose its power of ionisation. For this reason evidence of the 

 existence of an upper conductivity zero is to be looked for especially 

 in solutions of substances which like the acids are incapable of self- 

 ionisation. The acids are also especially suited for this purpose owing 

 to the fact that the mobility of the hydrogen ion increases relatively 

 slowly with rising temperature. Even at atmospheric temperatures 

 the temperature coefficients for the acids, though positive, are rela- 

 tively small, and the acids would therefore be the first to exhibit a 

 maximum of conductivity and a reversal in the sign of the tempera- 

 ture coefficient. This effect of decreasing ionisation was actually 

 observed by Arrhenius in the case of phosphoric and hypophosphorous 

 acids, which exhibit maxima of conductivity at 54° and 75° respec- 

 tively, and maxima of conductivity have also been observed by Saehf 

 in the case of aqueous solutions of copper sulphate. In the majority 

 of aqueous solutions the maximum of conductivity lies above the 



* ' Phil. Trans.,' 1900, vol. 194, pp. 321—360. 

 f < Koy. Soc. Proc.,' 1886, vol. 40, p. 268. 

 % ' Wied. Ann.,' 1891, vol. 43, p. 212. 



