168 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



constant. In the immediate neighborhood of the critical point, the 

 density of the solvent decreases very rapidly with increasing temperature, 

 whereas beyond the critical region the density of the solvent medium 

 remains fixed. The rapid decrease in conductance immediately below 

 the critical point is to be ascribed to the rapid decrease in the density of 

 the solvent medium. 



It is to be expected that the ionization and consequently the con- 

 ductance of solutions in the critical region will be governed largely by 

 the dielectric constant of the medium, and it may be inferred that those 

 liquids, which under ordinary conditions exhibit a very high dielectric 

 constant, will likewise exhibit a relatively high dielectric constant in the 

 critical region. In the case of sulphur dioxide and ammonia the dielec- 

 tric constant in the critical region is very low, whereas in the case of the 

 lower alcohols and water a relatively larger value of this constant is to 

 be expected. Water would be an ideal substance for the purpose of 

 studying the properties of electrolytic solutions in the critical region, 

 were it not for the difficulties attending conductance measurements in 

 this solvent at high temperatures. These difficulties, however, disappear 

 very largely in the case of the lower alcohols, although it is to be ex- 

 pected that the ionization in the critical region will be markedly lower 

 in these solvents than in water. 



In Table LX are given values of the specific conductance of solutions 

 of potassium iodide in methyl alcohol at a series of temperatures up to 

 252. 19 The critical point lies in the neighborhood of 240 C. The 

 reduced conductance values given in the last column are derived by 

 multiplying the specific conductance (second column) by the fraction 

 of the total volume of the tube occupied by the liquid (third column). 

 If the true critical phenomenon is to be observed, the tube must initially 

 be filled with an amount of liquid such that when the critical point is 

 reached the tube is just filled with liquid. Obviously, as the liquid 

 expands, the concentration of the solution decreases, and the corrected 

 values of the specific conductance therefore represent values of this 

 quantity on the assumption that the specific conductance varies as a 

 linear function of the concentration. This condition is probably not 

 fulfilled, but nevertheless represents an approximation somewhat nearer 

 the truth than the measured values of the specific conductance. More- 

 over, in the immediate neighborhood of the critical region, where the 

 volume of the liquid is almost equal to the entire volume of the tube, the 

 corrected value of the specific conductance corresponds very nearly with 

 the true value. If these corrected values are plotted against the tem- 



"Kraus, Pfy/8. Rev. 18, 40 and 89 (1904). 



