OTHER PROPERTIES OF ELECTROLYTIC SOLUTIONS 319 



in a given series increases with the order of the group. Thus, water 

 has a dielectric constant of approximately 80 and is an excellent ioniz- 

 ing agent, while hydrogen sulphide has a dielectric constant of 10 and 

 is a relatively poor ionizing agent. Ammonia at its boiling point has 

 a dielectric constant of 22 and is a moderately good ionizing agent. In 

 the seventh group, hydriodic acid has a dielectric constant of 2.9, hydrp- 

 bromic acid of 6.3, and hydrochloric acid of 9.5. As we approach the 

 derivatives of the upper members of this group, their dielectric constant 

 and their ionizing power increase. The dielectric constant of hydrogen 

 fluoride is not known but it is known to be an excellent ionizing agent. 

 Moissan, for example, prepared fluorine by the electrolysis of fluorides 

 in liquid hydrogen fluoride. 'It seems not improbable that the dielectric 

 constant of hydrofluoric acid is greater than that of water. At any rate, 

 in passing from ammonia to water, the dielectric constant increases from 

 22 to 80, and it is, therefore, not improbable that the dielectric constant 

 of hydrogen fluoride is higher than that of water. The dielectric con- 

 stant of many organic and inorganic substances containing oxygen, nitro- 

 gen, chlorine and sulphur is relatively high. Such substances in the 

 liquid state possess the power of dissolving salts and of forming conduct- 

 ing solutions with them. It is unnecessary to give here a detailed list of 

 these substances. 



With increasing temperature, the dielectric constant of all substances 

 decreases. The dielectric constants of a number of substances have been 

 measured through the critical point. 53 These include sulphur dioxide, 

 ether, ethylchloride, and hydrogen sulphide. For these substances the 

 dielectric constant just beyond the critical point is 2.1, 1.52, 4.68 and 2.7 

 respectively. A striking result, here, is the relatively high value of the 

 dielectric constant of ethylchloride at the critical point relative to its 

 value at lower temperatures. Thus at 59 its value is 6.29, which de- 

 creases to the value given above at 186. Evidently the variation of 

 the dielectric constant with the temperature depends largely upon the 

 nature of the solvent. Corresponding to the low value of the dielectric 

 constant of sulphur dioxide, the conductance of solutions of electrolytes in 

 this solvent falls to very low values. The same is true of ammonia solu- 

 tions, although in this solvent the conductance above the critical point 

 has a readily measurable value. The conductance of typical salts in 

 ethylchloride has not been measured, but that of mercuric chloride 

 solutions, whose ionization is usually relatively low, is greater than 

 that of solutions of typical electrolytes in sulphur dioxide under 

 corresponding conditions. Judging by the conductance of solutions 



"Eversheim, Ann. d. Phys. 8, 539 (1902) ; ibid., 13 f 492 (1904). 



