254 MR. W. K. BOUSFIELD AND DR. T. M. LOWliY ON THE ELECTRICAL 



the values for the coefficient of ionisation deduced by the two methods. It appears, 

 in fact, that the formula; in which AKBHKNIUS'S hypothesis has usually found 

 expression, can only be regarded as accurate when applied to solutions containing 

 more than 100 litres of water for each gram-molecule of dissolved salt. 



As a qualitative expression, however, the theory is applicable to electrolytes of all 

 kinds, and the development of exact numerical relationships is only hindered by the 

 difficulty of procuring the necessary experimental data, a difficulty that in many 

 cases can only be overcome by the discovery of new methods of investigation. The 

 essential postulate of AKRHENIUS' theory is that a salt may exist both in an active 

 and in an inactive form as regards conductive power, so that when brought into an 

 electrical circuit, only part of the total weight of electrolyte is as a rule directly 

 operative in carrying the current. This conception finds expression in the co-efficient 

 of ionisatiou, a, which represents the proportion of the salt which is thus active in 

 electrolytic conduction. Independently of any particular theory as to the exact 

 nature of the difference between the ionised and un-ionised part of the salt, the 

 formula K = ma. (w-j- <;) is universally applicable as expressing the fact that the 

 specific conductivity K of any binary compound is dependent on the mass in of the 

 compound contained in unit volume, the proportion ex. that is able to take part in the 

 transport of the current, and the velocities u and v with which the oppositely charged 

 ions move through the solution under the action of the electrical forces. In the case 

 of dilute aqueous solutions the numerical values of a, u, and t- are accurately known. 

 In the case of less dilute solutions they are approximately known. In the case of 

 concentrated solutions and in the case of fused salts they are altogether unknown. 



The present paper is essentially a contribution to the study of concentrated 

 aqueous solutions. We hope at some future date to extend the investigation to 

 dilute solutions, but the experiments now described deal almost exclusively with 

 solutions of normal concentration and above. The electrolyte chosen for investigation 

 was sodium hydroxide, which has the advantage, not possessed by many binary salts, 

 of being exceedingly soluble in water, so that even at C. it is possible to measure 

 the properties of a 50-per cent, solution, though at this temperature such a solution 

 is highly supersaturated. In common with others who have been engaged in the 

 study of concentrated aqueous solutions, we have been impressed with the fact that 

 combination between solvent and solute is an essential part of the process of solution, 

 and that the presence of hydrates must be recognised not only in the crystals which 

 separate when the solutions are cooled, but also in the solutions themselves. We 

 have not, however, .been able to reconcile our observations with the view, in itself 

 improbable, that this power of combination is no longer operative in dilute solutions, 

 and indeed have obtained evidence that if the reduction in volume which frequently 

 accompanies dissolution may be regarded as an index of the association of solvent and 

 solute, the degree of association reaches a maximum only at dilutions which are not 

 far removed from those required for complete iouisatiou. We believe, therefore, that 



