THEORIES OF ELECTROLYSIS. 309 



Ag I o - i normal Ag No, | i *o normal K CI with Ag CI | Ag, 

 in which silver electrodes are placed one in silver nitrate 

 solution, and the other in silver chloride and potassium 

 chloride, was calculated by Nernst to be 0*52 volt, and 

 observed by Ostwald to be 0*5 1 volt. 



The remarkable agreement shown between the results of 

 experiment and the consequences deduced from the dissocia- 

 tion theory in the cases of the thermal phenomena and the 

 laws of diffusion must be regarded as very strong evidence 

 in favour of the substantial truth of that theory. No such 

 agreement could arise unless there were at least a deep-seated 

 analogy between the actual facts and the hypothesis on which 

 the results have been worked out. It seems necessary to 

 accept the view that the ions of good electrolytes are, in 

 some way, free from each other for a considerable fractional 

 part of their existence. This does not, however, exclude 

 the possibility of the existence of molecular aggregates in 

 solutions. Evidence of the existence of such aggregates 

 has been collected by Pickering 1 and others, who have 

 shown that, in varying the concentration of a solution, the 

 physical properties often change at that point, at which its 

 constituents are present in proportions corresponding to 

 some simple molecular compound. Pickering made, for 

 instance, a careful determination of the densities of solu- 

 tions of sulphuric acid of different strengths, and drew a 

 curve to show his results. Changes of curvature appeared 

 at points corresponding to definite molecular proportions, 

 and some of the hydrates thus indicated were subsequently 

 isolated by crystallisation {e.g., H_,S0 4 . 4.H 2 0, HC1 . 3HX)). 

 These changes can more readily be seen if a new curve is 

 drawn between the concentration and the rate of change of 

 density with the concentration {i.e., the tangents at different 

 points of the first curve). By this process of " differentia- 

 tion," a series of straight lines is obtained, with breaks at 

 the positions where, in the first curve, changes of curvature 

 appeared. Similar figures were drawn for the electric con- 

 ductivity, expansion by heat, contraction on formation, heat 



1 Watts' Dictionary of Chemistry, Art. Solutions II. 



