DISCUSSION OF EVIDENCE. 159 



used to calculate the magnitude of the dissociation of the electrolytes 

 with which he worked, at the various dilutions of the solutions. 



Another method of measuring the dissociation of electrolytes, based 

 upon the change in the solubility of a salt on the addition of a second 

 salt with a common ion, was developed theoretically by Nernst 1 while 

 working in Ostwald's laboratory. When applied experimentally it 

 gave dissociations which were different from those obtained by the 

 conductivity method. The freezing-point method had not been used 

 at that time to measure dissociation. There was then a period when 

 there were but two methods for measuring dissociation, and these gave 

 widely different results. During this period the point was made 

 against the dissociation theory, that if dissociation took place in the 

 presence of water there was no means of determining its magnitude. 



At this time Ostwald so improved the freezing-point method that it 

 could be used to measure dissociation. He started me to work on the 

 application of this method, and we 2 measured the dissociation of a fairly 

 large number of salts. The results differed radically from those 

 obtained by the solubility method, but agreed fairly well with those 

 calculated from the conductivity method of Kohlrausch. It was 

 afterwards shown that an assumption had been made in applying the 

 solubility method, which, when corrected, enabled that method to give 

 essentially the same results as those obtained by the other two. 



A comparison of the data from the freezing-point method with those 

 from the conductivity method showed that dissociation as measured 

 by the former was slightly higher than by the latter. The meaning 

 of this discrepancy was at that time not understood. 



After it had been established, with reasonable certainty, that hydra- 

 tion takes place in aqueous solution, a possible explanation of this 

 apparent discrepancy presented itself. But before offering this expla- 

 nation it seemed desirable to do more experimental work, having this 

 point especially in mind. Pearce 3 carried out in my laboratory a very 

 careful piece of work, in which dissociation was measured by the 

 freezing-point method and also by the conductivity method, and the 

 two sets of results were compared. We worked with the chlorides of 

 calcium, strontium, magnesium, barium, cobalt, copper, and alumin- 

 ium; with the nitrates of calcium, magnesium, barium, cobalt, nickel, 

 and copper; with sodium bromide, and with hydrochloric, nitric, and 

 sulphuric acids. 



That hydration can explain the fact that dissociation as measured 

 by freezing-points is higher than as measured by conductivity can be 

 seen from the following. The combined water is removed from the 

 field of action as solvent ; only the uncombined water is acting as 

 solvent. Freezing-point lowering is proportional to the ratio between 



x Zeit. phys. Chem., 4, 1372 (1889). 3 Carnegie Inst. Wash. Pub. No. 180, p. 57. 



2 Ibid.,ll, 110, 529; 12, 633 (1893). 



