DISCUSSION OF EVIDENCE. 145 



APPROXIMATE COMPOSITION OF THE HYDRATES FORMED BY VARIOUS 



SUBSTANCES IN SOLUTION. 



The line of evidence just discussed seemed so strongly in favor of 

 the general correctness of the view that there is combination between 

 the dissolved substance and some of the water present, that Jones 

 and Getman 1 undertook to calculate the approximate composition of 

 the hydrates formed by the different substances, and by the same 

 substance at different dilutions. 



The experimental work consisted in determining the freezing-point 

 of the solution and, consequently, the depression of the freezing-point 

 of water produced by the dissolved substance at the concentration in 

 question. From the freezing-point lowering the molecular lowering 

 was calculated. 



The dissociation of the solution was measured by means of the 

 conductivity method. Knowing the dissociation, the theoretical 

 molecular lowering was calculated on the assumption that none of 

 the solvent was combined with the dissolved substance. The ratio 

 of the theoretical molecular lowering to the value found experimentally, 

 gave the proportion of all the water present that was uncombined. 

 The remainder of the water was, of course, combined with the dissolved 

 substance. The total amount of water present in any given solu- 

 tion could be readily determined. It was only necessary to take 

 the specific gravity of the solution by weighing a known volume of it. 

 Knowing the specific gravity and the concentration, it was, of course, 

 perfectly simple to determine the total amount of water in, say, a 

 liter of the solution. The total amount of water in the solution and 

 the percentage of combined water being known, the total amount 

 of combined water was known. Knowing the amount of dissolved 

 substance present in, say, a liter of the solution, and knowing the total 

 amount of water combined with it, it was perfectly simple, from 

 the molecular weights of the dissolved substance and the solvent, 

 to calculate how many molecules of water were combined with one 

 molecule of the dissolved substance. The results of such a calculation 

 are only approximations. In the first place, the conductivity method 

 of measuring dissociation is not accurate for concentrated solutions, 

 and there is no thoroughly accurate method known for this purpose. 

 The error here is, however, in all probability not very large. Another 

 source of error, which is probably larger, results from the assumption 

 that Raoult's law holds for concentrated solutions, i. e., that for con- 

 centrated solutions the lowering of the freezing-point is proportional 

 to the concentration. This is not strictly the case, and we do not 

 know at present how wide the deviation from Raoult's law is in con- 

 centrated solutions. 



Carnegie Inst. Wash. Pub. No. 60. 



