LOWERING OF SOLUBILITY 51 



a decision as to its molecular weight may be based on 

 Henry's law. But the separation may also be accomplished 

 by the addition of a liquid which does not mix with the 

 solution. Again two methods arise, according as we deal 

 with the solvent or dissolved body passing over into the 

 new liquid. In the first case we may expect the analogue 

 of the law of diminution of vapour pressure, in the second 

 that of Henry's law. 



Let us take first the lowering of solubility, which was 

 first applied to determine molecular weights by 

 Nernst l , and afterwards carried out in a very 

 simple manner by Tolloczko 2 . The whole 

 measurement of molecular weight could be carried 

 out in a flask (Fig. i o) of volume about 1 80 c.c. with 

 a long narrow neck (i cm. = 0-4385 c.c. ; content 

 13 c.c.) divided in half millimetres. As liquids, 

 ether and water were chosen, and the decrease of 

 solubility of ether in water brought about by Fig. 10. 

 any substance soluble in ether measured. 



To take an experiment in detail: a known quantity of 

 water saturated with ether at 18 -3 was used, containing, 

 according to a previous measurement of solubility, 10763 

 grams ether. This quantity of ether saturated with water 

 would occupy 34-967 scale divisions of the neck. The ethereal 

 layer occupied 4-86 scale divisions, and so weighed 1-4958 

 grams ; on addition of 0-0952 gram benzene it increased to 

 6-1 1, equivalent to 1-88 grams. 



As previously for the vapour pressure we get the 

 formula 



in which p and Ap are the pressure and decrease of 

 pressure respectively, n and N the number of dissolved 

 and dissolving molecules (the latter calculated from the 



1 Zeitschr. f. Phys. Chem. 6. i. 



2 1. c. 20. 389. 



D 2 



