PEESIDENTIAL ADDRESS — SECTION B. 101 



from Eamsay and Young's data. In Fig. 2 the value of the 

 osmotic pressures cannot be given, -as they have not been 

 experimentally determined. lu Fig. 3 any point outside of the 

 curve and to the right, as at a, corresponds to the state of 

 unsaturated alcohol vapour, whose temperature, specific 

 volume, and pressure are indicated — the last by the isobaric 

 line which passes through the point. In Fig. 2 any point out- 

 side the curve and to the right, as at a, must correspond to the 

 state of an unsaturated aqueous solution of aniline, whose 

 temperature and specilic solution-volume can be read, and 

 whose osmotic pressure could be indicated by an isobaric line 

 had we the data for plotting it. A little thought makes it 

 evident, too, that such isobaric lines would follow the same 

 general course as those shown in the alcohol diagram. 



Now consider what must be the effect of gradually decreasing 

 the volume of the unsaturated vapour in the one case and the 

 solution-volume of the aniline in the unsaturated aqueous solu- 

 tion in the other, while temperature is kept constant. In the 

 case of the vapour (Fig. 3) the point a will pass to the left 

 across lines of increasing pressure until the vapour becomes 

 saturated at b. Then, if the diminution of volume continue, a 

 portion of the vapour will condense to the liquid state, or be 

 transferred to c, while the rest remains saturated vapour at b. 

 With continued decrease of volume, the proportion condensed 

 will constantly increase, but there can be no alteration of 

 pressure till all is condensed ; and after that nothing but a very 

 slight diminution of volume is possible without a lowering of 

 temperature. Well, how are we to diminish the solution- 

 volume of our aniline in the unsaturated aqueous solutioii ? 

 Clearly, by depriving the solution of some of its water, so as to 

 leave the same quantity of aniline distributed throughout a 

 smaller space. And what will be the result of doing this while 

 temperature is kept constant ? Evidently, as in the other case, 

 the point a (Fig. 2) w'ill travel to the left across lines of in- 

 creasing osmotic pressure until it reaches b — that is, until the 

 solution is a saturated one ; and after that, if more water be 

 abstracted, some of the aniline will be thrown out or con- 

 densed, not as pure aniline, but as a saturated solution of 

 water in aniline ; so that two layers will now co-exist, the 

 aniline in one having the specific solution-volume represented 

 at b, and the aniline in the other having that represented at c. 

 This transference from b to c will continue, as water is 

 abstracted, until the ratio of residual water to aniline is just 

 enough to give the whole of the latter the specific solution- 

 volume shown at c. At this stage the water layer will dis- 

 appear, and only a saturated solution of water in aniline will 

 be left ; and after this only a very small volume-change can 

 possibly result from further abstraction of water, as the specific 



