292 



Dr. R. A. Lehfeldt on the 



an isothermal — that of 50° — is shown in fig. 2. The hori- 

 zontal part ends at the point A (63 per cent.), beyond which 

 the mixtures are homogeneous : the curve beyond A may 

 theoretically meet the horizontal line at a finite angle, but 

 that is certainly not distinguishable on the diagram. The 

 curve is in fact exactly similar, so far as the experiments 

 show, to that for 75°, and the pressure of the critical point, C 

 on the diagram, which lies between them (at 68°*4) appears to 

 make no difference whatever in this case— a case of great 

 disparity in the vapour-pressures of the two components. 



For comparison,, a few experiments were made with aniline 

 (not specially purified) and water. A mixture which consisted 

 of two layers, even at the highest temperature used in the 

 experiment, gave in the differential apparatus the following 

 results : — 



70° 

 -6-8 

 -0-0290 



7T — p 



(tt— p)U 



80° 



85° 



11-1 



-13-3 



-00315 



-0-0307 



Whilst the Beckmann apparatus 



gave at 100° :— 



Per cent, 

 aniline. 



Lowering of 

 boiling-point. 



7T-p. 



IT — p 



— ■ 



3-99 



0636 



-17-1 



00225 



7-68 



0921 



-24-6 



0-0324 



11-10 



0921 



-24-6 



00324 



The second column gives the observed fall in temperature 

 on adding aniline to the water ; the third column the rise of 

 pressure corresponding, at the rate of 26*8 millim. per degree. 

 Water at 100° is saturated by the addition of 6' 5 per cent, 

 of aniline, and it will be seen that the vapour-pressure rises 

 no further after that. The relative rise of vapour-pressure on 

 saturation is 0*0324 at 100°, in satisfactory agreement with 

 the numbers obtained by the differential gauge (0*0290, 

 0*0 515, 0*0307), a tendency to increase with temperature 

 being distinguishable here, as with phenol. Now suppose 

 the vapour-pressure of a saturated mixture to be obtained in 

 in this way : let the partial pressure of the water-vapour be 

 that of pure water reduced by the normal amount (Raoult's 

 law) due to the solution in it of the maximum quantity of 

 aniline : and let the partial pressure of aniline-vapour be that 

 of pure aniline reduced by the normal amount due to the 



