the Mutual Solubility of Liquids. 191 



34°*15 0. we found 764*3 mm. As the change of the vapour- 

 pressure of the three phases at the boiling-point is practically 

 the same as for pure ether, i. e. 26*7 mm. per degree, a pressure 

 of 760 mm. would have been obtained at a temperature of 

 35°-99C. The boiling-point of our ether was 34°-35 C. at 

 760 mm., so that the three-phase boiling-temperature was 

 0°36 lower than for pure ether. Beckmann found o- 32. 

 The agreement is very close, especially considering the ab- 

 solutely different methods by which the results were obtained. 

 We take this agreement as one of the proofs of the purity of 

 the substances used. 



Another test of purity is the comparison of the pressures 

 at the same temperature, but at different volume. In most 

 cases the pressure was lower at small than at large volume ; 

 this may be accounted for by a small fall in temperature in 

 the vapour-jacket in going up. We did not succeed in 

 getting rid of this fall ; the constant use of the electromagnet 

 prevented our protecting the tube as well as we might 

 otherwise have done. The differences in pressure are ac- 

 counted for by a fall in temperature of about 0°'13 C. 



The existence of a critical point for the upper liquid layer 

 and the vapour at 201° C. has already been mentioned. The 

 critical phenomena are very simple when two of three 

 phases become identical. There is one temperature, and one 

 only, where this can happen, and the critical phenomena are 

 therefore almost identical with those of a single substance, 

 and not with those of a mixture of two substances, when there 

 is not a second liquid. It was pointed out, however, that 

 ether and water showed some irregularity in the critical 

 phenomena appearing at temperatures below 201° C. down 

 to 198°. The influence of gravitation is of rather an intricate 

 character in the case of three phases, and we will not attempt 

 to discuss it on this occasion ; but the absence of similar 

 irregularities with other mixtures of pure substances which 

 we investigated, makes us think that the reason of the 

 irregular behaviour must be looked for in some impurity 

 caused by the action of the water on the glass, and not in 

 gravitation. 



Above 201° C. there are only two phases left ; the phe- 

 nomena now are the ordinary phenomena of mixtures. A 

 critical point, different for each mixture, may be looked for, 

 above which no condensation is possible, retrograde con- 

 densation at lower temperatures, and finally a temperature 

 (plait-point temperature) where the two remaining phases 

 become identical. None of these phenomena could be ob- 

 served with ether and water, as the opacity of the tube made 



