834 



iPhysiCS. — "On immhrinij in a hinary .'ii/stem /<>r w/i/'rh the three- 

 phase pressure is t/reafer than the sum of the vapour tensions 

 of the tiuo components." By Dr. F. E. C. Scheffer. (Commu- 

 nicated by Prof. J. D. VA^ der Waaj.s). 



(Communicated in the meeting of Sept. 26, 1914). 



1. In my investigation on the sy.stcm liexane-water the remarkable 

 phenomenon presented itself that the three-phase tension of tlie two 

 liquid layers by the side of gas appeared to be greater than the 

 sum of the vapour tensions of pure hexane and pure water ^j. When 

 the tensions of the pure substances at a definite temperature are 

 denoted by P, and P,. the three-phase pressure by P, then for 

 temperatures which are not too far from the critical end-point 



If we could speak of "partial pressures" for such an equilibi'ium, 

 this result would be impossible. A proof of this is found in van dkr 

 \Vaai.s--Kohnstamm's "Thermodynamik", which however is only 

 valid when the gas-laws") hold for the saturate vapours. When the 

 gas-laws do not hold for the gas phases, in other words if the gases 

 possess surface layers, the proof is not valid, and the statement that 

 the three-phase pressure must always be smaller than the sum of 

 the vapour tensions of the components, holds therefore only for 

 rarefied saturate vapours. 



In my papei- on the system hexane-water I have shown that the 

 contradiction with the second law of thermodynamics, which at first 

 sight may be supposed to exist in the observed phenomenon, is only 

 an apparent one. We might, namely, be incHned to reason as 

 follows: If tlie three-phase mixture possesses such a high pressure, 

 the pressure exerted by the water and hexane molecules, or at least 

 that of one of them will have to be greater than the pressure of 

 water vjipour, i-esp. hexane vapour over the pure components. If we 

 therefore bring the three-phase mixture I)v means of a semi-permeable 

 membrane into contact with pure hexaue and with pure water 

 under their own vapour pressure, hexane or water will pass through 

 the semi-permeable meuibranes from the three-phase mixture towards 

 the pure liquid. We should then get splitting up of the three-phase 

 mixture, whereas just on the contrary the so high three-phase pressure 

 sets in of its own accord from pure water and ))ure hexane. This 

 is in contlict with the second law of thermodynamics. 



1) These Proe. 16. i04. (1913). 



2) Thermodynamik. II. S. 476. 



