186 Dr. Kuenen on the Critical Phenomena of 



24. (2.) There is another difference between fig. 1 and 

 fig. 2. The border-curves of the mixtures do not all of them 

 lie between the vapour-pressure curves of C 2 H 6 and N 2 0. 

 Some of them lie above the N 2 curve. It follows that at 

 every temperature there is a maximum of pressure for 

 one of the mixtures. From the figure it appears that this 

 maximum lies somewhere between the mixtures the com- 

 position of which is 0*18 and 0*25, say +0*2. The border- 

 curves of the mixtures 0*18 and 0'25 differ only very slightly 

 and in the figure almost coincide. The meaning of this is 

 that the maximum belongs to almost the same mixture at all 

 temperatures represented in the figure. If the mixture for 

 which the pressure is a maximum varied quickly with change 

 of temperature, the border-curves of the mixtures in that 

 neighbourhood would intersect at perceptible angles. If we 

 tried to draw the curve giving the maximum pressure at any 

 temperature, it would lie a very little above the border- 

 curves for a? = 0*18 and # = 0*25, and be about parallel to 

 those. This curve we shall call the maximum-curve. 



25. There is another remarkable quality of the border- 

 curves in this region near the maximum, viz. the narrowness 

 of the border-curves, i. e. the very small increase of pressure 

 during the condensation. The value of this increase amounts 

 to 0*2-0*3 atm. for the first mixture, which is only a very little 

 more than what still remained in the pure nitrous oxide itself*. 

 We may therefore almost take it equal to zero. This agrees 

 with what follows from the theory of mixtures. In a 

 maximum-point (and also in a minimum-point) the two 

 coexisting phases have the same composition (x) though 

 different volumes (v), and there is no increase of pressure 

 during the condensation. In order to determine the exact 

 value of the maximum, a large number of mixtures lying about 

 # = 0'2 should be investigated : only the small admixture in 

 the components themselves would of course impede the fixing 

 of the maximum- curve. 



26. In itself the existence of a maximum (or minimum) 



* The increases for the second mixture (025) are somewhat higher, 

 viz. +0 - 6 atm., which must he due to a somewhat larger impurity of that 

 mixture. Probably the pressures of this mixture are therefore a little too 

 high at the same time. The maximum will therefore he nearer to the 

 first mixture (0-18). That is why I have put it equal to + O20. On the 

 whole the values for the increase of pressure are a little irregular, which is 

 quite natural considering the difficulty of these observations with 

 mixtures. From the observations one might, perhaps, conclude that the 

 maximum with rise of temperature shifts a little towards the N 2 ; 

 but as this seems rather uncertain, I prefer taking it constantlv equal 

 to 0-20. 



