Mixtures of Ethane and Nitrous Oxide. 187 



pressure is not a new phenomenon. Guthrie*, Konowalowf, 

 and others have found the same with different mixtures. 

 Only, so far as I know, a maximum (or minimum) has never 

 been traced up to the critical temperature. In Konowalow's 

 mixtures the maximum appears sometimes to shift with change 

 of temperature, and accordingly probably to disappear below 

 the critical temperature. The above results prove that this 

 need not be the case at all. This fact may be expressed in 

 this manner : the maximum- curve need not reach the vapour- 

 pressure curve of one of the substances. In my experiments 

 it reaches the plaitpoint-curve, near B. 



27. These results will be much better understood by 

 inspecting figs. 3, 4, 5. The curves in these figures give the 

 relation between the composition of the mixtures and the 

 volumes at which the condensation begins and at which it 

 ends (yid. § 1 sqq.), and may at the same time be considered as 

 the projection of the connodal curves of the plait in van der 

 Waals's surface upon the v-x plane. The figures belong to 

 the temperatures 20°, 25°, and 26° 0. respectively. 



28. If the phenomena were such as would correspond to 

 fig. 1, the plait with rise of temperature would get narrower 

 near the C 2 H 6 plane (x = l). At 32° C. the curve would 

 loose itself from the v-axis for x=l, whilst forming a plait- 

 point there ; above 32° C. it would withdraw towards the N 2 

 axis (x = 0), where it would disappear at 36° C. (critical 

 temperature of N 2 0). 



29. However, this is not what really happens. With rise 

 of temperature the plait becomes narrower somewhere near 

 the middle. Consequently, at a temperature far below 32° C, 

 near 25°-8 C, the plait divides into two parts. At that 

 moment two plaitpoints appear (P 1 and P 2 ) J. The two parts 

 into which the plait is divided henceforth withdraw towards 

 x=0 and x=l separately. At 32° C. the right-hand plait 

 disappears at x = l. At 36° C. the left-hand plait disappears 

 in # = 0. Above 36° C. there is no plait left : the surface is 

 convexo-convex henceforth, and all its points represent stable 

 phases. 



30. A maximum pressure (or minimum) manifests itself in 

 the v-x figure in the position of one of the lines connecting 



* Phil. Mag. [5] xviii. p. 510 et sqq. 



t Wied. Ann. xiv. p. 34. 



% In two interesting papers (Wien. Ber. xcviii. pp. 1154-91 ; Archives 

 Neerl xxiv. pp. 57-98, 295-368) Prof. Korteweg has derived the 

 geometrical properties of the plaits and plaitpoints. The case occurring 

 here, viz. the formation of two plaitpoints of the same kind at the same 

 time, is discussed on pp. 303-305. 



O 2 



