Maximum or Minimum Vapour-Pressure. 127 



than the maximum mixture (45 per cent.)* and solidify at 

 points between M and T ; the mixtures of the second group 

 contain more than 45 per cent, of ethane, and solidify at 

 points beyond M. It will be seen from Table VII. that the 

 50 per cent, mixture belongs to the second group, as it has 

 not solidified at — 65 0, 17 C: its pressure is at this tempera- 

 ture a little below the three-phase pressure, although the 

 scale of the diagram is too small to show this. The 30 per 

 cent, mixture, on the other hand, belongs to the first group, 

 and the maximum mixture must lie between the two as we 

 we have seen it actually does. The difference between 

 the pressures on the three-phase curve obtained with the two 

 mixtures at the lowest temperature, the boiling-point of solid 

 carbon dioxide, is due to the considerable slowness with which 

 the proper equilibrium sets in; the stirrer which we had 

 inside the tube having become quite immovable in the solid. 



In figs. 3 and 4 (p. 128) we give on a larger scale diagrams of 

 the relative positions of the three-phase curve and the con- 

 densation curves for mixtures belonging to the first and 

 second groups respectively. These diagrams can only be 

 properly understood by simultaneous consultation of figs. 5 

 and 6, which are intended to give an idea of the nature of 

 the volume-composition diagram at two different temperatures, 

 fig. 5 above and fig. 6 below the temperature corresponding 

 to the point M in fig. 2. 



Figs. 5 and 6 contain the projection of the vapour-liquid 

 plait with the maximum M. A part of this plait is cut off 

 (i. e. is made metastable) by the solidification figure. This 

 figure consists of (1) the vapour-solid plait which reduces to 

 a fan-shaped projection owing to the very small change of 

 volume of the solid ; (2) the three-phase triangle SLV ; 

 and (3) the solid-liquid plait, beyond SL, the properties of 

 which are unknown. The three-phase triangle leaves the 

 maximum free in fig. 5, and covers it in fig. 6, so that in 

 this latter case the maximum belongs to the unrealizable 

 (metastable) part. The metastable curves, as far as shown, 

 are dotted. 



It is now easily seen what condensation phenomena may 

 be expected at the various temperatures with the various 

 mixtures. As an instance, we point out that the mixture x 

 in fig. 5 will, on compression, first begin to solidify at a, then 

 liquid will appear at b : there are now three phases, and the 

 pressure becomes constant. On further diminution of volume 

 the solid will diminish and disappear at c, and ultimately 

 the vapour will all go at d y and the mixture will be completely 

 liquid. The corresponding points a, b, c, and d are also shown 



