274 SCIENCE PROGRESS. 



occurs with nitrous oxide and ethane for a mixture con- 

 taining about 1/5 N.O and 4/5 ethane, as was already 

 stated above. The fact was also mentioned that most of 

 the mixtures of those two gases have critical temperatures 

 below that of ethane. This is entirely explained by a 

 splitting up of the plait before the critical temperature of 

 ethane (32° C.) is attained, at about 26° C. Each of the 

 halves of the plaits has its plait-point. In one of the halves 

 the plait-point is situated as in fig. 3, i.e., nearer the x axis 

 than C. But in the other half, opposite the first, P lies 

 farther away from the x axis than the critical point C. 

 The critical phenomena which one may expect in that case 

 are different from those described in connection with fig. 3. 

 There are again the two temperatures /p and /(., but between 

 them compression will give rise to the appearance of a 

 second phase lighter than the mixture, therefore at the top 

 of the tube. This vapour phase will first increase and 

 subsequently decrease during compression. This new kind 

 of condensation may be called " retrograde condensation of 

 the second kind". Unfortunately, as was already stated, 

 the temperatures /p and 4 were so close that the critical 

 phenomena were the same as if the mixtures had been 

 single substances. 



An important case which has not been considered is that 

 in which the liquid phase itself is split up into two liquids, 

 as in mixtures of water and ether, or water and carbonic 

 acid, in other words the co-existence of two liquid phases 

 and even of three phases, two liquid and one vapour. In 

 cases of that kind the xp v x surface has a second plait with 

 properties similar to those of the vapour-liquid plait. If 

 the connodal curves of the two plaits intersect, the tangent 

 plane has three points of contact and therefore three phases 

 will be in equilibrium. At each temperature there is only 

 one pressure at which this triple coexistence is possible, 

 and in the p-t diagram these corresponding values of / and 

 / are represented by a curve. Besides this curve and the 

 two vapour-pressure curves there are the border curves for 

 the mixtures, which in this case have not the simple loop- 

 shape (fig. 4), but form a kind of double loop owing to the 



