CONDENSATION AND CRITICAL PHENOMENA. 267 



mixture 5/6. The density is found at the same time. The 

 same is true in Q^eneral. We find sets of two mixtures 

 which at the same temperature have the same saturation 

 pressure, the one when hquid, the other when in a state of 

 vapour. We may find in that way a number of pairs of 

 phases which may co-exist, say at io°C. and at different 

 pressures. 



A better insight is obtained by representino- graphically 

 the composition and density or volume of these co-existing 

 phases. Fig. 3 gives the diagram thus obtained. The 



Juj^ 3 



only values of x which are possible are those between 

 o and I and the diaoram is therefore enclosed between two 

 parallel v axes and the x axis. The diagram is supposed 

 to be drawn for lo'C. A curve is drawn through the co- 

 existing phases and the corresponding pairs are joined by 

 straight lines. To each of them belongs a diflferent pressure. 

 The pressure is lowest near x = i where in fact it aj)- 

 proaches the saturation pressure of pure carbonic acid. It 

 gets higher and higher towards the left hand side. The 

 corresponding phases approach each other and finally co- 

 incide at P. 



We may read the behaviour of any mixture of CO. and 

 air at 10 C. from fig. 3 as far as condensation is concerned. 

 A mixture for which x = 5/6 when compressed, i.e., the 

 total volume diminishing, shows liquid at d. The density 

 and composition of this liquid is given by point c ; on being 

 further compressed the two phases change. The straight 

 line must be imagined to move towards the left and it may 

 be proved that the relative quantities of vapour and liquid 



