266 SCIENCE PROGRESS. 



There is no sharp distinction between one and the other 

 any more than for pure substances. 



An important question which presents itself is to find co- 

 existing phases. In other words, what is the density (density 

 is the reciprocal of the volume of a given quantity of 

 mixture) and what is the composition of a liquid mixture in 

 equilibrium with a vapour mixture .■* The liquid which 

 during compression is formed when the condensation begins 

 (at d fig. i) has as a rule a different composition (as well as 

 density) from the gas mixture. For our mixture of 5/6 

 CO2 and 16 air the liquid will be much richer in CO, and 

 only contain a small admixture of air. During the process 

 of condensation the composition of both phases changes, as 

 a rule in the same direction. For our mixture both the 

 liquid and the vapour become poorer in carbonic acid, richer 

 in air (this is not a misprint!). In the end when the whole of 

 the mixture is changed into liquid its composition is again 

 5/6, equal to the original composition. Our diagram (fig. 

 i) only gives us the density of the mixture at d where it is 

 in equilibrium with a denser phase, a liquid, and e where the 

 same mixture, but now itself in a denser state, is in equili- 

 brium with a lighter phase, a vapour. But about the second 

 phase, the liquid one which appears at d on compression 

 and the vapour phase which disappears at c on compression 

 or reappears on increasing the volume, the diagram does not 

 say anything. We may find those phases however by com- 

 biningr the diaoram with those for other mixtures of the 

 same two substances. It is a result of thermodynamics, 

 confirmed by experiment, that at gixen temperature and 

 pressure there is only one equilibrum of two phases for a 

 mixture of two given substances. The relative quantities 

 of the two phases may be anything, but provided / and/ 

 do not change, the density and composition are always the 

 same. Suppose therefore we take the isothermals for the 

 same temperature (say io°C.) for a number of mixtures of 

 CO2 and air and select that mixture x^ for which the pressure 

 at the point c, i.e. in the liquid state, is equal to the pressure 

 at b for our mixture. According to the rule given above jf ' 

 will be the mixture which co -exists at io°C. with the 



