MOLECULAR CONSTITUTION OF BODIES. 425 



within a certain vessel. Hence, if it is possible for the medium to arrange 

 itself so that part has one density and part another, we cannot prevent it from 

 doing so. 



Now the points B and F represent two states of the medium in which 

 the pressure is the same but the density very different. The whole of the 

 medium may pass from the state B to the state F, not through the inter- 

 mediate states CDE, but by small successive portions passing directly from the 

 state B to the state F. In this way the successive states of the medium as 

 a whole will be represented by points on the straight line BF, the point B 

 representing it when entirely in the rarefied state, and F representing it when 

 entirely condensed. This is what takes place when a gas or vapour is liquefied. 



Under ordinary circumstances, therefore, the relation between pressure and 

 volume at constant temperature is represented by the broken line ABFG. If, 

 however, the medium when liquefied is carefully kept from contact with vapour, 

 it may be preserved in the liquid condition and brought into states represented 

 by the portion of the curve between F and E. It is also possible that methods 

 may be devised whereby the vapour may be prevented from condensing, and 

 brought into states represented by points in BC. 



The portion of the hypothetical curve from C to E represents states which 

 are essentially unstable, and which cannot therefore be realised. 



Now let us suppose the medium to pass from B to F along the hypo- 

 thetical curve BCDEF in a state always homogeneous, and to return along 

 the straight line FB in the form of a mixture of liquid and vapour. Since 

 the temperature has been constant throughout, no heat can have been trans- 

 formed into work. Now the heat transformed into work is represented by the 

 excess of the area FDE over BCD. Hence the condition which determines 

 the maximum pressure of the vapour at given temperature is that the line 

 BF cuts off equal areas from the curve above and below. 



The higher the temperature, the greater the part of the pressure which 

 depends on motion, as compared with that which depends on forces between 

 the particles. Hence, as the temperature rises, the dip in the curve becomes 

 less marked, and at a certain temperature the curve, instead of dipping, merely 

 becomes horizontal at a certain point, and then slopes upward as before. This 

 point is called the critical point. It has been determined for carbonic acid 

 by the masterly researches of Andrews. It corresponds to a definite temperature, 

 pressure and density. 



VOL. II. 54 



