CORRESPONDENCE OF MOLECULAR STATES 227 



orated the mercury column will rise as the tube is 

 raised, indicating a reduction in the pressure of the 

 vapor with increased volume. The volume and pres- 

 sure will then be related practically as for a perfect gas. 



Let us now make a plot to show these relations of P 

 and V for a substance which is maintained at a tem- 

 perature below its critical temperature, so that the 

 liquid and its vapor may coexist in equilibrium. Let 

 us start with the substance entirely in the form of 

 vapor. Let this condition be represented by the 

 point a of Fig. 27. If x 



the pressure is increased * 

 the volume decreases 

 practically inversely, 

 and successive states are 

 represented by points 

 along the hyperbolic 

 curve ab. At 6 the pres- 

 sure is such that there Vl Va v 

 is some condensation. 



This pressure is the vapor pressure of boiling for the 

 temperature of this isothermal. As we have just seen, 

 this pressure cannot be increased as long as the sub- 

 stance is maintained at the same temperature. An 

 attempt to increase the externally applied pressure 

 merely results in a decrease in volume. The successive 

 states of the substance are now represented by the 

 solid line bf. When the vapor is all condensed the 

 volume (represented at /) is merely that of the liquid. 

 Further increase in the pressure compresses the liquid, 

 as represented by the line fg. 



These changes may be carried out in the reverse 



