CORRESPONDENCE OF MOLECULAR STATES 231 



FIG. 27. 



Returning to the hypothetical isothermal of Fig. 

 27, we notice that for any pressure higher than that 

 corresponding to the state e and lower than that of c 

 there are three possible volumes, e.g. those marked 

 Vi, 02, and v 3 in the figure. In this figure are also plotted 

 isothermals for higher temperatures. Consider for 

 example that lettered 

 a'Vc'd'e'f'g'. At this 

 higher temperature the 

 molecules occupying a 

 volume corresponding 

 to a are at the higher 

 pressure corresponding 

 to a'. Because the tem- 

 perature is higher, that 

 is because of greater k.e., 

 the external pressure which must supplement the 

 attractions due to molecular potential energy in order 

 to cause condensation is greater. This is merely another 

 statement of the fact that the boiling point increases 

 with pressure. The volume of the liquid is now greater 

 than at the lower temperature because liquids expand 

 if the molecular k.e. is increased. 



The distance b'f is therefore less than bf, and the 

 dotted portions of the isothermal should be less pro- 

 nounced. The range of pressures for which three 

 volumes are possible is also reduced. The maximum 

 volume occupied by the liquid, corresponding to /, 

 increases as the temperature rises, while the minimum 

 volume which can be occupied by the molecules in a 

 stable gas condition, corresponding to 6, decreases 

 with the increase in temperature. (For some tempera- 



