308 



LIQUIDS AND GASES. 



briefly describe, iu mapping the actual position of the nurealizable por- 

 tions of the curve. They have the form pictured in this figure. The rise 

 trom the gaseous state is a gradual one, but the fall from the liquid state 

 is abrupt. 



36000 



Fig. 2. 



Consider the volume 14 cubic centimeters per gram on the figure. 

 The equi-volume vertical line cuts tlie isothennal lines for the temper- 

 atures 175°, 180°, 185°, 190°, and so on, at certain definite pressures, 

 which may be read from a properly-constructed diagram. We can map 

 the course of lines of equal volume, of which the instance given is one, 

 using temperatures as ordinates and pressures as absciss*. We can 

 thus find the relations of temperature to pressure for certain definite 

 volumes, which we may select to suit our convenience— say 2 c.c. per 

 gram; 3, 4, 5, 6, and so on, Now, all such lines are straight — that is, 

 the relation of pressure to temperature, at constant volume, is one of the 



