228 THE REALITIES OF MODERN SCIENCE 



order, by starting with the liquid in the state cor- 

 responding to g, and following the isothermal in the 

 order gfba by gradually reducing the pressure. In- 

 cidentally, it may be pointed out that in this reversible 

 cyclic process such work as we did in compressing the 

 substance is returned to us as it expands. Such 

 energy as was imparted to the surroundings during 

 compression is also returned during the expansion. 



It does not always happen that the change follows 

 exactly this isothermal, abfg. For example, it is pos- 

 sible with a pure liquid, free from dissolved air, in a 

 vessel with smooth walls, to obtain the following 

 phenomenon. Starting from the state indicated by 

 g the pressure is reduced. When / is reached where 

 boiling should occur, it may not occur. The pressure 

 may be reduced well below this value without ebullition 

 occurring. The V-P plot then follows the partially 

 dotted line gfe. Suddenly, however, ebullition occurs. 



A somewhat similar phenomenon may occur if we 

 start with the substance in the gaseous condition, as 

 represented at a. As the pressure is increased the 

 volume is decreased. This decrease may extend along 

 the dotted line abc well beyond the point b. The mole- 

 cules are thus compressed without aggregating into 

 drops of liquid. Suddenly condensation takes place 

 and the pressure drops to the boiling point pressure 

 corresponding to the temperature of the isothermal. 

 The state corresponding to b is called that of saturated 

 vapor. From a to b the vapor is below saturation, 

 that is, the molecular density of the vapor is less than 

 that of a vapor at the same temperature which is in 

 contact with its liquid. 



