CORRESPONDENCE OF MOLECULAR STATES 229 



Saturated vapor is in equilibrium with its liquid. 

 Unsaturated vapor is not, for it tends to increase in 

 number of molecules at the expense of the liquid. On 

 the other hand, supersaturated vapor, such as cor- 

 responds to states represented by the curve 6c, is in 

 a decidedly unstable equilibrium. 1 The slightest cause 

 will result in a disproportionate effect. Such a cause 

 may be furnished by inserting a drop of the liquid or 

 by particles of dust and other impurities. 



It has proved impossible as yet to penetrate very 

 far into the region of instability, whether above b or 

 below /. It is usual therefore to follow the suggestion 

 of James Thomson and draw a dotted line like cde to 

 represent for an absolutely pure substance the part of 

 the isothermal which we cannot follow experimentally. 

 Somewhere between 6 and / the substance, which is a 

 gas at b and liquid at /, must change from one to the 

 other. It seems reasonable to assume that such a 

 change would occur continuously in some such way as 

 that represented by cde. Thus the pressure rising 

 above b might ultimately reach a maximum at c. As 

 it decreases it might well fall too low, reaching a mini- 

 mum at e. Let us remember that the process of aggre- 

 gation into a liquid is a contraction of the substance, 

 that is, a motion of the molecules toward each other. 

 Since they have inertia, or mass, such a motion might 

 well result in an excessive contraction corresponding 

 to the state e. 



In the states represented by the line be the vapor is 



1 Similar phenomena of instability of state occur in the case of 

 liquids cooled below the freezing point. For their observation 

 laboratory conditions are usually required. 



