148 SECTION PHYSICS. 



measured by the excess of the energy in the phase (B), above what it 

 would have been if the magnitudes had increased from zero to the 

 values corresponding to the phase B, while the values of the intensi- 

 ties were those belonging to the phase (A). 



If the phase (B) is in all respects except that of absolute quantity of 

 matter the same as the phase (A), K is zero ; but when the phase (B) 

 differs from the phase (A), a portion of the matter in the phase (A) 

 will tend to pass into the phase (B) if K is negative, but not if it is 

 zero or positive. 



If the given phase (A) of the mass is such that the value of K is 

 positive or zero with respect to every other phase (B), then the phase 

 (A) is absolutely stable, and will not of itself pass into any other 

 phase. 



If, however, K is positive with respect to all phases which differ from 

 the phase (A) only by infinitesimal variations of the magnitudes, while 

 for a certain other phase, B, in which the magnitudes differ by finite 

 quantities from those of the phase (A), K is negative, then the question 

 whether the mass will pass from the phase (A) to the phase (B) will 

 depend on whether it can do so without any transportation of matter 

 through a finite distance, or, in other words, on whether matter in the 

 phase B is or is not in contact with the mass. 



In this case the phase (A) is stable in itself, but is liable to have its 

 stability destroyed by contact with the smallest portion of matter in 

 certain other phases. 



Finally, if K can be made negative by any infinitesimal variations 

 of the magnitudes of the system (A), the mass will be in unstable 

 equilibrium, and will of itself pass into some other phase. 



As no such unstable phase can continue in any finite mass for any 

 finite time, it can never become the subject of experiment ; but it is 

 of great importance in the theory of chemistry to know how these 

 unstable phases are related to those which are relatively or absolutely 

 stable. 



The absolutely stable phases are divided from the relatively stable 

 phases by a series of pairs of coexistent phases, for which the intensi- 

 ties p, t, p, c. are equal and K is zero. Thus water and steam at the 

 same temperature and pressure are coexistent phases. 



As one of the two coexistent phases is made to vary in a continuous 



