272 EQUILIBEIUM OF HETEROGENEOUS SUBSTANCES. 



discontinuity is accompanied by the vaporization or condensation 

 of a very large mass, according as the liquid or the vapor is the richer 

 in that component which is necessary for the formation of the surface 

 of discontinuity. 



If, instead of considering the amount of heat necessary to keep the 

 phases from altering while the surface of discontinuity is extended, 

 we consider the variation of temperature caused by the extension of 

 the surface while the pressure remains constant, it appears that this 

 variation of temperature changes sign with y\ y^y^y^* but 

 vanishes with this quantity, i.e., vanishes when the composition of the 

 phases becomes the same. This may be inferred from the statements 

 on page 99, or from a consideration of the figure on page 125. When 

 the composition of the homogeneous masses is initially absolutely 

 identical, the effect on the temperature of a finite extension or 

 contraction of the surface of discontinuity will be the same, either 

 of the two will lower or raise the temperature according as the 

 temperature is a maximum or minimum for constant pressure. 



The effect of the extension of a surface of discontinuity which is 

 most easily verified by experiment is the effect upon the tension 

 before complete equilibrium has been reestablished throughout the 

 adjacent masses. A fresh surface between coexistent phases may be 

 regarded in this connection as an extreme case of a recently extended 

 surface. When sufficient time has elapsed after the extension of a 

 surface originally in equilibrium between coexistent phases, the 

 superficial tension will evidently have sensibly its original value, 

 unless there are substances at the surface which are either not found 

 at all in the adjacent masses, or are found only in quantities com- 

 parable to those in which they exist at the surface. But a surface 

 newly formed or extended may have a very different tension. 



This will not be the case, however, when there is only a single 

 component substance, since all the processes necessary for equilibrium 

 are confined to a film of insensible thickness, and will require no 

 appreciable time for their completion. 



When there are two components, neither of which is confined 

 to the surface of discontinuity, the reestablishment of equilibrium 

 after the extension of the surface does not necessitate any processes 

 reaching into the interior of the masses except the transmission of 

 heat between the surface of discontinuity and the interior of the 

 masses. It appears from equation (593) that if the tension of the 

 surface diminishes with a rise of temperature, heat must be supplied 

 to the surface to maintain the temperature uniform when the surface 

 is extended, i.e., the effect of extending the surface is to cool it ; but 

 if the tension of any surface increases with the temperature, the 



