438 ./ W. Gibhs — Eqiiilibrmm of Heterogeneous Substances. 



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 be- 

 tween 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 ex- 

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

 the tension of any surface increases witli the temperature, the effect 

 of extending the surface will be to raise its temperature. In either 

 case, it will be observed, the immediate effect of extending the sur- 

 face is to increase its tension. A contraction of the surface will of 

 course have the opposite effect. But the time necessary for the re- 

 establishment of sensible thermal equilibrium after extension or con- 

 traction of the surface must in most cases be very short. 



In regard to the formation or extension of a surface between two 

 coexistent phases of more than two components, there are two ex- 

 treme cases which it is desirable to notice. When the superficial 

 density of each of the components is exceeding small compared with 

 its density in either of the homogeneous masses, the matter (as well 

 as the heat) necessary for the formation or extension of the normal 

 surface can be taken from the immediate vicinity of the surface with- 

 out sensibly changing the properties of the masses from which it is 

 taken. But if any one of these superficial densities has a considerable 

 value, while the density of the same component is very small in each 

 of the homogeneous masses, both absolutely and relatively to the 

 densities of the other components, the matter necessary for the for- 

 mation or extension of the normal surface must come from a consider- 

 able distance. Especially if we consider that a small difierence of 

 density of such a component in one of the homogeneous masses will 

 probably make a considerable difference in the value of the corres- 

 ponding potential [see eq. (217)], and that a small difference in the 

 value of the potential will make a considerable difference in the ten- 



