SURFACES OF DISCONTINUITY 527 



ground, so this field in the vapor will tend to retain molecules 

 in this layer in greater number than exist in an equal volume 

 elsewhere in the vapor; so that at the surface there is an excess 

 concentration in the vapor phase. Furthermore this "ad- 

 sorption" is accompanied by a decrease of the surface energy; 

 for the reader will recall the fact that any concentration of mole- 

 cules near the surface of the liquid tends to reduce the total 

 potential energy, since the nearer one molecule is to another, 

 outside the distance where repulsion begins, the smaller their 

 mutual potential energy. Again there is an analogy with the 

 mechanical conditions in the atmosphere, since any aggregation 

 of molecules of air in the lower levels produces a diminution of 

 potential energy as compared with a state of affairs in which 

 the molecules are more uniformly distributed in the atmosphere. 

 Indeed, when one is endeavoring to interpret thermodynamic 

 phenomena in terms of mechanical laws, we may expect to find 

 that any occurrence in which free energy tends to decrease is 

 to be explained by the mechanical fact that, in the passage of 

 an isolated dynamical system to a state of equilibrium, poten- 

 tial energy always tends to a minimum. 



V. The Dividing Surface 



10. Criterion for Locating the Surface of Tension 



We now return to the text of the treatise and consider one of 

 the most troublesome features of the earlier pages of this 

 section, viz., the location of the abstract dividing surface which 

 in the course of the reasoning replaces the non-homogeneous 

 film or region of discontinuity. The argument of Gibbs (I, 225- 

 228) leads to a criterion based on theoretical grounds for locat- 

 ing this surface in a precise fashion; yet, as will appear, it is 

 one which gives way in practice to other methods of placing the 

 surface more suitable for comparing the deductions from the 

 adsorption equation [508] with the results of experiments. 

 Nevertheless, as there are one or two points in the argument 

 which may require elucidation, we shall devote some considera- 

 tion to it. Fig. 3 will help to illustrate Gibbs' reasoning. He 

 chooses first an arbitrary position for the dividing surface which 



