SURFACES OF DISCONTINUITY 523 



and a similar argument demonstrates that the mercury must be 

 pushed down in the tube, in order to preserve conditions of 

 hydrostatic equilibrium. 



The writer feels that there exists so much misconception con- 

 cerning the surface tension of Hquids that the preceding elemen- 

 tary account may not be out of place at the outset of a commen- 

 tary on a portion of Gibbs' work which is so vitally concerned 

 with the concept of surface energy, with which the term ' 'surface 

 tension" has come to be practically synonymous. Before 

 proceeding, it may be desirable to take this opportunity to 

 clear up a misconception about another matter which experience 

 shows to occur often in this connection. Outside a spherical 

 mass of liquid the vapor pressure is less than the internal 

 pressure just inside the surface. It is quite easy, as the writer 

 knows from teaching experience, for the unwary student to pick 

 up the notion that the saturated vapor pressure outside a liquid 

 with a convex surface is therefore less than that outside a 

 plane surface; but, of course, the very reverse of this is true. 

 The capillary tube phenomena actually demonstrate this, as 

 well as the complementary fact that the saturated vapor pres- 

 sure above a concave surface is less than that above a plane 

 surface. The chapter on the vapor state in any good text of 

 physics contains the necessary details on this point. Moreover, 

 the matter can be argued out correctly from statistical con- 

 siderations. In any case the equations (7) and (8) show 

 that 



P - p > Po- Po, 



but unless we had some definite prior information concerning 

 the equality or inequality of P and Po we could draw no in- 

 ference from this as to the relation of p to po. Actually, as 

 stated just above, capillary experiments or statistical arguments 

 demonstrate that p > po, and so we can infer from this fact 

 that P > Po also. 



IV. Statistical Considerations 



8. The Finite Size of Molecules 



While the foregoing analysis is very instructive in giving some 

 insight into the true nature of the conditions at the surface of a 



