J. TF. Gibhs — Equilihrmm of Heterogeneous Sxihstances. 439 



sion [see eq. (508)], it will be evident that in this case a consider- 

 able time will be necessary after the formation of a fresh surface or 

 the extension of an old one for the reestablishraent of the normal 

 value of the superficial tension. In intermediate cases, the reestab- 

 lishment of the normal tension will take place with different degrees 

 of rapidity. 



But whatever the number of component substances, provided that 

 it is greater than one, and whether the reestablishment of equilibrium 

 is slow or rapid, extension of the surface will generally produce 

 increase and contraction decrease of the tension. It would evidently 

 be inconsistent with stability that the opposite effects should be pro- 

 duced. In general, therefore, a fresh surface between coexistent 

 phases has a greater tension than an old one.* By the use of fresh 

 surfaces, in experiments in caj^illarity, we may sometimes avoid the 

 effect of minute quantities of foreign substances, which may be 

 present without our knowledge or desire, in the fluids which meet at 

 the surface investigated. 



AVhen the establishment of equilibrium is rapid, the variation of 

 the tension from its normal value will be manifested especially during 

 the. extension or contraction of the surface, the phenomenon resem- 

 bling that of viscosity, except that the variations of tension arising 

 from variations in the densities at and about the surface Avill be the 

 same in all directions, while the variations of tension due to any 

 property of the surface really analogous to viscosity would be great- 

 est in the direction of the most rapid extension. 



We may here notice the diflerent action of traces in the homogene- 

 ous masses of those substances which increase the tension and of 

 those which diminish it. When the volume-densities of a component 

 are very small, its surface-density may have a considerable positive 

 value, but can only have a very minute negative one.f For the 

 vq-j^e when negative cannot exceed (niimerically) the product of the 

 greater volume-density by the thickness of the non-homogeneous 



* When, however, homogeneous masses which have not coexistent phases are 

 brought into contact, the superficial tension may increase with the course of time. 

 The superficial tension of a drop of alcohol and water placed in a large room will 

 increase as the potential for alcohol is equalized throughout the room, and is dimin- 

 ished in the vicinity of the surface of discontinuity. 



f It is here supposed that we have chosen for components such substances as are 

 incapable of resolution into other comjionents which are independently variable in the 

 homogeneous masses. In a mixture of alcohol and water, for example, the compo- 

 nents must be pure alcohol and pure water. 



Trans. Conn. Acad., Vol. III. 56 Jan., 1878. 



