EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 273 



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

 either case, it will be observed, the immediate effect of extending the 

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

 of course have the opposite effect. But the time necessary for the 

 reestablishment of sensible thermal equilibrium after extension or 

 contraction 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 

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

 density of each of the components is exceedingly 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 

 without sensibly changing the properties of the masses from which it 

 is taken. But if any one of these superficial densities has a consider- 

 able 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 

 formation or extension of the normal surface must come from a 

 considerable distance. Especially if we consider that a small 

 difference of density of such a component in one of the homogeneous 

 masses will probably make a considerable difference in the value of 

 the corresponding potential {see eq. (217)}, and that a small difference 

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

 the tension (see eq. (508)}, it will be evident that in this case a 

 considerable time will be necessary after the formation of a fresh 

 surface or the extension of an old one for the reestablishment of 

 the normal value of the superficial tension. In intermediate cases, 

 the reestablishment 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 

 produced. 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 capillarity, we may sometimes avoid the 

 effect of minute quantities of foreign substances, which may be 



* When, however, homogeneous masses which have net 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 diminished in the 

 vicinity of the surface of discontinuity. 



G. I. S 



