J. W. Gibbs — KquUtbrimn of Heterogeneous Substances. 4*79 



atmospliere, the effect of evaporation or condensation is not to be 

 neglected. In the first ease, the diniinntion of the thickness of the 

 film will be accelerated, in the second, retarded. In the case of the 

 film containing glycerine, it should be observed that the water con- 

 densed cannot in all respects replace the flaid carried down by the 

 internal current but that the two processes together will tend to 

 wash out the glycerine from the film. 



But Avhen a component which greatly diminishes the tension of the 

 film, although forming but a small fraction of its mass, (therefore 

 existing in excess at the surface,) is volatile, the effect of evaporation 

 and condensation may be considerable, even when the mean value of 

 the potential for that component is the same in the film as in the sur- 

 rounding atmosphere. To illustrate this, let us take the simple case 

 of two components *Sj and N^, as before. (See page 469.) It appears 

 from equation (508) that the potentials must vary in the film ^\\t\\ 

 the height z, since the tension does, and from (98) that these varia- 

 tions must (very nearly) satisfy the relation 



y^ and y2 denoting the densities of 8^ and iS.^ in the interior of the 

 film. The variation of the potential of 82 as we pass from one level 

 to another is therefore as much more rapid than that of aS'j, as its 

 density in the interior of the film is less. If then the resistances 

 restraining the e^'aporat^on, transmission thi'ough the atmosphere, 

 and condensation of the two substances are the same, these processes 

 will go on much more rapidly Avitli respect to 82- It will be 



observed that the values of — -^ and -~^ will have opposite signs, 



the tendency of *Sj being to pass down through the atmosphere, and 

 that of aS'o to pass up. Moreover, it may easily be shown that the 

 evaporation or condensation of /S'2 will produce a very much greater 

 effect than the evaporation or condensation of the same quantity of 

 Sy. These efi'ects are really of the same kind. For if condensation 

 of S„ takes place at the top of the film, it will cause a diminution of 

 tension, and thus occasion an extension of this part of the film, by 

 which its thickness y\W\ be reduced, as it would be by evaporation of 

 <S^j. We may infer that it is a general condition of the persistence of 

 liquid films, that the substance which causes the diminution of tension 

 in the upper parts of the film must not be volatile. 



But apart from any action of the atmosphere, we have seen that a 

 Trans. Conn. Acad., Vol. III. 61 April, 1878. 



