J. W. Gihbs — J^quillhrmni of Heterogeneous Substances. 477 



pipe. If the bubble is now laid upon a suitable support, the condi- 

 tion (CI 3) will be realized almost instantly. The bubble will then 

 tend toward conformity with condition (614), the lighter portions ris- 

 ing to the top, more or less slowly, according to the viscosity of the 

 tilm. The resulting ditterence of thickness between the upper and 

 the lower parts of the bubble is due partly to the greater tension to 

 which the upper parts are subject, and partly to a diifereuce in the 

 matter of which they are composed. When the film has only two 

 components of which the potentials are not determined by the con- 

 tiguous atmosphere, the laws which govern the arrangement of the 

 elements of the film may be very simply expressed. If we call these 

 components S^ and /S^, the latter denoting (as on page 469) that 

 which exists in excess at the surface, one element of the film will 

 tend toward the same level with another, or a higher, or a lower 

 level, according as the quantity of S2 bears the same ratio to the 

 quantity of S^ in the first element as in the second, or a greater, or a 

 less ratio. 



When a film, however formed, satisfies both the conditions (618) 

 and (614), its thickness being sufiicient for its interior to have the 

 properties of matter in mass, the interior will still be subject to the 

 slow current which we have already described, if it is truly fluid, how- 

 ever great its viscosity may be. It seems probable, however, that 

 this process is often totally arrested by a certain gelatinous consist- 

 ency of the mass in question, in virtue of which, although pi'actically 

 fluid in its behavior with reference to ordinary stresses, it may have 

 the properties of a solid with respect to such very small stresses as 

 those which are caused by gravity in the interior of a very thin film 

 which satisfies the conditions (613) and (614). 



However this may be, there is another cause which is often more 

 potent in producing changes in a film, when the conditions just men- 

 tioned are approximately satisfied, than the action of gravity on its 

 interior. This will be seen if we turn our attention to the edge 

 where the film is terminated. At such an edge we generally find a 

 liquid mass, continuous in phase with the interior of the film, which 

 is bounded by concave surfaces, and in which the pressure is therefore 

 less than in the interior of the film. This liquid mass therefore 

 exerts a strong suction upon the interior of the film, by which its 

 thickness is rapidly reduced. This eftect is best seen when a film 

 which has been formed in a ring is held in a vertical position. Unless 

 the film is very viscous, its diminished thickness near the edge causes 

 a rapid upward cui-rent on each side, Avhile the central portion slowly 



