48C J. W. Gihbs — Equilihrhirn of Ileteroijeneous iSubstances. 



film which is truly fluid in its interior is in getieral subject to a con- 

 tinual diminution of tliickness by the internal currents due to gravity 

 and the suction at its edge. Sooner or later, the interior will some- 

 where cease to have the properties of matter in mass. The film will 

 then probably become unstable with respect to a flux of the interior 

 (see page 478), the thinnest parts tending to become still more thin 

 (ajjart from any external cause) very mucli as if there were an 

 attraction between the surfaces of the film, insensible at greater dis- 

 tances, but becoming sensible when the thickness of the film is suffi- 

 ciently reduced. We should expect this to determine the rupture of 

 the film, and such is doubtless the case with most liquids. In a film 

 of soap-water, however, the rupture does not take place, and the 

 processes which go on can be watched. It is apparent even to a very 

 superficial observation that a film of which the tint is approaching 

 the black exhibits a remarkable instability. The continuous change 

 of tint is interrupted by the breaking out and rapid extension of 

 l)lack spots. That in the formation of these bright spots a separa- 

 tion of diflerent substances takes place, and not simply an extension 

 of a part of the film, is shown by the fact that the film is made 

 thicker at the edge of these spots. 



This is very distinctly seen in a plane vertical film, when a single 

 black spot breaks out and spreads rapidly over a considerable area 

 which was before of a neai'ly uniform tint approaching the black. 

 The edge of the black spot as it spreads is marked as it were by a 

 string of bright beads, which unite together on touching, and thus 

 becoming larger, glide down across the bands of color below. Under 

 favorable circumstances, there is often quite a shower of these bright 

 spots. They are evidently small spots very much thicker — appar- 

 ently many times thicker — than the part of the film out of which 

 they are formed. Now if the formation of the black spots were due 

 to a simple extension of the film, it is evident that no such appear- 

 ance would be presented. The thickening of the edge of the film 

 cannot be accovmted for by contraction. For an extension of the 

 upper portion of the film and contraction of the lower and thicker 

 portion, with descent of the intervening portions, would be far less 

 resisted by viscosity, and far more favored by gravity than such 

 extensions and contractions as would produce the appearances 

 described. But the rapid formation of a thin spot by an internal 

 current would cause an accumulation at the edge of the spot of the 

 material forming the interior of the film, and necessitate a thickening 

 of the film in that place. 



