312 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



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 black spots. 

 That in the formation of these black spots a separation of different 

 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 nearly 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 apparently 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 ex- 

 tension of the film, it is evident that no such appearance would 

 be presented. The thickening of the edge of the film cannot be 

 accounted 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. 



That which is most difficult to account for in the formation of 

 the black spots is the arrest of the process by which the film grows 

 thinner. It seems most natural to account for this, if possible, by 

 passive resistance to motion due to a very viscous or gelatinous 

 condition of the film. For it does not seem likely that the film, 

 after becoming unstable by the flux of matter from its interior, would 

 become stable (without the support of such resistance) by a continu- 

 ance of the same process. On the other hand, gelatinous properties 

 are very marked in soap-water which contains somewhat more soap 

 than is best for the formation of films, and it is entirely natural 

 that, even when such properties are wanting in the interior of a 

 mass or thick film of a liquid, they may still exist in the immediate 

 vicinity of the surface (where we know that the soap or some of 

 its components exists in excess), or throughout a film which is so 

 thin that the interior has ceased to have the properties of matter 



