668 



RICE 



ART. L 



the liquid pressure in the films between A and B, etc. is practi- 

 cally equal to that in the gas. This state of affairs causes the 

 "suction" referred to by Gibbs on page 309, and the liquid is 

 forced by this excess of pressure from the films into the channels, 

 thus assisting other influences such as gravity in the draining of 

 the films. When a film of soap solution is drawn up from a 

 mass of such solution at the mouth of a cup, we have a ring 

 shaped channel of this kind where the film meets the horizontal 



surface of the general mass and into this "Gibbs ring" there is a 

 considerable draining of the film by this suction and gravity. 



59. The Black Stage of a Soap Film 



In general a newly formed soap film passes through a regular 

 succession of changes. Recently, much more light has been 

 thrown on the nature of the succession by improvement in the 

 methods for preventing mechanical shock, sudden large changes 

 of temperature and, more especially, contamination of the solu- 

 tion. In this way it has been shown that the fundamental 

 change is the thinning down to the black stage, so that the black 

 stage is the only film in true equilibrium. It is true that it can 

 hardly be called a stable equilibrium in the accepted sense of 

 stability since the black stage is extremely susceptible to me- 



