566 RICE ART. L 



in the gas-liquid experiments, is negligible compared to 7/, so 

 that r2 exceeds r2(i) by (72' — 72")ri/7i'. Until we know some- 

 thing about Ti we cannot say whether this is going to improve 

 matters or not. We shall have occasion in the following 

 section to return to this point, which we leave for the present. 



IX. Gibbs' Equation and the Structure of Adsorbed Films 



24- Impermeable or Insoluble Films 



On pages 275, 276 Gibbs makes a very brief allusion to 

 "impermeable films" which may offer an obstacle to the passage 

 of some of the components from one phase to the other. "Such 

 may be the case, for example, when a film of oil is spread on a 

 surface of water, even when the film is too thin to exhibit the 

 properties of the oil in mass." The latter part of this sentence 

 is most significant in view of subsequent events. Gibbs con- 

 tents himself with pointing out that for any component which is 

 found on both sides of the film, but which cannot pass the film 

 itself, the potentials on either side cannot be proved to be equal, 

 and so in the adsorption equation, for example, a single term 

 such as —Tidni must be replaced by —Tidni — V^dn^, where 

 Fi and r2 refer to the surface excesses of the same component on 

 the two sides of the dividing surface and mi and /i2 indicate the 

 differing potentials in each adjacent phase. 



Soon after the existence of "surface tension" became known, 

 it was discovered that oil films on water reduced this property 

 very markedly. This is of course quite a different phenomenon 

 from the lowering by capillary-active soluble substances. It 

 was Rayleigh who began accurate experimental work on the 

 thickness of such oil films {Proc. Roy. Soc., 47, 364, (1890)). 

 Some very important results were discovered by Miss Pockels 

 who was the first to use the method of "barriers," which by rest- 

 ing just on the surface of a liquid in a trough and extending 

 over its whole width could be used to push a surface film in front 

 of them so that it could be compressed or extended in two 

 dimensions (Nature, 43, 437, (1891)). •She made the dis- 

 covery that provided the area of a film formed by a small given 

 quantity of oil exceeded a certain critical value the surface 



