Classification of Foaming Solutions. 719 



process of formation of the surface layer. The other is the 

 manner in which the surface layer contributes to the 

 durability of the liquid film. It is upon differences with 

 respect to these two points, that a classification of foaming 

 solutions may be based. 



The durability of thin films may be due to two causes : — 



(1) the power of local variation of the surface tension, 

 unaccompanied by any special superficial viscosity or rigidity, 



(2) extreme viscosity or rigidity of the surface layers. 

 Mixtures of acetic acid and water and soap solutions owe 

 their foaming power to the first cause ; aqueous solutions of 

 albumin, saponin and peptone, to the second cause. Schutt* 

 has shown that the surface layers of a soap solution are 

 devoid of any special viscous resistance to shear. I have 

 shown f that even in the case of a fresh surface of a solution 

 of saponin, there is a definite elastic resistance to shear, 

 though the process of elastic relaxation makes the surface 

 appear merely viscous when a steady shearing process is 

 applied. Towards a transient stress such as would be caused 

 Iby a slight disturbance, the surface layer would behave like 

 an elastic substance. 



We will next consider how the actual process of formation 

 •of the surface layer differs in different solutions. In a case 

 such as that of a mixture of acetic acid and water, equilibrium 

 between the surface layers and the body of the solution is 

 .attained rapidly (except in the case of very dilute solutions), so 

 that the process of extension of the surface may be conducted 

 in a thermodynamically reversible mannerj. In the case 

 of solutions of soap and saponin, the process seems to 

 be of a different nature. Milner§, from observations on 

 solutions of sodium oleate, concludes that the process is 

 .an irreversible one. In the case of solutions of saponin, 

 peptone and albumin, I have shown || that the process of 

 formation of the surface layer proceeds for several weeks 

 without any sign of equilibrium. The substance forming 

 the surface layer separates from the body of the solution at 

 a rate practically independent of the thickness of the surface 

 layer already formed. The process is thus thermodyna- 

 mically irreversible. In the case of solutions of albumin the 

 process seems to be also chemically irreversible, the film 

 substance being insoluble in water If. 



* Ann. der Phys. xiii. p. 712 (1904). 

 t Phil. Mag. x'i. p. 317 (1900). 



X See Donnan and Barker, Roy. Soc. Proc. A. lxxxv. p. 557 (1911). 

 § Phil. Mag-, xiii. p. 90 (1907)" || Phil. Mag. xvii. p. 500 (190W). 

 *f[ See Ranisden, Roy. Soc. Proc. lxxii. p. 150 (1903). 



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