74 PROTOPLASMIC ACTION AND NERVOUS ACTION 



properties of an emulsion system. Emulsions of oil in 

 alkaline water or soap solution are destroyed by adding 

 strong acid (HCl) which breaks down the soap films. 

 Similarly a foam structure may be destroyed mechani- 

 cally or by adding a surface-active substance of low vis- 

 cosity; thus a few drops of ether destroy a beer-foam, 

 a fact explained by Quincke as due to the displacement 

 of the material composing the surface lamellae.^ Similarly 

 a saponin solution to which sufficient alcohol is added 

 does not form a permanent foam; the addition of iso- 

 butyric acid to a saponin solution also prevents foaming, 

 but if alkali is added to neutralize the acid and form the 

 surface-inactive salt, foaming results.^ Many other 

 facts of a similar kind are well known. The conditions 

 of de-emulsification ('' cracking" of emulsions) deserve 

 careful study by biologists, for changes of this kind are 

 almost certainly concerned in many forms of protoplasmic 

 activity; e.g., secretion and the processes of activation, 

 stimulation, and cytolysis. 



In general, therefore, we may define the chief condi- 

 tion of stability in emulsion systems as the presence of 

 material, differing from that composing the two chief 

 phases, in the form of thin continuous layers or films 

 deposited or adsorbed at the boundary surfaces. The 

 thickness of these films may be extremely slight; when 

 a material is surface-active and is free to spread over 

 the surface separating the phases, conditions of equi- 

 librium may not be reached until the layer is only one 

 or two molecules thick. ^ Such a film, however, is 



' Quincke, Ann. Physik, XXXV (1888), 580. 



" Zawidski, Z. physik. Chem., XXXV (1900), 77. 



3 Cf. Langmuir, Journal of the American Chemical Society, XXXIX 

 (1917), 1848; cf. also Freundlich's Kapillarchemie, p. 278. 



