72 PROTOPLASMIC ACTION AND NERVOUS ACTION 



to rupture the films or lamellae are then small; i.e., 

 the natural tendency of the film material to minimize 

 its surface, or ^'draw together," is slight. But this 

 condition is not alone sufficient, as seen in the fact that 

 pure liquids of low surface-tension against air, like ether, 

 benzol, alcohol, etc., do not give permanent foams, 

 any more than does water. It is well known that 

 mixtures of alcohol and water foam more readily than 

 either liquid alone ; and this is especially true of mixtures 

 of water and a second liquid of great surface-activity 

 and high viscosity, such as amyl alcohol; hence the 

 important generalization that pure liquids do not foam — 

 do not form permanent disperse systems with air. Nor 

 do mixtures of two pure, mutually immiscible liquids 

 readily form permanent emulsions. Typically the pres- 

 ence of a third substance is necessary, and it is important 

 that this third substance should be of such a kind as to 

 lower the surface-tension at the boundary between the 

 phases, and also to impart to the surface layer a relatively 

 high viscosity or resistance to displacement. Under some 

 conditions this viscosity may be sufficient to impart to 

 the interfacial layer the properties of a solid film. In 

 general, a third substance is effective as an emulsifying 

 agent in proportion to its power of forming at the 

 boundary a film having these properties of low surface- 

 tension and high viscosity. Most substances which 

 form stable emulsions of oil in water (soap, proteins, 

 gums) are of this kind. The interfacial films or lamellae 

 then resist disruption and the disperse droplets are 

 prevented from fusing. If the film is considered as a 

 phase, most emulsions would be classed as three-phase 

 systems (triphasic). 



