MEMBRANES AND CELL-PROCESSES 135 



in droplets on the surface instead of spreading out in an extremely thin 

 continuous layer? A consideration of the conditions of surface-tension 

 at once explains this (Fig. 1). An oil-drop placed on the surface of 

 water is subjected to the pull of 

 three tensions, viz. : those at its air 



own two surfaces (t ± and t 2 ), 

 where it touches air and water, 

 respectively, and which tend to 

 round it off, and that of the 

 pure water at its margin (t s ), 

 which tends to spread it. But 

 the tensions t x and t 2 are together 



less than the tension t z ; the oil is thus rapidly drawn out over the sur- 

 face by the superior pull of the water-air tension at its margin. Hence 

 the water-air surface, that with high tension, disappears and is replaced 

 by a surface with lower tension. The total surface-energy has been di- 

 minished, part having been transformed into mechanical energy and 

 heat. If, instead of the case of a floating oil-drop, we take that of some 

 soluble substance which is produced locally within the water near the 

 surface — e. g., a soap or a protein, a solution of which has a lower ten- 

 sion than pure water — we find essentially the same phenomenon; the 

 substance is spread out over the surface, and this effect will continue so 

 long as the addition of further quantities of the substance to the sur- 

 face-layer continues to lower the surface-tension. The end-effect will 

 be to concentrate the substance at the phase-boundary. This phenom- 

 enon is the expression of a general law, the law of Willard Gibbs and 

 J. J. Thomson, which describes the part played by surface-energies in 

 the distribution of soluble substances in a polyphasic system. In the 

 present case, the process of surface-concentration will go on until some 

 equilibrium is reached, c. g., where the loss of substance from the sur- 

 face by diffusion balances its collection there under the influence of the 

 surface-energy. But in many cases, as with proteins, soaps and certain 

 lipoids, the substance separates at the surface as a continuous solid film 

 before this stage is reached. The formation of solid surface-films is 

 hence highly characteristic of the solutions of such substances. Casein 

 films form on warm milk, soap films form about droplets of rancid oil 

 in the presence of alkali, and protein films about drops of chloroform 

 or oil suspended in protein solutions. Thin solid membranes formed 

 in this manner at phase-boundaries are called "haptogen membranes." 

 In all of these instances we have to do with a surface-condensation, 

 known under certain conditions as " adsorption," of substances which 

 lower the surface-tension at the phase-boundary. Among the colloidal 

 constituents of protoplasm the proteins and the lipoids belong to this 

 class of substances. Hence it is not surprising that isolated portions of 

 living protoplasm should delimit themselves by membranes. The vari- 



