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 own two surfaces (${\displaystyle t_{1}}$ and ${\displaystyle t_{2}}$), Fig. 1. where it touches air and water, respectively, and which tend to round it off, and that of the pure water at its margin (${\displaystyle t_{3}}$), which tends to spread it. But the tensions ${\displaystyle t_{1}}$ and ${\displaystyle t_{2}}$ are together less than the tension ${\displaystyle t_{3}}$; the oil is thus rapidly drawn out over the surface 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 diminished, 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 tension 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 surface-layer continues to lower the surface-tension. The end-effect will be to concentrate the substance at the phase-boundary. This phenomenon 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, e. g., where the loss of substance from the surface 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-