Before dealing with the case of the living plasma-membrane, it will be necessary briefly to describe some of the methods by which artificial membranes, similar in many of their osmotic and other physical properties to plasma-membranes, may be prepared, and to consider some of the properties of these membranes. The tendency of the living cell (or living system) to surround itself with a membrane will then be seen to be in no sense a distinctively vital peculiarity, but one which it exhibits in common with a great many non-living systems. There is little doubt that the formation of membranes at the surface of small masses of living protoplasm is a particular instance of the general class of phenomena known collectively as "surface-processes"—processes, that is, occuring as manifestations of the special form of energy, surface-energy, which resides at the surface of separation between materials which do not freely intermix. Consider any material system consisting of various kinds of matter in various states of aggregation, i. e., what the physical chemists call a heterogeneous system, or a polyphasic system. Such a system may be analyzed into a certain number of components, each of which is physically and chemically homogeneous. Each such component is a phase. Oil-drops in a permanent emulsion form one phase, the water a second phase, the soap films at the surface of each droplet a third. Living protoplasm is a good instance of such a polyphasic system. It is—at least in certain forms, e. g., the protoplasm of egg-cells—an emulsion-like or foam-like mixture consisting of various fluid droplets or alveoli (which are supposedly droplets of oil or other fluid containing dissolved substances), separated by another fluid which is typically an aqueous colloidal solution of proteins and lipoids with various additional substances—salts, sugars, amino-acids—in solution. Each droplet or alveolus is a phase; so also is each colloidal particle, or each surface-film, or the interstitial suspension-medium or solvent. At the surface of contact between any two phases a certain tension exists, acting tangentially to the surface; this is the "surface-tension" which (if positive in value) tends to minimize the area of the surface. Each surface, or phase-boundary, is thus the seat of a particular form of energy, surface-energy, of which the intensity-factor is the surface-tension (T), the capacity-factor the total area of the surface (A). The total surface-energy (E) resident at any surface thus equals TA. The tension varies according to the nature of both of the contiguous phases: for water in contact with air it is ca. 75 dynes per linear centimeter, i. e., the pull of a ribbon-shaped portion of water-surface one centimeter wide is about one twelfth of a gram; for water in contact with oil it is ca. 23 dynes per centimeter; for oil in contact with air it is ca. 33 dynes. Now the distribution of the substances present in any such system is influenced in a remarkable manner by these surface-energies. Every one is familiar with the fact that oil spreads over the surface of pure water. This is a case in point: why does the oil not simply float
