Endosmose (Gr. within, and impulsion), the action exhibited by one of two fluids of different densities and composition in passing through an organic membrane which separates them, till they become both of the same density. Let a solution of sugar in a tube closed below with a slip of bladder tied across the end, and open above, be suspended in a vessel of water. The quantity of liquid in the tube is soon seen to increase by the passing through of the thinner fluid. It may even flow over and run down into the outer vessel, and so the action will go on until the membrane becomes partially decomposed, or until the two mixtures become uniform. Dutrochet, who first observed this phenomenon, found that the height to which the fluid would rise increased with the density of the thicker fluid. With a membrane 1 1/2 in. in diameter and sirup of the density 1.083, the liquid rose, in a tube 1/12 of an inch in diameter, more than 1 1/2 in. in 1 1/2 hours; with sirup of a density of 1.45 the fluid rose nearly 3 in.; and when the density was 1.228 the rise was 4 in. A considerable force is exerted in this movement; in sirup of density 1.3 Dutrochet estimated it to be equal to the pressure of 4 1/2 atmospheres.
If the flow is drawn inward, the action is called endosmose; if outward, exosmose. It is supposed to be on this principle that the sap ascends in trees and fluids are diffused through animal bodies. Lie-big, after describing some experiments, in which fluids were made to pass through nine membranes, to fill the vacant space left by evaporation of another fluid in a glass tube, remarks with reference to the application of the results to the processes taking place in the animal body as follows: "The surface of the body is the membrane from which evaporation goes constantly forward. In consequence of this evaporation, all the fluids of the body, in obedience to atmospheric pressure, experience motion in the direction toward the evaporating surface. This is obviously the chief cause of the passage of the nutritious fluids through the walls of the blood vessels, and the cause of their distribution through the body. We know now what important functions the skin (and lungs) fulfil through evaporation. It is a condition of nutrition, and the influence of a moist or dry air upon the health of the body, or of mechanical agitation by walking or running, which increases the perspiration, suggests itself." Interesting examples of this phenomenon are seen in the passage of the gases through membranes.
If a tumbler, filled with air and covered at top with a thin sheet of India rubber, be placed under a bell glass filled with hydrogen, the gas will soon penetrate the cover and mix with the air; and this action will go on till the India rubber bursts open from the increased bulk of the contents of the tumbler. If the tumbler contained hydrogen and the bell glass air, the India rubber would be pressed in by the escape of the gas, leaving the portion remaining of greatly reduced density.