Osmose, the interdiffusion of two liquids through a septum, usually of bladder or of parchment paper. If a bottle, filled with one liquid, be closed by parchment paper, and be completely immersed in a vessel containing another liquid, increase or decrease of the contents of the bottle will occur according as the liquid contained in the bottle passes out through the septum less quickly or more quickly than the other liquid passes inwards. When the contents are increased the phenomenon has been called endosmose; when they decrease it has been termed exosmose. The distinction is obviously not a scientific one; for a reversal of the positions of the liquids will cause a reversal of the osmotic process, so that the process which was formerly denominated exosmose must now be called endosmose, and vice versa. The phenomenon is one of extreme importance, for it is constantly taking place in living bodies—both animal and vegetable.
Nollet was the first to record the occurrence of osmose. He placed a vessel, filled with alcohol and closed with a piece of bladder, inside a larger vessel which was filled with water. The rapid entry of the water almost burst the bladder; and the opposite effect took place when the water was placed inside the inner vessel and the alcohol was placed outside it. Nollet did not pursue his observations any further. Dutrochet first made careful investigations into the subject, which has since received numerous practical applications—notably in the method of dialysis, which is due to Graham. The phenomenon consists merely in the interdiffusion of two liquids complicated by the mutual molecular actions which take place between the liquids and the material of the membrane. The rate of interdiffusion depends greatly upon the nature of the membrane; sometimes the direction of the osmose is affected when the membrane is altered. The action being essentially molecular, we can readily understand how sap may be raised to great heights in plants and trees against the action of gravity; for the molecular forces in a drop of water (say) are sufficiently powerful to hold the parts of the drop together against the gravitational attraction of the whole earth.
A process which is analogous to osmose occurs in the interdiffusion of two liquids through an intervening liquid layer. The difference between the rates of diffusion of colloids and crystalloids is even more marked when the substances are separated by parchment paper or animal membrane than when they diffuse directly into each other.