OSMOSE. The phenomena of liquid diffusion [DIFFUSION] become more complicated when the two liquids are separated by means of a porous diaphragm, because in such case there is adhesion between the liquids and the material of the diaphragm : nevertheless the liquids will mingle together in opposition to gravity. For example, if a funnel, furnished with a long narrow stem, and a piece of moistened bladder tied over its mouth, be filled with spirit of wine or with syrup, and be placed in a shallow vessel of water, the liquid will in a few hours rise in the tube, and even overflow. In this case there is a greater adhesion between water and the membrane than between spirit and the membrane; hence the water in contact with the lower surface of the membrane rises into it by capillary attraction, and gains the upper surface where diffusion of the water through the spirit sets in, and is promoted by the adhesion existing between the two liquids. In this way the liquid within the funnel increases in bulk, and fills and even overflows the tube. This flowing in of the liquid was termed by Dutrochet eadosmosis, from cvSoy " inwards," and waster " impulse. While this force is acting from without inwards, a small quantity of spirit is passing out into the water below, which process of flowing out is termed exosmosis. If two other liquids be selected the pheno mena will be produced, provided one liquid adhere to the bladder more perfectly than the other, and the two liquids have a certain degree of adhesion for each other.
Graham (` Phil. Trans.,' 1854) has investigated the subject, which is one of great interest, physiologically as well as chemically. The instrument used in his experiments is termed an osmometcr : it is formed of a bell-glass of five or six ounces, having a plate of perforated zinc over its open mouth, and outside this is securely tied a piece of fresh ox-bladder with the muscular coat removed ; or else an artificial membrane of calico soaked in white of egg, and dipped in boiling water to coagulate it. A tube of the diameter of the mouth of tho bell-glass, is fitted into an aperture at its top, and is graduated into millimetres, each degree being about Lth of an inch. A rise or fall in the liquid in the tube, amounting to 100 millimetres, represents the entrance or removal of a stratum of liquid millimetre thick over the surface of the membrane. The saline solution to be examined is placed in the jar, until it stands at a fixed mark in the tube. The jar is then placed on a tripod within a tall cylinder, into which distilled water is poured up to the level of the liquid in the tube, and this level is maintained during the experiment by adding or removing water from the outer jar. In this way it was found-1. That neutral organic substances, such as urea, gum arable, sugar of milk, gelatine, and salicin, exert little or no osmotic action. 2. That strictly neutral
salts, such as sulphate of magnesia, chloride of sodium, and chloride of barium, have no peculiar osmotic action, but follow nearly the same rate of diffusion as when no diaphragm is used. 3. That alkaline solutions, especially those of potash and soda, produce energetic osmotic results : dilute solutions containing not more than 2 per cent. of the salt, answered best ; but the effect was observed when 1 part of the salt was contained in 1000 of water; and while a large bulk of water entered the eemoineter, only a minute portion of the alkaline salt escaped into the water of the outer jar. Thus, with a solution of car bonate of potash (1 part salt to 1000 water) in the (manometer, the liquid wee in the stem 192 divisions In five hours, and for each grain of carbonate of potash that was diffused into the outer cyliuder more than 550 grains of water entered the otnnoineter. When, however, a solution containing 1 per cent. of carbonate of potash was used, only 63 grains of water entered the oemometer for each grain of carbonate that passed into the outer cylinder. When the liquid rises in the milometer it is termed poritier ()saw. 4. That dilute acids and solutions of acid salts produce a current in the opposite direction, so that the column falls in the stem ; this is known as negatire mason. Osmose is remarkably shown in salts which admit of division into a basic salt and a free acid. Acetate of alumina, nitrate of lead, chloride of zinc, and the salts of the sesquioxides of chromium and iron are good examples. In such cases the acid diffuses outwards, and the inner surface uf the membrane remains in a basic condition while the outer surface is acid—conditions which are highly favourable to rapid positive osmose. 5. That in all cases of osmotic actions (except where alcohol and cane•sugar are con cerned) the material of the diaphragm is chemically acted on, and if the diaphra,gui be made of a material not capable of decomposition by the liquids, such as gypsum, unbaked clay, tanned leather, cumpreesed charcoal, although sufficiently porous, there is little or no osmotic action. The most favourable cases are those in which the chemical action on the diaphragm is different on the two sides both in degree and kind, as when one surface is acid and the other alkaline. 6. That two salts, when mixed, often exert an osmotic action very different from what they exercise separately. Thus, sulphate of potash has a feeble positive osmose, but a very minute addition of carbonate of potash increases its osmotic forco fivefold, and a minute trace of hydro chloric acid arrests it. Chloride of sodium also reduces osmotic action in other salts.