NOTICES OF PAPERS CONTAINED IN OTHER JOURNALS. BY HENRY WATTS, B.A., F.C.S. On Osmotic Force.' By Thomas Graham, F.R.S., &c. THE expression "Osmotic Force" (from wouòs, impulsio) has reference to the endosmose and exosmose of Dutrochet. The force of liquid diffusibility will still act if we interpose between the two liquids a porous sheet of animal membrane or of unglazed earthenware; for the pores of such a septum are occupied by water, and we continue to have an uninterrupted liquid communication between the water on one side of the septum and the saline solution on the other side. To impel by pressure any liquid through the pores of such a septum may be extremely difficult, from the interference of frictional resistance and the attraction of capillarity. But these last forces act on masses and not on molecules, and the ultimate particles of water and salt which alone diffuse, appear really to permeate the channels of the porous septum with little or no impediment. A comparative experiment on diffusion, with and without septa, is easily made by means of a wide-mouthed phial, which is filled completely with saline solution and then immersed in water, in one experiment with the mouth of the phial open, and in the other experiment with the mouth covered by membrane. In a fixed time, such as seven days, a certain quantity of salt leaves the phial by diffusion. This quantity was reduced to onehalf when the strong and thick membrane of the ox-gullet was used to cover the mouth of the phial; and it was not affected in a sensible degree by passing through a thinner membrane, consisting of ox-bladder with the outer muscular coat removed. In the last experiment the actual diffusates were 0.631 grm. common salt in the absence of the membrane, and 0.636 grm. common salt with the membrane interposed, which may be considered as the same quantity. The diffusion of a salt appears to take place, therefore, without difficulty or loss through the substance of a thin membrane, although the mechanical flow of a liquid may be nearly stopped by such an obstacle. It is well to bear in mind the last fact in the consideration of what * Phil. Trans. 1854, 177. is seen in an endosmotic experiment. An open glass tube, with one end expanded into a bell form and covered by tight membrane, forms a vessel which may be filled with a saline solution and immersed in a jar of pure water. The volume of liquid in this osmometer soon begins to increase and is observed to rise in the tube, while the simultaneous appearance of salt in the water of the jar may easily be verified. M. Dutrochet described the result as the movement of two unequal streams through the membrane in opposite directions, the smaller stream being that of the saline solution flowing outwards, and the larger that of pure water flowing inwards. The double current bas been always puzzling, but the expression of the fact becomes more conceivable when we say (as we may do truly) that the molecules of the salt travel outwards by diffusion through the porous membrane. It is not the whole saline liquid which moves outwards, but merely the molecules of salt, their water of solution being passive. The inward current of water, on the other hand, appears to be a true sensible stream or a current carrying masses. The passage outwards of the salt is inevitable, and being fully accounted for by diffusibility, requires no further explanation. It is the water current which requires consideration, and for which a cause must be found. This flow of water through the membrane I shall speak of as osmose, and the unknown power producing it as the osmotic force. It is a force of great intensity, capable of supporting a column of water many feet in height, as shown in Dutrochet's well-known experiments, and to which naturalists are generally disposed to ascribe a wide sphere of action, both in the vegetable and animal kingdoms. Cannot liquid diffusion itself, it may first be asked, contribute to produce osmose? Diffusion is always a double phenomenon, and while molecules of salt pass in one direction through the membrane, molecules of water no doubt pass by diffusion in the opposite direction at the same time, and replace the saline molecules in the osmometer. Water also is probably a liquid of a high degree of diffusibility; at least it appears to diffuse four times more rapidly than alcohol, and four or six times more rapidly, therefore, than the less diffusive salts. A possible consequence of such inequality of diffusion is, that while one grain of a certain salt diffuses out of the osmometer, four or six grains of water may diffuse into the osmometer. Liquid diffusion, I believe, generally tends to increase the volume of liquid in the osmometer, and a portion, if not the whole, of the small osmose of chloride of sodium, sulphate of magnesia, alcohol, sugar, and many other organic substances, may be due to the relatively low diffusibility of such liquefied bodies compared with the diffusibility of water. But many substances, it will immediately appear, are replaced in experiments of endosmose, not by four or six, but by several hundred times their volume of water, and manifestly some other force besides diffusion is at work in the osmometer. An explanation of osmose has been looked for in capillarity by Poisson, Magnus, and by Dutrochet himself. Combining diffusion with this idea, we might imagine that the pure water which first occupies the pores of the septum suffers a sudden and great loss of its capillarity-force when the salt of the osmometer enters the pores by diffusion and mixes with the water they contain. Experiments published by Dutrochet give a capillary ascension to pure water of 12 millimeters, and to a solution of common salt, of density 1.12, 6·14 millimeters, or only one-half of the former ascension. If a porous septum, occupied by such a saline solution, had the same solution in contact with one surface, and pure water in contact with the other surface (the actual condition of the septum in an osmotic experiment), the pure water should enter its pores from its high capillary attraction,. and, like a solid piston, force out the saline solution from them: the saline solution so displaced would go to swell the liquid within the osmometer. When the pure water, now again occupying the pores, came in time to acquire salt by diffusion, the displacement would be repeated, and a continuous osmose or flow of water inwards be in fact established. This explanation is attended with certain physical difficulties, but it is unnecessary to discuss these, as the experimental basis of the hypothesis is unsound. The great inequality of capillarity assumed among aqueous fluids does not exist. Many saline solutions which give rise to the highest osmose are, I find, undistinguishable in capillarity from pure water itself. To obtain constant results with saline solutions, the capillary tube must be retained for some minutes in the saline solution at a boiling temperature, and afterwards be cooled without removal from the liquid, otherwise the indications are singularly irregular and most fallacious. The near equality in capillarity of solutions of the most different composition is very apparent in my observations, which are placed together in the following series of capillary ascensions : Capillary ascension of several liquids in the same glass tube. Millimeters. Water, at 58° F. Water, at 66° 17.75 17.55 Carbonate of potash, 0.25 per cent., in water, at 63° 17.2 17.55 Carbonate of soda, 1 per cent., at 61° 17.55 Carbonate of soda, 10 per cent., at 55° Sulphate of potash, 1 per cent., at 58° 17.15 Sulphate of potash, saturated solution, at 58° Sulphate of soda, 1 per cent., at 55° 17.75 Sulphate of soda, 10 per cent., at 58° 16.95 Hydrochloric acid, 1 per cent., at 63° 17.5 Millimeters. Sulphuric acid, 0.1 per cent., at 63° Sulphuric acid, 1 per cent., at 63°. Sulphuric acid, undiluted (HO SO3), at 63° Ammonia, 12 per cent. (0.943 sp. gr.), at 66° Alcohol, 0.8 per cent. (0.9985 sp. gr.), at 60° Alcohol falls in the greatest degree below water in capillarity, yet the former substance is one of the least remarkable for the power to occasion osmose. FIG. 1. The newer facts to be related also increase the difficulties of the capillary theory of osmose. My own experiments on osmose were made with both mineral and organic septa. 1. A convenient earthenware or baked clay osmometer is easily formed by fitting a glass tube and cover to the mouth of the porous cylinder, often used as a cell in Grove's battery, as in Fig. 1; the cylinder was generally five inches in depth by 1.7 inch in width, inside measure, and was capable of holding about six ounces of water. Gutta percha is much preferable to brass as the material for the cap or cover. The glass tube above was also comparatively wide, being 0.6 inch or 15 millimeters in diameter, and was divided into millimeters. than 6 inches in length. Each of the divisions or degrees amounted approximatively to th part of the capacity of the clay cylinder. It was not mona In conducting an experiment, the cylinder, always previously moistened with pure water, was filled with any saline solution to the base of the glass tube, and immediately placed in a jar of distilled water, of which the level was kept adjusted to the height of the liquid in the tube of the osmometer throughout the whole experiment, so as to prevent inequality of hydrostatic pressure. The volume of water in the jar was comparatively large, fifty to eighty ounces. The rise or fall of the liquid in the tube was noted hourly for five hours. This rise commenced immediately, and was pretty uniform in amount for each hour during the short period of the experiment. The object aimed at was to observe the osmose of the solution before its composition was materially altered by dilution and the escape of salt by diffusion. The quantity of salt diffused from the osmometer into the water-jar during the experiment was also observed. After every experiment the osmometer was washed out by distilled water, which was allowed to permeate the porous walls of the cylinder, under the pressure of a column of water of about 30 inches in height, for eighteen hours. All the experiments were made at a temperature between 56° and 64°. The clay osmometer attained a considerable degree of uniformity in its action, when the same saline solution was diffused from it once in each of two or three successive days, with a washing between each experiment. A single observation is not much to be relied upon, as the first experiment often differs considerably from the others. One per cent. solutions were always used when the proportion of salt is not specified. Much larger proportions of salt have hitherto been generally employed, but it was early observed that the osmose absolutely greatest is obtained with small proportions of salts in solution. One part of salt to 400 water gives a higher osmose in earthenware than any other proportion for the great majority of substances. Osmose appeared, indeed, to be peculiarly the phenomenon of dilute solutions. With the same proportion (1 per cent.) of different substances, the osmose varied from 0 to 80 degrees. Occasionally, instead of a rise of liquid in the tube, a fall was observed; the fall may be spoken of as negative osmose, to distinguish it from the rise or positive osmose. Soluble substances of every description were tried, and find a place in the following classes: 1. Substances of small osmotic power in porous earthenware; osmose under 20 of the millimeter degrees (ms.) This class appears to include nearly all neutral organic substances, such as alcohol, pyroxylic spirit, sugar, glucose, mannite, salicin, amygdalin, salts of quinine and morphine, tannin, urea; also certain active chemical substances, which are not salts nor acids; chlorine water, bromine water. The great proportion of neutral salts of the earths and metals proper also belong to the same class, such as chloride of sodium, of which the positive osmose was greatest in a solution containing no more than 0.125 per cent., being 19 ms. with that proportion of salt, but falling off and often becoming slightly negative with 1 per cent. |