Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/235

217 DIFFUSION 217 the encounters between the molecules of these components. The value of this resistance, however, depends, in liquids, on more complicated conditions than in gases, and for the present we mast regard it as a function of all the physical properties of the mixture at the given place, that is to say, its temperature and pressure, and the proportions of the different components of the mixture. The coefficient of interdiffusion of two liquids must therefore be considered as depending on all the physical properties of the mixture according to laws which can be ascertained only by experiment. Thus Fick has determined the coefficient of diffusion for common salt in water to be O OOOOllG, and Voit has found that of cane-sugar to be 00000365. It appears from these numbers that in a vessel of the same size the process of diffusion of liquids requires a .greater number of days to reach a given stage than the process of diffusion of gases in the same vessel requires seconds. When we wish to mix two liquids, it is not sufficient to place them in the same vessel, for if the vessel is, say, a metre in depth, the lighter liquid will lie above the denser, and it will be many years before the mixture becomes even sensibly uniform. We therefore stir the two liquids to gether, that is to say, we move a solid body through the vessel, first one way, then another, so as to make the liquid contents eddy about in as complicated a manner as possible. The effect of this is that the two liquids, which originally formed two thick horizontal layers, one above the other, are now disposed in thin and excessively convoluted strata, which, if they could be spread out, would cover an im mense area. The effect of the stirring is thus to increase the area over which the process of diffusion can go on, and to diminish the distance between the diffusing liquids; and since the time required for diffusion varies as the square of the thickness of the layers, it is evident that by a moderate amount of stirring the process of mixture which would otherwise require years may be completed in a few seconds. That the process is not instantaneous is easily ascertained by observing that for some time after the stir ring the mixture appears full of streaks, which cause it to lose its transparency. This arises from the different indices of refraction of different portions of the mixture which have been brought near each other by stirring. The surfaces of separation are so drawn out and convoluted, that the whole mass has a woolly appearance, for no ray of light can pass through it without being turned many times out of its path. Graham observed that the diffusion both of liquids and gases takes place through porous solid bodies, such as plugs of plaster of Paris or plates of pressed plumbago, at a rate not very much less than when no such body is interposed, and this even when the solid partition is amply sufficient to check all ordinary currents, and even to sustain a con siderable difference of pressure on its opposite sides. But there is another class of cases in which a liquid or a gas can pass through a diaphragm, which is not, in the or dinary sense, porous. For instance, when carbonic acid gas is confined in a soap bubble it rapidly ^escapes. The gas is absorbed at the inner surface of the bubble, and forms a solution of carbonic acid in water. This solution diffuses from the inner surface of the bubble, where it is strongest, to the outer surface, v?here it is in contact with air, and the carbonic acid evaporates and diffuses out into the atmosphere. It is also found that hydrogen and other gases can pass through a layer of caoutchouc. Graham showed that it is not through pores, in the ordinary sense, that the motion takes place, for the ratios are determined by the chemical relations between the gases and the caout chouc, or the liquid film. According to Graham s theory, the caoutchouc is a colloid substance, that is, one which is capable of combining, in a temporary and very loose manner, with indeterminate proportions of certain other substances, just as glue will form a jelly with various proportions of water. Another class of substances, which Graham called crystalloid, are distinguished from these by being always of definite com position, and not admitting of these temporary associations. When a colloid body has in different parts of its mass different proportions of water, alcohol, or solutions of crys talloid bodies, diffusion takes place through the colloid body, though no part of it can be shown to be in the liquid state. On the other hand, a solution of a colloid substance is almost incapable of diffusion through a porous solid, or another colloid body. Thus, if a solution of gum and salt in water is placed in contact with a solid jelly of gelatine and alcohol, alcohol will be .diffused into the gum, and salt and water will be diffused into the gelatine, but the gum and the gelatine will not diffuse into each other. There are certain metals whose relations to certain gases Graham explained by this theory. For instance, hydrogen can be made to pass through iron and palladium at a high temperature, and carbonic oxide can be made to pass through iron. The gases form colloidal unions with the metals, and are diffused through them as water is diffused through a jelly. Root has lately found that hydrogen can pass through platinum, even at ordinary temperatures, By taking advantage of the different velocities with which different liquids and gases pass through parchment- paper and other solid bodies, Graham was enabled to effect many remarkable analyses. He called this method the method of Dialysis. Diffusion and Evaporation, Condensation, Solution, and Absorption. The rate of evaporation of liquids is determined prin cipally by the rate of diffusion of the vapour through the air or other gas which lies above the liquid. Indeed, the coefficient of diffusion of the vapour of a liquid through air can be determined in a rough but easy manner by placing a little of the liquid in a test tube, and observing the rate at which its weight diminishes by evaporation day by day. For at the surface of the liquid the density of the vapour is that corresponding to the temperature, whereas at the mouth of the test tube the air is nearly pure. Hence, if p be the pressure of the vapour corresponding to the tem perature, and p = Jcp, and if m be the mass evaporated in time t, and diffused into the air through a distance h, 1 then khm = 7T- This method is not, of course, applicable to vapours which are rarer than the superincumbent gas. The solution of a salt in a liquid goes on in the same way, and so does the absorption of a gas by a liquid. These processes are all accelerated by currents, for the reason already explained. The processes of evaporation and condensation go on much more rapidly when no air or other non-condensible gas is present. Hence the importance of the air-pump in the steam engine. Relation between Diffusion of Matter and Diffusion of Heat. The same motion of agitation of the molecules of gases which causes two gases to diffuse through each other also 1 h should be taken equal to the height of the tube above the surface of the liquid, together with about f of the diameter of the tube. See Clerk Maxwell s Electricity, Art. 809. VII. 28