DIFFUSION, the gradual dispersion of particles of one liquid or gas among those of another—or of the particles of a solid in a liquid holding it in solution. It is of the greatest importance in terrestrial physics, being the cause of the uniform composition of the atmosphere at all elevations, and one of the causes of the speedy dissipation of noxious gases and vapors in the open air, and of the nearly uniform saltness of the sea, eto, so necessary to animal and vegetable life.
We shall consider the above cases in detail, and in addition the transfusion, as it has been called, which occurs when different gases or liquids are separated from each other by a porous plate or membrane. The principal experiments on this subject are those of Graham and Bunsen—to be found in the Philosophical Transactions or Graham's Chemistry, and in Bunsen's Gasometry.
1. Diffusion of Gases. —If two flasks be filled, one with hydrogen, 'the other with chlorine, and connected by a long tube fitted into their necks by corks—in whatever position the compound apparatus be placed it will be found that the gases mutually interpenetrate—in this particular case the color of the chlorine enables us to follow by the eye the course of the diffusion. When the mixture has attained its permanent state, each of the gases is found to be uniformly diffused through the whole containingspace, precisely as it would have been had the other not been present. In fact, the presence of a second gas seems merely to affect the time which the first takes to distribute itself equably throughout the vessel, and in no other way to influence the final result. (Dal ton, long ago, suggested the analogy of the passage of water among stones in the bed of a river.) The pressure of the mixture is the suns of the pressures corresponding to each of the gases, if separately occupying the space which they jointly fill; and the same is true of a mixture id any proportions of any number of long, at all events, as they. do not act chemically upon each other.
Precisely the same is true of vapors. If, for instance, a few drops of ether be injected into an exhausted receiver, there will be an almost instantaneous conversion of a definite quantity into vapor, so that its tension shall have a certain value depending on the tem perature alone. If air be present in any quantity whatever, the vaporization will pro ceed more slowly, but the final amount converted into vapor will be the same as in the former case. A familiar illustration of this is afforded by the dew-point, which is a temperature merely, having no connection with the height of the barometer.
Next let us take the case of a gas forced by difference of pressure, from one vessel to another through a very small hole in a thin metallic plate—one of the vessels, for instance, being full of the gas, and the other connected with an air-pump kept con tinually in action. Experiment and theory, such as it is, agree in giving in this case, for the velocity with which the different gases pass through the orifibe, under similar cir cumstances as to pressure, a result inversely proportional to the square root of the den sity of each gas. Now, if, instead of the plate with the small hole, we substitute a thin layer of bladder or other membrane, or a thin disk of plaster of Paris, it appears from experiment that the results are sensibly the same. Thus, if we have the same gas at different pressures on opposite sides of such a layer or disk, the rate of passage of the gas through it, from greater to less pressure, will, for the same pressures, but different gases, vary according to the above law; and,,morcover, the presence of .a second gas will in no degree modify the rate of transfusion of the first. If, therefore, a glass tube, say an inch in diameter, and 2 or 3 ft. long, have a diaphragm of plaster of Paris formed near one end, and that end ground flat, so as to be perfectly closed by a glass plate, it may be filled with hydrogen by displacement, its other and lower extremity being plunged into water, and care beingtaken not to wet the diaphragm. If the glass plate be now removed, transfusion will take place—hydrogen passing out as if into a vacuum, and the constituent gases of air entering also as if into a vacuum. On account of the comparative lightness of hydrogen, the velocity with which it escapes will be considerably greater than that with which the others enter; so that the immediate effect will be a rise of the water in the tube. After a short time, the whole of the hydrogen escapes, and the tube will contain only air. The proportion of the volume of the latter to that of the hydrogen may be calculated from the above law—remembering that the hydrogen is practically diffusing into a vacuum all along, and the air entering by the pressure of the atmosphere in excesAyer that of the air in the tube. Allowing for/the unavoidable errors of experiment, tfWveritications of these results are very satisfactory. It has been attempted to deduce these laws as consequences of the dynamical theory of gases. See a remarkable paper by Maxwell in the Philosophical Magazine for 1860.