34. Adhesion of Gases to Liquids.-When a liquid is poured from one vessel to another, the gases of the air adhere to the descending stream, are carried downward, and a portion of them remain combined with it. The force to be overcome by this adhesion is the elasticity of the gases, or the mutual repulsion of their particles. Pressure and cold lower the elastic force, and therefore favor absorption. As the temperature rises, adhesion is diminished, and hence the readiest means of driving out a gas from solution is by boiling. 35. Adhesion of Gases to Solids. If iron filings are gently dusted over the surface of water, they float, though iron is eight times heavier than water. This is because of the adhesion and condensation of a layer of air upon their surface, which prevents the water from wetting them. The condensed air around the particles forms a capillary cavity, and thus displaces a large volume of the liquid in comparison with that of the solid. Insects walk upon water and skim over its surface, because the air adhering to their feet forms capillary cavities, and prevents them from becoming wetted. 36. Diffusion. Whenever the cohesive force subsisting between the molecules of any body is exceeded by the adhesive force subsisting between its molecules and those of another body, the cohesion of one or both bodies is overcome, their molecules separate, and become evenly intermixed. The bodies in this case are said to be dissolved in, or diffused through, one another, the process by which their particles become intermingled being termed diffusion. When diffusion takes place between bodies in unlike states of aggregation, one of the two, under the influence of adhesive attraction, assumes the state of the other. Thus, a solid or liquid, in order to become diffused through a gas, must first assume the gaseous state; gases and solids, to be come diffused through liquids, the liquid state; and gases and liquids, to become diffused through solids, the solid state of aggregation. The term diffusion is generally limited to the molecular union or intermingling of bodies already in a like condition of aggregation. FIG. 18. 37. Diffusion of Gases.-The molecules of gases exercise upon each other no cohesive attraction. Consequently, all gases when brought in contact will intermix, or diffuse through each other uniformly, and in all proportions, the process setting in even in opposition to their specific gravities. Thus, if two jars be connected by a narrow tube (Fig. 13), and the lower filled with carbon dioxide, the upper containing hydrogen, diffusion takes place through the narrow passage. The light hydrogen descends, and the carbon dioxide, though twenty times heavier, rises, and they become equally mingled in both jars. Our atmosphere owes its stability to this principle, its constituents being perfectly intermingled. The baneful products of respiration, combustion, and decay, instead of accumulating, are incessantly dissolved away and dispersed in the atmospheric ocean. Diffusion of Gases. 38. Rate of Diffusion of Gases.-All gases do not, however, diffuse with equal facility. There is a very simple relation between the density of gases and the rapidity of their diffusion, which is expressed by saying that the diffusive power of gases varies inversely as the square root of their densities. 39. Osmose of Gases.—If a vessel be divided into two portions by a diaphragm or partition of dry plaster of Paris or some other porous substance, and each half filled with a different gas, diffusion will immediately commence. The rate of diffusion is governed by the law already men tioned, so long as tue porous plate be very thin, but, when the plate is thick, the law observed is different. A distinction must also be carefully drawn between real diffusion through small apertures and the apparently similar passage of gases through membranous diaphragms, such as caoutchouc or bladder. In this mode of passage, which is called FIG. 14. branes. osmose, the rate of interchange depends partly on the relative diffusibilities of the gases, partly on the different degrees of adhesion exerted by the membrane, the gas which adheres most powerfully penetrating the diaphragm most easily. A sheet of India-rubber tied tightly over the mouth of a Passage of Gases through Mem- wide-mouthed jar containing hydrogen is soon pressed inward, even to bursting. If the jar be filled with air, and placed in an atmosphere of hydrogen, the swelling and bursting take place outward (Fig. 14). If the membrane is moist, the result is likewise affected by the different solubilities of the gases in the water or other liquid which wets it. Though the diffusive power of carbon dioxide is small compared with that of air, yet it easily passes into the latter through wet bladder. This process appears to be brought into play in atmospheric respiration. There is air on one side of the moist lung-membrane, and blood on the other; oxygen is transmitted from the air to the blood, and carbon dioxide from the blood to the air. 40. Diffusion of Liquids and Solids through Gases.—The diffusion of liquids through gases is a phenomenon of common observation. Water, as well as other liquids, at all temperatures, gives off vapors, which diffuse through the air as fast as they are formed. Solids in some cases do the same thing-ice, for example, evaporating very fast when in contact with a current of dry air. The law which gov erns these diffusions is indentical with that under which other gaseous bodies intermingle. 41. Diffusion of Liquids. That the molecules of liquids cohere may be seen in the formation and persistence of drops. But, though thus held together by cohesive force, the amount of its action in liquids is never sufficient to unite large masses. The adhesive attraction of the molecules of dissimilar liquids, on the other hand, is in many cases very considerable. Diffusion of liquids through each other, though not universal as that of gases, may be observed in many cases. Thus, if a colored fluid, heavier than water-as, for example, ink-be placed in the bottom of a tall glass jar filled with water, taking care not to mix the two liquids by agitation, they will, after a time, be found commingled. FIG. 15. 42. Rate of Diffusion of Liquids.-Different liquids under entirely like conditions diffuse with very unequal velocity. According to Graham, who placed small jars, filled with liquids to be tested, in larger ones containing distilled water, as in Fig. 15, and determined the amount of the inner solution that diffused into water in a given time, substances were found to differ greatly in diffusibility, chlorohydric acid proving to be the most diffusible. The equal diffusion of several solutions took place in the following times: Chlorohydric acid, 1; common salt, 2.33; sugar, 7; albumen, 49; caramel, 98. Substances thus tested are called diffusates. Diffusion of Diffusion is generally found to take place more rapidly at high than at low temperatures. It is particularly rapid with solutions of crystallized substances, like sugar, salt, etc., and slowest with those of non-crystalline bodies, which, like gelatine, gum, etc., are capable of forming jellies. The substances of great diffusibility have accordingly been designated as crystalloids, those of low diffusibility as colloids. Crystalloid bodies form solutions which are mobile, the solutions of colloids are viscid. When solutions of colloids are in contact, they hardly diffuse through each other, while the solutions of crystalloids not only diffuse with rapidity through the solutions of other crystalloids, but also through those of colloids. FIG. 13. 43. Osmose of Liquids. When a piece of moistened bladder is tied tightly over the end of a tube placed in a vessel of water, and then filled with alcohol up to the level of the outer liquid, the fluid in the tube will shortly begin to ascend, and may rise to a considerable height (Fig. 16). The external water passes through the membrane and mixes with the alcohol, while at the same time a feeble current of alcohol flows the other way and commingles with the water. When different liquids are separated by a membrane in this manner, the one is transmitted fastest which wets the barrier most perfectly. Dutrochet, who first drew attention to this matter, named the inflowing current endosmose, and the outflowing one exosmose; but these terms are lately less employed, and the phenomena are now known simply as osmose, from a Greek word signifying impulsion. The osmose of liquids is due partly to their adhesive attractions for each other, and partly to the difference of their adhesive attractions for the membrane or diaphragm, the pores of which act as short capillary tubes. Osmose of Liquids. 44. Diffusion of Gases through Liquids. Absorption.-The diffusion of gases through liquids is called absorption. It is a phenomenon often noticed, the most common liquid, water, being possessed of high absorptive power. The power of absorption of liquids varies for different gases; pressure or cold increases it, heat diminishes it. The effect of pressure is often employed to induce absorp |