Page:Elementary Text-book of Physics (Anthony, 1897).djvu/118

104 unit time is $$\kappa$$. The quantity $$\kappa$$, called the coefficient of diffusion, is, therefore, the quantity of salt that passes in unit time through unit area of a horizontal plane when the difierence of concentration is unity. Coefficients of diffusion increase with the temperature, and are found not to be entirely independent of the degree of concentration.

As implied above, the units of mass and length employed in these measurements are respectively the gram and the centimetre; but, since in most cases the quantity of salt that diffuses in one second is extremely small, it is usual to employ the day as the unit time.

91. Diffusion through Porous Bodies.—It was found by Graham that diffusion takes place through porous solids, such as unglazed earthenware or plaster, almost as though the liquids were in direct contact, and that a very considerable difference of pressure can be established between the two faces of the porous body while the rate of diffusion remains nearly constant.

92. Diffusion through Membranes.—If the membrane through which diffusion occurs be of a type represented by animal or vegetable tissue, the resulting phenomena, though in some respects similar, are subject to quite different laws. Colloid substances pass through the membrane very slowly, while crystalloid substances pass more freely. It is to be noted that the membrane is not a mere passive medium, as is the case with the porous substances already considered, but takes an active part in the process; and consequently one of the liquids frequently passes into the other more rapidly than would be the case if the surfaces of the liquids were directly in contact.

If the membrane separate two crystalloids, it often happens that both substances pass through, but at different rates. In accordance with the usage of Dutrochet, we may say there is endosmose of the liquid which passes more rapidly to the other liquid, and exosmose of the latter to the former. The whole process is frequently called osmosis. If the membrane be stretched over the end of a tube, into which the more rapid current sets, and the tube be