=Imbibition and Swelling of Gels.=--When substances which are natural gels, such as gelatin, agar-agar, various gums, etc., are submerged in water, they imbibe considerable quantities of the liquid and the cells become distended so that the mass of the material swells up very considerably. This swelling will take place even against enormous pressures. For example, it has been found that the dry gel from sea-weeds will swell to 330 per cent of its dry volume, if immersed in water under ordinary atmospheric pressure; but that it will increase by 16 per cent of its own volume when moistened, if under a pressure of 42 atmospheres. During the swelling of gels by imbibition of water, the total volume of the system (i.e., that of the original dry gel plus that of the water absorbed) becomes less. For example, a mixture of gelatin and water will, after the gelatin has swelled to its utmost limit, occupy 2 per cent less space than the total volume of the original gelatin and water. It has been computed that a pressure equivalent to that of 400 atmospheres would be necessary to compress the water to an extent representing this shrinkage in volume. On the other hand, gels when exposed to the air lose water by evaporation, shrink in volume, and finally become hard inelastic solids, as in the case of the familiar forms of glue, gelatin, agar-agar, gum arabic, etc. The difference in the relation of gels and that of non-colloidal solids to water may be illustrated by the different action of peas, beans, etc., and of a common brick, when immersed in water. Each of these substances, under these conditions, absorbs, or "imbibes," water; but the peas and beans swell to more than twice their original size and become soft and elastic, while the brick undergoes no change in size, elasticity, or ductility. In all cases of colloidal swelling, the swollen body possesses much less cohesion, and greater ductility, than it had before swelling. The essential difference in the two types of imbibition is that in the case of the non-swelling substances the cohesion, or internal attraction of the molecules of the material, is too great to permit them to be forced apart by the water; while in colloidal swelling, the particles are forced apart to such an extent as to make the tissue soft and elastic. It is possible, of course, to make this separation go still further, until there is an actual segregation of the molecules, when a true solution is produced; for example, gum arabic when first treated with water swells into a stiff gel, then into a soft gel, and finally completely dissolves into a true solution.