Page:Elementary Text-book of Physics (Anthony, 1897).djvu/116

102 '''87. Porous Bodies.'''—Porous bodies may be considered as assemblages of more or less irregular capillary tubes. Thus the explanation of many natural phenomena—as the wetting of a sponge, the rise of the oil in the wick of a lamp—follows directly from the preceding discussion.

88. Solution and Absorption.—Many solid bodies, immersed in a liquid, after awhile disappear as solids, and are taken up by the liquid. This process is called solution. The quantity of any body which a unit quantity of a given liquid will dissolve at a given temperature, is called its solubility in that liquid at that temperature. The solubility of a given solid varies greatly for different liquids, in many cases being so small as to be inappreciable.

One liquid may also be dissolved in another, the degree of solubility diflEering very much for different liquids. At ordinary temperatures many liquids are practically insoluble in others, but there is reason to believe that as the liquids approach their critical points (§ 223), their solubilities in other liquids increase, and that at their critical points any liquid is soluble in all others in any proportion.

Gases are also taken into solution by liquids. The process is usually called absorption. The quantity of gas dissolved in any liquid depends upon the temperature, and varies directly with the pressure. The solubility of any gas at a given temperature and at standard pressure is called its coefficient of absorption at that temperature.

Gases, in general, adhere strongly to the surfaces of solids with which they are in contact. This adhesion is so great, that the gases are sometimes condensed so as to form a dense layer which probably penetrates to some depth below the surface of the solid. The process is called the absorption of gases by solids. When the solid is porous, its exposed surface is greatly extended, and hence much larger quantities of gas are condensed on it than would otherwise be the case. When this condensation occurs there is in