SOLUTION. A substance is said to undergo solution, or to become dissolved, when the force of adhesion between it and a liquid in which it is h»rnersed is sufficient to over come the force of cohesion between the solid particles. Thus sugar or salt is dissolved by water, camphor, or resin by spirit of wine, and silver by mercury. The liquid which effects the solution is termed the solvent, or sometimes the menstruu-m; and some solu tions have special names—for example, the term syrup is applied to a solution of sugar in water, and tincture to a solution of a solid in alcohol. If a solid body be introduced in'successive small portions into a definite quantity of a liquid capable of dissolving it, the first portions disappear the most rapidly, and each successive portion dissolves more slowly than its until a point is reached at which the liquid ceases to possess any further solvent power. When this occurs the forces of cohesion and adhesion ate balanced, and the liquid is said to be saturated. Solution is promoted by increasing the extent of surface in a solid, or by reducing it to powder. An elevation of temperature, by diminishing cohesion, ivi11 ,generally also increase the solvent power of he liquid; but there are exceptions to this rule—as for instance. in the case of ,lime and its salts, water just above the freezing-point dissolving nearly twice as much lilne as it does when boil mg. A compound of lime and sugar, very soluble in cold water, is separated from the solution almost completely if heated to boiling. But. the most remarkable case of the kind occurs in sulphate of soda (Glanber's salt),which in its crystalline form dissolves in about ten times its weight of ice-cold water, and rapidly becomes more soluble as the temperature rises until it reaches 91°: from this point until the solution boils the solu bility slightly decreasos. the boiling liquid only retaining about fou•-tlfths of thr tity which was dissolved at 91°. Carbonate and sekuiatc of soda, and sulphate of iron, exhibit the same peculiarity in a less marked degree. "These anomalous results may
be partly explained," says Dr. Miller, "by the consideration that heat diminishes the force of adhesion as well as that of cohesion. Generally speaking, cohesion is the more rapidly diminished of the two, although not uniformly so; and in these cases it would appear that the adhesive force decreases in a greater ratio than the cohesion of the saline particles" (Chetnica/ Physics, 3d ed. 1808, p. 72). The accompanying diagram shows the unequal solubility of vari (Ina of the ontrmInn ..... Balls in water of different temperatures. The lines of solubility cut the verticals raised from points indicat ing the temperature upon the lower horizontal line, at heights proportional to the quantities of salt dis solved by 100 parts of water. For example, 100 parts of water dissolve— at 32°, 8 parts, at 122°, 17 parts, and at 212°, 26 parts of sulphate of potash. Water which has been sat urated with one substance ---.that is, which refuses to dissolve any more of that substance, will often con tinue to dissolve others. In true or rumple solwtion the properties, both of the solid and the solvent, are retained. When, 'however, any chemical action ensues between the solid and the liquid, the result ing solution commonly presents' perfectly new and distinct features; as, for example, v lien the metals are dissolved by acids, or oils by the alkalies (as in soap-making). For tl.e solubility of the gases in water we must refer to the article GASES.
The uses of solution in laboratory processes are numerous. By the difference in Oa give of their solubility we can separate one substance from another; and by dissolving a body- we can purify it, either by filtration or crystallization. Moreover, when it is rt quired that two bodies shall react on one another, they do so with incomparably more force in their dissolved than in their solid state.