tion, as, for example, when water is impregnated with carbon dioxide to form the common beverage known as sodawater. When a mixture of different gases is brought in contact with a liquid, the absorptive power of the latter for each gas contained in the mixture will, however, be only proportional to the pressure of that gas, and not to that exercised by all the gases present.

45. Diffusion of Solids through Liquids.—The diffusion of solids through liquids, which is termed solution, is familiarly known. In this case the solid, assuming itself the fluid state, disappears, mixing uniformly with the liquid, which remains transparent. The solid is then said to have been dissolved by it, and the liquid employed is called the solvent. A liquid which dissolves one substance may refuse to dissolve another, while substances insoluble in one liquid are dissolved in others. A distinction must, however, be drawn between solution which depends entirely on diffusion and solution which is owing in part to chemical change. In the former case, as when sugar is dissolved in water, the sugar may be again obtained in an unaltered condition by the vaporization of the water, while in the latter instance, as when zinc is dissolved in sulphuric acid, the vaporization of the excess of the solvent will not yield the solid zinc, but an entirely different substance known as zinc sulphate.

46. Conditions favorable to Solution.-Whatever weakens cohesion favors solution. Thus, by powdering a substance, cohesion is partially destroyed and the surface increased; solution is consequently promoted. Heat, in most cases, contributes powerfully to solution, its effect being, as is supposed, to weaken cohesion by increasing the distance between the particles of the solid; yet there are marked exceptions. Water just above the freezingpoint dissolves twice as much lime as at the boiling-point, while the solubility of common salt seems hardly affected by temperature. Some substances increase in solubility

regularly as the temperature increases; in many cases the solubility increases faster than the temperature, and in others it rises with the increasing heat to a certain point, and then declines, while the temperature continues to ascend.

47. Saturation.-A liquid is said to be saturated when it has taken up as large a quantity of a solid as it can dissclve; in which case the force of cohesion between the particles of the solid is equaled by the adhesion of the solid and liquid to each other. The solvent power of liquids varies much. Water is the great solvent, and so general and important is its use that, in speaking simply of the solubility of a body, water is always understood.

48. Separation of Solids from Solution.-If the adhesive attraction between the solvent and the dissolved solid can be overcome, cohesive attraction resumes its sway, and reunites the molecules of the solid. This change may be effected in various ways-as, when the solvent is removed by evaporation, or, when another liquid, having no chemical effect upon the solid, is mixed with the solution. When a solution is evaporated, the solid is deposited either during the process, or remains at its close. The former is generally the case with crystalloid, the latter with colloid bodies. When the solid is separated by the addition of another liquid, the separation is due to the insolubility of the solid in the liquid added. Thus, if water be mixed with a solution of camphor in alcohol, the camphor separates as a white cloud, at first rendering the liquid turbid, but, after some time, depositing on the bottom of the vessel. The instantaneous separation of a solid from a clear liquid is termed precipitation, and, the deposit formed, a precipitate. As most frequently observed, however, precipitation is not due only to a reversal of solution, but also involves various forms of chemical action.

49. Diffusion of Solids.-The cohesive attraction subsisting between the molecules of any solid is much greater

than the like attraction between the molecules of liquids. Their molecules being so much less mobile, diffusion cannot take place directly between solid substances; but, when they have been first diffused through liquids, and, the resulting solutions being mingled together, the mixed liquids are exposed to conditions under which the solids are compelled to again separate from the solvent, they will in some cases remain blended with, dissolved in, or diffused through, one another. This takes place, for example, when mixed solutions of magnesic sulphate and zinc sulphate in water are evaporated, and likewise with the mixed solutions of many other salts.

50. Diffusion of Gases through Solids. Occlusion.—The fact that gases adhere to solids has already been noticed. Under some conditions, certain solids absorb large quantities of gases, which appear to be truly diffused through the mass of the solid. Thus, the metals iron, platinum, and palladium, have the power of taking up various gases; the last-named metal is said to be capable of uniting in this way, at ordinary temperatures, with several hundred times its own bulk of hydrogen gas. Although not directly demonstrable by experiment, it is maintained that the hydrogen, having undergone intense condensation, must be in a state of solidity. This diffusion of gases through solids is termed "occlusion." No phenomena bearing the character of true diffusions of liquids through solids have so far been noticed.

§ 4. Crystallization.

51.-Under various conditions, and particularly when bodies pass from the liquid or gaseous state to the solid state, their molecules tend to arrange themselves in regular geometrical forms termed crystals, of which Fig. 17 may be taken as an example. The substances in which this tendency is marked are said to be crystallizable, and

FIG. 17.

the process of their formation is called crystallization. Many substances, however, do not crystallize. They are, in that case, said to be amorphous, their molecular condition being distinguished as amorphism. Water, salt, sugar, are examples of crystallizable, gum and glass of amorphous bodies.

52. Crystals in Nature.-Nature teems with crystals. When it snows, the heavens shower them down, and ice is a mass of crystals, only Crystal of Quartz. so blended that we cannot distinguish them. Geology teaches that the materials of the globe were formerly in a melted state, so that in the slow process of solidification the opportunity was offered on the grandest scale for the formation of crystals. Hence vast rocky systems have their constituents crystallized, and are known as the crystalline rocks. Metallic ores are nearly all crystallized, and immense regions of granite are but mountains composed of crystals, varying in size from particles that can only be distinguished by the aid of the microscope up to masses sometimes weighing several hundred pounds.

53. Artificial Crystals.-Crystals may be artificially produced in various ways, as-from solutions, by the slow cooling of bodies in a state of fusion, by the condensation of gases, or even by rearrangement of the molecules of solids. When chemical action produces bodies not before present, they very frequently make their appearance in the form of crystals.

54. Crystals by Solution.-It has already been stated that the solvent power of liquids for any solid body is generally greater at high than at low temperatures. When, therefore, a hot, saturated solution of any such substanceas, for example, alum in water-is allowed to cool, a portion of the solid separates, and, in doing so, assumes the form of crystals. The liquid which remains after their formation has ceased is called the mother-lye, or mother-liquor.

FIG. 18.

55. Crystals by Fusion.-Nearly all bodies when cooled after melting take the crystalline form, though this may not be at first perceptible. The spaces left between the crystals which first form are completely filled up by the portions which solidify afterward, so that fracture reveals only a general crystalline structure, as may be observed in broken cast-iron and zinc. Common sheet-tin is beautifully crystallized, though it is not apparent. If with weak acid we wash cff the thin surface-film of metal, which had cooled too rapidly to crystallize, the structure will be revealed of a beautiful feathered appearance. To obtain crystals by fusion, the excess of liquid must be removed from around those which are first formed. In this way beautiful sulphur-crystals are produced. If a quantity of this substance be melted, and then allowed to cool till a crust forms upon the surface and sides of the vessel, crystals will be formed within, which may be seen either by breaking the vessel (Fig. 18), or by piercing the crust and draining off the intericr liquid.

Sulphur-Crystals.

56. By Sublimation.-Solid substances vaporized (sublimed) may be condensed in the crystalline form, as iodine, sulphur, arsenic. Camphor thus vaporizes and condenses in brilliant crystals upon the sides of apothecaries' jars by the rise and fall of common temperatures.

57. Crystallization in the Solid State.-The strong tendency of molecules to assume crystalline shape is manifested even in solids. Thus sugar-candy, at first transparent and amorphous, after some time becomes opaque and crystalline. Glass, by long-continued heat, though it does not melt, becomes also opaque and crystalline Réaumur's porcelain). Brass and silver, when first cast, are tough and uncrystalline, but, when repeatedly heated and cooled, they become brittle, and show traces of crystallization. Even the little liberty the particles obtain by the motions of