SNOW; FUSING AND FREEZING POINTS; HEAT. It appears from the researches. of Arego and Fresnel, that notwithstanding the gradual dilatation of water below 39', its refractive power on light continues to increase regularly, as though it contracted. Its density at 60', and at the level of the sea, is taken at 1.000, and forms the standard of comparison for all solids and liquids, hydrogen being similarly taken as the standard of comparison for gases and vapors. Distilled water is 815 times heavier than air; a cubic inch weighs, in air at 62', with the barometer at 30 inches, 252.438 grains, and in wen°, 252.722 grains, the grain being of the avoirdupois pound. See AvoIR DUPOIS, GALLON. For all practical pm•poses, water may be considered as incompres sible; but very accurate experiments have shown that it does yield to a, slight extent when the pressure employed is very great; the diminution of volume for each atmos phere of pressure being about 51-millionths of the whole.—See Miller's Chemical Physics, Sit ed. p. 41. Water evaporates at all temperatures, and under the ordinary pressure of the atmosphere, boils at 212°, passing off in the form» of steam, which, in its state of greatest density at 212', compared with air at the same temperature, and with an equal elastic force, has a spec. gray, of 0.025. In this condition it may be represented as con taining. in every two volumes, two volumes of hydrogen and one volume of oxygen.
See BOILING, STEAM, VAPOR.
Water is the most universal solvent with which the chemist is acquainted, and its operations in this respect are equally apparent, although on very different scales, on the surface of the globe and in the laboratory. This solvent action is usually much increased by heat, so that a hot aqueous saturated solution deposits a portion of the dissolved matter on cooling. Some substances are so soluble iu water, that they extract its vapor from the atmosphere, and dissolve themselves in it. Moreover, when water is heated in a strong closed vessel to a temperature abov that of the ordinary boiling-point, 212°, its solvent powers are much increased. Pieces of plate and glass, acted upon for four months by water at (in a steam-boiler), were found by the late prof. Turner to be reduced to a white mass of silica, destitute of alkali; while stalactites of siliceous mat ter, more than an inch in length, hung from the little wire cage which inclosed the glass —an experiment illustrating the action which goes on in the springs of Iceland, which deposit siliceous sinter. All gases are soluble in water, but water dissolves very unequal quantities of different gases, and very unequal quantities of the same gas at different temperatures. Some gases are so extremely soluble in this fluid, that it is
necessary to collect them over mercury. For example, at 32°, 1 volume of water dis solves somewhat less than of its volume of hydrogen, and exactly -ell of its volume of nitrogen, while it dissolves 506 and 1050 volumes of hydrochloric acid and ammonia gases; and while at 32° water dissolves 1.8 times its volume of carbonic acid, it dissolves only half that volume of the gas at 60'.
Water enters into combination with acids, bases, and salts. When an acid has once been allowed to combine with water, the latter can seldom be entirely removed unless by the intervention of a powerful base, which displaces the water, and allows of its removal by heat. For example, if sulphuric acid be largely diluted with water, and exposed to heat, watery vapor alone at first passes off; but as the temperature is raised to about 600°, a point is reached at which acid and water distil over together. The liquid at this stage of concentration is found to be composed of one equivalent of acid and one of water The further separation of the water can only be effected by the addition of a base, as potash, oxide of lead, etc. Water which, in this case, supplies the place of a base, is called basic water, and the compound is called a hydrate, or is said to be hydrated. Similarly, water combines with strong bases, such as potash and soda, and heat can only succeed in reducing a mixture of potash and water to a con ditibn represented by one equivalent of each (HO,K0); and this last equivalent of water can only be removed by the addition of an acid. In this case, the water in combination with the base acts the parts of an acid. These compounds also are hydrates. In these cases of acids and bases, the one equivalent of water cannot be removed without completely altering the chemical character of the body. (See, for instance, in the article Sux.. PIIURIC ACID, the difference between the properties of hydrated sulphuric acid and slit phuric anhydride.) In the case of many salts, however, a certain of the water entering, so to speak, loosely into their composition may be expelled by beat without altering the properties of the salt. The water capable of being thus got rid of is•called. water of crystallization, and is taken up by the salt in the act of crystallizing. The form of the salt depends upon this water of crystallization. In chemical fortuuke, this variety of water is represented by Aq instead of by HO. For example, in the formula for rhombic phosphate of soda-2Na0,110,P0,1-24Aq—the HO represents an equivalent of basic water, while 24Aq represents 24 equivalents of water of crystallization.