1309. When burnt lime in exposed to the atmosphere, in a certain time it becomes combined with carbonic acid gas. Quicklime, when first made, is caustic and burning to the tongue, renders vegetable blues green, and is soluble in water; but when combined with carbonic acid, it loses all these properties, its solubility, and its taste: it regains its power of efferveseing, and becomes the same chemical substance as chalk or limestone. Very few limestones or chalks consist entirely of lime and carbonic acid. The statuary marbles, and certain of the rhomboidal spars, are almost the only pure species; and the different properties of limestones, both as manures and cements, depend upon the nature of the substances contained in the limestone; for the true calcareous element, the carbonate of lime, is uniformly the same in nature, properties, and effects, and consists of 45 parts of carbonic acid and 56 parts of lime in 100 parts. When a limestone does not copiously effervesce in acids, and is sufficiently hard to scratch glass, it contains siliceous, and probably aluminous earth: when it is deep brown or red, or strongly coloured with any of the shades of brown or yellow, it contains oxide of iron; when it is not sufficiently hard to scratch glass, but effervesces slowly, and makes the acid in which it effervesces milky, it contains magnesia; and when it is black, and emits a fetid smell if rubbed, it contains coaly or bituminous matter. When newly-burnt lime is exposed to the air, it soon falls into powder: in this case it is called slaked lime; and the same effect is immediately produced by throwing water upon it, when it heats violently, and the water disappears. Slaked lime is merely a combination of lime with about one third of its weight of water; i.e. seventy-five parts of lime absorb twenty-five parts of water, and form a definite compound called by chemists hydrate of lime; and when hydrate of lime becomes carbonate of lime by long exposure to the air, the water is expelled, and the carbonic acid gas takes its place. Lime moistened with sea-water is more efficacious than common lime.