LIME is the oxide of the metal calcium (q.v.), and is known in chemistry as one of the alkaline earths. Its symbol is Ca°, its equivalent is 28, and its specific gravity is 3.18. Iu a state of purity it is a white caustic powder, with an alkaline reaction, and so infusible as to resist even the heat of the oxhydrogen jet. See DnummoNn LIGHT. It is obtained by heating pure carbonate of lime (as, for instance, Carrara marble or Ice land spar) to full redness, when the carbonic a.cid is expelled and lime is left. Com mercial lime, which is obtained by burning common limestone in a kiln, is usually very far from pure. This compound (Ca0) is known as quicklime, or, from the ordinary method of obtaining it, as burned lime, to distinguish it from the hydrate of lime or slaked lime, which is represented by the formula CaO,H0. On pouring water on quicklime, there is an augmentation of bulk, and the two enter into combination; and if the pro portion of water be not too great, a light,,white, dry powder is formed, and a great heat is evolved. On exposing the hydrate to a red heat, the water is expelled, and quicklime is left.
If quicklime, instead of being treated with water, is simply exposed to the air, it , slowly attracts both aqueous vapor and carbonic acid, and becomes what is termed air slaked, the resulting compound in this case being a powder which is a mixture (or pos sibly a combination) of carbonate and hydrate of lime.
Lime is about twice as soluble in cold as in boiling water, but even cold water only takes up about vid-u of its weight of lime. This solution is known as lime-water,. and is much employed both as a medicine and as a test for carbonic acid, which instantly renders it turbid, in consequence of the carbonate of lime that is formed being more insoluble even than lime itself. It must, of course, be kept carefully guarded from the atmosphere, the carbonic acid of which would rapidly affect it. If, in the preparation of slaked lime, considerably more water is used than is necessary to form the hydrate, a white semi-fluid matter is produced, which is termed milk of lime. On allowing it to
stand, there is a deposition of hydrate of lime, above which is lime-water.
The use of lime in the preparation of mortars and cements is described in the articles on these subjects. Lime is also largely employed as a manure (see below), and in the purification of coal-gas, in the preparation of hides for tanning, for various laboratory processes (from its power of attracting water), etc. Its medicinal uses are noticed below.
The following are the most important of the salts of lime. Sulphate of lime (CaO,S03) occurs free from water in the mineral anhydrite, but is much more abundant in combi nation with two equivalents of water in selenite, and in the different varieties of gypsum and alabaster. See Gypsum.
Carbonate of lime (CaO,CO3) is abundantly present in both the inorganic and organic kingdoms. In the inorganic kingdom it occurs in a crystalline form in Iceland spar, Aragonite, and marble—in which it is found in minute granular crystals--while in the amorphous condition it forms the different varieties of limestone, chalk, etc. It is always present in the ashes of plants, but here it is, at all events, in part the result of the combustion of citrates, acetates, malates, etc., of liinc. It is the main constituent of the shells of crustaceans and mollusks, and occurs in considerable quantity in the bones of man and other vertebrates. Carbonate of lime, held in solution by free carbonic acid, is also present in most spring and river waters, and in sea-water. Stalactites, stalagmites, tufa, and travertin are all composed of this salt, deposited from calcareous waters. Certain forms of carbonate of lime—the Portland and other oolites, some of the mag nesian limestones, etc.—are of extreme value for building purposes, and the various uses of the finer marbles (q.v.) are too well known to require comment.