Lime is the monoxide of the metal Calcium (q.v.), and is known in chemistry as one of the alkaline earths. Its symbol is CaO, its equivalent is 56, and its specific gravity is 3.08. In a state of purity it is a white caustic powder, with an alkaline reaction, and so non-fusible as to resist even the heat of the oxyhydrogen flame (see LIME-LIGHT). It is obtained by heating pure carbonate of lime (as, for instance, white Carrara marble or Iceland spar) to full redness, when the carbonic acid is expelled and lime is left. This compound, CaO, 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 . On pouring water on quicklime there is an augmentation of bulk, and the two enter energetically into combination; and, if the proportion 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 vapour and carbonic acid, and becomes what is termed air-slaked, the resulting compound in this case being a powder which is a mixture of carbonate and hydrate of lime. Owing to this property quicklime is employed to prevent instruments and other objects from being rusted or otherwise injured by damp. A jar is partly filled with lime and placed beside the articles in a glass case or box.
Lime is about twice as soluble in cold as in boiling water, but even cold water only takes up about 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 insoluble. 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 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. Milk of lime is much used as a whitewash.
Lime prepared for building and other purposes by burning limestones in kilns often contains a considerable amount of impurity. But certain kinds of slightly impure are better than pure lime for making mortar. On the other hand, the lime which enters into the composition of plate and sheet glass, and which is used in some chemical industries, requires to be obtained from a nearly pure limestone. Chalk and white marble consist of almost pure carbonate of lime, but many of even the dark coloured limestones from different geological formations do not contain more than from 2 to 5 per cent. of foreign bodies, and these when burned generally yield a lime sufficiently pure for most purposes. Some limestones, again, contain from 20 to 30 per cent. of impurities, which commonly consist of silica, clay, magnesia, oxide of iron, and other bodies. These impure kinds often yield excellent hydraulic lime, which is very generally made by burning a limestone containing from 12 to 20 per cent. of silica, or of clay in which silica predominates. A less valuable hydraulic lime is prepared from a limestone containing a considerable amount of magnesia as well as clay. According to the absence or presence of foreign bodies, their nature and extent, limes are classed as (1) rich, fat, or pure lime; (2) impure or poor lime; and (3) hydraulic lime (see CEMENTS). When the percentage of magnesium carbonate in a limestone is high it is called a magnesian limestone, and this requires less fuel to burn it than a pure or nearly pure limestone. See DOLOMITE.
Besides the uses of lime noticed above, it is employed in the purification of coal-gas, in the unhairing of hides for tanning, in the preparation of stearic acid for candlemaking, for causticising alkalies, in the smelting of some metals, &c. Lime precipitates organic impurities from vegetable solutions containing sugar.
The following are the most important of the salts of lime. Sulphate of lime (calcium sulphate) is found native free from water, , as the mineral Anhydrite (q.v.), but more abundantly in the hydrated form, , as Gypsum. Sulphate of lime is a constituent of sea-water, and is also frequently present in drinking-water. For laboratory use a solution of sulphate of lime is made by shaking up the powder of burnt gypsum in water. See CALCIUM, and GYPSUM.
Carbonate of lime (calcium carbonate, ) 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; while in the amorphous condition it forms the different varieties of common limestone, chalk, &c. It is always present in the ashes of plants, and it is the main constituent of the shells of crustaceans and molluscs, and occurs in considerable quantity in the bones of man and other vertebrates. See LIMESTONE; and BUILDING STONE, MARBLE.
Chloride of Calcium, , is a remarkably deliquescent substance and one of the most soluble of salts on account of its great attraction for moisture. In the solid state it is much used for drying gases, and the pipes of freezing machines are filled with a solution of it to convey the low temperature produced to the cooling vessels.
There is a combination of lime with an organic acid—viz. oxalate of lime—which is of great importance in pathology as a frequent constituent of urinary calculi and sediments; for a description of it, see OXALIC ACID.
There are several compounds of phosphoric acid and lime, of which the most important is the tribasic phosphate of lime (tricalcium orthophosphate), sometimes termed bone phosphate, from its being the chief ingredient of bones. This phosphate is represented by the formula , and occurs not only in bones, but also in the minerals apatite and phosphorite, and in the rounded nodules termed Coprolites (q.v.). It forms four-fifths of the ash of well-burned bone, the remaining fifth being chiefly carbonate of lime. This ash is known as bone-earth, and is employed as a manure and in the preparation of phosphorus, &c.
The soluble salts of lime give no precipitate with caustic alkalies, but yield a white precipitate with their carbonates. These reactions are also common to the salts of barium and strontium. Solution of sulphate of lime gives a white precipitate with the salts of barium and strontium. The most delicate test for lime is oxalate of ammonia, which, even in very dilute solutions, throws down a white precipitate of oxalate of lime. This precipitate is insoluble, except in mineral acids.
For the substance commonly designated as chloride of lime, see BLEACHING POWDER. For lime as manure, see MANURES.
Lime-compounds in Materia Medica.—Quicklime, in association with potash, either as the Potassacum calce, or as Vienna Paste, is occasionally used as a caustic. Lime-water, mixed with an equal quantity or an excess of milk, is one of our best remedies for the vomiting dependent on irritability of the stomach. From half an ounce to two or three ounces may be thus taken three or four times a day. Its use as a constituent of Carron oil in burns is noticed in the article LINIMENTS. Chalk, or carbonate of lime, when freed from the impurities with which it is often associated, is used as a dusting-powder in moist excoriations, ulcers, &c.; and, in the form of chalk mixture and compound powder of chalk, is a popular remedy in various forms of diarrhoea. A mixture of an ounce of precipitated carbonate of lime and a quarter of an ounce of finely-powdered camphor is sold as Camphorated Cretaceous Tooth-powder.