One of the primitive earths; and since the discovery by Sir H. Davy of its metallic base, which he denominated calcium, it is regarded by chemists as the oxide of calcium; that important substance commonly called lime being found to be a combination of calcium and oxygen. The nature of lime is proved by the phenomena of the combustion of calcium; the metal changing into the earth with the absorption of the oxygen gas. Lime is soluble in 450 parts of water, according to Sir H. Davy, and in 760 parts according to other chemists. The solution is called lime water, which is limpid, but has an acid taste, and turns vegetable blues to green. If lime water be allowed to stand, a scum called the cream of lime forms on its surface; and if this be removed another follows, till by this means the whole of the lime maybe separated from the water. If the lime be not skimmed, the cream, after having acquired a certain thickness, precipitates and falls to the bottom. Pure lime, or calcareous earth, is never found native; but in combination with acids, particularly the carbonic, it exists in prodigious quantities. Marble, limestone, and chalk, are all carbonates of lime; gypsum is a sulphate of lime.

Berzelius attempted to determine the prime equivalent of calcium, from the proportion in which it combines with oxygen to form lime: on which Dr. Ure remarks that " his results can be regarded only as approximations, in consequence of the diffi-culties of the experiment. The prime equivalent of lime, or oxide of calcium, can be determined to rigid precision by my instrument for analyzing the carbonates. By this means I find that 100 parts of carbonate of lime consist of 43.60 of carbonic acid + 56.4 lime; whence the prime equivalent proportions are 2.75 acid + 3.562 base."

The operation called burning lime, consists in exposing marble, limestone, chalk, oyster shells, or any other carbonate of lime, for some time to a white heat, by which means the carbonic acid and water contained in these substances are expelled; and the earth which has the peculiar characters assigned to lime, is left behind in a mass which has little coherence, and is therefore easily reduced to powder. It is usually called quick-lime after calcination. Newly prepared it absorbs water with great avidity; it will absorb one-fourth of its weight of that fluid, and still remain perfectly dry. If a sufficient quantity of water be poured upon it, the lime falls into powder; some of the water is converted into vapour by the disengaged caloric of that part which unites with the lime; this is called the slacking of lime: if the quantity slacked be considerable, and performed in a dark place, light will be observed as well as heat.

The kilns for burning lime are of a great variety of forms, according to the kind of fuel used, and the manner in which they are to be wrought. Some persons affirm that the best form of a lime-kiln is that of an egg placed upon its narrow end, having part of its broader end struck off, and its sides somewhat compressed towards the lower extremity; the ground plate, or bottom of the kiln, being nearly an oval, with an eye or draft hole towards each end of it. It is supposed that two advantages are gained by this form over that of the spreading inverted cone (also much used). By the upper part of the kiln being contracted, the heat does not fly off so freely as it does in the spreading cone; on the contrary, it thereby receives a degree of reverberation which adds to its intensity. But the other, and more valuable effect is said to be this; when the cooled lime is drawn out at the bottom of the furnace, the ignited mass in the upper parts of it settles down freely and evenly into the central parts of the kiln.

One of the best kilns that we are acquainted with is Heathorn's patent kiln, combined with the manufacture of coke, and described under the article Iron. The frustrum of a cone is a form of kiln much used; and it may be some advantage to hollow or arch out the upper part, which is frequently done. In many parts of the south of England, lime is prepared from the calcination of chalk in kilns sunk in the earth, of the form of inverted cones, and lined with brick; the base of the cone is about 10 feet in diameter, and about 14 feet deep. It is calculated that a kiln of this kind will yield 150 bushels of lime in 24 hours. When the chalk is dry, about 5 bushels of it may be burned with 1 bushel of coal; but when damp, or in the winter, not more than 4 bushels by 1 bushel of coal. In Yorkshire, and some other places where coal is abundant, calcareous slate and limestone are burned in great pieces stratified with coal* in these cases the consumption of coal is equal to more than a third part of the lime produced. The waste of fuel in this process renders it very ineligible where coals are dear.

The saving of fuel in the use of kilns is apparent from the previous accounts, but that saving, according to Mr. Rawson, may be considerably increased by inclosing the kiln at the top, and building a chimney over it; and it seems to follow that the higher that chimney is the better.

Some lime-burners prefer peat to coal for the fuel; but that preference has probably arisen from an injudicious management of coal. Mr. Dodson asserts peat to be more economical than coal; that coal, by its excessive heat, causes the limestone to run into solid lumps, which it never does with peat, as it keeps them in an open state and admits the air freely. That the process of burning goes on more slowly with coal, and does not produce half the quantity of lime. This inconsistency requires no comment; nevertheless peat is a very useful fuel fur the purpose, and an excellent substitute for coal where the latter is scarcer or dearer. All kinds of lime exposed to the air recover nearly their original weight, except chalk lime, which, although long exposed, never recovers more than seven-eighths of its original weight. Some limestones, as Portland-stone, yield a very white lime; others, as chalk and roe-stone, a lime with a yellowish cast; the latter is best adapted for mixing with tarras, puzzolano, or Parker's cement, for buildings under water.

It has long been said by lime-burners, that if limestone be imperfectly burned in the first instance, no further exposure of it to the fire will produce quick-lime. This assertion, which it was supposed was the offspring of ignorance, has been confirmed by M. Vicat, in a valuable treatise lately published by him on mortar and cements. Such lime, which is technically termed dead lime, does not slake with water, but upon being ground and made into a paste with water, differs from common mortar by setting under water.

Whiting is a fine carbonate of lime, made in some places by grinding soft chalk in a mill, separating the finer particles by washing them over in water, letting the water settle, and making up the sediment into loaves, which are exposed to the air to dry. There are numerous manufactories beside the river Thames, where whiting is thus prepared, the loaves being exposed on shelves in lofty sheds, which form, as it were, the vertical external walls of the buildings. In some places whiting is made from lime by slaking it with a little water, then grinding it in a mill with water, exposing the lime water to the air for some time to absorb the carbonic acid from the atmosphere, washing over the sediment, making it into loaves, and drying them. When made into small loaves it is called Spanish white; and if in small drops, prepared chalk; the creta preparata of the apothecaries. It is principally used as a white paint, either alone, or mixed with white lead; the inferior priced white lead has a large proportion of whiting mixed with it.

Spanish white and prepared chalk are likewise extensively used to saturate acids in liquids in various chemical and manufacturing operations.

Lime with water form a paste of but little cohesion; for common mortar is mixed with rough sand to give it firmness; but the mortar for the outermost covering of in-door work, is mixed with hair to give it cohesion without lessening its capability of receiving a smooth surface. As lime absorbs carbonic acid gas as well as water from the atmosphere, it should be made into mortar before it has imbibed any considerable portion of it, otherwise it will be of little value. It is by the absorption of carbonic acid that mortar acquires hardness, its lime being slowly converted again into the state of limestone; but the hardness will not be perfect unless undisturbed from its commencement; when this circumstance is observed it soon acquires a moderate degree of hardness, but ages are probably required for it to attain its maximum. The silex or sand mixed with lime operates by hastening its crystallization. Lime, though infusible alone, promotes the fusion of all the other earths, and is extensively used in smelting iron ores; it serves as a flux to the alumine and silex which the ores of that metal contain.

Marl, which is of so much value in agriculture, consists of a mixture of lime and clay, and it is the calcareous part of its composition to which its value is owing; if the quantity of lime in it do not exceed 30 per cent. it is worthless. Every good soil contains a portion of carbonate of lime, which materially assists in retaining the moisture necessary to active vegetation. Limestone containing much magnesia is unfit to afford lime for the farmer's use; it may be known from good limestone by its being much longer in dissolving in acids. Lime is used by the soap manufacturer to render his soda caustic: it enters into the composition of glass, which it renders less liable to attract moisture, and less brittle than it would otherwise be. It is employed in the manufacture of glue to prevent its becoming flexible by the ready absorption of moisture. It is used by the tanner to facilitate the removal of the hair from skins. It is used by the sugar refiner to absorb the acid, which would prevent the sugar from crystallizing. A solution of lime is employed to cleanse feathers from their animal oil, and render them sweet and fit for use. Acids dissolve pure lime with effervescence, but heat is evolved during the solution.

Water impregnated with carbonic acid will dissolve a much larger quantity of it than before; and when deprived of this acid by exposure to the air, the lime it held in solution is precipitated; hence the formation of stalactites and incrustations found in caverns. The crystals of the solutions of lime in acids form what are called spars. The beautiful spar called fluor spar, or Derbyshire spar, is a fluate of line, that is, a combination of lime and the fluoric acid. Combined with muriatic acid, large quantities of lime are held in solution by the waters of the ocean. Combined with sulphuric acid lime forms gypsum; gypsum, when calcined by a moderate heat, is called plaster of Paris. Combined with the oxymuriatic acid, or chlorine, it forms chloride of lime, the famous salt used in Bleaching, (which see.) Combined with phosphoric acid, lime forms the solid parts of the bones of all animals. The shells of testaceous animals consist chiefly of carbonate of lime cemented by a small portion of animal glue; while those of crustaceous animals always contain more or less of phosphate of lime, which approximates them to the nature of bone.