LIME 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 cakiwn ; 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 950 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 may be 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, er oxide of calcium, can be determined to rigid precision by my instrument for analyzing the car bonates. 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 + 3.502 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 con verted 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 consider able, 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 beat form of a lime-kihi 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 spreadmg 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 ftwnace, 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 Heathorrfs patent kiln, combined with the manufacture of coke, and described under the article hoz. The frustruui 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 inany 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. Ramon, may be con siderably 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 for the purpose, and an excellent substitute for coal where the latter is scarcer or dearer. All hinds 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 terms, 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.