Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/669

 LIME LIME is the name of the strongly basic monoxide CaO of the metal calcium. This base is widely diffused throughout the three kingdoms of nature iu the form of salts, of which the carbonate CaCO 3 and the hydrated sulphate CaS0 4 .2H 2 O are by far the most abundant. Both are found in the mineral kingdom in a variety of forms. Of native carbonates of lime, calc-spar (Iceland spar), though comparatively rare, may be mentioned first as representing the purest native form of the compound. It generally presents itself in the form of well-developed transparent colourless rhombohedra, which possess to a remarkable degree the property of producing double refraction of light, whereupon is founded its applica tion in the construction of certain optical instruments. Of the varieties of massive or crystalline carbonate of lime, those which, through the fineness of their grain and other qualities, lend themselves for the purposes of the sculptor go by the name of marble, while the remainder are embraced under the generic term of limestone. This name, however, is understood to exclude chalk, a soft, amorphous variety which, according to Ehrenberg, consists mainly of Foraminifera shells. All limestones contain at least traces of magnesia. When this foreign base is present in considerable proportion the rock is termed &quot;dolomite&quot; (see MAGNESIUM). Among the native forms of (hydrated) sulphate of lime the mineral &quot; selenite &quot; (glacies Marias) corresponds to Iceland spar among the carbonates. It forms colourless transparent clino-rhombic prisms, generally united into &quot; twins,&quot; and flattened down into plates readily cleavable along planes parallel to the surface. Hardness ranges from T5 to 2; the specific gravity is 2 3. Far more common than selenite are the massive varieties known as ALABASTER (see vol. i. p. 439) and ordinary GYPSUM (vol. xi. p. 337). Both sulphate and carbonate of lime, apart from their occurrence as independent minerals, are almost universally diffused throughout the earth s crust, and in the waters of the ocean. Now the sulphate is appreciably soluble in even pure water, while the carbonate, though practically insoluble in pure, is quite decidedly soluble in carbonic acid water. As all atmospheric water must necessarily hold carbonic acid gas in absorption, most natural waters, and certainly all deep-well waters, are contaminated with more or less of bicarbonate or sulphate of lime, or with both. When such a water is being boiled, there is an escape of the free and the loosely combined carbonic acid, and the carbonate of lime comes down as a loose precipitate or as a &quot; crust&quot;; and, when the water is sufficiently concentrated by evaporation, the sulphate likewise is partly deposited. The decomposition of the &quot;bicarbonate&quot; in fact takes place, though slowly, even at ordinary temperatures, when the water iu which it is held in solution is exposed to the atmosphere. It is in this manner that stalagmites and stalactites frequently seen within rock-caverns are produced, and there is no difficulty in accounting for the grotesque and fantastic forms which the latter often exhibit. Quicklime. The native carbonate always serves as the starting-point in the preparation of calcium compounds. From it the oxide CaO, known as quicklime or caustic lime, is produced industrially by heating limestone or marble in kilns, between layers of fuel, which in the United Kingdom is generally coal. The carbonic acid goes away with the gaseous products of combustion, and the oxide remains in unfused lumps of the form of the original stones. Lime, when pure, is an amorphous white solid, which is absolutely infusible and non volatile : and on this account, when raised to high temperatures, it emits a brilliant white light (&quot; lime-light &quot;). The commercial article is generally grey or otherwise discoloured by the presence of foreign metallic oxides. The decomposition that goes on in a limekiln is not brought about by the effect of heat alone. Gay-Lussac found loug ago that carbonate of lime, when heated to intense redness in a closely covered crucible, loses its carbonic acid only very slowly, while the acid goes oiF readily even at somewhat lower temperatures when a current of steam is passed over the heated limestone. This may be accounted for by assuming that the steam, in the first instance, produces hydrate from the carbonate of lime, which latter then at once breaks up into its two components. More probably, however, the steam acts only by producing a quasi vacuum, that is, by clear ing out the carbonic acid which, if allowed to stagnate even at high temperatures, would react on the quicklime produced, thus pre venting the decomposition of a portion of the carbonate. Quicklime acts readily and energetically on water, with evolution of much heat (269 units per unit weight of lime, Berthelot) and formation of a bulky white powder of the hydrate CaOH 2 O or Ca(OH) 2. This powder readily mixes with water into a smooth paste, which may be diluted to a milky liquid milk of lime. This, when filtered through paper, yields &quot; lime-water,&quot; a strongly alkaline liquid con taining about -nj^th of its weight of lime (calculated as CaO). When boiled it deposits a part of its dissolved lime as such, and when exposed to ordinary air it quickly draws a skin of carbonate of lime. Hence its application as a reagent for carbonic acid, and the extensive use of milk of lime (whitewash) as a cheap white pigment in wall- painting. Lime paste, as every one knows, is most extensively used as a mortar or cement for bricks and stones in building. For this purpose it is always mixed with a certain proportion of sand. This admixture in all probability was originally intended only to save lime and prevent shrinking. But it is now generally assumed to have a chemical function, causing the formation of a hard silicate of lime pervading and thus strengthening the mortar. Some chemists deny the practical importance though not the occurrence of this silication ; what admits of no doubt is that the hardening of mortar involves the very gradual conversion of the original hydrate into car bonate of lime. Under the name of plaster, a fine smooth paste of lime and sand, with short hair to increase the tenacity of the mixture, is a most important material for coating the internal walls and roofs of ordinary buildings. Hydraulic Cements. Ordinary mortar, on account of the solubility of lime in water, is unfit for aquatic mascnry; for this purpose hydraulic cements must be used. Of these there are a great variety, which, however, mostly agree in this that they consist of calcined mixtures of limestone and clay (preferably alkaliferous clay) and other silicates. By calcining such mixtures at temperatures short of that at which a glass would be produced, the lime becomes caustic, and part of the caustic lime, by uniting with the clay (and silicate generally), forms a silicate sufficiently basic to be disintegrate by acids and even by water. When such cement, as a powder, is mix^ed with water, the lime acts upon the silicate of alkali and the gelatinous silica-hydrate transitorily produced, and with the silica and alumina and oxide of iron unites into a hard, waterproof, very complex, silicate mixture. H. Ste Claire Deville having found that magnesia has hydraulic properties, hydraulic cements have been made by calcining dolomites of the proper composition so far as to decompose only the carbonate of magnesia (into MgO and CO.,). See CEMENT, vol. v. p. 328. Lime, being the cheapest of powerful bases, is largely used in chemical manufacturing. It serves for the caustic- izing of soda, for the preparation of ammonia from ammonia salts, and for the manufacture of bleaching powder. It also enters into the composition of certain kinds of glass, and is used (as lime or as carbonate), in the making of soda ash. Lime Salts. These can in general be prepared by the saturation of the respective acids with lime hydrate. Thus the (pure) car-