Nicholson 1795/I

William Nicholson, A Dictionary of Chemistry, Exhibiting the present State of the Theory and Practice of that Science, its Application to Natural Philosophy, the Processes of Manufatures, Metallurgy, and numerous other Arts dependant on the Properties and Habitudes of Bodies, in the Mineral, Vegetalbe, and Animal Kindoms… I, London [George G. Robinson – John Robinson] 1795.

pp. 127–130

Antimony

ANTIMONY. The word antimony is always used in commerce to denote a metallic ore, consisting of sulphur combined with the semi-metal which is usually called regulus of antimony. But as it appears proper and convenient that the metallic substance itself should be distinguished by a simple term, instead of deriving its name from one of its ores, many chemical writers use the word antimony to denote the metallic substance or regulus, and in this acceptation it is used in the present article.

Antimony is of a silvery white colour, very brittle, and of a plated or scaly texture. Its specific gravity is moderate. Soon after ignition it melts, and by a continuance of the heat it becomes calcined, and rises in white fumes, which may afterwards be volatilized a second time, or fused into a hyacinthine glass, according to the management of the heat: they are called argentine flowers of regulus of antimony. In closed vessels, the regulus rises totally without decomposition. This metallic substance is not subject to rust by exposure to air, though its surface becomes tarnished by that means. Its calces are soluble in water; and in that respect they resemble the calx of arsenic, probably by an approach towards the acid state.

Vitriolic acid, boiled upon the regulus of antimony, calcines the greater part, so as to render it insoluble, the acid being at the same time decomposed. Much vitriolic acid air escapes, and towards the end a small quantity of sulphur is sublimed. By washing the residue in water, a vitriolic salt of antimony is separated from the calx, which does not crystallize.

Nitrous acid very readily attacks antimony in the cold. Most part of the metal is calcined by this action; but a portion is dissolved, and affords deliquescent crystals, decomposable by heat. The calx of antimony formed by this acid is very white, and difficult of reduction.

Continued digestion is required for the solution of regulus of antimony in the marine acid. A considerable quantity is, however, at length dissolved, which affords very deliquescent crystals. This salt melts by the application of heat, and is decomposed by distilled water, in the same manner as the butter of antimony, from which it does not much differ.

Dephlogisticated or aerated marine acid dissolves the regulus of antimony with great facility.

Aqua regia, composed of seven parts nitrous and one marine acid, dissolves it very readily, but lets fall a portion of white calx as it cools. The solvent power of either of the three ancient mineral acids on this semi-metal appears to be increased by mixture with any one of the others.

Earthy substances do not act on the regulus of antimony in the dry way. Its calx however enters readily into the composition of glass, to which it imparts more or less of an hyacinthine colour. When fused with vitriolated tartar it converts it partly into hepar or liver of sulphur, which dissolves a portion of the calx of antimony; that is to say, the vital air of the acid calcines the regulus of antimony, while part of the acid becomes converted into sulphur, either simply by the loss of its vital air, or else by that loss, together with the acquisition of phlogiston from the regulus.

Nitre detonates very readily with the regulus of antimony; when equal parts of these substances are projected into a red hot crucible, the residue of calx and alkali is known by the name of diaphoretic antimony. When the saline part is washed out by hot water, the residue is called washed diaphoretic antimony. The water used in the washing contains a portion of the calx suspended by the alkali. This may be precipitated by the addition of an acid, and has been distinguished by the name of ceruse of antimony.

When regulus of antimony is pulverized, and accurately mixed with about twice its weight of corrosive sublimate, a mutual action takes place with the production of heat; and if the mixture be distilled with a gentle fire, a thick fluid comes over, which congeals in the receiver, or in the neck of the retort, and is called butter of antimony. The residue consists of revived mercury, and some regulus and calx of antimony. In this experiment, the dephlogisticated marine acid combines with the antimony, while the mercury is revived, as may be easily explained on either of the two theories of chemistry. If the combination of regulus of antimony and sulphur be used instead of the regulus itself, the mercury will be obtained in the form of cinnabar, at a much greater heat than is required to sublime the butter of antimony.

When butter of antimony is thrown into pure water, an abundant white precipitate or calx falls down, which is a violent emetic, and is known by the name of powder of algaroth.

Nitrous acid dissolves the butter of antimony. The solution, which does not appear to differ greatly from the solution of the regulus in aqua regia, soon deposits a portion of calx. When an equal weight of nitrous acid has been three times distilled to dryness from butter of antimony, the residue, after ignition, is called bezoar mineral, and seems to be little more than a calx of the metal.

Sulphur combines very readily with the regulus of antimony, and forms a substance differing in no respect from the mineral or ore to which the name of antimony is exclusively appropriated. One part of sulphur completely mineralizes four of the regulus.

Liver of sulphur dissolves the regulus of antimony, and affords an orange-coloured precipitate upon the addition of an acid.

Antimony, or the regulus combined with sulphur, was a favourite object of research in the experiments of the alchemists; in consequence of which its properties are much better known than those of the pure regulus. If this substance be heated, it melts, and a considerable portion of the sulphur flies off, at the same time that the regulus becomes calcined, and rises in white vapours. A gentler heat, less than is necessary to fuse it, converts it into a grey calx; this calx contains a portion of sulphur. If it be urged by a stronger heat, it melts into the form of glass, which is more or less transparent, according to the degree of calcination of the metal and the dissipation of the sulphur. When it contains much sulphur, the glass is fusible, opake, and of a dark red colour; whence it has been called liver of antimony.

When acids are applied to crude antimony, they dissolve the regulus, and leave the sulphur. The nitrous acid is best adapted to this solution.

Diaphoretic antimony is most commonly and advantageously prepared by detonating the crude antimony with nitre instead of the regulus; the only difference being, that more nitre is required for the detonation, and that the residue contains vitriolated tartar as well as alkali and calx.

There are several preparations, consisting of combinations of antimony with an alkali, in which the proportions of the ingredients and the state of the calx are very different, according to the nature and management of the processes. Many of these have been highly praised in medicine, at the same time that they have been as strongly exclaimed against for their ill effects. Both these assertions appear to have been well-founded. It is sufficiently proved, that antimonial medicines have produced the happiest effects, and are justly entitled to be considered as very powerful remedies; but, on the other hand, it is equally certain that their great efficacy must have required greater attention in their first exhibition than perhaps may have been paid; and the complicated nature of many of the processes must have rendered it very difficult to produce substances possessing exactly the same properties, or proportion of component parts, at all times.

If antimony be treated with a fixed alkali, either by fusion and subsequent dilution in boiling water, or by simple ebullition, a precipitate is afforded by cooling, which is called kermes mineral, formerly used in medicine. It is thought to consist of the calx of antimony in combination with a portion of sulphur; but its component parts have not been accurately determined, and its properties differ according to the various methods used in preparing it.

The antimonial preparations most commonly used at present are, antimonial wine, and emetic tartar. These, like every other preparation of this semi-metal, are prepared in a variety of ways. The wine may be had by infusing pulverized glass of antimony in Spanish white wine for some days, and then filtering the clear fluid through paper. A very minute portion of the calx is taken up; and this is greater or less, according as the wine is more or less acid; and perhaps according to the temperature of the weather at the time of administering it. On this account it is found necessary to give this medicine cautiously, and by small portions at a time, when it is intended that it shall act as an emetic. The emetic or antimoniated tartar of the London college is thus prepared: – Take of crude antimony and nitre equal parts, separately reduced to powder, mix them, and inject them into a crucible heated to whiteness, that the mixture may melt after deflagration; pour it out, and reserve the yellow mass beneath the scoriæ, under the name of crocus of antimony; reduce this to a very subtle powder; boil it in water, and wash the powder repeatedly in warm water till it becomes perfectly infipid; then take equal parts, by weight, of the washed crocus of antimony and of crystals of tartar, and boil them together in three pints of water for every pound of the mixture, during half an hour; filter the liquor, and after due evaporation set it by to crystallize. – This is the antimoniated tartar. It is a triple salt, consisting of the acid of tartar, united to vegetable alkali, and antimony partially calcined, and is considered as a safe medicine, whose properties and effects are more constant and milder than most other antimonial remedies.

It has not been clearly determined on what circumstances the medical effects of antimony depend. The saline preparations of mercury and other metals are supposed to derive their causticity from their tendency to become reduced to the metallic state; in consequence of which they corrode and decompose other combustible substances: but whether this be the case with antimony is doubtful. It appears necessary, however, that antimony should be in the saline state, in order that it may act upon the animal system. When the regulus is made into those small balls or pills, which, on account of the little change they undergo in passing through the human body, have been called perpetual pills, its purgative action is more or less violent according to the quantity of acid it meets with; and, in the preparations of this semi-metal, their effects seem likewise to be governed by the same circumstance joined to their respective degrees of solubility. The nearly pure and insoluble calx, produced by detonation with a large proportion of nitre, is almost ineffectual; whereas the more soluble calces or combinations are more or less active, according to their respective natures. Hence it appears to follow, that the simplest saline combinations are the most likely to produce effects constantly similar; and that most of the calces and combinations, so highly extolled in the earlier age of chemistry, are attended with dangerous uncertainty in their operation.

Antimony combines with most other metallic substances, and produces mixtures whose properties have been attended to under their respective titles.

Antimony is found either native, in masses of the regulus, composed of shining irregular plates; or calciform, in white crystallized filaments; or combined with sulphur, in the dark blueish or grey friable mineral called antimony, consisting most commonly of brilliant filaments disposed parallel to each other; or, lastly, combined with sulphur and arsenic, in an ore which greatly resembles the foregoing, except that it is of a red or reddish colour.

Native regulus of antimony, or its calx, may be assayed by nitrous acid, which dissolves whatever arsenic it may contain, and only calcines the antimony. The sulphureous antimonial ores are most conveniently analyzed by aqua regia, which takes up the calx, and leaves the sulphur, which may be separated by filtration. The remaining solvent may be examined with the usual precipitants. In the dry way, antimony is separated from its stony parts by fusion in a moderate heat, nearly in the same manner as bismuth, and may be reduced by slowly roasting it, till it becomes converted into a grey calx, which may be briskly fused with twice its weight of black flux.

Antimony and its regulus are chiefly used in medicine, and in some metallic alloys, such as that used for printers’ types, small shot, &c.

pp. 256–258

Cinnabar

CINNABAR. An ore of mercury, consisting of that metal united with sulphur. This combination is also made by art. The native cinnabar is of different shades, from yellowish to a deep red, and even black. It is found either in hard or friable masses, irregularly figured or crystallized in cubes sometimes, though rarely transparent, and its texture is either radiated, striated, scaly, or granular. One hundred parts contain pretty regularly eighty parts mercury and twenty sulphur. In closed vessels it sublimes by heat, but in open vessels it is decomposed.

The principal cinnabar mines which are wrought in Europe are those of the Palatinate and those of Spain. In the Palatinate the ore is decomposed by mixing it with one third of its weight of lime, and distillation in iron cucurbits one inch thick, three feet nine inches long, one foot wide, with an aperture of five inches. These vessels are disposed in a gallery; forty-eight being arranged in two parallel lines, a second row above the first. An earthen pot is one third part filled with water, adapted to the neck of each cucurbit, and accurately luted on. The gallery is heated by a fire lighted at the two ends, and several apertures, formed in the upper part, serve the purpose of chimneys. The heat is kept up for ten or twelve hours before the process is finished.

The above process was also followed at Almaden in Spain, till the year 1647, when the following was adopted on account of its greater simplicity and economy. The furnace is twelve feet high, and four feet and a half diameter within. At the distance of five feet and a half from the ground is an arched floor, upon which the ore is deposited, and a fire is kindled in the ash-hole. The sublimed mercury escapes through twelve apertures, formed in the upper part of the laboratory. To these apertures rows of aludels inserted one in the other are adjusted, and disposed parallel upon a terrace, which terminates in a small building separated into as many chambers as there are files of aludels. Each chamber has a cavity in the middle, to receive the small quantity of mercury which may arrive to that distance.

Every furnace contains two hundred quintals of cinnabar, and the fire is kept up for three days. The sulphur which burns is disengaged in the form of sulphureous acid, and escapes through small chimneys made in each chamber. Every repetition of the process affords from twenty-five to sixty quintals of mercury.

The mine of Almaden has been wrought from time immemorial. Its veins are from three to fourteen feet in breadth; and their breadth is even larger where they join.

Artificial cinnabar is known in commerce by the name of vermilion, and ought to be used in all cases wherein this compound is meant to be applied to medical purposes, on account of its greater purity. The manufacture of this pigment has long been in the hands of the Hollanders, who kept it a secret; and as there is some difficulty in perfectly succeeding in the process, chemical writers have given various methods of performing it; most of which, according to Mr. Tuckert, are inaccurate. This gentleman has given a full account of the method used at Amsterdam, which I find extracted from Crell’s Chemical Journal, and inserted in the fourth volume of the Annales de Chimie. Its contents are as follow:

The manufactory, at which Mr. Tuckert several times assisted, in the preparation of artificial cinnabar, is without the Utrecht port at Amsterdam, and is one of the most considerable in Holland. Forty-eight thousand pounds of vermilion are annually made in three furnaces by four workmen, besides other mercurial preparations.

The ethiops mineral is first prepared by mixing together 150 pounds of sulphur with 1080 pounds of pure mercury, and then exposing the mixture to a moderate heat in a flat-bottomed polished iron vessel, one foot in depth, and two feet and a half in diameter: its form is that of a chocolate machine. Mr. Tuckert does not enter into the particular manipulations of the Hollanders in this part of the process, because the methods of producing the black combination of mercury with sulphur are well known.

The mercurial ethiops thus prepared is in the next place pounded, in order that it may more readily be put into small earthen bottles, capable of holding each about 24 ounces of water. Thirty or forty of these bottles are filled to be in readiness for the subsequent operation.

In the next place three large pots, or sublimatory vessels, made of clay and very pure sand, are taken. These vessels are previously covered with a coating of lute, which is suffered to become perfectly solid and dry before the vessel is used. Mr. Tuckert refers to a German translation of a work of Mr. Machy for the figure of these vessels, as well as for the composition of the lute. As we have not that translation, it is impossible for us to give any further information on these heads than may be gathered from the rest of his memoir. These pots are placed over three furnaces, upon iron circles. The sublimatory vessels may be of different sizes, and the furnaces are constructed in such a manner as that the flame circulates freely round the vessels to two thirds of their height.

When the vessels are duly placed in their furnaces, a moderate fire is first lighted, which is gradually raised until they become red hot. The fuel is turf, or rather peat, commonly used throughout the United Provinces. As soon as the vessels are red hot, a bottle of the ethiops is poured into the first, another into the second, and another into the third. In the subsequent progress of the operation, two, three, and perhaps more bottles may be poured in at a time; but this depends on the strength of the inflammation exhibited by the ethiops after its introduction, the flame of which sometimes rises to the height of four, and even six feet. When this is a little diminished, the mouth of the vessel is covered with a plate of iron, one foot square, and an inch and a half thick, which perfectly closes it. In this way, during thirty-four hours, the whole of the prepared matter is introduced into the three pots; that is to say, for each pot three hundred and sixty pounds of mercury, and fifty of sulphur.

After all the ethiops has been introduced, the fire is duly kept up; and when the whole sublimation has taken place, it is suffered to go out, which requires thirty-six hours from first to last. The workmen know when the fire is too strong or too weak by the appearance of the flame when the iron cover is taken off: if too strong, the flame rises to the height of several feet; if, on the contrary, it be too weak, the flame barely appears playing about the edges of the pot. The proper degree of heat is when, upon taking off the cover, a brisk flame appears, but does not rise more than three or four inches above the opening.

In the last thirty-six hours, the mass was stirred every quarter or half hour with an iron triangle, to accelerate the sublimation. The workmen do this with so much courage, that Mr. Tuckert was every time apprehensive they would fall into the vessels.

When the whole is cool, the vessels are taken out by means of iron circles, which prevent their breaking. The cinnabar is taken out by breaking the vessel. Each vessel constantly affords four hundred pounds of cinnabar, the loss of original weight in each being ten pounds.

The cinnabar does not attach itself to the plates of iron, because they are so frequently taken off, excepting towards the end, when the vessels were left untouched. These plates are not in the least corroded.

The workmen were Germans, and were paid as follows. The foreman, besides his lodging, received twelve guilders a week, which amount to twenty-two shillings and nine-pence of our money; the second received ten guilders, or eighteen shillings and three pence; the third and the fourth received six or seven guilders. They relieve each other night and day, every twelve hours, except holidays and Sundays, so that two men are always at work.

The workmen assured Mr. Tuckert, that they did not remember any accident having happened in the manufactory, though the foreman had been employed for thirteen years; and that, in case of an accident, the whole loss would consist in the broken pot. These four workmen likewise manufactured on the same premises the corrosive sublimate of mercury, and the red calx called red precipitate. The foreman delivers monthly to Mr. Brand the cinnabar and other products of the manufactory, and receives in exchange sulphur, mercury, martial vitriol, common salt, and nitre.

pp. 262–258

Cobalt

COBALT is a semi-metal, of a whitish grey or steel colour, hard and brittle; of a dull, close-grained fracture, and moderate specific gravity. It is rather more difficult of fusion than copper; does not easily become calcined; and its calx is of so deep a blue colour as to appear black. The most remarkable and most valuable property of this metallic substance is, that its calx, when fused with borax, or with alkali and sand, produces a blue glass, known by the name of smalt. The action of air soon tarnishes cobalt; but water has little or no effect upon it.

Concentrated and boiling vitriolic acid, distilled nearly to dryness, combines with this semi-metal. Much vitriolic acid air flies off; and the cobalt is in part calcined, and in part converted into a crystallizable salt, soluble in water, and precipitable by lime and by alkalis in the form of a rose-coloured powder or calx. Diluted vitriolic acid acts upon the calx of cobalt, and forms the same salt.

Nitrous acid dissolves cobalt by the assistance of a moderate heat. Nitrous air is disengaged, and the solution affords deliquescent crystals by evaporation, which do not detonate on ignited coals, but boil up and leave a red calx. Lime and the alkalis precipitate the solution; and, if the alkali be added in excess, it dissolves the precipitate.

The marine acid has scarcely any action on cobalt, unless it be boiling; in which case it dissolves a small portion. It dissolves the calx more readily, with which it forms a red brown fluid, that becomes green when heated. This solution affords deliquescent crystals by evaporation.

Aqua regia dissolves cobalt more easily than the marine, though not so readily as the nitrous acid. This solution is well known as one of the most celebrated sympathetic inks afforded by chemistry. If it be diluted with a sufficient quantity of water to prevent its action upon paper, and then used to write with, the letters are invisible as soon as the clear solution has become dry; but, if the paper be held to the fire for a short time, they appear of a fine green colour; which again disappears by removing it, and suffering it to cool again. If the heat be continued too long after the letters appear, it will render them permanent. This effect seems to be analogous to that which obtains in the marine solution: but none of the efficient causes of change of colour in this, or any other chemical phenomenon, have been hitherto explained.

The acid of borax does not act immediately on cobalt, in the humid way: but borax itself, added to either of the foregoing solutions, effects a decomposition by double affinity; the alkali uniting with the solvent acid, while the acid of borax seizes the cobalt, and forms a scarcely soluble compound, which falls down.

The acid of sugar precipitates cobalt from its solutions, in the form of a pale rose-coloured powder.

Whether alkalis or earths combine with this metal directly, by the intervention of water, has not been determined.

Cobalt does not act on neutral salts in general. It detonates feebly with nitre, when projected into a red hot crucible, with twice or thrice its weight of that salt. The metal becomes calcined by the action of the nitre; but the changes in both substances require farther examination.

Sal ammoniac is not decomposed by cobalt.

Sulphur does not unite with cobalt but with difficulty. Liver of sulphur combines more readily with it. The action of phosphorus, or its acid, on this substance, remains to be ascertained.

This semi-metal unites by fusion with most of the metals and semi-metals, as has before been noticed. Silver, lead, and bismuth, do not mix with it; and zinc does not but with great difficulty.

Cobalt is found native in alloy with arsenic and iron, and of a steel-grained appearance when broken; or in a calciform state, of a black colour, either pulverulent or indurated; or combined with arsenical acid, in the flowers of cobalt, of a red colour; or, lastly, united to sulphur and iron, with or without arsenic, of various shades of redness. Bismuth, nickel, and other substances are contained in these ores. They may in general be distinguished by solution in aqua regia; with which, after dilution with water, they form the sympathetic ink above described.

The native cobalt, and its calciform or sulphureous ores, may be examined by solution in aqua regia, and evaporation to dryness; after which, the calcined cobalt may be dissolved by vinegar. When this calx is precipitated by mild mineral alkali, the regulus may be accounted for, by allowing one hundred grains for every hundred and sixty grains of precipitate. The other component parts of the residue, not taken up by the vinegar, may be ascertained by the usual methods. The red arsenical cobalt ore, which contains arsenical acid, may be decomposed by vitriolic acid; and the disengaged arsenical acid will be taken up by highly rectified spirit of wine; after which, the combination of vitriolic acid and cobalt may be dissolved in water, and precipitated by mild alkali: or the ore itself may be dissolved in water, sharpened by an acid; and the calx be then precipitated by the alkali.

In the dry way, the ores of cobalt, after previous pounding, washing, and roasting, may be fused with three times their weight of black flux, in a lined and covered crucible, by the heat of a smith’s forge. The tingeing power of cobalt ores may be assayed by fusion with three parts of fixed alkali, and five of powdered flint or glass. The alkali must be put first into the crucible, next the flint, and, above all, the roasted ore. When cobalt ores, containing bismuth, are reduced, this semi-metal usually occupies the lower part of the crucible, and may be separated from it by a blow with a hammer; or at least by eliquation, or melting, on account of its greater fusibility.

Cobalt is found in several parts of Europe, but most plentifully on the southern border of France and in Saxony. The ore is usually broken into pieces about the size of a hen’s egg, and the stony parts picked out. The sorted mineral is then pounded in mills, and sifted through wire sieves. By washing in water, the lighter parts are carried off; and the remainder is calcined in a furnace resembling an oven, wherein it is heated by the action of the reverberated flame of wood which plays upon it. In this situation, it is occasionally stirred with long iron rakes; and emits fumes, consisting chiefly of arsenic, which is collected in a long horizontal chimney, built for that purpose. If the ore contains bismuth, this fusible semi-metal is collected at the bottom of the furnace. The cobalt, after a sufficient torrefaction, remains in the form of a dark grey calx, called zaffre. The zaffre of commerce always contains twice or thrice its weight of powdered flints. The flint is pulverized for this and other purposes by means of previous ignition, and quenching in water, which renders it friable. Smalt is a blue glass, composed of one part of the calcined cobalt, fused with two of the flint powder, and one of pot-ash. The use of this metallic substance is confined chiefly to the production of the blue glass for enamels, and other purposes. Powder and stone blue, used by laundresses, is a preparation made by the Dutch from the coarse smalt.

p. 267

Colcothar

COLCOTHAR. The brown red calx of iron which remains after the distillation of the acid from martial vitriol: it is used for polishing glass and other substances by artists, who call it crocus, or crocus martis.

pp. 300–301

Earth, Calcareous

EARTH, CALCAREOUS. Calcareous earth, or lime, predominates in most stones which are soft enough to be scratched with a knife. These are chalk, limestone, marble, spars, gypsum, or plaster-stone, and various others. As the lime is most frequently combined with fixed air, it is usual for mineralists to drop a small quantity of nitrous acid upon the stones they are desirous of classing; and if it froths by the escape of the fixed air, they conclude that lime enters into the composition. To obtain pure calcareous earth, powdered chalk must be repeatedly boiled in water, which will deprive it of the saline impurities it frequently contains. It must then be dissolved in distilled vinegar, and precipitated by the addition of concrete volatile alkali. The precipitate, when well washed and dried, will consist of lime united to fixed air; the latter of which may be driven off by heat, if necessary.

If chalk, marble, limestone, spar, or any other specimens of this earth, containing fixed air, be exposed to continued ignition, they give out fixed air and water, to the amount of near half their weight. The remainder, consisting chiefly of lime, has a strong tendency to combination, and attracts water very powerfully. The addition of water to lime produces a very considerable heat, attended with noise, and agitation of the parts, which break asunder; and a phosphoric light is seen, if the experiment be made in the dark. Lime thus saturated with water is said to be slaked. Water dissolves about one seven-hundredth part of its weight of lime, and is then called lime-water. This solution has an acrid taste, and turns syrup of violets to a green colour. If lime-water be exposed to the open air, the lime attracts fixed air, and is by that means converted into chalk; which, not being soluble in water, forms a crust on the surface, formerly called cream of lime, that, when of a certain thickness, breaks, and falls to the bottom: and in this way the whole of the lime will in time be separated. If the fire has been too violent in the burning of lime, the stone becomes hard, sonorous, and incapable of absorbing water with the requisite degree of avidity. This effect seems to arise from part of the calcareous earth having entered into fusion with the clay, flint, or other contaminating earths, with which it forms a glass that covers and defends the rest.

The paste of lime and water, called mortar, has a degree of adhesion and ductility, though much less than clay. When dry, it is more or less friable, like chalk. A mixture of sand, or broken earthen vessels, greatly increases its firmness, which it seems to effect by rendering it more difficult for the parts to be removed with respect to each other. When mortar is left to dry by the gradual evaporation of its superfluous water, it is very long before it obtains its utmost degree of firmness. But if dry quick-lime be mixed with mortar, it gradually absorbs the superfluous water, and the mass becomes solid in a very short time.

Gypsum, or plaster of Paris, consists of lime united to the vitriolic acid, together with water. If this substance be exposed to a moderate heat, part of the water is driven off with an appearance resembling ebullition. The dry powder which remains may be mixed with water to the consistence of thin paste, and poured into a mould; and soon afterwards it suddenly becomes solid; at the same time that it is a little heated, and its bulk somewhat increased. This effect may be explained by observing that the particles of the gypsum are at first simply wetted by the water, in the same manner as happens with clay; and for that reason no other effect takes place, than the production of an imperfect degree of fluidity, from the motion of the parts among each other being facilitated: but when the water, by the gradual progress of the action between it and the dried gypsum, becomes combined in the same manner as before the calcination, it is absorbed, and enters into the composition of a solid body; the imperfect fluidity arising from the presence of uncombined water disappears, heat is developed, and the whole mass takes the solid form.

If the heat be strong, or continued any considerable time after the appearance of ebullition has ceased, the selenite will be partly decomposed by the loss of some of its vitriolic acid, and the plaster will be unfit for this purpose. The use of plaster in casting small statues, medallions, and other ornaments, is well known.

From various pertinent observations, Chaptal has shewn that gypsum is formed by the gradual decomposition of pyrites, which form vitriolic acid. This being carried off by water, takes up lime in its course, and the combination is afterwards deposited in consequence of the spontaneous evaporation of the water.

Calcareous earth, though infusible in the strongest heats of our furnaces, is nevertheless a very powerful flux with regard to mixtures of the other earths. These are all fusible by a proper addition of calcareous earth. Compounds are still more fusible; for any three of the five well-known earths may be fused into perfect glass, if they be mixed together in equal portions, provided the calcareous be one of them.

The earthy part of animals is chiefly, if not altogether, calcareous: in most cases it is united with phosphoric acid, but frequently with fixed air. See EARTH, ANIMAL.

p. 448

Lime

LIME. Is made by exposing chalk or other native combinations of calcareous earth and fixed air, to ignition, in a furnace properly adapted for that purpose, called a lime-kiln. The heat must be of considerable intensity, and continued twelve or fifteen hours. A less time will be sufficient if the heat be greater, or a longer if it be more moderate. The effect of this process is to drive off the fixed air and water, which compose about half the weight of such stones. Calcareous earth thus treated, is said to be in a caustic state, from its disposition to combine with and destroy the organization of animal substances, by forming a soap with their fat parts. As calcareous earth is infusible by the heat of a furnace, there would be no danger from too violent a heat if the specimens of chalk or lime-stone were pure; but as this is seldom the case, an extreme degree of heat produces a commencement of vitrification in the compound stone, and enables it to preserve its solidity when attempted to be made into mortar. This is called over burned lime.

If the calcination be perfectly accomplished, the quick lime will have a very strong tendency to combine with water. When this fluid is thrown upon a mass of quick lime, it becomes hot, cracks and splits with noise, emits boiling hot vapour, falls to pieces, and is in a very little time totally reduced to a fine powder. This experiment is even attended with light, if performed in the dark. Quick lime exposed to the air attracts, in the course of time, a sufficient quantity of water to pulverize it. It is then called lime slaked in the air. It resumes very little fixed air by this exposure. The paste of lime mixed with sand to give it firmness, is well known for its utility in the art of building. It is disposed to resume its original state and become stone again, by a long course of drying and re-absorption of fixed air. The drying may be greatly accelerated by adding one quarter of the weight of mortar, of quick lime in powder, a very short time before the mixture is used. This effect is similar to what takes place in the use of the natural compound called plaster of Paris, which consists of gypsum, and a small portion of uncombined lime. See EARTH CALCAREOUS.

p. 531

Ochre

OCHRE. A ferruginous earth, or ore of iron, commonly of a yellow, brown, or red colour. It is used as a pigment. The colour of such specimens as are dark, may be rendered of a brighter red by calcination; which alters the state of the metal. Ochres appear to have been produced by the decomposition of the martial pyrites, which consist of sulphur and iron. By the combined action of the air and water, the sulphur becomes acidified, and forms vitriol, the iron of which may be deposited upon calcareous earths, which seize the acid: or more commonly, by the more complete calcination of the iron, by the air, which then becomes less soluble and falls down, as is seen in a solution of common vitriol left in an open vessel. In many places the iron is extracted from this ore.