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parates it readily and completely from water; but no degree of cold or pressure has yet exhibited this acid in a dense or concentrated state of fluidity.

Carbonic acid gas is much denser than common air, and for this reason occupies the lower parts of such mines or caverns as contain materials which afford it by decomposition. The miners call it chokedamp. The Grotto del Cano, in the kingdom of Naples, has been famous for ages, on account of the effects of a stratum of fixed air which covers its bottom. It is a oave or hole in the side of a mountain, near the lake Agnano, measuring not more than eighteen feet from its entrance to the inner extremity; where if a dog or other animal that holds down its head be thrust, it is immediately killed by inhaling this noxious fluid.

If a

Carbonic acid gas is emitted in large quantities by bodies in the state of the vinous fermentation, (see FERMENTATION), and on account of its great weight, it occupies the apparently empty space or upper part of the vessels in which the fermenting process is going on. A variety of striking experiments may be made in this stratum of elastic fluid. Lighted paper, or a candle dipped into it, is immediately extinguished; and the smoke remaining in the carbonic acid gas renders its surface visible, which may be thrown into waves by agitation like water. dish of water be immersed in this gas, and briskly agitated, it soon becomes impregnated, and obtains the pungent taste of Pyrmont water. In consequence of the weight of the carbonic acid gas, it may be well corked, may used convey to lifted out in a pitcher, or bottle, which, if great distances, or it may be drawn out of a vessel by a cock like a liquid. The effects produced by pouring this invisible fluid from one vessel to another, have a very singular appearance; if a candle or small animal be placed in a deep vessel, the former becomes extinct, and the latter expires in a few seconds, after the carbonic acid gas is poured upon them, though the eye is incapable of distinguishing any thing that is poured. If, however it be poured into a vessel full of air, in the sunshine, its density being so much greater than that of the air, renders it slightly visible by the undulations and streaks it forms in this fluid, as it descends through it. Carbonic acid reddens infusion of litmus; but the redness vanishes by exposure to the air, as the acid flies off. It has a peculiar sharp taste, which may be perceived over vats in which wine or beer is fermenting, as also in sparkling Champaign, and the brisker kinds of cider. Light passing through it is refracted by it, but does not effect any sensible alteration in it,

though it appears, from experiment, that it favours the separation of its principles by other substances. It will not unite with an overdose of oxygen, of which it contains 72 parts in 100, the other 28 being pure carbon. It not only destroys life, but the heart and muscle of animals killed by it lose all their irritability, so as to be insensible to the stimulus of galvanism.

Carbonic acid is dilated by heat, but not otherwise altered by it. It is not acted upon by oxygen, or any of the simple combustibles. Charcoal absorbs it, but gives it out again unchanged, at ordinary temperatures; but when this gaseous acid is made to traverse charcoal ignited in a tube, it is converted into carbonic oxide. Phosphorus is insoluble in carbonic acid gas; but, as already observed, is capable of decomposing it by compound affinity, when assisted by sufficient heat; and Priestley and Cruickshank have shown that iron, zinc, and several other metals, are capable of producing the same effect. If carbonic acid be mixed with sulphuretted, phosphuretted, or carburetted gas, it renders them less combustible, or destroys their combustibility entirely, but produces no other sensible change. Such mixtures occur in various analyses, and particularly in the products of the decomposition of vegetable and animal substances. The inflammable air of marshes is frequently carburetted hydrogen intimately mixed with carbonic acid gas, and the sulphuretted hydrogen gas obtained from mineral waters is very often mixed with it.

Carbonic acid appears from various exable utility in promoting vegetation. It is periments of Ingenhousz to be of considerprobably decomposed by the organs of plants, its base furnishing part at least of the carbon that is so abundant in the vegetable kingdom, and its oxygen contributing to replenish the atmosphere with that necessary support of life, which is continually diminished by the respiration of animals and other causes.

* The most exact experiments on the neutral carbonates concur to prove, that the prime equivalent of carbonic acid is 2.75; and that it consists of one prime of carbon - 0.75+ 2.0 oxygen. This proportion is most exactly deduced from a comparison of the specific gravities of carbonic acid gas and oxygen; for it is well ascertained that the latter, by its combination with charcoal, and conversion into the former, does not change its volume. Now, 100 cubic inches of oxygen weigh 33.8 gr. and 100 cubic inches of carbonic acid 46.5, showing the weight of combined charcoal in that quantity to be 12.7. But the oxide of carbon contains only half the quantity of oxygen which Carbonic acid does; and we hence infer, that the oxide of carbon consists of one prime of oxygen united to one of carbon. This a priori judgment is confirmed by the weight 2.75 deduced from the carbonates, as the prime equivalent of carbonic acid. Therefore we have this proportion:

If 33.8 represent two primes of oxygen, or 2; 12.7 will represent one of carbon. 33.8: 2.:: 12.7: 0.751, being, as above, the prime equivalent or first combining proportion of carbon. If the specific gravity of atmospheric air be called 1.0000, that of carbonic acid will be 1.5236.

We have seen that water absorbs about its volume of this acid gas, and thereby acquires a specific gravity of 1.0015. On freezing it, the gas is as completely expelled as by boiling. By artificial pressure with forcing pumps, water may be made to aborb two or three times its bulk of carbonic acid. When there is also added a little potash or soda, it becomes the aërated or carbonated alkaline waters, a pleasant beverage, and not inactive remedy in several complaints, particularly dyspepsia, hiccup, up, and disorders of the kidneys. Alcohol condenses twice its volume of carbonic acid. The most beautiful analytical experiment with carbonic acid, is the combustion of potassium in it, the formation of potash, and the deposition of charcoal. Nothing shows the power of chemical research in a more favourable light, than the extraction of an invisible gas from Parian marble or crystallized spar, and its resolution by such an experiment into oxygen and carbon; in the proportions above stated, 5 gr. of potassium should be used for 3 cubic inches of gas. If less be employed, the gas will not all be decomposed, but a part will be absorbed by the potash. From the above quantities, 3-8ths of a grain of charcoal will be obtained. If a porcelain tube, containing a coil of fine iron wire, be ignited in a furnace, and if carbonic acid be passed backwards and forwards by means of a full and empty bladder, attached to the ends of the tube, the gas will be converted into carbonic oxide, and the iron will be oxi

dized.*

In point of affinity for the earths and alkalis, carbonic acid stands apparently low in the scale. Before its true nature was known, its compounds with them were not considered as salts, but as the earths and alkalis themselves, only distinguished by the names of mild, or effervescent, from their qualities of effervescing with acids, and wanting causticity.

The carbonates are characterized by effervescing with almost all the acids, even the acetic, when they evolve their gas⚫ cous acid, which passed into lime water by

a tube, deprives it of its taste, and converts it into chalk and pure water.

The carbonate of barytes was formed artificially by Bergman and Scheele in 1776; but Dr. Withering first found it native at Alston Moor in Cumberland in 1783. From this circumstance it has been termed Witherite by Werner. It has been likewise called aërated heavy spar, aërated baroselenite, aërated heavy earth or barytes, barolite, &c. Its crystals have been observed to assume four different forms, double six-sided, and double four-sided pyramids; six-sided columns terminated by a pyramid with the same number of faces, and small radiated crystals, half an inch in length, and very thin, appearing to be hexagonal prisms, rounded toward the point. The hexaedral prism is presumed to be its primitive form. Its specific gravity, when native, is 4.331, when prepared artificially it scarcely exceeds 3.763.

It may be prepared by exposing a solution of pure barytes to the atmosphere, when it will be covered with a pellicle of this salt by absorbing carbonic acid; or carbonic acid may be received into this solution, in which it will immediately form a copious precipitate; or a solution of nitrate or muriate of barytes may be precipitated by a solution of the carbonate of potash, soda, or ammonia. The precipitate, in either of these cases, being well washed, will be found to be very pure carbonate of barytes. It may likewise be procured by decomposing the native sul. phate of barytes by the carbonate of potash, or of soda, in the dry way, with the assistance of fire; but in this way the sulphate of barytes is never completely decomposed, and some of it remains mixed with the carbonate.

The carbonate of barytes is soluble only in 4304 times its weight of cold water, and 2304 of boiling water, and this requires a long time; but water saturated with carbonic acid dissolves 1-830th. It is not altered by exposure to the air, but is decomposed by the application of a very violent heat, either in a black lead crucible, or when formed into a paste with charcoal powder. Sulphuric acid, in a concentrated state, or diluted with three or four parts of water, does not separate the carbonic acid with effervescence, unless assisted by heat. Muriatic acid does not act upon it likewise, unless diluted with water, or assisted by heat. And nitric acid does not act upon it at all, unless diluted. It has no sensible taste, yet it is extremely poisonous.

As this salt has lately been found, in large quantities, near Murton in Cumber. land, and some other places in the vicinity, it might probably be introduced inte

manufactures with advantage, as for ex tracting the bases of several salts.

Hope

in the

It is composed of 2.75 parts of acid, and 9.75 of barytes. Its prime equivalent is therefore the sum of these numbers = 12.5. Carbonate of strontian was first pointed out as distinct from the preceding species by Dr. Crawford in 1790, but Dr. Hope 790 gave the first accurate account of it Edinburgh Transactions. It has been found native in Scotland, at Strontian in Argyllshire, and at Leadhills. It is usually in fine striated needless or prisms, that appear to be hexaëdral, semitransparent, and of a white colour slightly tinged with green. It is insipid; requires 1536 parts of boiling water to dissolve it; is not altered by exposure to the air; but when strongly heated in a crucible loses part of its acid; and this decomposition is facilitated by making it into a paste with charcoal powder. When the fire is strongly urged, it attacks the crucible, and melts into a glass, resembling the colour of chrysolite, or pyramidal phosphate of lime. If thrown in powder on well kindled coals, or the flame of a candle, it exhibits red sparks. The same phenomenon occurs, if it be treated with the blow-pipe, which fuses it into an opaque vitreous globule, that falls to powder in the open air. Its specific gravity is only 3.66, in which it differs strikingly from the carbonate of barytes; as it does, in not being poisonous, according to the experiments made by Pelletier on various animals.

It consists of 6.5 strontian + 2.75 carbonic acid = 9.25.

Carbonate of lime exists in great abundance in nature, variously mixed with other bodies, under the names of marble, chalk, limestone, stalactites, &c. in which it is of more important and extensive use than any other of the salts, except perhaps the muriate of soda. It is often found crystallized, and perfectly transparent. The primitive form of its crystals is the rhomboidal prism, with angles of 101 and 784. Its integrant particles have the same form. Beside this, however, many varieties of its crystals have been discovered and described by mineralogists. The specific gravity of the marbles is from 2.65 to 2.85; of the crystallized carbonates, about 2.7; of the stalactites, from 2.32 to 2.47: of the limestones, from 1.39 to 2.72.

It has scarcely any taste; is insoluble in pure water, but water saturated with carbonic acid takes up 1-1500th, though as the acid flies off this is precipitated. It suffers little or no alteration on exposure to the air. When heated it decrepitates, its water flies off, and lastly its acid; but this requires a pretty strong heat. By this process it is burned into lime.

It is composed of 3.56 lime + 2.75 car

bonic acid = 6.31; or in 100 parts of 56.4 lime and 43.6 acid.

The carbonate or rather subcarbonate of potash was long known by the name of vegetable alkali. It was also called fixed nitre, salt of tartar, salt of wormwood, &c. according to the different modes in which it was procured; and was supposed to retain something of the virtues of the substance from which it was extracted. This error, has been some time exploded, but the knowledge of its true nature is of more recent date.

As water at the usual temperature of the air dissolves rather more than its weight of this salt, we have thus a ready mode of detecting its adulterations in general; and as it is often of consequence in manufactures, to know how much alkali a particular specimen contains, this may be ascertained by the quantity of sulphuric acid it will saturate.

This salt is deliquescent.

It consists of 5.94 potash + 2.75 carbonic acid 8.69.

=

The bi-carbonate of potash crystallizes, according to Fourcroy, in square prisms, the apices of which are quadrangular pyramids. According to Pelletier they are tetraedral rhomboidal prisms, with diedral summits. The complete crystal has eight faces, two hexagons, two rectangles, and four rhombs. It has a urinous but not caustic taste; changes the sirup of violets green; boiling water dissolves five-sixths of its weight, and cold water one-fourth; alcohol, even when hot, will not dissolve more than 1-1200th. Its specific gravity is 2.012.

When it is very pure and well crystallized it effloresces on exposure to a dry atmosphere, though it was formerly considered as deliquescent. The fact is, that the common salt of tartar of the shops is a compound of this carbonate and pure potash; the latter of which, being very deliquescent, attracts the moisture of the air till the whole is dissolved. From its smooth feel, and the manner in which it was prepared, the old chemists called this solution oil of tartar per deliquium.

9

The bi-carbonate of potash melts with a gentle heat, loses its water of crystallization, amounting to T용어, Tåg, and gives out a portion of its carbonic acid; though no degree of heat will expel the whole of the acid. Thus, as the carbonate of potash is always prepared by incineration of vegetable substances, and lixiviation, it must be in the intermediate state; or that of a carbonate with excess of alkali: and to obtain the true carbonate we must saturate this salt with carbonic acid, which is best done by passing the acid in the state of gas through a solution of the salt in twice its weight of water; or, if we want the potash pure, we must have recourse to lime, to separate that portion of acid which fire will not expel.

Another mode, recommended by Berthollet, and which may be of use on some occasions, is to add solid carbonate of ammonia to a solution of potash not saturated, and distil the mixture: when the ammonia may be obtained in the form of gas, or caustic liquor, while the carbonate crystallizes in the retort.

The bi-carbonate usually called supercarbonate by the apothecaries, consists of 2 primes of carbonic acid = 5.500, 1 of potash= 5.940, and 1 of water = 1.125, in all 12.565.

The carbonate of soda has likewise been long known, and distinguished from the preceding by the name of mineral alkali. In commerce it is usually called barilla or soda; in which state, however, it always contains of a mixture of earthy bodies and usually common salt. It may be purified, by dissolving it in a small portion of water, filtering the solution, evaporating at a low heat, and skimming off the crystals of muriate of soda as they form on its surface. When these cease to form, the solution may be suffered to cool, and the carbonate of soda will crystallize. To obtain this salt perfectly pure Klaproth dissolves common carbonate of soda in water, and saturates this solution with nitric acid, taking care that the acid is a little in excess. He then separates the sulphuric acid by nitrate of barytes, and the muriatic acid by nitrate of silver. The fluid thus purified he evaporates to dryness, fuses the nitrate of soda obtained, and decomposes it by detonation with charcoal. He then lixiviates the residue, and crystallizes the carbonate of soda. If it be adulterated with potash, tartaric acid will form a precipitate in a pretty strong solution of it.

pando

It is found abundantly in nature. In Egypt, where it is collected from the surface of the earth, particularly after the desiccation of temporary lakes, it has been known from time immemorial by the name of nitrum, natron, or natrum. This it has been proposed to retain; and accordingly the London college has adopted the term natron Dr. Bostock of Liverpool lately found, that the efflorescence, which copiously covered the decaying parts of the plaster of the salt water baths in that town, consisted of carbonate of soda. A carbonate of soda exported from Tripoli, which is called Trona from the name of the place where it is found, and analyzed by Klaproth, contained of soda 37 parts, carbonic acid 38, water of crystallization 22.5, sulphate of soda 2. This does not effloresce. A great deal is prepared in Spain by incinerating the maritime plant salsola; and it is manufactured in this country, as

well as in France, from different species of sea-weeds. It is likewise found in mineral waters; and also in some animal fluids.

Its

It crystallizes in irregular or rhomboidal decaedrons, formed by two quadrangular pyramids, truncated very near their bases. Frequently it exhibits only rhomboidal laminæ. Its specific gravity is 1.3591. taste is urinous, and slightly acrid, without being caustic. It changes blue vegetable colours to a green. It is soluble in less than its weight of boiling water, and twice its weight of cold. It is one of the most efflorescent salts known, falling completely to powder in no long time. On the application of heat it is soon rendered fluid from the great quantity of its water of crystallization; but is dried by a continuance of the heat, and then melts. It is somewhat more fusible than the carbonate of potash, promotes the fusion of earths in a greater degree, and forms a glass of better quality. Like that, it is very tenacious of a certain portion of its carbonic acid. It consists in its dry state of 3.94 soda,+ 2.75 acid, = 6.69.

* But the crystals contain 10 prime proportions of water. They are composed of 22 soda,+ 15.3 carbonic acid,+ 62.7 water in 100 parts, or of 1 prime of soda = 3.94, 1 of carb. acid = 2.75, and 10 of water = 11.25, in whole 17.94.

The bi-carbonate of soda may be prepared by saturating the solution of the preceding salt with carbonic acid gas, and then evaporating with a very gentle heat to dryness, when a white irregular saline mass is obtained. The salt is not crystallizable. Its constituents are 3.94 soda, + 5.50 carb. acid, + 1.125 water, = 10.565; or in 100 parts 37.4 soda,+ 52 acid,+ 10.6 water. The intermediate native compound, the African trona, consists, according to Mr. R. Phillips, of 3 carbonic acid, + 2 soda, + 4 water; or in 100 parts 38 soda, + 40 acid, +22 water.* See the article SODA.

ardus

The carbonate of magnesia, in a state of imperfect saturation with the acid, has been used in medicine for some time under the simple name of magnesia. It is prepared by precipitation from the sulphate of magne magnesia by means of carbonate of potash. Equal parts of sulphate of magnesia and carbonate of potash, each dissolved in its own weight of boiling water, are filtered and mixed together hot; the sulphate of potash is separated by copious washing with water; and the carbonate of magnesia is then left to drain, and afterwards spread thin on paper, and carried to the drying stove. When once dried it will be in friable white cakes, or a fine powder.

Another mode of preparing it in the

grate will be found under the article AM

ΜΟΝΙΑ.

To obtain carbonate of magnesia saturated with acid, a solution of sulphate of magnesia may be mixed cold with a solution of carbonate of potash; and at the expiration of a few hours, as the superfluous carbonic acid that held it in solution flies off, the carbonate of magnesia will crystallize in very regular transparent prisms of six equal sides. It may be equally obtained by dissolving magnesia in water impregnated with carbonic acid, and exposing the solution to the open air. Dr. Thomson says, the most regular crystals will be obtained by mixing together 125 parts of sulphate of magnesia and 136 parts of carbonate of soda, both dissolved in water, filtering the solution, and then setting it aside for two or three days.

These crystals soon lose their transparency, and become covered with a white powder. Exposed to the fire in a crucible, they decrepitate slightly, lose their water and acid, fall to powder, and are reduced to one-fourth of the original weight. When the common carbonate is calcined in the great, it appears as if boiling, from the extrication of carbonic acid; a small portion ascends like a vapour, and is deposited in a white powder on the cold bodies with which it comes into contact; and in a dark place, towards the end of the operation, it shines with a bluish phosphoric light. It thus loses half its weight, and the magnesia is left quite pure.

As the magnesia magnesia of the shops is sometimes adulterated with chalk, this may be detected by the addition of a little sulphuric acid diluted with 8 or 10 times its weight of water, as this will form with the magnesia a very soluble salt, while the sulphate of lime will remain undissolved. Calcined magnesia should dissolve in this dilute acid without any effervescence.

The crystallized carbonate dissolves in forty-eight times its weight of cold water; the common carbonate requires at least ten times as much, and first forms a paste with a small quantity of the fluid.

Guyton Morveau has lately found the carbonate of magnesia native, near Castella-Monte, in a stone considered there as a clay very rich in alumina. It is amorphous, as white as ceruse, and as compact as the hardest chalk; does not sensibly adhere to the tongue; and has no argillaceous smell. Its specific gravity, when all the bubbles of air it contains have escaped, is 2.612. In the fire it lost 0.585 of its weight, and became sufficiently hard to scratch Bohemian glass slightly. On analysis it was found to contain magnesia 26.3, silex 14.2, carbonic acid 46, water 12, iron an inappreciable quantity.

The carbonate of ammonia, once vulgarly [5]

VOL. I.

known by the name of volatile sal ammoniac, and abroad by that of English volatile salt, because it was first prepared in this country, was commonly called mild volatile alkali, before its true nature was known.

When very pure it is in a crystalline form, but seldom very regular. Its crystals are so small, that it is difficult to determine their figure. Bergmann describes them as acute octaëdrons, the four angles of which are truncated. Romé de Lisle had compressed tetraedral prisms, terminated by a diëdral summit. Bergmann obtained his by saturating warm water with the salt, stopping the bottle closely, and exposing it to great cold. The crystals commonly produced by sublimation are little bundles of needles, or very slender prisms, so arranged as to represent herborizations, fern leaves, or feathers. The taste and smell of this salt are the same with those of pure ammonia, but much weaker. It turns the colour of violets green, and that of turmeric brown. It is soluble in rather more than twice its weight of cold water, and in its own weight of hot water, but a boiling heat volatilizes it. When pure, and thoroughly saturated, it is not perceptibly alterable in the air; but when it has an excess of ammonia, it softens and grows moist. It cannot be doubted, however, that it is soluble in air; for if left in an open vessel, it gradually diminishes in weight, and its peculiar smell is diffused to a certain distance. Heat readily sublimes, but does not decompose it.

It has been prepared by the destructive distillation of animal substances, and some others, in large iron pots, with a fire increased by degrees to a strong red heat, the aqueous liquor that first comes over being removed, that the salt might not be dissolved in it. Thus we had the sait of hartshorn, salt of soot, essential salt of vipers, &c. If the salt were dissolved in the water, it was called spirit of the substance from which it was obtained. Thus, however, it was much contaminated by a fetid animal oil, from which it required to be subsequently purified, and is much better fabricated by mixing one part of muriate of ammonia and two of carbonate of lime, both as dry as possible, and subliming in an earthen retort.

Sir H. Davy has shown that its component parts vary, according to the manner of preparing it. The lower the temperature at which it is formed, the greater the proportion of acid and water. Thus, if formed at the temperature of 300°, it contains more than fifty per cent of alkali; if at 60°, not more than twenty per cent.

* There are three or four definite com. pounds of carbonic acid and ammonia. The 1st is the solid sub-carbonate of the shops. It consists of 55 carbonic acid, 30

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