CAR

CAR ( 239 )

coal, which was not extinguished without muchdifficulty.

The ufes of charcoal are very extenfive andimportant. Its greateft confumption is as anarticle of fuel both for domeftic purpofes andin the working of metals. It is of indifpenfableneceffity in the laboratory both as a fuel andan ingredient in all kinds of reducing fluxes.It is an eflential conftituent part of gunpowder,and is the bafis of almoft all black paints andvarniffies. It is employed by mathematical in-ftrument-makers and others in polilhing brafsand copper, and is ufed in many cafes withgreat advantage as a limple and efficacious clari-fier and purifier.

CARBONIC ACID.

CARBONIC ACID GAS. Fixed Air. AerialAcid. Mephitic Air. Kohlenfaure , Ger. AcideCarbonique , Fr.

Carbonic acid is known to us only in itsgafeous ftate when uncombined, and it is thenthe fixed air of Cavendifli and Prieftley, theaerial acid of Bergman, the mephitic air of otherchemifts. It was the firft gas in which acid pro-perties were clearly difcovered, and many of itsproperties as a gas were known long before itsexiftence was fufpected in many of the com-pounds of which it forms a part. It is knownto be an acid by the united characters of red-dening vegetable blues, neutralizing alkalies andalkaline earths, and being formed by the unionof a combuftible bafe with oxygen.

Its alkaline, earthy, and metallic compoundsare called carbonate , and will be defcribed underthat head ; the properties of carbonic acid gas,and carbonic acid united with water, belong tothe prefent article.

The fources of this acid are immenfe, andwidely diffufed throughout the natural world.The chief are the following :

1. The atmofphere always contains a fmallportion, which feldom varies except in the im-mediate vicinity of places where refpiration andcombuftion are going on in the large way, andis about one hundredth part. Hence it is thatalkalies and lime lofe their caufticity in the openair, by extrading its fmall proportion of car-bonic acid.

2. Many mineral waters rife from the earthmore or lefs ftrongly impregnated with this acid,much of which flies off in the gafeous form as foon? s expofed to the atmofphere. The brifk froth-in g vv aters of Seltzer are examples of this kind.

3. Every procefs in which coal, wood, or anyOther carbonaceous fubftance is confumed, is one

which generates this acid gas; and hence thereis no vegetable matter on the face of the earthwhich does not give this acid largely when burnt.

4. The refpiration of animals refembleS com-buftion in this refped, and hence there is aquantity of carbonic acid gas unceafingly pour-ing into the atmofphere from the lungs of everyliving animal.

5. The vegetation of plants is, under fome cir-cumftances, a procefs in which carbonic acid isgenerated, but is fubjed to more variations frontexternal caufes than refpiration.

6. The fpontaneous decompofition of veget-able and animal matter alfo generates carbonicacid, more efpecially the curious procefs ofvinous fermentation, from which many of themoil important properties of this acid were firflobferved.

7. An immenfe ftore of this acid is containedin the natural carbonats, more efpecially all thevarieties of lime-ftone, chalk, and calcareousftones, many of which contain as much as 40per cent, of their weight of this acid.

Other fmaller fources exift which do not re-quire enumeration.

Carbonic acid gas, or fixed air, when pro-cured by modes to be prefently mentioned, hasthe following properties. It is quite permanentat any temperature, nor can it, without com-bination, be exhibited by cold or preffure in anyother than a gafeous form. It is abfolutelyfatal to animal life, any living creature immerfedin it perifhing in as fhort a time as it would byabfolute fuffocation or interruption of refpira-tion. Thus, fmall animals, fuch as mice, drop-ped into a veflel of fixed air die almoft imme-diately; dogs become convulfed and fenfelefs inabout a minute, but recover in the open air iftaken out very ipeedily; men and large animalsnearly the fame, according to' their powers oflife; frogs and cold-blooded animals who havealways in their lungs a fupply of air for manyminutes, live in it without much inconveniencetill that internal fupply is exhaufted, after whichthey perifli. Fixed air is incapable of main-taining combuftion for a moment. If a lightedtaper is let down into a jar full of this air,the taper is extinguilhed as foon it reaches thelevel of the gas, as completely as if dipped intowater. Even a fmall admixture of this air withcommon air prevents combuftion; for Mr.Cavendifli found, that a candle dipped in a jarholding only one-ninth of fixed air went outimmediately. 1

Carbonic acid gas is the heavieft of all the

• Phil.Tranf. for 1766.