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known gaffes except the fulphureous. Its weightcompared to that of common air is very nea rlyas 3 to 2 in all circumftances, hence its fpecificgravity will be about .001806, and the weightof a cubic inch at 6o° and 29.5 inches bar. willbe about 0.456 grains. On account of itslieavinefs, when generated, it falls rapidly to theground, and fpreads laterally over the furface,though after a. time it mixes uniformly with theatmofphere. This is well fhewn at the Grottodel Cani, a cave in the vicinity of Vefuvius,through the floor of which a quantity of fixedair is conftantly rifing from fome fubterraneanrefervoir. A man Handing upright in it feels noinconvenience from the noxious gas, as it onlyrifes to about the height of his knee, but if adog or other fmall animal enters, it is prefentlyfuffocated.

Carbonic acid gas is procured for experimentgenerally from limeftone, chalk, marble, or anyof the carbonats of lime, either by heat, or morecommonly by any of the ftronger acids. The di-luted fulphuric or muriatic are the acids moftlyemployed. The latter is perhaps preferable, asthe muriat of lime refulting from the mixtureremains diffolved, whereas the fulphat of limefrom its little folubility cryftallizes in the mafs,and fomewhat impedes the procefs. Withfmall pieces of marble, and moderately ftrongmuriatic acid, a conftant and pretty uniformftream. of gas is given out till the acid is fatu-rated, or the whole of the marble diffolved.Marble yields in this way from 40 to 45 percent, of its weight of carbonic acid gas, fo that100 grains will yield between 90 and 100 cubicinches. The difengagement of carbonic acidfrom any of the carbonats, whether alkaline,earthy, or metallic, is accompanied by a froth-ing or effervefcence, occafioned by the carbonicacid affuming the gafeous form, or becomingcarbonic acid gas, when detached from its com-bination by the affinity of the ftronger acid tothe bafe of the carbonat.

Heat is another method of obtaining the car-bonic. acid from the earthy carbonats. Chalk ormarble enclofed in an earthen retort, and ftronglyheated, readily gives out this gas: carbonat ofmagnefia witn ftill greater cafe. The gas thusprocured when united with water produces a lefspungent liquid than that prepared by acids, fothat probably the latter carries, off with it al'mall portion of the acid ufed, and therefore isnot fo pure.

All fubftances undergoing the vinous fermen-

tation alfo give a copious fupply of this gasywhich from its weight long remains hoveringover the furface of the fermenting mafs. There-fore fubftances placed in the vats of a brew-boufe juft over the fermenting liquor, are there-by kept immerfed in an atmofphere of carbonicacid gas, which however is not quite pure, be-ing mixed with a fenfible quantity of alcohol.See Fermentation.

Carbonic acid gas is readily abforbed by water,and this combination is found in a very high de-gree in feveral of the moft efteemed MineralWaters . Carbonated water, when the pro-portion of carbonic acid is confiderable, has avery agreeable, brilk, pungent tafte and fmell,bubbles and froths ftrongly when expofed to theair, particularly when gently heated; and it isin this combination that the acid properties ofcarbonic acid are the moft fenfible, but whennearly at the freezing point the peculiar tafteis hardly perceptible, though cold water willabforb more of the gas than warm. b This na-tural combination may be very well imitated byart, and fince the firft experiments of Bergman,Prieftley, and Cavendifh, many very ingeniouscontrivances have been invented to effedt thiscombination in very large proportions; as thecarbonated water in various modifications, hasbecome a very fafhionable and falutary articlefor diet and medicine.

The abforption of carbonic acid gas by wateris fhewn in the fimpleft manner by the follow-ing experiment: fill a phial quite full of coldwater, invert it over a cupful of the fame, andthrow up a quantity of the gas from a bottlewith a curved tube, in which marble duft andmuriatic or fulphuric acid have been put. Asthe gas rifes into the inverted bottle the wateris proportionally difplaced, and when about halffull of gas, flop the phial ftrongly with thefinger, and fhake together violently the gas andthe water. A ftrong abforption will immedi-ately be felt by the external preffure on thefinger, and the water will be found fenfibly to.tafte of carbonic acid.

Agitation therefore is one method of affiftingthe abforption, and ftrong preffure is anotherequally powerful. Thefe are combined in a.high degree in the forcing-pump apparatus ufedfor preparing, artificial mineral waters in the-large way. The temperature alfo of the watermuch affects its ablorbing power; for Mr.-Cavendifli early in the enquiry found that cold,water abforbs much more than warm,