CARBONIC acid gas is the first elastic aeriform fluid that was known. We find that the ancients were in some measure acquainted with it. Van Helmont called it the gas of Must, or of the vintage, or gas sylvestre.

We are indebted to Dr. Black of Edinburgh for the knowledge of some of the most remarkable properties of this fluid. In the year 1755 he discovered the affinity between this gas and the alkalies and Bergman, in 1772, proved that it was an acid.

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PROPERTIES OF CARBONIC ACID GAS.

Carbonic acid gas is invisible. It extinguishes flame. It is fatal to animal life. It exerts powerful effects on living vegetables. Its taste is pungent and acid. Its energy, as an acid, is but feeble, although distinct and certain. Neither light nor caloric seems to produce any distinct effect upon it, except that the latter dilates it. It mixes without combining with oxygen gas. It unites with water slowly. These two fluids, after considerable agitation, at last combine, and form a sub-acid fluid. The colder

Heat

the water, and the greater the pressure applied, the more carbonic acid gas will be absorbed. The water impregnated with it sparkles upon agitation; it has a pungent, acidulous taste, and reddens tincture of litmus. again disengages the gas from the water. This gas precipitates lime, strontia, and barytes, from their solutions in water. It is greedily attracted by all the alkalies. It undergoes no alteration by light. Its specific weight is to that of atmospheric air as 1500 to 1000. It may be poured out of one vessel into another. It is not acted upon by oxygen, nor is it altered by any of the simple combustible bodies at common 'temperatures; but phosphorus, iron, and lime, are capable of decomposing it when assisted by heat.

METHODS OF OBTAINING CARBONIC

ACID GAS.

Of all the gases, carbonic acid gas is that, perhaps, which is diffused in the greatest abundance throughout nature. It is found in three different states: 1st, In that of gas; 2dly, In that of mixture; and 3dly, In that of combination. The various processes of obtaining it are the following:

1. Put into a common gas-bottle, or retort, a little marble, chalk, or lime-stone, and pour on it sulphuric acid, diluted with about six times its weight of water; an effervescence will ensue, and carbonic acid gas will be liberated, which those who have an opportunity may collect over mercury; but a mercurial apparatus is not, absolutely necessary, since the gas may be collected over water, if it is to be used immediately when procured.

RATIONALE....In this instance the carbonic acid is disengaged from the state of combination, and reduced to the aeriform state of gas. The marble, lime-stone, or chalk, consists of this acid and lime on presenting to it sulphuric acid, a decomposition takes place the sulphuric acid has a greater affinity to the lime than the carbonic acid has; it therefore unites to it, and forms sulphate of lime, disengaging at the same time the carbonic acid in the state of gas at the temperature of our atmosphere.

REMARK....Carbonic acid gas may in this manner be disengaged from all its combinations with alkalies; by

using indifferently any other dense acid, possessing a superior affinity to the alkali in the common acceptation of the word.

2. It may likewise be obtained from the same substances by the action of caloric.

For this purpose reduce marble or chalk to powder, introduce it into a gun barrel, which must be placed across the universal furnace, adapt a bent tube to its lower extremity, and insert it below a receiver in the pneumatic apparatus. Maintain a strong heat till the barrel is brought to a state of ignition, and at that temperature carbonic acid gas will be liberated in abundance.

RATIONALE....In this case a decomposition of the marble or carbonate of lime takes place, on account of the action of caloric, which at a high temperature breaks the affinity of the carbonic acid and lime; it unites with the first, and leaves the lime behind in that state which is generally called quick lime.

3. Carbonic acid gas may also be obtained by burning charcoal in oxygen gas. We have already mentioned this, but we shall here give directions how it is best effected.

Take a bell glass, filled with oxygen gas, resting inverted in a basin of mercury; pass up into it some bits of new made charcoal, with some touch paper affixed to them; set fire to them by means of a lens collecting the sun's rays, and carbonic acid will be produced by the combustion of the charcoal.

The rationale of this experiment has been given, page

230.

Carbonic acid gas is often found occupying the lower parts of mines, caverns, tombs, and such other subterraneous places as contain materials for producing it. It is called choke, or chalk damp. The grotto del Cane, near Naples, has long been famous for the quantity of carbonic acid gas produced there, which runs out at the opening like a stream of water. The quantity of carbonic acid gas generated in this cavern is so great, that a dog or any other animal is immediately killed if his nose be thrust into it.*

* But as by its weight it does not rise speedily, a man's head is above it, and therefore though a dog will be effected by it in that grotto, a man will not by merely going in. T.C.

The carbonic acid, existing naturally in a state of gas, may be collected by filling bottles with water and emptying them in the atmosphere of this gas; the gas takes the place of the water, and fills the bottles, which must then be corked.

Carbonic acid gas is likewise formed during fermentation; on account of its great weight, it occupies the apparently empty space, or upper part of the vessel in which the fermenting process is going on. It may in this case

be collected in a manner similar to that above.

Carbonic acid gas is also obtained during respiration, during the reduction of metallic oxyds, and during the deflagration of nitrates with combustible bodies, as shall be noticed in another part of this work. It contains 13 parts carbon, 34 parts oxygen.

SECT. II.

EXPERIMENTAL PROOFS OF THE PROPERTIES OF CARBONIC ACID GAS.

EXPERIMENT I.

Carbonic acid gas is unfit for combustion.

To prove this, set a vessel filled with this gas with its mouth upwards, and let down a lighted candle; the candle will be immediately extinguished, and the smoke remaining in the gas will render its surface visible, which by agitation may be thrown into undulating waves.

The following experiment will show this property in a pleasing manner.

EXPERIMENT II.

Take three glass tubes, of equal heights, and fill the first with atmospheric air, the second with carbonic acid gas, and the third with oxygen gas; plunge successively, and with swiftness, a lighted taper into these three tubes; first into that containing atmospheric air, then into the tube filled with carbonic acid gas, and lastly into that filled

with oxygen gas. In the tube filled with atmospheric air, the taper will burn with its usual brightness; in that filled with carbonic acid gas, the flame will be suddenly extinguished, but the glowing wick will be re-lighted in the tube containing oxygen gas, and burn with a dazzling splendour.

RATIONALE....This experiment confirms a fact already established; namely, that oxygen gas is much fitter for combustion than atmospheric air; and is also a most com. plete proof that carbonic acid gas extinguishes light.

REMARK....This property of carbonic acid gas may be advantageously made use of to learn whether the air in a newly sunk well, in subterraneous caverns, pits, mines, vaults, cellars, and the like, may be approached without danger.

EXPERIMENT III.

Carbonic acid gas is fatal to animal life.

Put a mouse or other small animal into a vessel filled with this gas, and cover the vessel, to prevent the contact of atmospheric air. The animal will die in a minute or

two.

REMARK....It is this gas which has produced so many fatal accidents, at the opening of cellars or vaults, in which wine, cider, or beer, has been suffered to ferment.

The famous lake of Averno, where Virgil placed the entrance of hell, yields so large a quantity of carbonic acid gas, that birds cannot fly close over it with impunity.

EXPERIMENT IV.

Carbonic acid gas is heavier than atmospheric air.*

Let a long glass tube proceeding from a gas-bottle in which carbonic acid gas is disengaging be twice bent at right angles. Let the open end of the longest leg reach to the bottom of a jar, perfectly dry within, and standing with its mouth uppermost. The carbonic acid disengaged will expel the atmospheric air from the jar, on account of its greater specific gravity. When the jar is filled with the gas (which may be known by a lighted candle being *Atmospheric air being 1, carbonic acid gas according to Biot and Arago, is 1,5196. T. C.