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METHODS OF OBTAINING CARBONIC ACID GAS.

bonic acid in the state of gas, at the temperature of our atmos-phere.

Remark. ..Carbonic acid gas may in this manner be disen-gaged from all its combinations with alcalies ; by using indiffe-rently any other dense acid, possessing a superior affinity to thealcali in the common acceptation of the word.

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

For this purpose reduce marble or chalk to powder, intro-duce it into a gun-barrel, which must be placed across a furnace,adapt a bent tube to its lower extremity and insert it below a re-ceiver in the pneumatic apparatus. Maintain a strong heat tillthe barrel is brought to a state of ignition, and at that tempera-ture carbonic acid gas will be liberated in abundance.

Rationale. ..In this case a decomposition of the marble orcarbonate of lime takes place on account of the action of caloric,which at a high temperature breaks the affinity of the carbonicacid and lime.; it unites with the first, and leaves the lime berhind in that state which is generally called quick-lime.

3. Carbonic acid gas may also be obtained by burning char-coal in oxigen gas. We have already mentioned this, butwe shall here give directions how it is best effected.

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

The rationale of this experiment has been given page 1()0.

Carbonic acid gas is often found occupying the lower parts ofmines, caverns, tombs, and such other subterraneous places ascontain materials for producing it. It is called choke, or chalk-damp. The grotto del Cane, near Naples, has long been famousfor the quantity of carbonic acid gas produced there, which runsout at the opening like a stream of -water. The quantity of car-bonic acid gas generated in this cavern is so great, that a dog or