CARBONIC ACID.
191
(except in the case above mentioned), and by its forming, with a solutionof lime or of baryta, a white precipitate, soluble in acetic acid.
The carbonates effervesce on the addition of acetic or hydrochloricacid. The evolved gas is known to be carbonic acid by the charactersbefore stated. The soluble carbonates form white precipitates in solu-tions of lime or baryta
Before combustion. After.
Composition. —By burning charcoal in onevolume or 16 parts, by weight, of oxygen gas,we procure one volume or 22 parts, by weight,of carbonic acid gas.
Eq. Eq.wt. Per ct. Saussure. Vol. Sp.gT.or Vol.Sp.gr.
Carbon .Oxygen . .
. 2 .
16 .
. 72-73
. 27 04 to 27*38 jCarb. vapour . 0*5orl. 72 96 to 72 * 62 !Oxygen gas. . 1 . .
0-4166
1-llli
Carb . Oxide gas 1 . 0-9722Oxygen gas . . 0-5 0-5555
CarbonicAcidl
22 .
100-00 .
lOO’OO . 10000 Carb. Acid gas 1 . . .
Carbonic Acid Water {Aqua Acidi Carbonici ).—At the ordinarytemperature and pressure of the atmosphere, one volume of water absorbsone volume of carbonic acid gas, and acquires a sp. gr. of T0018. Bydoubling the pressure the quantity of gas absorbed is doubled, and so onfor other degrees of pressure ; for Dr. Henry has shewn that the quantityof gas forced into water is directly as the pressure. In the UnitedStates Pharmacopoeia five volumes of gas are ordered to be condensedin one of water. This solution is called Artificial Seltzer Water. Thebottled soda ivater, and soda water from the fountain or pump, are, ingeneral, merely aqueous solutions of carbonic acid. Carbonic acidwater is a sparkling liquid, reddening litmus, having a pungent, aciduloustaste, and causing a white precipitate with lime water, which precipitateis re-dissolved by an excess of carbonic acid water.
Physiological Effects, [a.) On Vegetables.— Carbonic acid gas isinjurious to seeds, and diminishes or stops their germination according tothe quantity of acid present. An aqueous solution of carbonic acidapplied to the roots of plants promotes vegetation. An atmosphere con-taining not more than l of its volume of carbonic acid promotes thevegetation of plants exposed to the solar rays, but is injurious to thosewhich grow in the shade (Saussure, Recherches, p. 25, et seq.)
{b.) On dead animal matter .—Carbonic acid gas retards the putrefac-tion of animal substances.
(c.) On animals .—The respiration of carbonic acid gas is deleteriousand fatal to all classes of animals. That it operates in these cases as apositive poison, and not merely by excluding oxygen, as some have sup-posed, seems to be proved by two facts:—firstly, that an atmospherecomposed of 79 parts of carbonic acid and 21 of oxygen acts as apoison, although there is as much oxygen present as there is in atmos-pheric air; and, secondly, that one bronchial tube of the land-tortoisemay be tied, without any serious injury to the animal; but if, instead oftying it, the animal be made to inhale carbonic acid gas by it, deathtakes place in a few hours (Christison , Treatise on Poisons, p.745, 3rdedit.) The impression produced on the pulmonary extremities of thepar vagum, by the carbonic acid in the lungs, is supposed by some phy-siologists to be the ordinary stimulus to inspiration. According to theexperiments of Nysten ( Recherches , p. 88), this gas may be injected intothe venous system in large quantity, without stopping the circulation, and