302

ELEMENTS OP MATERIA MEDICA.

Its specific gravity is 1-5277. It is neither combustible nor a supporter of com-bustion, except in the case of potassium, which, when heated in this gas, takesfire, the producta of the combustion being carbon and carbonate of potash. Itextinguishes most burning bodies when introduced into it in the ignited condi-tion. It reddens litmus feebl) r .

Liquefaciion. —Under a pressure of 36 atmospheres at 32°, carbonic acid is alimpid, colourless liquid, which is insoluble in water and in the fat oils, but issoluble in all proportions in alcohol, ether, oil of turpentine, and carburet of sul-phur. Its refractive power is much less than that of water. Its expansibility byheat is greater than that of gases; for when heated from 32° to 86° F., its bulfcincreases from 20 to 29 volumes, while the pressure of its vapour augraents from36 to 73 atmospheres. 1

SoMdification.< —By an intense degree of cold, liquid carbonic acid may besolidified. Thus, when the pressure is removed from this liquid by opening astopcock in the condensing vessel, the cold produced by the expansion is sogreat, that a quantity of solid carbonic acid is fofmed. In this State it is so in-tensely cold, that mercury, liquid chlorine, liquid cyanogen, &c, may be readilyfrozen by it. Being a very bad conductor of heat, it may be handled with irtt-punity.

Characlerislics. —Carbonic acid gas is recognised by its feebly reddening lit-mus, by being incombustible and a non-supporter of combustion, (except in thecase above mentioned,) and by its forming, with a Solution of lime or of baryta;a white precipitate, soluble in acetic acid.

The mrbonal.es effervesce on the addition of hydrochloric acid. The evolvedgas is known to be carbonic acid by the characters before stated. The solublemono-carbonates form white precipitates in Solutions of lime or baryta, solublein excess of carbonic acid.Before combustion. After. Composition. —By burning charcoal in one vo-

lume or 16 parts, by weight, of oxygen gas, weprocure one volume or 22 parts, by weight, of car-bonic acid gas.

2eq.Oxyden.= 16

and 1 eq.Carbon= 6

leq. CarbAcid gas.= 22

Jiloms.

Carbon...... 1

Oxygen...... 2

Eq.

Wt.

016

Per

Cent.

27-2772-73

Sßugsure.

27-04 to 37-3!72-96 to 72-62

Vol.Carb vapour. 0*5 or ..Oxygen gas... 1......

Sp.0-4160

im"i

Vol. Sp. gr.Oarb. Oxide gas 1 .. 0-9722Oxygen gas___0-5 . 0-5555

Carbonic Acid 1 .. 22 . 10000 . 10000 . 10000 Carb. Acid gas 1...... 1-5277 Carb. Acid gas. 1 .. 1-5277

Physiological Effects, a.. On Vegetables. —Carbonic acid gas is inju-rious to seeds, and diminishes or stops their germination. An aqueous Solutionof carbonic acid applied to the roots of plants promotes Vegetation. An atmo-sphere containing not more than l-8th of its volume of carbonic acid promotesthe Vegetation of plants exposed to the solar rays, but is injurious to those whichgrow in the shade. (Saussure, Recherchen, p. 25, et seq. Paris, 1804.) Thecarbon of plants is derived from carbonic acid, which they take in from the atmo-sphere, decompose, retain the carbon, and evolve (partially or wholly, aecording-to ciicumstances) the oxygen. Humus nourishes plants by presenting a slowand lasting source of carbonic acid which is absorbed by the roots. 3

ß. On Jinimah. —The respiratinn of carbonic acid is deleterious and fatal toall classes of animals. It operates as a narcotic or stupefacient poison. That itis a positive poison, and does not act merely by exeluding oxygen, as some havesupposed, seems to be proved by three facts:—firstly, an atmosphere composedof 79 parts of carbonic acid and 21 of oxygen acts as a poison, although there

1 At tbe Ecole de Pharmacie, in Paris, the apparatus employed in the condensatinn of the gas hurst, and(lestroyeil the fre.pariitrur (sec Journal de Pharmacie, t. xxvii ; and London Medical Gazatte, April 16, 1841.)

■> I.iebig-s Orga.uic Ckcmistru. in its Jjpplieation lo An-rieullnre and Physiology, edited by L. Playfair, Ph. D.r.ond. 1-340. f u