190

ELEMENTS OF MATERIA MEDJCA.

It is produced in the burning of limestone (carbonate of lime) at limekilns, and by the combustion of charcoal, coal, wood, coal gas, the fire-damp of coal mines, and other combustibles containing carbon.

(b.) In the organized kingdom .— Carbonic acid gas is exhaled byplants in dark or shady places, and hence is met with in green-liouses,especially during the night. Animals develope it in the process ofrespiration, and, therefore, in crowded rooms, with imperfect ventilation,accidents have sometimes happened from the accumulated carbonic acid.It is produced by the decomposition of organic matters, as during fer-mentation of saccharine fluids, and in the destructive distillation ofanimal substances: hence the danger of descending into brewers’ vats.Free or combined carbonic acid is found in the blood, urine, bones, &c.

Preparation. —Carbonic acid gas may be procured in various ways,but for ordinary purposes is usually obtained by the action of some acid(usually hydrochloric) on carbonate of lime (as white marble). It ismost readily prepared in a tubulated glass retort, and may be collectedover water. The ordinary hydrochloric acid of the shops should bediluted with four or five times its volume of water. By the reaction ofone equivalent, or 37 parts of hydrochloric acid, on one equivalent, or 50parts of carbonate of lime, we obtain one equivalent, or 50 parts ofchloride of calcium, one equivalent, or 9 parts of water, and one equi-valent, or 22 parts of carbonic acid.

INGREDIENTS USED.

r 1 eq. Carbon. Acid ... 22

Properties. —At ordinary temperatures and pressures carbonic acidis gaseous. In this form it is invisible, irrespirable, has a faint odour,and a sharp taste. Its specific gravity is P5277. It is neither com-bustible nor a supporter of combustion, except in the case of potassium,which, when heated in this gas, takes fire, the products of the combus-tion being carbon and carbonate of potash. It extinguishes most burn-ing bodies when introduced into it in the ignited condition. It reddenslitmus feebly.

Under a pressure of 36 atmospheres at 32°, carbonic acid is a limpid,colourless liquid, -which is insoluble in water and in the fat oils, but issoluble in all proportions in alcohol, ether, oil of turpentine, and carburetof sulphur. Its refractive power is much less than that of water. Itsexpansibility by heat is greater than that of gases ; for when heatedfrom 32° F. to 86° F., its bulk increases from 20 to 29 volumes, while thepressure of its vapour augments from 36 to 73 atmospheres.

By an intense degree of cold, liquid carbonic acid may be solidified.Thus, when the pressure is removed from this liquid by opening a stop-cock in the condensing vessel, the cold produced by the expansion is sogreat, that a quantity of solid carbonic acid is formed. In this state itis so intensely cold, that mercury, liquid chlorine, liquid cyanogen, &c.may be readily frozen by it. Being a very bad conductor of heat, itmay be handled with impunity.

Characteristics.— Carbonic acid gas is recognised by its feebly red-dening litmus, by being incombustible and a non-supporter of combustion

RESULTS.

-1 eq. Carbon. Acid 221 1 eq. Water ... 9

-I eq. Chlor.Calc™. 56