a, Flask containing the sample of carbonate for examination, stopped by a closely fitting cork, through which passes the bent tube c.
b, A small tube, sufficiently long to maintain a slanting position without falling, filled with sulphuric or hydrochloric acid.
c, A bent tube, connecting the flask with d.
d, Horizontal tube, filled with small fragments of fused or dried chloride of calcium, with a fine orifice at the extremity e.
CARBONIC ACID. H2CO3. True carbonic acid has not yet been obtained in any satisfactory condition, although the solution of carbonic anhydride (often called carbonic acid), or anhydrous carbonic acid, is generally regarded as such. It forms with bases an important series of salts, called the carbonates, by double decomposition.
CARBONIC ANHYDRIDE. CO2. Syn. Carbonic acid, Carbon dioxide, Fixed air, Choke damp; Acide carbonique, Fr.; Kohlen säure, Ger. A compound formed by the chemical union of carbon and oxygen.
Hist. Van Helmont recognised carbonic acid as a peculiar gas. Dr Black, in 1757, proved that it was a constituent of limestone, and gave it the name of fixed air; he also showed that the causticity of alkalies depended on its absence. Bergmann first described it as an acid, applying to it the term aërial acid. Lavoisier, in 1776, established its true nature, and gave it the name it now bears. Faraday, in 1823, by pressure at an extremely low temperature, reduced carbonic acid to a liquid, and a few years later Thiloria and Brunel obtained it in the solid form.
Nat. Hist. Carbonic acid is a constituent of the atmosphere, its presence being essential to the existence of vegetable life on the globe. It issues from the earth in many situations, as the Grotto del Cane in Italy, the Valley of Poison in Java, and near the Lake of Laach in Germany. It gives to many mineral springs their sparkling brilliancy, and is held in solution by all natural waters. Combined with the bases, lime and magnesia especially, it exists in large quantities in the crust of the earth. It is the chief product of combustion, and one of the products of fermentation. It is always being exhaled by animals in the process of respiration, and in smaller quantities by plants at night or in the shade. It forms the terrible “choke-damp” or “after-damp” of the coal mines. It is the gas disengaged during the effervescence of soda water and other aërated drinks, and the cause of the freshness of newly-drawn beer.
Prep. Hydrochloric acid, 1 part, diluted with water, 4 or 5 parts, is poured upon fragments of white marble, previously placed in a suitable generating apparatus.[237]
[237] A large flask, provided with a bent glass tube for conveying the gas, and a tube-funnel for introducing the acid, is the most convenient form of apparatus. A tubulated retort may be used, but the generating flask or bottle is to be preferred.