CARBONIC ACID, FIXED AIR, or CIIOICE-DA?P, is a substance occurring free as a gas in the atmosphere, to the extent of 1 volume to 2,500 of air, and also in combination with a variety of substances, It is most easily prepared for experimental purposes front chips of marble, water, and hydrochloric acid, which are placed in a gas bottle with tubes. • The hydrochloric acid (11C1) acts on the marble and forms chloride of calcium (CaC1), water (HO), and carbonic acid which escapes as gas with effervescence, and may be conducted by a proper tube under the mouths of jars filled with water and placed on a pneumatic trough. Where C. A. is required in large quantities, it is prepared in a leaden vessel from chalk and sulphuric acid (S02) diluted with water, when sulphate of lime (CaO,SO,) is formed, and C. A. escapes as gas.
The atomic weight or equivalent of C. A. is 22; it is a clear, colorless gas, with a pleasant acidulous smell and taste. Under great pressure and cold, it can be condensed into a liquid, and even a solid resembling snow in appearance. Under ordinary atmos pheric pressure, C. A. dissolves in water to the extent of 2 volumes of gas in 3 of water; but under increased pressure, a very much larger amount of gas is taken up by the water, and in this way the various kinds of Ai:MATED WATERS (q.v.) are prepared. The gas is more than half as heavy again as ordinary air, being 1529. It is incombustible, and a non-supporter of combustion, at once extinguishing a lighted candle, gas jet, or even a piece of burning phosphorus, when these are placed in a jar filled with the gas, or even in a mixture of C. A. and air. This power of putting out flame and fire has been turned to account in the extinguishing of burning coal-mines, where, all the openings to the mine being properly secured, C. A., in the form of the spent air from an ordi nary coal-furnace, has been passed into the mine, with the result of successfully stop ping the fire. It is irrespirable in a concentrated form, producing spasm of the glottis, which prevents the admission of the gas into the system; and when mixed with air, it can be breathed without suspicion, and then acts as a narcotic poison, even when pres ent only to the extent of 4 or 5 per cent of the air. The deadly effects of C. A. are observed, in the combustion of charcoal, coal, or coal-gas, in chauffers, furnaces, or in fireplaces with the dampers down, when the .deadly fumes of C. A. steal more or less quickly over the inmates of the room, and they almost unconsciously become its vic tims—thus unknowingly following the course of the Parisian suicide, who purposely lights a charcoal fire in the center of his room, and prepares for death ; and in over crowded rooms where the C. A., exhaled from the lungs of each inmate at every breath, poisons the air of the apartment, and day by day slowly but surely robs the robust of health, and ultimately of life. In such cases as the Black Hole of Calcutta (q.v.), where there was scarcely any outlet for the poisonous gas, only a few hours may be required to complete the catastrophe.
Though poisonous when inhaled by the lungs, C. A. is rather refreshing when taken into the stomach. Thus, aerated beverages of all kinds—beer, champagne, and car bonated mineral waters—owe their refreshing and invigorating qualities to the presence of C. A.; and if the gas be allowed to escape, they become almost tasteless, stale, and mawkish.
Besides abounding everywhere in the atmosphere, C. A. is largely evolved from fissures in the earth, especially in volcanic districts. In the poison or Upas valley of Java, which is a valley of an oval form, about I- of a mile in circumference, and 80 to 35 ft. deep,•the carbonic rises to a height of about 18 ft. from the surface, and the whole bottom of the valley is devoid of vegetable and animal life, and is strewn here and there with the bleached bones of man and other animals that have unluckily stepped within the deadly circle. A dog thrown in dies in 14 seconds; and birds attempting to fly across the valley, instantly drop down dead. In the neighborhood of the lake of Laach, in Rhenish Prussia, the amount of C. A. evolved every day has been estimated at 600,000 lbs. weight. In a state of combination, C. A. forms an ingredient in a great number of minerals called carbonates, such as chalk, limestone of various kinds black-band iron-stone (carbonate of iron, FeO,CO2), malachite (carbonate of copper, CuO,H0,-KuO,CO2), etc. C. A. is the principal product of combustion; the carbon of the burning substance (coal, candle, coal-gas, wood, paper, etc.) uniting with the oxygen of the atmosphere, and forming C. A. It is also a product of respiration (q.v.), and is evolved more or less largely by all animals, not only by the mouth, but in exhalations from the skin, and is present in blood, urine, etc. It is evolved during the fermentation (q.v.) of beer, wine, etc., and often remains in brewers' vats when the liquor has been drawn off. During the decay of vegetable and animal matter, C. A. is produced, and in explosions of fire-damp in coal mines, it is formed in large quantity, and fills the underground passages.
C. A. forms the largest ingredient in the food of vegetables, and is therefore abstracted in large quantity from the air by plants. It enters into combination with the majority of the oxides of the metals and other compounds, to form a class of salts called carbonates, several of which have been referred to. C. A. when present in a vessel in quantity may be recognized by the power of extinguishing a lighted candle, or by not burning itself. C. A. in the form of gas may be readily recognized in the atmosphere by exposing a little lime-water in a saucer, or other shallow vessel, when the lime (Ca0) abstracting the C. A. from the air, a white film of carbonate of lime or chalk is formed on the surface of the liquid. A solution of baryta. (q.v.) in water is more delicate in its action on the C. A. of the air, and more readily indicates its presence.