CARBONIC ACID. (Fa., Acide carboniqu,e ; GER., It" Ildensiiure.) Formula CO,.
Although carbon dioxide in its natural state possesses no acid properties, and therefore the designation " carbonic acid" as applied to it is somewhat misleading, it seems desirable, under the popular acceptation of the term, to describe the nature, properties, and chief sources of the four forms of the oxide with which we are acquainted, viz. the gaseous, solid, and liquefied anhydride, and the tutu, ous solution of the gas which alone is the true carbonic acid. It may be said at the outset that there are few bodies more important than carbon dioxide to which so inadequate attention has been given by chemists and physicists. Known and utilized from the daysof Paracelsus, its composition, even, was not demonstrated until Lavoisier turned his attention to the subject. It was then estab lished that the true proportions were as nearly as possible 28 per cent. of carbon, and 72 per cent. of oxygen. Carbon dioxide has been known by a variety of names :—carbonie acid ; fixed air (so ealh d by Dr. Black, who, in 1757, separated it as a definite constituent of limestone, and from its absorption by solutions of caustic alkalies); mephitic air (front its noxious and destructil e proper ties); and carbonic anhydride, or carbon hydride ; koblensaures gas ; kohlensiture; gaz sylvestre ; spiritus sylvestris.
Gaseous carbon dioxide is a colourless body, with a slight sweetish taste, and is one and a half times heavier than air. It is not inflammable, nor does it support combustion, though potassitun and sodium decompose it—the former when heated to redness. Forming a food for growing vegetation it is yet exceedingly destructive of animal life it hen inhaled. When perfectly dry, it does not redden litinus, and, in fact, possesses no acid properties. It is soluble iu about its own weight of water at ordinary temperatures, forming the aqueous carbonic acid. Carbonic anhydride is an exceedingly stable body, being decomposable at only a very great heat ; the solution is as unstable.
Few bodies are so widely diffused as gaseous carbon dioxide. It is a constituent of the air to the extent of 0.4 per cent. by volume, it exists in expired air up to 3.5 per cent., and iu the air of crowded rooms to 0-5 or 0.78 per cont., through the oxidation of the carbon contaiocd in the body, and is necessary to the growth of vegetation. It is produced in the ordinary process of oxida tion or combustion of all substances containing earbon, in the various processes of fermentation, and in the decay of animal and vegetable matter. Front the craters of volcanoes, front fissures in the ground, in mines, wells, and quarries, the gas is continually evolved in enormous quantities. It is produced by the decomposition of the earbonates either by heat or by ths action of stronger acids. Finally, it is a constituent of the natural efferveseing waters of Vichy, Spa, Kissingen, Ems, Carls bad, &c., 8re. The amount of the gas present iu the best known of these natural springs is as Carbonic anhydride may be readily formed by heating carbonate of lime to redness in an iron, brick, or earthenware retort or furnace. The usual method of preparation, however, is by de
composing marble or chalk by dilute hydrochloric or sulphuric acid. Hydrochloric aeid is the better agent, as the insoluble sulphate of lime which is formed when sulphuric acid is used coats the chalk and hinders further decomposition. The residue sets very hard aLso, and is diffieult to remove from the generating vessel. Chloride of calcium forms a useful bye-product, and is readily soluble.
Under ordinary conditions as s, gas, carbon dioxide may be readily liquefied by varying tbe temperature and pressure. For this purpose, at 0° (32° F.) a pressure of thirty-six atmospheres is required ; as the temperature is raised, greater pressure is required, until the gas refuses to be liquefied under any pressure. This point—the " critical point," 9,8 it has been designated—is rather over 32°. Under the ordinary pressure of the atmosphere a temperature of — 87° (which may he produced by evaporating liquefied ammonia over sulphuric acid in vacuo) is required to effect liquefaction. The body obtained by these methods is a thin, colourless liquid, lighter than water, and four times rnore expansible than air. It is miscible in all proportions with ether, aleohol, naphtha, and turpt ntine, but floats on the surface of water. Its specific gravity at — 10" is 0-9951, at 0°, 0-9470, and at + 20°, 0.8266. The:•e figures show the extraordinary expansion of the liquid upon increasing the temperature, its coe eient of expansion being greater than that of any other body. The boiling poiut of liquid carbon anhydride is — 78.2° under a pressure of 760 mme. Its tension at different temperatures is shown by the following table :— The spontaneous dehydration of liquefied carbon anhydride, and the readiness with which it is converted into the gaseous compound upon variation of pressure and temperature, has led to its adoption as a motive power. Little beyond experiment bas yet been done in this direction, but if the difficulty of first cost could be got over, or if some ready way could be found of recovering the gas and re-liquefying it, so as to make a continuous operation, there seems to be little reason why it should not to some considerable extent supersede the use of steam. So far about 7 lb. of coal are required to produce a sufficient quantity of "carboleum," as this substance has been called, to do the work of one horse-power per hour. It has been prepared at Newport, Rhode Island—at the United States naval station—for the purpose of driving torpedoes, but the difficulties both of cost and extensive plant seem at present to be insuperable, a steam-engine to work the compressing pump and an arrangement of freezing mixture being required. Given a sufficient pressure, of course the freezing arrangement might be done away with, but the compressing of the air in the receiver up to 801b. per square inch would be necessary.