CARBON DIOXIDE, CARBONIC ACID GAS, or CARBONIC ANHYDRIDE, COI, is formed whenever carbon is burned in the presence of excess of oxygen or air. It is a colorless, odorless gas about 1.53 times as heavy as air, bulk for bulk, and soluble to a considerable extent in cold water, especially when subjected to pressure. Its solution pos sesses feebly acid properties, and has a pecul iarly pungent taste, on account of which the aqueous solution of the acid is greatly used as a constituent of various beverages. The effervescence accompanying the opening of a bottle of beer, soda-water or champagne is due to the escape of the carbon dioxide that was previously held in solution under pressure. Carbon dioxide occurs in great abundance in nature, both free and in combination with vari ous elements in the form of carbonates. Car bonate of lime, CaCO., is one of the most com mon carbonates. It is formed when the gas is allowed to bubble up through a solution of lime water and exists in nature in vast masses as limestone and marble. (Other carbonates are described under the metals that constitute their bases). Carbon dioxide is a constant con stituent of the atmosphere (see AIR), occur ring even at the tops of mountains and in the air collected by balloons at great height. It is generated by the combustion of fuel, by respira tion, by fermentation and by the decay of ani mal and vegetable matter. In some localities, too, immense quantities of the gas are emitted from the ground, or from mineral springs and wells, as at Saratoga Springs in the United States, and in the Grotto del Cane, the Cave of Montjoly in Auvergne, in the valley of Wehr, in the Eifel and at many other places in Europe. It is being simultaneously abstracted from the air by plants, which in the sunlight decompose the gas, timing the carbon that it contains, and setting the oxygen free. Carbon dioxide has but feeble affinity for the bases with which it combines, and is readily displaced by almost any other acid. In preparing the gas for experimental purposes the usual method is to add a dilute mineral acid to pulverized mar ble or other carbonate, the carbon dioxide then being liberated continuously and in large quan tities. On a large scale carbon dioxide is made
by heating limestone to redness in closed re torts, at the bottom of which superheated steam is blown in. This passes up through the heated limestone carrying with it the liberated car bon dioxide, and through outlets at the top into coolers and compressors. Besides its large use in the manufacture of aerated drinks, carbon dioxide is used in sugar factories to clarify the cane juice after treatment with lime. It is also used to preserve wines from deterioration by certain molds, and other organisms which set up acetic fermentation. Wine thus treated is distinctly improved in quality. Carbon dioxide is the active principle in baking pow ders, being liberated from the soda carbonate by the acid constituent of the powder, and in its efforts to escape from the dough produces the lightening effect.
The critical temperature of carbon dioxide is about 88° F., and at any temperature lower than this it can be reduced to a liquid by the application of pressure. Liquid carbon dioxide is colorless. It will not mix with water, but dissolves readily in alcohol, ether and volatile oils. When the pressure is released, part of the liquid vaporizes rapidly, and the remainder solidifies through the production of intense cold. Solid carbon dioxide is a white mass resembling snow. It remains for some time open to the air without melting. Its in terior temperature, however, as shown by a thermometer sunk into the mass, F. Its melting point Poisoning by this gas frequently results in closed rooms crowded with people. The symp toms may be very slight, consisting of a mild indisposition, or they may be severe—head ache, nausea, vomiting, etc. In poisoning in the severer grades there is cyanosis, coma and unconsciousness. Carbon dioxide is not in it self a fatal poison; it becomes so, however, in the absence of a sufficient supply of oxygen, death being produced by simple asphyxiation.