CARBONIC-ACID GAS, or CARBON DIOXIDE (carbon -f- dioxide, from Gk. Bi-, di-, double oxide), CnoKE-DAmie, or VI :KED Ala. A gaseous compound of carbon and oxygen represented by the formula CO,. It occurs in the free state as a constituent of atmospheric air, and iu solu tion in sea-water and mineral springs. It is largely evolved from fissures in the earth. espe cially in volcanic districts: in certain localities in 'Java and in the neighborhood of Lake Lam-h, near the Rhine, the amount of carbon dioxide evolved is so great that birds attempting to fly across the poisonous spots drop dead. The fa mous Dog Grotto, near Naples. is likewise filled to a certain height with carbonic-acid gas. by which dogs brought into the grotto are rendered insensible iu a few seconds. The experiment is often performed, on payment of a small fee, for the amusement of travelers. The amount of car bonic acid normally contained in atmospheric is relatively small—I0.000 volumes of air contain about 3 volumes of carbon dioxide; or, which is the same. 10,000 parts by weight of air contain about 4.5 parts by weight of carbon dioxide. Animals constantly add to this by respiration: plants. on the contrary, absorb the gas, which they transform, with the aid of light, into oxidizable food-matter. As a result, the amount of carbonic acid in the atmosphere tends to remain constant. However, the principal cause of the constancy of the composition of our atmosphere lies in the fact that the water of the ocean contains immense quantities of free carbonic acid. If the amount of the latter in the air should rise above the normal. the excess would be dissolved in the sea : conversely, a cer tain amount of the gas would escape from the water if the amount in the air should fall below the normal. Slight variations, however. have been observed. Thus. in elevited places the amount of carbonic acid is usually smaller than 111.3 r the level of the sea. In the vicinity of forests, too, especially in summer, the air con tains somewhat less carbonic acid than the air in cities.
The average amount of carbonic acid produced in twenty-four hours by man is about 900 grains, the amount given out during the day being con siderably greater than that produced during sleep. It has been recently suggested that the air in closed rooms may be continually purified and renewed by the use of sodium peroxide, a substance which has the pi-mail-1.y of absorbing carbonic acid and of giving an equivalent amount of pure oxygen. total amount of free carbonic acid in nature is obviously very great. But even larger quantities, probably, exist combined in the forms of carbonates, such as chalk and limestone, forming part of the solid crust of the earth. For experimental purposes carbon dioxide is most easily prepared by the action of dilute hydrochloric acid on chips of marble (calcium carbonate). The large quanti ties of carbonic acid employed in the arts are prepared in several different ways. Usually di lute sulphuric acid is made to act on some porous variety of calcium carbonate, such as chalk. In the manufacture of mineral waters. magnetite or the densest varieties of dolomite are employed, as they are liable to contain a smaller amount of organic matter than other materials. The carbonic acid used in sugar-manufactories for precipitating lime is made by burning charcoal; the gas evolved from lime-kilns, or that produced by fermentation, is largely used for the same purpose. Carbonic acid is also extensively used in the manufacture of soda. The gas emplcyed in making artificial mineral waters (q.v.) must be carefully purified by passing it through solutions of potassium permanganate, which re tains all organic impurities without attacking the gas itself. Carbonic acid does not support combustion; it is, therefore, used, highly com pressed in iron cylinders. as a fire-extinguisher.
Besides the methods mentioned above, carbonic acid may be prepared by simply heating metal lic carbonates; thus, in determining nitrogen, in organic analysis, the air is best expelled from the combustion-tube by means of carbonic acid made by heating sodium bicarbonate or mag nesium carbonate. Pure carbon dioxide is a colorless gas about one and a half times as heavy as air; it has a feeble odor and a slightly acid taste. Owing to its high density. it does not rapidly diffuse through air, and may be poured like a liquid from one vessel into an other. When absorbed by plants. its earbon is transformed into carbohydrates, while the whole of its oxygen is returned to the atmosphere. When Mtroduced into the stomach of man (say in the form of some mineral water). it has a refreshing effect, and promotes digestion. If inhaled, however, it is supposed to decompose the haimoglobin of the blood and to enter into combinati011 With the colored product of decom position called haimoch•omogen. It has been shown. nevertheless. that the animal organism is eapable of adapting itself in a peculiar way to the action of carbonic acid; birds confined 1%ithin air-tight jars are capable of living in an atmosphere in which an animal introduced di rectly front fresh air would die in a very short time. The simplest way of testing the air in a room is to introduce a burning candle: if it continues to 1)111-11 quietly and with a bright flame. the air is pure and respirable. The amount of carbonic acid in rooms should not exceed two grams to the cubic meter of air. Water contain ing free carbonic acid is eapable of attack many substanees that pure water cannot dissolve. :Many rock formations have been de stroyed by the action of such water on the car bonate of lime contained in them. Under crdi nary conditions of pressure and temperature, carbon dioxide dissolves in water in accordance with the law of Henry; that is to say, the amount of gas absorbed is proportional to the pressure. This shows that the existence of the hypothetical compound of Water and carbon di oxide in the free state is highly improbable, though its derivatives, the carbonates, are among the substances commonly met with in nature. At 0° C. carbon dioxide is liquefied under a pressure of 31i atmospheres. The critical temperature urns ascertained by Andrews, and, more recently, by Dewar. The latter investigator found that above 31.9° C. a pressure of 77 atmospheres is necessary and sufficient to liquefy it. Liquid carbonic acid is now largely used in the arts. A mixture of liquid carbonic acid and ether. if rapidly evaporated, attains a temperature of about 100° C. below the freezing-pond of water. Chemically, car bonic acid is a dibasic acid capable of forming salts in which metallic elements are substitutol either for one or for both of the hydrogen atoms. These salts, called carbonates, are readily decom posed by most other acids known in chemistry, carbonic acid being one of the weakest acids known. Even the so-called acid carbonates of sodium and potassium have a feebly alkaline re action• carbonic acid being incapable of neu tralizing the powerful alkaline properties of sodium or potassium hydroxide. Carbonic acid is the earliest known gas. Paracelsus and Van Belmont, who lived in the Sixteenth Century, knew that the gas produced in burning charcoal was identical with that evolved by limestone heated to a high temperature. About the middle of the Eighteenth Century .Joseph Black isolated it in a perfectly pure state. In the latter part of the Eighteenth Century Priestley discovered it in the air, and Lavoisier showed that the same gas was produced during the combustion and decay of vegetable and animal matter, during respiration, etc. Faraday was the first to liquefy it. Sec CHEMISTRY.