Carbonic Acid Gas (synonymes, carbon dioxide, carbonic anhydride), a gas discovered in 1757 by Dr. Black, and called by him fixed air. He detected it in limestone and magnesia, from which he found it could be expelled by heat and the acids, and also noticed that it was produced by combustion, fermentation, and respiration. Lavoisier demonstrated its composition synthetically by burning carbon in oxygen, and obtaining this product. It was analyzed by Smithson Tennant, by causing it, as evolved from heated limestone, to be decomposed by the vapor of phosphorus passing over it; carbon was deposited in a light black powder, and the oxygen combined with the phosphorus. The composition of this gas is:

Carbon, 1 atom.......=12,or percent........27.27

Oxygen, 2 atoms......=32, " ........7273

Its chemical equivalent then = 44, and it is represented by the symbol C02. The volume of the oxygen it contains is the same as that of the compound produced. Compared with air, its weight is as 1.529 to 1. It may be poured almost like water from one jar into another, displacing the air before mixing with it, as may be shown by its extinguishing a light placed in the lower vessel. It is without color, but has a decided sour taste, and a pungent odor. Its feeble acid reaction is shown in transiently changing litmus paper red. Flame is immediately extinguished when it is mixed with air in the proportion of 1 part to 4. Unmixed with air, it is entirely irrespirable; it is rejected with violent spasms of the glottis. In the atmosphere it is universally diffused in proportion exceeding 4 volumes in 10,000 by measure, even at the greatest height reached by man. It is this small quantity which furnishes to growing plants the carbon of their solid structures; and as the supply is diminished by this enormous absorption, the combustion and decay of organic bodies, and the respiration of animals, ever make good the deficiency. The great weight of this gas tends to keep it in the low places where it is generated, though, like other gases, it has also the tendency to mix with atmospheric air.

Hence it is always prudent, before descending into badly ventilated wells, to let a candle down to prove the presence or absence of the gas. It is related by Dr. Christison that cases have occurred of men becoming instantly insensible, even when the light burned. This may be owing to some peculiarity of the mixture of gases not understood, probably to the presence of carbonic oxide; for the writer has descended into air so impure that a candle could not possibly be lighted in it, and remained with another person long enough to make many ineffectual attempts to ignite it, and this with no other effect than a severe headache. In mines it is a very common thing for the men to continue their work in an atmosphere so foul that their candles go out, and are then relighted from the fire still in the wick, by swinging them quickly through the air, when they burn a little while and go out, and are again relighted in the same way. The son of Berthollet, the chemist, who destroyed himself by inhaling the fumes from burning charcoal, writing down his sensations at the time, remarked that the candle was soon extinguished. The lamp continued to burn, and was flickering as he became himself powerless to record more.

Persons made insensible by inhaling this gas may be restored by immediately dashing cold water over them. This is the practice pursued at the famous Grotto del Cane at Naples, in order to restore the dogs which, for the gratification of visitors, are exposed to the fumes of the gas, into which they are dipped as into an insensible bath. Such natural accumulations of this gas are not very rare, though much that is evolved from the earth is absorbed by the waters it meets, some of which are almost as highly charged with it as the " mineral " waters of the shops. When the air of wells is too impure to enter, it may be driven out by the ordinary modes of ventilation, by agitating the column for some time in any way, by the explosion of powder, or, as suggested and practised by Prof. Hubbard, by lowering a vessel containing ignited charcoal nearly to the bottom. Incandescent coals have the property of absorbing many times their bulk of this gas, and when cooled they may be raised up, reignited, and lowered again. A well in which a candle would not burn within 26 feet of the bottom, was thus purified in the course of an afternoon. - Water readily absorbs carbonic acid gas, from which it may be freed by boiling, freezing, or being placed under the exhausted receiver of an air pump.

Under the ordinary pressure of the atmosphere, and at a temperature of 60°, water takes up its own volume of the gas, and according as the pressure is increased, so is the bulk of the gas forced into the water. It gives a pungent, pleasant, slightly acid taste, and the sparkling effervescence seen in bottled liquors in which it has been generated. The gas obtained from powdered carbonate of lime or limestone, exposed to the action of hydrochloric or sulphuric acid, is used to saturate water for drinking. It is generated in strong metallic vessels, capable of sustaining a pressure of four or five atmospheres or more. This is the " mineral water " or " soda water" of the apothecaries - both improper names, as it contains neither soda nor other mineral substance. Exposed to the air, the greater part of the gas soon escapes, and when thoroughly expelled by boiling, the water has an insipid taste. Pure lime water detects its presence in solution, becoming immediately turbid; as the lime seizes upon the gas, and is converted into an insoluble white carbonate. But if the gas is greatly in excess, a portion of this is redissolved. Not only is limestone soluble in water impregnated with this gas, but metallic bodies are also acted upon by it and converted into carbonates.

As some of these are soluble and possess poisonous qualities, regard should always be had to this in the use of leaden pipes and vessels used for conveying and containing water, which by any means may be impregnated with the gas; and the copper gas generators of the druggists should especially be protected by a lining of tin, glass, or porcelain. - By subjecting carbonic acid gas to powerful pressure, Prof. Faraday succeeded in obtaining it in a liquid form. Thilorier repeated the experiments, and congealed the condensed gas into a solid form like snow. The pressure used for this purpose is that of 36 to 40 atmospheres. Sulphuric acid is made to react upon bicarbonate of soda in strong cast-iron cylinders, and the gas is passed through very small metallic pipes into a reservoir placed in a freezing mixture. In this it solidifies. In one of the early experiments of Thilorier, in a course of public lectures at Paris, the apparatus of cast iron exploded under the enormous pressure, and one of the assistants was so much injured that he died in a few hours.

It was observed by Thilorier that when the liquid gas was allowed to escape into a brass box through a small tube, the cold produced by the sudden evaporation of one portion was so intense that it served to congeal the remainder of the gas. This snowy product, remelted and resolidified, becomes a clear crystalline solid like ice. Having a low conducting power, it is not so volatile as the liquid gas; and though its real temperature is more than 100° below the zero point of Fahrenheit's thermometer, it does not cause a strong sensation of cold. Mixed with ether and then evaporated under an exhausted receiver, the greatest degree of cold ever known was obtained by Prof. Faraday. The spirit thermometer sank to 166° below zero. With the control of such a congealing temperature and the application of pressures varying from 27 to 58 atmospheres, Prof. Faraday succeeded in converting several of the compound gases into liquids and colorless transparent solids. An illustration of the intense cold produced by tho evaporation in the open air of the solid gas and ether is given in the freezing of ten pounds of mercury in less than eight minutes by contact with these substances upon its surface.

A large lump of the gas was kept for a minute in a red-hot crucible, and a pound of mercury was immediately afterward frozen with it. The vapor given off from the solid gas possesses a higher tension than that from any other substance; and, unlike the vapor from other bodies, it is developed by lowering instead of raising the temperature. - According to modern notation, carbonic acid gas is more properly to be called carbonic anhydride, or carbon dioxide. No definite hydrated carbonic acid is known, the anhydride, both in the form of gas and in its denser conditions of liquid and solid, being, as its name indicates, free from water; but it appears convertible into a true acid by solution in water, C02 + H20, yielding H2CO3; this in turn combines with bases to form the well known series of salts called carbonates. (See Carbonates).