CARBONIC ACID. There are three com pounds of carbon and oxygen, Carbonic Oxide, Carbonic Acid, and Oxalic Acid. Carbonic acid is the most important of the three. It exists largely in nature. It is in comparatively small quantity in the gaseous state in the atmosphere ; it is in solution in most spring water, and in some called mineral waters to a considerable degree; but it is in solid combi nation that it is found in the largest quantity, forming nearly 44 per cent. of nil limestones and marbles, besides occurring in less quan tity united with other earths and metallic oxides. Carbonic acid is produced by fermen tation; by the process of respiration ; by animal and vegetable putrefaction; and by combustion, whether of oil, wax, tallow, vegetable matter, or coal.
In its uncombined state carbonic acid is a gas, colourless and transparent, and therefore invisible ; it has an acid and slightly astringent taste ; it extinguishes burning bodies, and is fatal to animals. On account of its great density, it diffuses slowly in the air, and hence it is apt to remain long in fermenting vats, old wells, &c., and has frequently produced fatal effects upon persons descending into them. Atmospheric air may however contain 1-20th of its volume of this gas, and be re spired without becoming hurtful. On account
of its great weight, it may be poured from vessel to vessel, as is shown by its extinguish ing a taper repeatedly. Though gaseous at common temperatures, Faraday has showed that carbonic acid can be brought to the liquid state by intense cold and pressure: it is then a limpid colourless fluid, which M. Thilorier rendered even solid, in which state it is one of the most powerful refrigerating substances known. Carbonic acid is an ingredient in many mineral waters, to which it imparts effervescing qualities.
Carbonic acid gas is apt to be formed and accumulate in mines, particularly coal-mines, where it is termed choke-damp, and whence have arisen fearful colliery accidents ; it is also found in old draw-wells.
Carbonic acid plays so important a part in medicine, in chemical manufactures, and in every day life, that it would he impossible even to enumerate its characteristic functions. CARBONIFEROUS SYSTEM is the geo logical name for the great group of strata which includes nearly all the valuable coal yet discovered. We are concerned with it here only in respect to COAL.