an acid , not existing separately, which, combined with positive or basic atoms or radicals, forms carbonates. In common language the term is very generally applied to a compound of carbon and oxygen, , more correctly called . It is a colorless, heavy, irrespirable gas, extinguishing flame, and when breathed destroys life. It can be reduced to a liquid and solid form by intense pressure. It is produced in the fermentation of liquors, and by the combustion and decomposition of organic substances, or other substances containing carbon. It is formed in the explosion of fire damp in mines, and is hence called ; it is also know as , and . Water will absorb its own volume of it, and more than this under pressure, and in this state becomes the common soda water of the shops, and the carbonated water of natural springs. Combined with lime it constitutes limestone, or common marble and chalk. Plants imbibe it for their nutrition and growth, the carbon being retained and the oxygen given out.

a colorless gas, , of a light odor, called more correctly . It is almost the only definitely known compound in which carbon seems to be divalent. It is a product of the incomplete combustion of carbon, and is an abundant constituent of water gas. It is fatal to animal life, extinguishes combustion, and burns with a pale blue flame, forming carbon dioxide.