One of the products of the combustion of carbon, also known as carbon dioxide. When coal or wood burns brightly in the fire, carbonic acid is produced, and it may be prepared artificially by acting on chalk or marble with hydrochloric acid. The air we breathe contains oxygen, which enters the blood, combines with the waste carbon of the tissues, and is breathed out again as carbonic acid. This substance is given out in large quantities from volcanoes, and from the ground in volcanic regions. It is also producedduring the fermentation of wine and beer. It is one of the permanent gases of the atmosphere, in the proportion of about 4 volumes in 10,000. Plants derive it from the atmosphere and supply it to animals.