The principal compound of carbon and oxygen is carbon dioxid, often
called carbonic acid gas. This gas is always present in the air. It
issues from the earth in many places, particularly in the neighborhood
of volcanoes. With it many mineral waters are naturally charged.
[Sidenote: How carbon dioxid enters the air]
Carbon dioxid is constantly formed by many natural processes. Every
animal that breathes gives off carbon dioxid from its lungs. This gas
is also formed whenever ordinary combustible materials are burned. The
natural processes of decay of both vegetable and animal matter tend to
convert the carbon contained therein into carbon dioxid, which is thrown
off and absorbed into the air. The process of alcoholic fermentation,
and similar processes, also give rise to the formation of this gas.
When fruits ripen, fall, and decay, the sugar, which all fruit-juices
contain, is changed to alcohol and carbon dioxid.