volcanoes, and it is always present in small quantities in the
atmosphere; it is found dissolved in well and river waters, and it is a
product of the respiration of animals. Brewers also are well aware of
the existence of this body, for it is evolved in enormous quantities
during the alcoholic fermentation of saccharine fluids. When
carbonaceous substances are burnt the bulk of the carbon is converted
into carbonic acid, and thus our furnaces and fireplaces are continually
emitting enormous quantities of carbonic acid into the atmosphere. With
these different sources of supply it might reasonably be thought that
carbonic acid would be gradually accumulating in our atmosphere; the
breathing of animals, the eruption of volcanoes, the combustion of
fuel, and the fermentation of sugar, are ever going on, and to a
fast-increasing extent with the progress of civilization, and yet the
proportion of carbonic acid in our atmosphere is no greater now than it
was at the earliest time when exact chemical research determined its
presence and quantity. A counteracting influence is always at work;
nature has beautifully provided for this by causing plants to absorb
carbonic acid, holding some of the carbon, and allowing the oxygen to
escape again into the atmosphere to restore the equilibrium of purity.
This mutual evolution and absorption of carbonic acid is continually
going on; occasionally there may be either an excess or a deficiency in
a particular place, but fortunately any irregularity in this respect is
soon overcome, and the air retains its original composition, otherwise
animal life on the face of the globe would be doomed to gradual but sure
extinction.

Carbonic acid can be prepared for experimental purposes by causing
dilute hydrochloric acid to act upon fragments of marble placed in a
bottle with two necks, into one neck of which a funnel passing through a
cork is fixed, and into the other a bent tube for conveying the gas into