This gas is the chief product of combustion; our candles and fires are continually pouring it forth into the atmosphere, animals expire it from their lungs, and it is produced in every case of putrefaction and fermentation.
Carbonic acid, so fatal to animal life, is essential to the life of plants; indeed the existence of the whole vegetable kingdom depends on the presence of this gas in the atmosphere. Carbonic acid is a compound of oxygen and carbon or charcoal, which substance is the principal constituent of all plants. Every green leaf may be compared to a little chemical laboratory, in which the carbonic acid of the air is decomposed, the carbon being retained by the plant, while the pure oxygen is cast forth into the atmosphere.
Vegetables absorb the carbon which is exhaled in combination with oxygen by animals, and the two great divisions of organized beings are thus indissolubly connected by the interchange of substances necessary to their existence.
The old fable of the Hamadryads who presided over the trees of the forest, and who died when the trees were cut down, shadowed forth a deep truth. In the fairy-tales of science we read that the lives, not merely of wood-nymphs, but of all living creatures, are dependent on trees and herbs!
The atmosphere invariably contains a minute portion of ammonia, another compound body, its constituents being nitrogen and a gas called hydro-
