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CHEMICAL ACTION OF

and emit the oxygen. So they absorb the same gasfrom water, when it is separated from that fluid bythe action of light. The burning of a candle, or thebreathing of animals, in confined air, produces somuch of this gas, that neither of these operations cango on beyond a certain time ; but the air so conta-minated serves as food for vegetables, whose leaves,assisted by light, soon restore the oxygen, or, inother words, purify the air again. This beautifuldiscovery, for the main principles of which we areindebted to the celebrated Dr. Priestley, shows amutual dependance of the animal and vegetablekingdoms on each other, which had never been sus-pected before his time. Comparative experimentsupon the lower tribes of these kingdoms have notyet been made, but they would probably afford us anew test for distinguishing them. The air so copi-ously purified by a Conferva, one of the most infe-rior in the scale of plants, may be very extensivelyuseful to the innumerable tribes of animated beingswhich inhabit the same waters. The abundant air-bubbles which have long ago given even a botanicalname to one supposed species, Conferva bullosa, areprobably a source of life and health to whole nationsof aquatic insects, worms and polypes, whenever thesun shines.

In the dark, plants give out carbon and absorboxygen; but the proportion of the latter is small,compared to what they exhale by day, as must like-