This is the balance which we imitate in the Aquarium.  It is the whole law of life, the whole scheme of Nature, the whole equilibrium of our organic world, inclosed in a bottle.

For the rapid evolution of oxygen by plants the action of sunlight is required.  That evolution becomes very feeble, or ceases entirely, in the darkness of the night.  Some authorities assert even that carbonic acid is given off during the latter period.  So, too, they claim that there are two distinct processes carried on by the leaves of plants,—­namely, respiration and digestion:  that the first is analogous to the same process in animals; and that by it oxygen is absorbed from, and carbonic acid returned to the atmosphere, though to a limited degree:  and that digestion consists in the decomposition of carbonic acid by the green tissues of the leaves under the stimulus of the light, the fixation of solid carbon, and the evolution of pure oxygen.  The theory of distinct respiration has been somewhat doubted by the highest botanical authority of this country; but the theory of digestion is indisputable.  And it is no less certain that all forms of vegetation give to the air much more free oxygen than they take from it, and much less carbonic acid, as their carbonaceous composition shows.  If fresh leaves are placed in a bell-glass containing air charged with seven or eight per cent. of carbonic acid, and exposed to the light of the sun, it will be found that a large proportion of the carbonic acid will have disappeared, and will be replaced by pure oxygen.  But this change will not be effected in the dark, nor by any degree of artificial light.  Under water the oxygen evolved from healthy vegetation can be readily collected as it rises, as has been repeatedly proved.

Why carbonic acid is, to a limited degree, given off by the plant in the night, is merely because the vital process, or the fixation of carbon and evolution of oxygen, ceases when the light is withdrawn.  The plant is only in a passive state.  Ordinary chemical forces resume their sway, and the oxygen of the air combines with the newly deposited carbon to reproduce a little carbonic acid.  But this must be placed to the account of decomposing, not of growing vegetation; for by so much as plants grow, they decompose carbonic acid and give its oxygen to the air, or, in other words, purify the air.

It has been found by experiment, that every six pounds of carbon in existing plants has withdrawn twenty-two pounds of carbonic acid gas from the atmosphere, and replaced it with sixteen pounds of oxygen gas, occupying the same bulk.  And when we consider the amount of carbon that is contained in the tissues of living, and of extinct vegetation also, in the form of peat and coal, we may have some idea of the vast body of oxygen which the vegetable kingdom has added to the atmosphere.

And it is also to be considered, that this is the only means we know of whereby free oxygen is given to supply the quantity constantly consumed in respiration, combustion, and other vast and endless oxygen-using processes.  It follows, therefore, that animals are dependent upon plants for their pure oxygen, as well as for their food.  But the vegetable kingdom might exist independently of the animal; since plants may derive enough carbon from the soil, enriched by the decaying members of their own race.