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Plants take up this carbonic acid from the atmosphere, mould the carbon into their own substance, and set free the oxygen, once more returning it to the atmosphere. Animals reverse the process, taking up the oxygen, and giving out carbonic acid for the nourishment of plants," and so as Keats beautifully says,—
"Keeping up an interchange of favours
Like good men in the truth of their behaviours."
It is to the want of an acquaintance with these first principles that so many of the failures in attempting to form Aquaria may be attributed; the mere filling them with animals will not suffice,—and secondly to an abominable system of overstocking,—specimen after specimen dying from an insufficiency of those gasses which are essential to life.
How this necessary balance may be attained, we shall endeavour to show as we progress,—meanwhile we can go in detail through the various matters required for the establishment of an Aquarium. Its form must depend very much on the taste or caprice of the owner; the square one with glass sides is perhaps the most common, but we question whether it is the most desirable, for Mr. Warrington has discovered that it is absolutely necessary the sun's light should fall directly from the surface—on those Algae, which have been removed from the deeper recesses of the ocean, where they were but partially exposed to light, to ornament the tank and afford sustenance to its inhabitants. The tank used by this gentleman is four-sided,—the back and sides being composed of slate,—the back gradually sloping upwards from the
