While these devour the banquet death has spread for them, while forests of confervoid threads rise in silken tufts like microscopic savannas, Nature is passing portions of the ichthyic débris through her laboratory, and the very source of life for which they pined and perished—oxygen—is poured in in large measure, and the corruption is quickly changed to sweetness. Of the once sportive fishes some portions have become air, other portions have become water, but the chief of their bulk lives already in the vegetation which hides their grave, and the moving throng with which that vegetation is peopled. God's purpose, in the working of the laws in obedience to which these changes have taken place, is manifestly to keep ever true that balance of life and death of which He holds the beam in His own hands.
But my aquarium which has not thus been interfered with presents already a similar scene of life and bustle. When first supplied, the milky-looking water was abundantly full of gaseous matters, and every part of the rough rockwork was, for a time studded with silvery globules. The fishes consumed all that in the process of breathing. As the water passed through their gills, the oxygen was absorbed; that oxygen, by a process of refined chemistry, and perhaps by the help of iron also, gave their gills a bright-red color, gave their blood its red color too, and, by other processes not less refined, sustained he balance of life's functions within them, for without it they must perish. We believe that not the airiest particle of earth, atmosphere, or water, nor the most minute globule of condensed moisture, nor the most infinitesimal point of meteoric dust, can ever be lost, at least during Time, from the fabric of the universe. My fishes tell me that the oxygen they absorb from the water they again return to it, but in another form. They inspire oxygen and expire carbonic acid, just as a man does, and every other living creature that moveth upon the face of all the earth. Is it within the reach of human power, even when reason, imagination, and fancy combine together as a bold triad to look direct upon a fact, to appreciate that principle of terrestrial life by which animal and vegetable organisms reciprocally labor to maintain the balance of atmospheric purity? The carbonic acid given off by the animal is poison to it, if it accumulate while the supply of oxygen is cut short. It was carbonic acid as much as absence of oxygen that killed our fishes just now, for, though inhabitants of water, they were not the less suffocated. Therefore I see why, in the tank that has been left alone, plants have cast anchor on the glass walls, the brown pebbles, and the gray blocks of sandstone-rock. My fishes breathe and breathe. If their numbers are properly proportioned to the area they occupy, they will never exhaust the water of oxygen, never render it fetid with carbonic acid, so long as one necessity of vegetable life—light—is allowed to use its active influence to paint the plants green, even as oxygen gives a sanguine hue to the gills or lungs of the fishes. To those plants, the carbonic acid, which the fishes
