If we now retrace our steps along the border of the plantation, which forms a deep green background for countless dahlias, and moreover screens the garden from the biting east, we shall, by turning to the right hand, come upon the Aquarium, the latest and most attractive sight in the gardens. How cool and delicious! Around us we perceive slices of the deep sea-bed and the rapid river. Were we mermen we could not examine more at ease the rich pavement of the ocean set with strange and living flowers. In the midst of the green walls of water which surround us, mimic caves, waving with sea-weed and other marine plants, afford shelter and lurking-holes for bright fish which stare and dart, or for shambling crustaceæ which creep over the pebbly bottom. Against the dark verdure of these submerged rocks, the sea-anemone rears its orange base tipped with flower-like fans, or hangs its snake-like tentacles, writhing as the head-dress of Medusa. But we must look narrowly into each nook and under every stone, if we wish to realize the amount of animal life, which here puts on such strange vegetable forms. Let us consider well for a few minutes one of the tanks running down the middle of the building. For months all the minute animal and vegetable life has been multiplying and decaying, and yet the water remains pure and bright. The explanation of this phenomenon affords one of the most beautiful examples of the manner in which nature on a grand scale holds the balance true between her powers. If we were to put these little bright-eyed fish alive into the crystal tank, in a week's time they would die, because they would have withdrawn all the oxygen it originally contained, and contaminated it with the poisonous carbonic acid gas exhaled from their lungs. To prevent this, the philosopher hangs these mimic caves with verdant seaweed, and plants the bottom with graceful marine grasses. If the spectator looks narrowly at the latter, he finds them fringed with bright silver bells: these bells contain oxygen, which the plants have eliminated from their tissues under the action of light, having previously consumed the carbonic acid gas thrown out by the fishes and zoophytes. Thus plants and animals are indispensable to the preservation of each other's life. But even now we have not told the entire causes which produce the crystal clearness of the water. The vegetable element grows too fast, and if left to itself the sides of the tank would be covered with a confervoid growth, which would speedily obscure its inmates from our view.