"Now, if a fish-globe be not overcrowded with fishes, we have only to throw in a goodly handful of some water-weed,--such as the _Callitriche_, for instance,--and a new set of chemical operations commences at once, and it becomes unnecessary to change the water. The reason of this is easily explained. Plants absorb oxygen as animals do; but they also absorb carbonic acid, and from the carbonic add thus absorbed they remove the pure carbon, and convert it into vegetable tissue, giving out the free oxygen either to the water or the air, as the case may be. Hence, in a vessel containing water-plants in a state of healthy growth, the plants exhale more oxygen than they absorb, and thus replace that which the fishes require for maintaining healthy respiration. Any one who will observe the plants in an aquarium, when the sun shines through the tank, will see the leaves studded with bright beads, some of them sending up continuous streams of minute bubbles. These beads and bubbles are pure oxygen, which the plants distil from the water itself, in order to obtain its hydrogen, and from carbonic acid, in order to obtain its carbon."[A] [Footnote A:_The Book of the Aquarium_, by Sidney Hibbert.] Thus the water, if the due proportion of its animal and vegetable tenants be observed, need never be changed. This is the true Aquarium, which aims to imitate the balance of Nature. By this balance the whole organic world is kept living and healthy. For animals are dependent upon the vegetable kingdom not only for all their food, but also for the purification of the air, which they all breathe, either in the atmosphere or in the water. The divine simplicity of this stupendous scheme may well challenge our admiration. Each living thing, animal or plant, uses what the other rejects, and gives back to the air what the other needs. The balance must be perfect, or all life would expire, and vanish from the earth. 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.