Page:Popular Science Monthly Volume 5.djvu/708

688 wherever established as a pecuniary investment, it has never failed to yield the most profitable results. Thus the aquarium at Hamburg has proved an immense pecuniary success; and that at Brighton, although beginning its existence so recently as August, 1872, has nevertheless already made a gratifying return to its proprietors.



The aquarium further serves to illustrate an important biological truth—one of the most subtile relations between the animal and vegetable kingdoms. That truth is, that the two kingdoms exert complementary influences upon the atmosphere we breathe. Plants inhale carbon and exhale oxygen; animals do the reverse. Strike out all the plants from existence, and we should, poisoned by our own breath, die in heaps, with other animals; while, if all the animals could, at one blow, be swept away from the earth into space, the plants would be destroyed by the want of carbon. And now the aquarium, which, properly speaking, is an artificial sea, or lake, possessing all the conditions necessary to the maintenance of aquatic life, both animal and vegetable, beautifully illustrates this truth. Who has not observed that fish, confined in water without plants, quickly die, unless the water be repeatedly changed? The fish die because, having breathed out all the oxygen of the water, as there is nothing in it to produce any more, they become poisoned with the suffocating carbon. But, when the plants also are put in, they take up the carbon from the fish and go on producing oxygen all the while, so there is no longer any necessity for changing the water. The fact that marine aquaria do not require the introduction of plants has been supposed by some observers to furnish a contradiction of the truth just stated. But the contradiction is apparent only, not real. Sea, or "salt" water, as it is usually called, contains a great quantity of little germs or "spores,"