4 THE AQUAVIVARIUM. 



easily be done in a tube inverted over water, and 

 test them, we shall find they consist of pure 

 oxygen gas. The leaves of all growing plants are 

 constantly engaged in giving off oxygen gas ; here, 

 then, is the source, not only of oxygen to the 

 atmosphere, replacing what animals take away in 

 breathing, but also to the water in which plants 

 live. We now begin to see why it is that fish 

 will live in water where plants grow, and die 

 where they are not. But this is not the whole 

 of the functions performed by plants ; in the atmo- 

 sphere they would not save animal life from destruc- 

 tion if they only restored the oxygen. There is the 

 carbonic acid. This gas, so destructive of animal 

 life, is constantly being produced by animals. It is 

 the great distinguishing feature between animals 

 and plants, that animals take up oxygen and throw 

 out carbonic acid ; the animals would therefore be 

 poisoned by their own secretion, but for the fact, 

 that what is the poison of the animal kingdom is 

 the food of plants. They live on carbonic acid ; 

 they abstract it from the air and the soil, they 

 absorb it from the water. It is in this way that 

 plants purify the water. The carbonic acid is 

 composed of carbon and of oxygen, and plants 

 have the power of separating and using the carbon 

 for forming the tissues of which they are composed, 

 and letting go the oxygen gas. 



It is thus that we find that our jar of water with 

 plants and animals is truly a microcosm, a minia- 

 ture world, in which all the great changes go on 

 which are necessary to the life of man, and the 

 maintenance of animals and plants on the surface 

 of the earth. 



But these changes, although the most essential, 

 are not all that go on in plants and animals, and 

 constitute a mutual relation between them. Thus, 



