72 THE BOOK OF NATURE STUDY 



made by pouring water on a little quicklime, and after a time 

 pouring off the clear layers of fluid, leaving the undissolved residue 

 behind. Take this clear fluid, and by means of a glass tube allow 

 the pupils to breathe into it. It rapidly becomes turbid owing 

 to the formation of carbonate of lime, due to the carbonic acid 

 of the breath. We thus reach the conclusion that animals in 

 breathing produce carbonic acid, while green plants in sunlight 

 evolve oxygen. This is the basal principle of the aquarium, for 

 if we succeed in stocking an aquarium with animals and plants 

 in the right proportions, then as fast as the animals produce 

 carbonic acid in the process of breathing, the plants will seize 

 this and, breaking it up in their tissues, will give off the oxygen 

 which the animals require, and thus there will be an eternal balance 

 between the two. Further, while plants retain their waste within 

 their bodies, or only get rid of it very slowly, animals, with rare 

 exceptions, have special organs whose function it is to remove 

 nitrogenous waste from the body and discharge it to the exterior. 

 Now the nitrogenous waste of animals is one great source of food 

 to plants. The protoplasm or living substance of plants consists 

 of the same elements as that of animals, but whereas animals 

 require solid food, such as, for instance, is furnished by plants, 

 green plants, on the other hand, make a considerable part of their 

 food from the carbonic acid of the air, which they build up into 

 starch or sugar, and take in the remainder in the form of salts, 

 dissolved in water. Theoretically, then, we could establish an 

 aquarium containing animals and plants which would be in a 

 permanent condition of equilibrium. The animals would eat the 

 plants, they would pass out into the water nitrogenous and other 

 waste products, and also carbonic acid. The plants would absorb 

 the carbonic acid, would break it up, returning the oxygen to 

 the water for the respiration of the animals, and building up the 

 carbon which they had retained, with the elements of water, into 

 starch. With this starch added to the nitrates and other sub- 

 stances absorbed from the water, they would build up new organic 

 substance, and so reproduce fast enough to make up for the loss 

 due to the amount of plant life consumed by the animals. Now 

 this conception is a sufficiently striking one to make it worth 

 while to keep going, for at least a time, a simple form of aquarium, 



