2 THE AQUAVIVARIUM. 



be secured, such an arrangement of plants and 

 animals must be made in the vessel of water as we 

 find made upon the surface of the earth. There 

 must be such a relation between them that the one 

 may supply the other with what it wants. If we 

 take a jar of spring or river water, and put into it 

 some gold fish, they die in the course of a few days, 

 unless the water is changed ; but if we put them 

 into cold boiled water they die in a few minutes, 

 and no amount of fresh boiled water will keep them 

 alive. If, now, we put in some plants which natu- 

 rally grow in the water, we shall find that our 

 fishes will live in it without a change of the water. 

 These, then, are the problems we have to solve : 

 What caused the death of the fishes in the spring 

 and boiled water 1 and, Why do they live in the 

 water with plants ? Having explained these phe- 

 nomena, we shall see how they bear on the con- 

 struction of our Aquavivarium. 



The fish, although it lives in water, has as much 

 need of fresh air as animals that live and breathe 

 in the atmosphere. If we put a bird, or a mouse, 

 under a glass jar, it dies in the course of a few 

 minutes : and there are two causes for its death 

 first, it needs a fresh supply of oxygen to aerate its 

 blood, which is not supplied in the closed glass jar ; 

 second, it is necessary that the carbonic acid gas, 

 which is generated in its system, and which acts 

 upon it as a poison, should be got rid of, which 

 cannot be effected in the closed glass jar. Thus the 

 animal dies because two necessary processes of its 

 life are not carried on. 



The fish, it is true, has not lungs into which it 

 takes air, but it has gills, which are adapted for 

 absorbing air from the water in which it lives. 

 All water, then, which is to support the life of 

 animals breathing by means of gills, must contain 



