132 THE FRESH-WATER AQUARIUM. 
and others of England, and the late Mr. Cotting, of Boston, 
have done much towards forwarding the interests of the 
aquarium. The whole secret of the success of the aquarium 
lies in the exactness with which we imitate nature in arrang- 
ing and disposing our collections ; but let us understand first 
of all that what is meant by the term an aquarium is a col- 
lection of water plants and animals, so arranged in suitable 
ratio that it shall be perfectly self-supporting. We do not 
expect, then, that the water will have to be changed until 
after long periods, if at all; the plants and animals should 
flourish as well as if in their native locality. 
How then is this balance of forces to be attained? This 
leads us to examine the philosophy of the aquarium, which 
is simply this: The element in water which the fishes live 
on by breathing is free oxygen, which, as the water is fanned 
through the gills or lungs of the fish, comes in contact with 
the walls of its vessels, and arterializes the blood; all water 
contains a certain amount of this oxygen, sufficient to keep 
a fish alive for a short time, but if no means are taken to 
create a fresh supply, it will become exhausted sooner or 
later, and an escape of carbonic acid will render the water 
poisonous to the fish. In plants on the other hand we have 
an agent taking up the carbonic acid in the water, and re- 
solving it into carbon and oxygen, the former of which it 
- converts into its substance, while it expels the latter from 
every part of its tissue, especially from the leaves in the 
form of minute bubbles, plainly seen in healthy plants, and 
so often compared to drops of quicksilver in appearance. It 
is true that plants absorb oxygen also as fishes do, but they 
give out so much more than they absorb, that this is of slight 
account. 
Another oxygen producing agent, as was shown by Liebig, 
is to be found in the almost microscopic forms of animal life 
which abound in water which has stood for some time eX- 
posed to the air. These animaleule seem to form another 
link in the chain which binds together all kinds of animal - 

