8 THE FRESH-WATER AQUARIUM. 
plants and animals contribute mutually to each other’s 
support in the preservation of the purity of the air. 
What happens when we put half-a-dozen gold fish into a 
globe? The fishes gulp in water and expel it at the gills. 
As it passes through the gills, whatever free oxygen the 
water contains is absorbed, and carbonic acid given in its 
place ; and in course of time the free oxygen of the water 
is exhausted, the water becomes stale, and at last poison- 
ous, from excess of carbonic acid. If the water is not 
changed the fishes come to the surface and gulp atmos- 
pheric air. But, though they naturally breathe air as we 
do, yet they are formed to extract it from the water; and 
when compelled to take air from the surface, the gills, or 
lungs, soon get inflamed, and death at last puts an end to 
their sufferings. 
Nowif a gold-fish globe be not over-crowded with fishes 
we have only to throw in a goodly handful of some water 
weed—such as the Callitriche, for instance—and a new 
set of chemical operations commences at once, and it 
becomes unnecessary to change the water. The reason of 
this is easily explained. Plants absorb oxygen as animals 
do; but they also absorb carbonic acid, and from the car- 
bonic acid thus absorbed, they remove the pure carbon, 
and convert it into vegetable tissue, giving out the free 
oxygen either to the water or the air, as the case may 
be. Hence, in a vessel containing water plants in a state 
of healthy growth, the plants exhale more oxygen than 
they absorb, and thus replace that which the fishes require 
for maintaining healthy respiration. Any one who will 
observe the healthy plants in an aquarium, when the sun 
