THE FRESH-WATER AQUARIUM. 9 
shines through the tank, will see the leaves studded with 
bright beads, some of them sending up continuous streams 
of minute bubbles. These beads and bubbles are pure 
oxygen, which the plants distil from the water itself, in 
order to obtain its hydrogen, and from carbonic acid, in 
order to obtain its carbon. 
There is one more feature, which no writer on the 
aquarium has yet noticed, namely—when a tank is pro- 
perly stocked, the water soon gets crowded with infusorial 
animalcule, which swarm among the plants, and on the 
sides of the glass in countless thousands, visible only by 
the aid of the microscope. These are in accordance with 
a natural law ; the presence of vegetable matter in water 
always induces them. But observe their value: they con- 
tribute to the sustenance of the smaller fishes, by supply- 
ing them with food; and, strangely enough, the researches 
of modern chemists have proved that these minute 
creatures respire in much the same way as plants. While 
all other animals absorb oxygen, and perish if the supply 
of that gas is withdrawn, these minute organisms absorb 
carbonic acid, and give out oxygen in abundance. This 
has been proved by Professor Liebig, who collected several 
jars of oxygen from tanks containing infusoria only. Every 
one who has had experience in the management of tanks 
must have noticed that the water ina tank which has 
been established some months will sustain a much greater 
amount of animal life than one of the same dimensions, 
but recently stocked. The presence of infusoria in im- 
mense numbers is one of the reasons for this. 
So far I have endeavoured to explain the theory of the 
