MATERIAL FOR STUDY. 
9 
Another method of purifying the air in water consists 
in forcing a stream of air 
through it. This is not prac- 
ticable in most schoolrooms. 
Pouring fresh water against 
the side of the aquarium in 
such a way that many bub- 
bles of air are caught in 
the descending stream is a 
common and easy method. 
Large aquaria frequently have 
a constant supply of running 
water with a regular outflow. 
Such aquaria are hardly neces- 
sary in most schools. Small  py¢, 7.—An American Pond- 
rectangular glass vessels and 
common battery-jars answer 
every purpose. 
weed. 
Of course water 
Fic. 8.—The Horn- 
wort. 
lost by evaporation must be re- 
placed. With the aquatic animals 
mentioned here it is a good plan 
to depend partly on plants to 
change the air in the water, but 
in addition to this it is better to 
remove the greater part of the 
water from time to time and to 
replace it by a fresh supply. An 
easy way to accomplish this is to 
have all the aquaria placed on a 
shelf a little higher than the faucet 
from which water is to be supplied. 
A hose can be attached to the faucet 
for the purpose of filling the aquaria. 
In order to empty the vessels, it 
is only necessary to unscrew the 
hose from the faucet while it is still 
filled with water, being careful to 
Fic. 9.—A Duck- 
weed. 
keep the end of the hose in thé aquarium under water. 
