168 
SECOND BOOK. 
answer. During the time of sunlight they are 
continually taking in this gas, and separating the 
carbon and the oxygen of which it is formed. They 
keep the carbon to build up their leaves, stems, and 
other parts, and send the oxygen out again. Thus 
plants and animals 
between them keep 
balance of 
up the 
oxygen 
acid in 
and carbonic 
the air. 
8. Carbonic-acid gas 
is very poisonous. If 
an animal breathes 
much of it, it dies, 
although it may have 
plenty of oxygen. 
9. The vapour in 
the air comes from 
the various bodies of 
water on the earth, 
from the breath of 
animals, and from the 
leaves of plants. The quantity that is present at 
any time depends upon the warmth of the air and 
the direction of the winds. The warmer the air, the 
more vapour it can hold. 
io. In the higher and cooler regions of the air the 
vapour gets chilled, and then the moisture forms 
clouds. When these are still further cooled, by cold 
currents of air meeting them, the water can be no 
Solid Impurities in the Atmosphere. 
