THE GENERAL FEATURES OF THE EARTH 15 
change takes place much more slowly. So, also, the 
active oxygen of the air is constantly engaged in the 
decomposition of the rocks, causing changes which 
we know as oxidation. This action is well illustrated 
in the rusting of iron. 
In the air there is a small, amount (about 0.03 %) of 
carbonic acid gas, which the plants use to obtain the 
carbon needed in their growth. Charged with this’ 
rain water becomes a weak acid, of the same kind 
as soda water, though far weaker. When in this con- 
dition, water may dissolve some rocks and minerals, 
and may cause changes in others. 
A small quantity of evaporated water, or water vapor, 
is constantly present in the atmosphere, and under 
favorable conditions this may be condensed into rain. 
One of these conditions is the presence of minute solid 
particles of dust, which are everywhere floating in the 
air. So by this means the rain falls, rivers are formed, 
and springs appear, while an entire set of important 
changes is begun. 
The atmosphere is a great engine with many func- 
tions and many duties to perform, and the heat of 
the sun is the fuel. 
The Ocean. — Covering about three-quarters of the 
surtace of the earth are the oceans, with an area of 
about 145,000,000 square miles. Near the shores of 
the continents they are shallow, but in some places, 
