FUNDAMENTAL CONSIDERATIONS 7 
' vapour present varies considerably at different times and 
in different places. Atmospheric moisture plays an im- 
portant role in climate (Chapter I.). 
(b) Carbonic Acid Gas.—This is a compound of carbon 
and oxygen (CO,). The amount of the gas present in 
the air is small, but the importance of this quantity is 
manifest when we consider that it forms the ultimate 
source not only of all food, but also, perhaps, of every 
other organic compound in nature. We have already 
referred to the role played by carbonic acid gas in photo- 
synthesis. In this process, carbonic acid gas is with- 
drawn from the air, its oxygen is liberated as a gas, and 
its carbon fixed in the starch which is produced. The 
energy required to carry out the process is obtained from 
sunlight. In spite of the fact that carbonic acid gas is 
continually being withdrawn during photosynthesis, the 
quantity present in the air remains constant. This is 
because the atmospheric supplies are always being replen- 
ished from other sources—e.g., respiration (Chapter VIII.). 
(c) Oxygen.—All living things breathe, plants and 
animals alike. During respiration oxygen is taken in, 
food is destroyed, and carbonic acid gas given out. The 
nutritive body most commonly destroyed in this way is 
starch, and in this case the process is the reverse of 
photosynthesis. Energy is imprisoned in food. When 
food is broken down by respiration, energy is liberated, 
and becomes available for the performance of physio- 
logical work. The part played by oxygen in the process 
is chemical ; it attacks nutritive substances and breaks 
them down into simpler bodies, one of which is always 
carbonic acid gas. Oxygen, therefore, plays the part of 
an energy-liberator, and, with rare exceptions, energy in 
living bodies is liberated in no other way. 
(d) Nitrogen.—Atmospheric nitrogen is of no direct 
use to ordinary plants, which derive their nitrogenous 
supplies from mineral salts present in the soil. But all 
the nitrogen fixed in the soil has been ultimately derived 
from the atmosphere. This interesting problem of 
nitrogenous circulation is considered in Chapter X. 
