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.). 



(6) Carbonic Acid Gas. — This is a compound of carbon 

 and oxygen (CO2). 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 continually being withdrawn during photosynthesis, the 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. — AU hving 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 Uberated in no other way. 



[d) Nitrogen. — Atmospheric nitrogen is of no direct 

 use to ordinary plants, which derive their nitrogenous 

 suppHes 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. 



