316 THE GROWTH OF PLANTS AND ANIMALS 



nature cannot be used by animals but must first be built 

 into other compounds by plants. 



Carbon dioxide (CO a ). Carbon dioxide consists by 

 measure of one part of the solid, carbon, and two parts 

 of the gas, oxygen, and is present in fresh air only in 

 minute quantities. The burning of coal, wood and oil, 

 and the decay of animal and vegetable substances mean 

 that the oxygen of the air is uniting with the carbon in 

 these substances to form carbon dioxide. It is also gen- 

 erated in the bodies of animals when the food which 

 they eat unites with the oxygen which they breathe. In 

 animals, it is a waste product given off by the breath, 

 but for plants it is a most -important building material 

 or food. 



In the remote geologic ages before plants had taken 

 the carbon dioxide from the air and stored up the im- 

 mense quantities of carbon now found in coal and oil 

 fields, the amount of carbon dioxide in the air was much 

 greater than at present. It is probable, however, that 

 the burning of the large quantities of coal and wood now 

 in use is increasing the proportion of carbon dioxide in 

 the air. The air of cities has distinctly more of this 

 gas (0.06 per cent) than that of the country (0.04 per 

 cent), while the sea air contains even less (0.025). 



Carbon monoxide (CO). Carbon monoxide is an 

 invisible gas which is very poisonous for both plants 

 and animals. It is produced when there is too little 

 oxygen present to furnish the two parts of oxygen to 

 one of carbon necessary for complete combustion into 

 carbon dioxide, or when the temperature is too low 

 to permit of complete oxidation, as in the case of wood 

 or charcoal burning without a flame. It is commonly 

 found in large amounts in illuminating gas, especially 

 in the so-called " water gas," and in the fumes from 



