THE COMPOSITION OF THE AIR 99 



them as it occurs free in the air. Certain very low and 

 minute forms of life known as bacteria have the power to 

 take nitrogen from the air and to prepare it for the use of 

 plants. The nitrogen must be chemically compounded with 

 other substances before it can be used either by animals or 

 plants as food. 



Plants need carbon dioxide as much as animals need 

 oxygen. The growth of a plant is due to the power it has 

 of tearing apart the carbon dioxide by the help of the sun 

 and of building the carbon into its structure. It returns 

 the oxygen to the air to be used again by the animals and 

 the plants. By far the greater part of plants is made from 

 the carbon which they get from carbon dioxide. 



Animals have not the bodily power of breaking down 

 carbon dioxide to obtain oxygen from it ; consequently they 

 smother in this gas. Since men and other animals are con- 

 stantly using up the oxygen in the surrounding atmosphere 

 and are breathing out carbon dioxide, the rooms where they 

 stay must be properly ventilated. 



Carbon dioxide is heavier than air and has a tendency to 

 accumulate in wells and unventilated mines. Workmen 

 caught in this gas are smothered exactly as if by drowning. 

 Frequently in coal-mine explosions so much carbon dioxide 

 is formed that but little free oxygen remains ; and so miners 

 often escape an explosion only to be smothered by the carbon 

 dioxide (choke damp, as they call it). Before going down 

 into a well or cistern, careful workmen always lower a lighted 

 candle to test for the presence of carbon dioxide. If this is 

 present in large quantities the candle is extinguished. 



In some places, such as Dog Grotto near Naples, Italy, 

 and Death Gulch in Yellowstone Park, carbon dioxide is 

 being steadily emitted from the ground. Since these places 



