CARBON DIOXIDE 81 



is one example of the good which bacteria do for us. 

 Later we shall learn that they help us in many ways. 

 The larger plants cannot take in the nitrogen until it 

 has been changed by the bacteria, and many of the plants 

 could not live if it were not for the bacteria. 



The little lumps upon the roots of the plant in the 

 illustration are called nodules. Each nodule is the 

 home of bacteria. The illustration is the root of the 

 garden-pea, but there are nodules upon the roots 

 of many other plants and trees. Whenever you see these 

 nodules you will know that there are many helpful 

 bacteria within them. 



The kind of food which the plants form from the 

 material supplied by the bacteria is the source of our 

 strength. When we study plants and food we shall learn 

 more about these bacteria and their work. 

 32. Carbon Dioxide. 



The amount of carbon dioxide in the air varies 

 according to the location. Near factories there is a larger 

 amount than in the open country, or on the ocean, while 

 in a room where there are many persons the amount may 

 become very great. This shows that fires produce it and 

 animals exhale it. In the open country, air contains 

 only three or four parts of carbon dioxide in ten thousand 

 parts. The gas is a combination of oxygen and carbon. 

 Coal, coke, and charcoal, all of which are fuels, are chiefly 

 carbon. If the air of the room contains eight to ten parts 

 of carbon dioxide in ten thousand parts of air it is unfit 

 for us to breathe. Since exhaled air contains four hun- 

 dred parts in ten thousand, it can easily be seen that there 

 should be, not only a supply of fresh air, but the stale air 

 should be removed continually. In addition to carbon 



Elem. Sci. 6 



