122 A YEAR IN SCIENCE 



readily with other elements. Because it is so inactive 

 its chief function in relation to life is often said to be 

 "to dilute the oxygen." Since nitrogen constitutes 

 more than three-fourths of the weight of the air, the 

 pressure of the air, the force of the wind, the flight of 

 birds are largely possible because of it. 



Another important use of nitrogen is as a plant food. 

 Most plants use the nitrogen compounds which are in 

 the soil. These in solution are taken into the plant 

 through its roots. When plants are grown in the same 

 place year after year, they take out so much of the 

 nitrogenous matter as to decrease the fertility of the 

 soil. If the soil is lacking in nitrogen, no plant will 

 thrive. One large family of plants, of which peas, 

 beans, alfalfa, lentils, and a number of others are men- 

 bers, have on their roots little nodules in which certain 

 kinds of bacteria live. These bacteria possess the 

 power of absorbing the nitrogen from the air and com- 

 bining it with the oxygen and some of the salts of the 

 earth to form nitrates. These nitrates are very valu- 

 able as plant foods. These plants are now extensively 

 grown not only for their value as crops, but also 

 because they enrich the soil by adding nitrogen to it. 



Oxygen from the air is consumed all the time by ani- 

 mals and plants in breathing. Without it they could 

 not live. Oxygen, we have already learned, combines 

 readily with most other elements. By its chemical 

 union with other elements we know that heat and 

 sometimes light are produced. This heat, developed by 



