The Atmosphere. 27 



Gases of the air. Dry, pure air is essentially a mixture of 

 gases. A gas differs from the more familiar forms of matter, 

 as liquids and solids, in that its particles are much farther re- 

 moved from one another, or as we say, it has less density. This 

 relation is illustrated by the different forms which water may 

 assume. When the solid substance known as ice is heated, its 

 particles spread farther apart until it no longer has sufficient co- 

 hesive power to retain its shape. It then melts and becomes the 

 liquid known as water. Sufficient further heating, by separating 

 the particles of water still farther apart, transforms it to the 

 state of an invisible gas known as steam, which becomes a con- 

 stituent of the gaseous atmosphere. When steam comes in con- 

 tact with cold solid objects, or even with cold air, it contracts 

 or condenses to visible water vapor. The gases of the air main- 

 tain their rarified form under all ordinary conditions. They can 

 be converted, however, like the air itself, to liquids, and even 

 to solids, by subjecting them simultaneously to very low tem- 

 peratures and high pressures. 



Nitrogen. This is the most considerable constituent of the 

 air and amounts to more than three-quarters of the total weight, 

 or about 30,000 tons over every acre of land. It is characterized 

 by extreme inertness. When combined in chemical compounds 

 it is frequently held with difficulty. High power explosives de- 

 pend for their value upon the ready and sudden release of a 

 large volume of gaseous nitrogen from less bulky compounds as 

 nitro-cellulose and nitro-glycerine. Since nitrogen is an essential 

 constituent of compounds of the greatest importance in the living 

 cells of plants and animals, its ready escape from such com- 

 pounds has presented one of the greatest problems of agriculture. 



Relation of nitrogen to plant growth. The work of several 

 able investigators has proved conclusively that higher plants can- 

 not draw directly upon the great stores of nitrogen in the air for 

 their supply of this element. 



In 1855 the French chemist Boussingault announced the re- 

 sults of a series of carefully performed experiments to determine 



