CHEMICAL WORK OF THE ATMOSPHERE 391 



planted on sandy soil to stop the shifting of sand (Fig. 260) ', 

 and in the dry, sandy regions of Utah and Colorado, poplar 

 trees are grouped around farm buildings and dwellings as a 

 protection against sand storms. A great many plants thrive 

 under desert conditions, and 

 attempts are being made to 

 introduce these plants in 

 regions where there is a 

 danger from advancing and 

 shifting dunes. 



Chemical work of the at- 

 mosphere. The oxygen, 

 carbon dioxide, and water 

 vapor that are in the at- 

 mosphere cause changes in 

 the appearance and charac- 

 ter of building stones, rocks, 

 and soils. Newly built 

 houses soon lose their fresh- 

 ness and become toned down 

 or weathered. Monuments 

 lose their polish and take on 

 a weather-beaten appear- 

 ance. This is because the oxygen, carbon dioxide, and moisture 

 of the atmosphere produce chemical changes in wood, stone, 

 and metal. Many rocks and soils contain iron. Oxygen and 

 water vapor unite with iron and cause rust. A knife blade 

 which rusts acquires a reddish or yellowish color; in the same 

 way the iron which rusts in rock and soil acquires a reddish 

 color. 



Many soils and rocks owe their pleasing color to the oxi- 

 dation or rusting of the iron which they contain. Just as 

 rusty nails slowly crumble and fall to pieces, so rocks, which 



FIG. 260. Beach grass planted to prevent 

 the drifting of sand. 



