46 PRODUCTIVE FARMING 



The Action of Air and Temperature Changes. The action 

 of the air and the changes in temperature, which together we 

 call weathering, is of many kinds. The heat of the sun causes 

 the rock to expand. As all of the substances which make up 

 a rock do not expand the same amount, the rock breaks and 

 the particles flake off the surface. In regions where the soil 

 is not protected by vegetation the wind becomes an import- 

 ant soil former (Fig. 24). The particles of soil are caught up 

 by the wind and hurled against rocks and against cliffs, and 

 grind the surface of the rocks and undermine the cliffs. This 



FIG. 25a. A little stream falling many feet will wear away the hardest rock. 

 The fine particles help to make up the soil. (Plant Industry.) 



FIG. 256. A muddy water-fall. Soil is carried by swift current to the low land. 



little agent does a rea. y large work. Then there is a chemical 

 action of the air, in which certain substances in the air, as 

 oxygen and carbonic acid, unite with certain substances in 

 the rock and cause it to decompose. Such an action of the 

 air is termed oxidation and is a slow burning or decay. 



A good example of weathering may be observed in the 

 rapid crumbling of shale on exposure to the air. In such 

 cases the air, temperature, and water each play an important 

 part. 



EXERCISE. The Action of Air on Iron. A familiar 



