TILLING AND DRAINING THE SOIL. 37 



CHAPTER IX. 



TILLING AND DRAINING THE SOIL. 



WEATHERING. If we leave a piece of iron exposed to the 

 damp air it soon becomes rusty ; if we keep it in a dry place or 

 put it under water so that the air cannot reach it, it will not 

 become rusty so soon. Vegetables left in a damp cellar, or 

 thrown out on the ground, soon decay. Pieces of wood, if left 

 long enough, will rot and decay, first becoming brown and 

 later on crumbling to a fine, black substance, the same as the 

 humus of the soil. Harder substances, such as bones, will in 

 time decay and wear away. An old brick when picked up is 

 found to have lost its sharp corners and edges and to have be- 

 come smaller than when first made. As we examine object 

 after object, we find that there are very few things that do not 

 become changed through the effect of the air, dew, rain, frost, 

 snow, and ice. In a previous chapter we have referred to the 

 oxygen and the carbonic acid gas of the air. These are the 

 two substances in the air that cause many of the changes 

 rotting the stumps, charring old leaves and roots and branches, 

 wearing away the boulders in the field, and dissolving lime 

 out of the rocks. 



If you thrust a stick into the coals it will catch fire and burn. 

 Blow out the blaze and you have a charred stick. If you 

 throw another stick of the same kind out on the ground, or 

 bury it just under the soil, after many months it will be found 

 to become brown and then almost black it has become 

 charred also, but it has taken a long time. The oxygen of thj 

 air has burnt up some of it in both cases. If we go to an old 



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