THE FORMATION OF SOIL 19 



present in the atmosphere to the extent of 

 about twenty-one per cent., has the power 

 of uniting chemically with most substances, 

 the result in many cases being the de- 

 struction of the body thus affected. The 

 destructive action of oxygen is illustrated 

 in the process of ordinary combustion, as, 

 for example, where a heap of thorns is con- 

 sumed by fire But destruction though 

 slower is no less complete where the process 

 is not associated with visible fire. Iron, for 

 instance, when exposed to the atmosphere, 

 corrodes and becomes rusty, the conversion 

 of metallic iron into rust being essentially 

 of the same character as the destruction of a 

 heap of brushwood by fire. Whereas iron 

 is a highly resistant body, the rust which 

 oxygen forms is easily crumbled and is 

 markedly non-resistant. Not only may pure 

 substances like iron combine with oxygen 

 and be converted into what chemists call 

 oxides, but certain compounds, e.g. sulphides, 

 may unite with more or less oxygen and be 

 converted into other compounds, as, for 

 instance, sulphates. In stones and rocks 

 there are many substances which can com- 

 bine with oxygen, and the compound so 

 formed is in most cases easily disintegrated, 

 as compared with the original material. If 



