MOULD. 117 



The foregoing account of the sources of in- 

 organic substances in soils, shows that the soil 

 acquires from rocks alumina, silica or sand, 

 phosphoric acid, sulphuric acid, lime, magnesia, 

 oxide of iron, potash, soda; all being indispensable 

 ingredients for the growth of plants. These sub- 

 stances are all termed mineral or inorganic. 



Mould will now be considered. It is popu- 

 larly supposed to be the organic portion of soils. 

 If the leaves that fall from the trees, and the 

 vegetation that dies yearly on the approach of 

 winter, did not rot or decay, or become resolved 

 into their original elements, the accumulation 

 would interfere with the subsequent growth of 

 plants. The decomposition of organic remains 

 is governed by fixed laws. All plants after they 

 die undergo two processes of decomposition ; first, 

 fermentation, then putrefaction or decay. These 

 changes are somewhat analogous to those pro- 

 duced by a smouldering fire, and the product 

 of this decay is very appropriately termed mould. 

 This process of decay or mouldering is the im- 

 perceptible union of the oxygen of the air with 

 the carbon of the plant ; but it proceeds so slowly 

 that it produces only in a very slight degree the 

 effects of ordinary combustion, that is light and 

 heat; still the results are the same, with the ex- 

 ception that it is not carried to the same extent. 



The complete burning of vegetation leaves 



