WOOD. 351 



mulations of vegetable matter, which have been buried 

 in the earth during previous ages of its existence. As 

 a consequence of pressure, the material takes a different 

 form from that already described, and is found, after 

 ages have elapsed, as bituminous coal. 

 Howisanthra- 881. ANTHRACITE COAL. Where bitu- 

 dte formed? mous coa i has been subjected to great heat, 

 more carbon and hydrogen are expelled, and anthracite 

 coal remains. A similar change takes place where bi- 

 tuminous coal is heated by artificial means. The coke 

 which remains, is, like anthracite coal, nearly pure car- 

 bon. 



882. PRODUCTION OF HUMUS. Humus, 



What is hu- 

 mus? How is or the vegetable mould of forests, is formed 



it produced? by the decay of wood Qr yegetable matter 



in the air. Such decay is a species of slow combus- 

 tion. The carbon is more slowly consumed than the 

 other constituents. The vegetable mould or humus 

 which remains after the partial decay, is, therefore, like 

 peat, much richer in carbon than the material from 

 which it was produced. It is variable in composition, 

 according to the progress of the decay. The access 

 of oxygen from without being unlimited, it is found to 

 remain in equal atomic proportion with the hydrogen. 



What is the 883. WHITE ROTTEN WOOD. White 



composition of roUe n wood, which forms in stumps and 



wliite rotten 



wood? the interior of trees where there is abun- 



dant moisture and deficient access of air, has a different 

 composition. The water present becomes chemically 

 combined, and the product may be regarded as differ- 

 ing from the former, somewhat as a hydrated oxide 

 differs from the oxide itself. 



