46 ORIGIN AND ACTION OF HUMUS. 



ceases upon the disappearance of the oxygen. If 

 the carbonic acid is removed, and oxygen replaced, 

 its decay recommences, that is, it again converts 

 oxygen into carbonic acid. Woody fibre consists of 

 carbon and the elements of water ; and if we judge 

 only from the products formed during its decom- 

 position, and from those formed by pure charcoal, 

 burned at a high temperature, we might conclude 

 that the causes were the same in both : the decay 

 of woody fibre proceeds, therefore, as if no hydro- 

 gen or oxygen entered into its composition. 



A very long time is required for the completion 

 of this process of combustion, and the presence of 

 water is necessary for its maintenance : alkalies 

 promote it, but acids retard it ; all antiseptic sub- 

 stances, such as sulphurous acid, the mercurial 

 salts, empyreumatic oils, &c. cause its complete 

 cessation* 



Woody fibre, in a state of decay, is the substance 

 called humus*. 



The property of woody fibre to convert surround- 

 ing oxygen gas into carbonic acid diminishes in 

 proportion as its decay advances, and at last a 

 certain quantity of a brown coaly-looking substance 

 remains, in which this property is entirely wanting. 

 This substance is called mould ; it is the product of 

 the complete decay of woody fibre. Mould consti- 



* The humic acid of chemists is a product of the decomposition of 

 humus by alkalies; it does not exist in the humus of vegetable physio- 

 logists. 



