258 HUMUS. 



acid and water. The first change which they under- 

 go, when exposed to air and moisture in circum- 

 stances favorable to decay, is a diminution of the 

 quantity of oxygen and hydrogen, and likewise, 

 though in a smaller relative proportion, of the carbon 

 which they contain. 



672. The result of this is a change of color some- 

 what similar to that produced by slightly burning or 

 singing them by heat: they become more or less 

 brown, according to the degree to which this change 

 has proceeded. Whilst in this state, they constitute 

 what is called IIUMUS, the name by which chemists 

 distinguish all kinds of vegetable matter when in a 

 state of decay, and more especially woody fibre, which 

 indeed usually constitutes a very large proportion of 

 the bulk of plants. 



673. This process of decomposition proceeds slowly 

 for a considerable time, the relative proportion, of 

 the oxygen and hydrogen to the carbon gradually 

 decreasing, carbonic acid and water being all the 

 time formed, until, at last, the humus, consisting of 

 carbon with only a small proportion of oxygen and 

 hydrogen, ceases to undergo any further change ; it 

 then is termed mould by chemists. 



674. In order to understand this process, which is 

 one of great importance in connection with the growth 

 of plants, it must be remembered that woody fibre 

 and most vegetable substances consist of carbon, oxy- 

 gen, arid hydrogen. Though the elements of these 

 substances are united together in nearly the proper- 



