DECAY OF IIUMUS. 259 



tions to form carbon and water, "vve must nevertheless 

 clearly understand that they are not composed of car- 

 bon and water, but that they are distinct compounds 

 of the three elements; and therefore, of course, that 

 the water produced during their decay is not merely 

 set at liberty by a process of decomposition, but is 

 formed by the hydrogen which is given off during 

 decay, entering into combination with oxygen. 



675. When woody fibre and similar vegetable sub- 

 stances are exposed to air and moisture, they decay; 

 the chemical affinity which binds their elements to- 

 gether seems to be weakened, and the elements 

 acquire a tendency to form new and more simple 

 combinations. The oxygen and a portion of the 

 carbon unite and constitute carbonic acid ; whilst a 

 portion of the hydrogen combining with oxygen from 

 the air, gives rise to the formation of water. 



676. Tliis explains the use of air in facilitating the 

 decay of these substances ; for it is evident that if 

 free oxygen be required to combine with the hydro- 

 gen, this process cannot go on without a constant 

 supply of fresh air. The rapidity of the decay of 

 vegetable matters in the soil, therefore, depends 

 greatly on the porosity or closeness of the soil, per- 

 mitting a more or less perfect action of air. 



677. In soils containing a large proportion of clay 

 or alumina, the decay of vegetable matter or humus 

 proceeds very slowly, because the close, dense nature 

 of the soil greatly retards the free access of air ; 

 though in other respects, such soils are decidedly 



