104

DECAY OP HUMUS.

of great importance in connexion witli the growth ofplants, it must be remembered that woody fibre andmost vegetable substances consist of carbon, oxygen,and hydrogen. Though the elements of these sub-stances are united together in nearly the proportions toform carbon and water, we must nevertheless clearlyunderstand that they are not composed of carbon andwater, but that they are distinct compounds of thethree elements ; and, therefore, of course, that the waterproduced during their decay, is not merely set at libertyby a process of decomposition, but is formed by thehydrogen which is given off during decay, entering intocombination with oxygen.

319. When woody fibre and similar vegetable sub-stances are exposed to air and moisture, they decay;the chemical affinity which binds their elements togetherseems to be weakened, and the elements acquire a ten-dency to form new combinations. The oxygen and aportion of the carbon unite and constitute carbonic acid ;whilst a portion of the hydrogen combining with oxygenfrom the air, gives rise to the formation of water.

320. This explains the use of air in facilitating thedecay of these substances; for it is evident that if freeoxygen be required to combine with the hydrogen, thisprocess cannot go on without a constant supply of freshair. The rapidity of the decay of vegetable matters inthe soil, therefore, depends greatly on the porosity orcloseness of the soil, permitting a more or less perfectaction of air.