102 THE CHEMISTRY OF CREATION. 



may repeat, called humus. When decay has 

 proceeded to a certain length, the constituents 

 have become so entirely altered and recon- 

 structed, that this decomposition no longer 

 takes place. No more oxygen is absorbed, and 

 no more carbonic acid is discharged. In this 

 condition mould is analogous to peat, which 

 may be defined to be vegetable fibre which has 

 undergone comparatively complete decay, only 

 the decay proceeds on somewhat different prin- 

 ciples to that of ordinary vegetable mould, or 

 humus. 



This ultimate cessation of changes, however, 

 never in reality takes place in nature, because 

 every Autumn witnesses a fresh accession of 

 decomposing material to the soil, in the shape 

 of withered roots, stems, and leaves. The 

 mould of our gardens, fields, and woods, is con- 

 sequently always in process of change; absorb- 

 ing oxygen, emitting carbonic acid, restoring 

 the carbon of pknts to the soil, and fulfilling its 

 most important office, in causing the separation 

 of the mineral ingredients, previously contained 

 in the decaying plants, by the thorough disor- 

 ganization of structure and tissue which takes 

 place. 



An important question here arises Of what 

 use is the decaying woody fibre to vegetation? 

 Is the fibre thus comminuted, and reduced to 



