30 OF THE ASSIMILATION OF CARBON. 



purposes for which they are destined in its nutrition. 

 The composition and uses of such substances cannot, 

 however, be studied with advantage, until we have 

 considered the manner in w^hich the organic matter 

 is obtained by plants. 



Some virgin soils, such as those of America, con- 

 tain vegetable matter in large proportion ; and as 

 these have been found eminently adapted for the 

 cultivation of most plants, the organic matter con- 

 tained in them has naturally been recognised as the 

 cause of their fertility. To this matter, the term 

 " vegetable mould '^ or humus has been applied* 

 Indeed, this peculiar substance appears to play such 

 an important part in the phenomena of vegetation, 

 that vegetable physiologists have been induced to 

 ascribe the fertility of every soil to its presence. It 

 is believed by many to be the principal nutriment of 

 plants, and is supposed to be extracted by them 

 from the soil in which they grow. It is itself the 

 product of the decay of vegetable matter, and must 

 therefore contain many of the constituents which 

 are found in plants during life. Its action will 

 therefore be examined in considering whence these 

 constituents are derived. 



CHAPTER II. 



OF THE ASSIMILATION OF CARBON. 



COMPOSITION OF HUMUS. 



The humus, to which allusion has been made, is 

 described by chemists as a brown substance easily 

 soluble in alkalies, but only slightly so in water, and 

 produced during the decomposition of vegetable 

 matters by the action of acids or alkalies. It has, 

 however, received various names according to the 

 different external characters and chemical properties 

 which it presents. Thus, ulmin, humic acid, coal of 



