ASSIMILATION OF CARBON. 119 



cheaper form, the substances that will furnish 

 the different elements removed in the grain is 

 apparent. 



Humus is the technical term used to designate 

 the mould or brown earthy part of soils. As this 

 substance has partially lost the power of as- 

 similating oxygen, and giving forth carbonic 

 acid gas, it is almost a fixed substance. Its 

 special office is that of a mechanical medium for 

 the absorption and retention of heat, moisture, 

 and fertilizing gases ; also that of a mechanical 

 support to the structure of the plant. 



The bulk or mass of vegetation is not fur- 

 nished by the soil. Generally the largest part 

 of the mass is water or its elements; a large part 

 also is carbon. Many ingenious experiments 

 have been made to determine the source of car- 

 bon in vegetation. They have resulted in estab- 

 lishing the fact that it is furnished almost en- 

 tirely, if not wholly, by the carbonic acid in the 

 atmosphere and soil. It is true that humic acid 

 a compound of humus and oxygen, containing 

 58 per cent, of carbon will form soluble com- 

 pounds with potash, soda and lime. Both Mal- 

 aguti and Sprengel say that 1 Ib. of lime com- 

 bines with 121bs. of humic acid, and thus every 

 pound of lime thus combined might be the ve- 

 hicle to furnish 7 Ibs. of carbon to the plant 

 But as only 15s Ibs. of lime enter into the com- 



