Sii CAEBONIC AdlD. 



thrive upon. The first has been already very slightly adverted 

 to (pp. 38 and 29), but must now be more fully examined. 



Nothing can be taken into the system of a plant while in a 

 solid state. To be suited to absorption, it is indispensable that 

 matter should be gaseous, or fluid, or soluble in water. 



The most important gaseous substances are — 1, Carbonic 

 Acid ; 2, Nitrogen ; others may be practically disregarded.' 



Carbonic acid. When a plant is exposed to high heat, it is 

 soon reduced, however delicate it may be, to a brown or black 

 substance. That substance is charcoal, which constitutes by 

 far the larger part of all vegetable structure. Charcoal is assimi- 

 lated by plants from carbonic acid, in which all atmospheric air 

 abounds ; the carbon or charcoal is separated by vital force, 

 and the oxygen is liberated. 



Carbonic acid is formed slowly by all animal and vegetable 

 substances undergoing decay in the presence of moisture ; 

 hence in part the manuring value of decaying leaves, of vege- 

 table mould, of the excrements of animals, &c. 



It has been doubted, indeed, whether it ia by the formation of carbonic 

 acid, that decaying vegetable matter acts beneficially, and it has been 

 imagined that what ia called hixmic acid, formed from decaying mould 

 by the action of alkalies is taken up directly by plants as food. 

 Opinion is generally unfavourable to this theory ; and since we know 

 from the experiments of De Saussure and others, and it is not indeed 

 denied, that dead vegetable matter disappears in consequence of its 

 gradually combining with oxygen, and forming carbonic acid, there is 

 no necessity for looking to some supposed action of humates ia order 

 to account for the manuring value of vegetable substances in a state of 

 decay. 



It has been found that charcoal itself is highly beneficial 

 when introduced into the soil, and it has been inferred that even 

 charcoal acts by its property of assuming a gaseous form when 

 combined with oxygen. But chemists believe that it is rather 

 by virtue of its porosity, whence it derives the jpower of con- 

 densing gaseous matter, and slowly parting with it again, that 

 charcoal acts beneficially. According to Mitscherlich, "the 

 cells of wood-charcoal have a diameter of about -rjVo of an 

 inch, and if a cubic inch consisted entirely of cells, their 

 united surface would amount to 100 square feet. By expe- 



