OF SOILS. 29 



not have been produced by the immediate agency of 

 vegetable matter. Carbonic acid gas performs a 

 most important part in the process of vegetable 

 nutrition, the consideration of which belongs to 

 another part of the work. 



Carbonic acid, water, and ammonia (a compound 

 of hydrogen and nitrogen) are the final products of 

 the decay of animal and vegetable matter. In an 

 isolated condition, they usually exist in the gaseous 

 form. Hence, on their formation, they must escape 

 into the atmosphere. But ammonia has not hitherto 

 been enumerated amongst the constituents of the 

 air, although, according to our view, it can never be 

 absent. The reason of this is, that it exists in 

 extremely minute quantity in the amount of air usu- 

 ally subjected to experiment in chemical analysis; 

 it has consequently escaped detection. But rain 

 which falls through a large extent of air, carries 

 down in solution all that remains in suspension in it. 

 Now ammonia always exists in rain-water, and from 

 this fact we must conclude that it is invariably pres- 

 ent in the atmosphere. Nor can we be surprised at 

 its presence when we consider that many volcanoes 

 now in activity emit large quantities of it.* This 

 subject will, however, be discussed more fully in 

 another part of the work. 



Such are the principal constituents of the atmo- 

 sphere from which plants derive their nourishment; 

 for although other matters are supposed to exist in 

 it in minute quantity, yet they do not exercise any 

 influence on vegetation, nor has even their presence 

 been satisfactorily demonstrated. 



OF SOILS. 



A soil may be considered a magazine of inorganic 

 matters, which are prepared by the plant to suit the 



* The annual evolution of carbonic acid from springs and fissures in the 

 ancient volcanic district of the Eifel, on the Rhine, has been estimated by 

 Bischof, at not less than 100,000 tons, containing 27,000 tons of carbon. 



3* 



