50 



Nitrogen, the organic element which now remains 

 to be considered, is present in its pure state in our 

 atmosphere, and constitutes two-thirds of it ; but 

 it is such an inert substance in this pure state that 

 it will not combine with any other substance. Most 

 exhaustive experiments have conclusively proved 

 that plants are incapable of assimilating it in its 

 pure state, and the only known sources from which 

 plants derive it are Ammonia, a compound of 

 nitrogen and hydrogen ; and Nitric Acid, a combi- 

 nation of the same element with oxygen. 



Ammonia is principally absorbed by the roots, it 

 being highly soluble in water ; and though it is 

 present in the atmospheric air in such small 

 quantities that we can barely detect it with our 

 finest reagents, yet we can trace it distinctly in all 

 rain-water. The balance of its percentage in our 

 atmosphere is preserved in the same way as 

 carbonic acid by the decay of vegetable and animal 

 matter, and by the eruptions of volcanoes. 

 Ammonia and nitric acid are compounds very 

 important to the farmer, being the only sources 

 which supply to our edible crops nitrogen, the 

 relative proportion of which stands in close connec- 

 tion with their nutritive qualities ; and therefore 

 they cannot be too highly valued. 



Nitrate of Potash and Soda, when in contact with 

 decaying vegetable matter, will also yield ammonia ; 

 hence their importance as chemical manures. 



