14 WALL'S MANUAL 



of sucli animal and vegetable compounds us contain 

 nitrogen. It is absorbed and retained by the clay 

 and humus or mold in the soil. Some plants are 

 believed to absorb it through their leaves, from the 

 air; but in most cases it enters through the roots 

 from the soil. 



Although nitrogen is so abundant as the chief con- 

 stituent of the atmosphere, yet it rarely or never 

 enters directly, in its pure gaseous form, into the 

 plant. In contact with decaying vegetable matter in 

 the soil, nitrogen, in the form of air, unites with 

 hydrogen, forming ammonia. The ammonia formed 

 in this way, as well as that formed by the decay of 

 animal matters, are again decomposed by strong bases, 

 such as lime, potash, soda, etc. j the nitrogen becom- 

 ing oxydized, forms nitric acid (aquafortis), while the 

 hydrogen combining with oxygon, becomes water. 



The nitric acid, or aquafortis, generated as above, 

 combines with whatever bases may be present in the 

 soil, forming nitrates. Thus are formed the nitrate of 

 potash, of soda, of lime, etc. Nitre, or saltpetre, is 

 thus often formed in cultivated lands, whence it passes 

 into the juices of plants. This fact can be illustrated 

 thus : it is a w T cll known fact that beets and tobacco 

 grown upon strongly manured land, and also rank 

 plants growing on manure heaps, such as henbane, 

 thorn-apple, etc., contain so much nitre, that when 

 dried they emit sparks, when burning, like paper 

 w r hich has been dipped into a solution of saltpetre. 



Nitric acid is also naturally formed by the passage 

 of electricity through the atmosphere. The air con- 

 sists of nitrogen and oxygen mixed together. When 

 an electric current is passed through a quantity of 



