OP THE NITROGEN OF PLANTS. 85 



residue of decayed plants would, after some years, 

 be found in its place. 



All animal excrements emit carbonic acid and am- 

 monia, as long as nitrogen exists in them. In every 

 stage of their putrefaction an escape of ammonia 

 from them may be induced by moistening them 

 with a potash ley ; the ammonia being apparent 

 to the senses by a peculiar smell, and by the dense 

 white vapour which arises when a solid body moist- 

 ened with an acid is brought near it. This ammo- 

 nia evolved from manure is imbibed by the soil 

 either in solution in water, or in the gaseous form, 

 and plants thus receive a larger supply of nitrogen 

 than is afforded to them by the atmosphere. 



But it is much less the quantity of ammonia, 

 yielded to a soil by animal excrements, than the 

 form in which it is presented by them, that causes 

 their great influence on its fertility. Wild plants 

 obtain more nitrogen from the atmosphere in the 

 form of ammonia than they require for their growth, 

 for the water which evaporates through their leaves 

 and blossoms, emits, after some time, a putrid smell, 

 a peculiarity possessed only by such bodies as 

 contain nitrogen. Cultivated plants receive the 

 same quantity of nitrogen from the atmosphere as 

 trees, shrubs, and other wild plants ; but this is 

 not sufficient for the purposes of agriculture. 

 Agriculture differs essentially from the cultivation 

 of forests, inasmuch as its principal object consists 

 in the production of nitrogen under any form capa- 



