il("U PLANTS DhKlVi; Fi:()M liJl. \;:. '!■) 



■\ liicti render tlieni valuable as food for man and the inferior 

 mimals. 



9. The most singular character of nitrogen is the indispo- 

 sition which it exhibits to enter into combination with other 

 bodies. The other simple undecompounded bodies, like oxy- 

 gen, dT. have a constant tendency to unite together, but 

 resisting what appears to be a general law, this gas can 

 with difficulty be forced into combination ; so that it is con- 

 sidered that, as existing in the air, it serves merely to modify 

 the energetic properties of oxygen, but takes no part in con- 

 tributing to the formation either of plants or animals. How, 

 then, is this element, which is so unwilling to unite with 

 other bodies, and yet so indispensable to animal and vegetable 

 existence, obtained by plants ? This important question will 

 be answered as we proceed. 



10. In addition to oxygen and nitrogen, the preponderating 

 ingredients of the atmosphere, there are invariably diffused 

 through it exceedingly minute portions of two other gaseous 

 bodies designated ammonia and carbonic acid.* These bodies, 

 though existing in the air in quantities which render them 

 almost imperceptible, you will find are of the greatest im- 

 portance, and may be regarded as pre-eminently required for 

 the nourishment of plants. The first of these bodies, am- 

 monia, is, like oxygen and nitrogen, a kind of an*. It is, 

 however, not like them simple, but has by the chemist been 

 discovered to consist of two of the simple elements, one of 

 them the unsocial element nitrogen, which we lately noticed, 

 and which, in this compound, has united itself with a gas 

 which we have yet to describe, named hydrogen. In ammo- 

 nia, nitrogen loses all the inertness that distinguished it 

 in its separate state, and takes under various forms a most 

 active part in several important operations which I shall 

 have occasion to notice. 



11. Before proceeding further, it is necessary that you 

 should understand the meaning of some terms which I will 

 have occasion to employ, and which you will frequently meet 

 in reading works on agricultme. You have probably observed 



• Dr. Clarke gives the following statement, in round nmnbers, of 

 ti> . ,.,..... wition of the atmosphere: — 



Nitrogen 1900 volumes. 



Oxygen 500 do. 



Carbonic acid i <!"• 



