52 SCIENTIFIC AGRICULTURE. 



with acid vapors which it meets in the atmosphere, and forms 

 other compounds. 



The salts of ammonia are decomposed by lime, magnesia, 

 potash and soda, and the ammonia is set free in the gaseous 

 state : the ease with which compounds of ammonia are decom- 

 posed, constitutes one of its most valuable properties, and 

 renders it peculiarly adapted to the various offices it performs 

 in the processes of vegetation. In the air, the, soil, or the 

 interior of plants, it is easily decomposed by electricity and the 

 alkaline bases before named. 



" The hydrogen it contains in so large quantity, [says Prof. 

 Johnston,] is ready to separate itself from the nitrogen in the 

 interior of the plant, and, in concert with the other organic 

 elements introduced by the roots or the leaves, to aid in pro- 

 ducing the different solid bodies of which the several parts of 

 plants are made up. The nitrogen also becomes fixed in the 

 colored petals of the flowers, in the seeds, and in other parts, 

 of which it appears to constitute a necessary ingredient, passes 

 off in the form of new compounds, in the insensible perspira- 

 tion or odoriferous exhalations of the plant, or returning with 

 the downward circulation, is thrown off by the root into the 

 soil from which it was originally derived." The transforma- 

 tions which actually take place in the interior of plants, is not 

 yet perfectly understood, although many of them can be 

 clearly explained. The agency of ammonia and its various 

 compounds, in the promotion of vegetation, is both powerful 

 and important, and will be explained more fully in a subse- 

 quent chapter, as will also its formation and sources. 



