ON MANURES. 113 



It is, therefore, proposed to consider these 

 substances in the following order : — 



Ammonia. — Already under the head " nitro- 

 gen," (p. 60) this substance has been fully 

 discussed, and the important part it performs 

 in vegetable economy fully described, and 

 to this article therefore we beg to refer the 

 reader. 



The chemical analysis of ammonia yields 

 three volumes of hydrogen and one of nitro- 

 gen, and it is to the presence of this latter sub- 

 stance that the effects produced on vegetables 

 are to be attributed. This combination is one 

 of the most simple compounds of nitrogen that 

 exists, and yet certain conditions are necessary 

 for its union with hydrogen. It is not at all 

 necessary here to detail what these conditions 

 are, it is only necessary to repeat what we have 

 previously stated, that the nitrogen of the atmo- 

 sphere does not in any way combine with hy- 

 drogen to form ammonia; it is only formed 

 11 



