80 AMMONIA. 



and when carbon, or substances containing it, are 

 burnt, they combine only with the oxygen, and never 

 with the nitrogen of the air : so that it would appear 

 as if the chief use of nitrogen in the air was to dilute 

 the oxygen, and prevent it from combining too ra- 

 pidly with carbon, and other substances. 



147. Under some circumstances, however, nitrogen 

 does combine with other elements, and its compounds 

 are amongst the most curious and important sub- 

 stances we know. When animal or vegetable matters 

 containing nitrogen decay, we find that it, like the 

 carbon, is not set free in its simple and uncombined 

 form, but that during decay it combines with a por- 

 tion of hydrogen. 



148. Therefore, in addition to water and carbonic 

 acid, the two principal substances arising from the 

 decay of ordinary vegetable matters, we find a pun- 

 gent, strong-smelling gas, composed of nitrogen and 

 hydrogen, which is called Ammonia. 



149. This substance, though a transparent invisible 

 gas, is, like potash and soda, a base; like them it has 

 a strong affinity for acids, and when combined with 

 them neutralizes their powers; and, therefore, as it is 

 evolved at the same time with carbonic acid, it com- 

 bines with that acid and produces a solid salt, which 

 is called a carbonate of ammonia, just as the com- 

 pound of carbonic acid <ind lime is called a carbonate 

 of lime. 



150. Ammonia is always produced when animal or 

 vegetable substances containing nitrogen are decom- 



