32

AMMONIA.

the air : so that it would appear as if the chief use ofnitrogen in the air was to dilute the oxygen and pre-vent it from combining too rapidly with carbon andother substances.

92. Under some circumstances, however, nitrogendoes combine with other elements, and its compoundsare amongst the most curious and important substanceswe know. When animal or vegetable matters contain-ing nitrogen decay, we find that it, like the carbon, isnot set free in its simple and uncombined form, but thatduring decay it combines with a portion of hydrogen.

93. Therefore in addition to water and carbonic acidthe two principal substances arising from the decay ofordinary vegetable matters, we find a pungent, strong-smelling gas, composed of nitrogen and hydrogen,which is called ammonia.

94. This substance, though a gas, is, like potash andsoda, a base; like them it has a strong affinity for acids,and when combined with them neutralises their powers ;and, therefore, as it is evolved at the same time withcarbonic acid, it combines with that acid and producesa solid salt, which is called a carbonate of ammonia, justas the compound of carbonic acid and lime is called acarbonate of lime.

95. Ammonia is always produced when animal orvegetable substances containing nitrogen are decom-posed, whether by combustion or by decay; in boththese cases carbonate of ammonia is formed, which is avery volatile salt, and consequently is carried away by