AMMONIA. 81 



posed, whether by combustion or by decay ; in both 

 these cases carbonate of ammonia is formed, which is 

 a very volatile salt, and consequently is carried away 

 by the air, as fast as it is formed. Now water ab- 

 sorbs large quantities of ammonia, which, as well as 

 all its compounds, is easily soluble in that fluid ; and' 

 consequently the ammonia and salts of ammonia, 

 formed during these processes of decomposition, are 

 never retained in the air, but are dissolved and brought 

 down to the surface of the earth, by rain. 



151. Ammonia was formerly called the volatile 

 alkali, because in many properties it closely resembles 

 the alkalies potash and soda, which are distinguished 

 from all other bases, by that name. Ammonia, how- 

 ever, differs from the two other alkalies in being a 

 gas, whilst they are both solid, and almost all its 

 compounds are more or less volatile, or capable of 

 being converted into vapor by heat ; whilst the com- 

 pounds of potash and soda are all fixed, or quite in- 

 capable of being raised into vapor by any ordinary 

 application of heat. 



152. Ammonia has also a less powerful affinity for 

 acids than the two other alkalies have, and conse- 

 quently it is easily expelled from its compounds, by 

 either potash or soda, as well as by lime and several 

 other bases, which set free the ammonia in the gas- 

 eous form, and unite themselves with the acid pre- 

 viously in combination with the ammonia. 



153. Ammonia, in its pure state, is a colorless, in- 

 visible, and very pungent-smelling gas, readily soluble 



