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the air as fast as it is formed. Now water absorbslarge quantities of ammonia, which, as well as all itscompounds, is easily soluble in that fluid ; and conse-quently the ammonia and salts of ammonia formedduring these processes of decomposition are neverretained in the air, but are dissolved and broughtdown to the surface of the earth by rain.
96. Ammonia was formerly called the volatile alkali,because in properties it very closely resembles potashand soda, which are distinguished from all other basesby being called alkalies. Ammonia, however, differsfrom the two other alkalies in being a gas, whilst theyare both solid, and almost all its compounds are moreor less volatile, or capable of being converted intovapour by heat ; whilst the compounds of potash andsoda are all fixed, or quite incapable of being raisedinto vapour by any ordinary application of heat.
97. Ammonia has also a less powerful affinity foracids than the two other alkalies have, and consequentlyit is easily expelled from its compounds by either potashor soda, as well as by lime and several other bases,which set free the ammonia in the gaseous form andunite themselves with the acid previously in combina-tion with the ammonia. (149—150.)
98. Ammonia is not the only compound containingnitrogen which is formed during the decomposition ofanimal substances; for although nitrogen and'oxygenappear to have no affinity for each other, as they existtogether in a state of mixture in the air, and yet do not