xiii THE DECOMPOSITION OF THE ALKALIS 269 



But when a basic oxide combines with an acid hydride, 

 water is formed in addition to the salt, e.g. 



litharge + muriatic gas -- > lead chloride + water. 



On the other hand, when a basic hydride combines with 

 an acid oxide ivater is absorbed 1 in the formation of the salt. 

 Thus ammonia does not unite with sulphuric anhydride to 

 form a salt, but 



ammonia -f water + > ammonium sulphate. 



The water is needed to convert the ammonia into am- 

 monium oxide, which can then unite with the acid oxide to 

 form a salt : alternatively, it might be used to convert the 

 acid oxide (sulphuric anhydride) into a hydro-acid (sulphuric 

 acid) which can then unite directly with ammonia to form 

 a salt, thus : 



rr, . , ammonium , sulphuric ammonium 



Twooxute + ar / ydride - ' su!phate . 



sulphuric ammonium 



Twohydndes: ammonia + su , phate 



It is evident from these considerations that the ammonium 

 salts of oxy-acids are quaternary compounds, containing 

 both hydrogen and oxygen, in addition to nitrogen and (e.g.) 

 sulphur. 



The case of ammonium nitrate is specially interesting, as 

 the acid oxide and basic hydride are both compounds of 

 nitrogen. When the salt is heated, the hydrogen of the 

 "ammonium" unites with oxygen from the acid to form 

 water, and the nitrogen escapes in the form of nitrous oxide. 

 In the case of ammonium nitrite the decomposition is 

 almost explosive, the salt being decomposed completely 

 into water and nitrogen. 



1 Sal-volatile, which Priestley prepared from ammonia and carbonic 

 anhydride, -is an exception ; the product is not ammonium carbonate, 

 but ammonium carbamate, which forms a carbonate only when dissolved 

 in water. 



