286 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



in just the same way that oxy-salts were formed by combining 

 two oxides, e.g. 



PbO + SO 3 -> PbSO 4 



(Lith- (Sulphuric (Lead 

 arge.) anhydride.) sulphate.) 



In 1808, Seebeck at Jena, and Berzelius and Pontin at 

 Stockholm, discovered that an amalgam could be produced by 

 passing an electric current through ammonia in the presence of 

 mercury. The amalgam is decomposed by heat into mercury, 

 hydrogen (i vol.} and ammonia (2 vols.). As ammonia (2 vols} 

 contains hydrogen (3 vols.} and nitrogen (i vol.\ the pseudo- 

 metal in the amalgam contains nitrogen (i vol.} combined with 

 hydrogen (4 vols} ; Davy in 1808, proposed to call it ammonium. 

 Amp&re, in 1816, showed that the salts of ammonia could be 

 regarded as strictly analogous with the salts of the metals if am- 

 monium were regarded as a " compound metal," which united 

 directly with halogens to form salts such as sal-ammoniac or 

 ammonium chloride : 



NH 3 +HC1 = NH 4 C1 or (NH 4 ) Cl 



(Am- (Hydrogen (Sal- (Ammonium 



monia.) chloride.) ammoniac.) chloride.) 



A compound of ammonia with half its volume of water vapour 

 constituted an ammonium oxide, which could unite with oxy- 

 acids (anhydrides) to form oxy-salts, thus : 



2NH 3 + H 2 O -> (NH 4 ) 2 O 



(Ammonium oxide.) 



(NH 4 ) 2 + 80s - (NH 4 ) 2 S0 4 



(Ammonium (Sulphuric (Ammonium 

 oxide.) anhydride.) sulphate.) 



The oxy-salts of ammonia contain both oxygen and hydrogen. 

 In some cases these may be driven off together in the form of 

 water, e.g. : 



NH 4 NO 3 -> 2H 2 O + N 2 O 



(Ammonium (Water) (Nitrous 



Nitrate.) oxide.) 



NH 4 NO 2 -> 2H 2 O + N 2 



(Ammonium (Water) (Nitrogen.) 



nitrite.) 



The decomposition of ammonium nitrate, the "nitrum 

 flammans" of the alchemists, had been known from an early 

 period and was used by Berthollet in 1785 to prove the presence 



