260 PRINCIPLES OF CHEMISTRY 



compounds, which we will proceed to describe. Only let us first show 

 that the passage of ammonia into the oxygen compounds of nitrogen 

 up to nitric acid, as well as the converse preparation of ammonia 

 from nitric acid, are reactions which proceed directly and easily under 

 many circumstances. In nature the matter is complicated by a 

 number of influences and circumstances, but in the law the rela- 

 tions are presented in their simplest aspect. The bond between this 

 simplicity of laws and this complexity of phenomena forms the essence 

 of a scientific understanding of things. 



It is easy to prove the possibility of the oxidation of ammonia into 

 nitric acid by passing a mixture of ammonia and air over heated 

 spongy platinum. This causes the oxidation of the ammonia, nitric 

 acid being formed, which partially combines with the excess of 

 ammonia. 



The converse passage of nitric acid into ammonia is accomplished 

 by the action of hydrogen at the moment of its evolution. 28 Thus 

 metallic aluminium, evolving hydrogen from caustic soda, is able to 

 completely convert nitric acid added to the mixture (really as a salt, 

 because the alkali gives a salt with the nitric acid) into ammonia, 

 NH0 3 + 8H=NH 3 + 3H. 2 O. 



The compounds of nitrogen with oxygen present an excellent 

 example of the law of multiple proportions, because they contain for 14 

 parts by weight of nitrogen 8, 16, 24, 32, and 40 parts, respectively, by 

 weight of oxygen. The composition of these compounds is as follows : 



N 2 O, nitrous oxide ; hydrate NHO. 

 N 2 O 2 , nitric oxide, NO. 

 N 2 O 3 , nitrous anhydride ; hydrate NHO 2 . 

 N 2 O 4 , peroxide of nitrogen, NO 2 . 

 N 2 O <5 , nitric anhydride ; hydrate NH0 3 . 



Of these compounds, 29 nitrous and nitric oxides, peroxide of nitrogen, 

 and nitric acid, NHO 3 , are characterised as being the most stable. The 

 lower oxides, when coining into contact with the higher, may give the 

 intermediate forms ; for instance, NO and NO 2 form N 2 O 3 , and the 

 intermediate oxides may, in splitting up, give a higher and lower oxide. 



28 The formation of ammonia is remarked in many cases of oxidation by means of 

 nitric acid. This substance is even formed in the action of nitric aci4 on tin, especially 

 if dilute acid be employed in the cold. A still more considerable amount of ammonia is 

 obtained if, in the action of nitric acid, there are conditions directly tending to the evolu- 

 tion of hydrogen, which then reduces the acid to ammonia ; for instance, in the action 

 of zinc on a mixture of nitric and sulphuric acids. 



29 According to the determinations of Favre, Thomsen, and more especially of Berthelot, 

 on thermochemical data, it follows that in the formation of such quantities of the oxides 

 of nitrogen as express their formulae, if gaseou^nitrogen and oxygen be taken as the , 



