CCXMI'Ol'NDS OF NITROGEN WITH II YIK< xiKN AND ()XV(iK.V 269 



even struck, decompose with an explosion, owing to the pressure of the 

 vapours and gases formed free nitrogen, carbonic anhydride, and 

 aqueous vapour. In the explosion of iiitro-compoimds much heat is 

 evolved, as in the combustion of gunpowder or detonating gas, and in 

 this case the force of explosion in a closed space is great, because from 

 a solid or liquid nitro-compound occupying a small space there proceed 

 vapours and gases whose elasticity is great not only from the small 

 space in which they are formed, but owing to the high temperature 

 corresponding to the combustion of the nitro-compound. 38 



The combustion of nitro-compounds, as well as that which nitrates 

 bring about (in gunpowder), originates in the weakness of the bond 

 which holds together the oxygen and nitrogen in nitric acid itself, as 

 well as in all the oxygen compounds of nitrogen. If the vapour of 

 nitric acid is passed through an even moderately heated glass tube, the 

 formation of dark-brown fumes of the lower oxides of nitrogen and the 

 separation of free oxygen may be observed 2NH0 3 =H 2 O + 2NO 2 + O. 

 The decomposition is complete at a white heat that is, nitrogen is 

 formed, 2NH0 3 =H 2 O + N 2 -f 5 . Hence it is easily understood 

 that nitric acid may part with its oxygen to a number of substances 

 capable of being oxidised. 39 It is consequently an oxidising agent. 

 Charcoal, as we have already seen, burns in nitric acid ; phosphorus, 

 sulphur, iodine, and the majority of metals also decompose nitric acid, 



means of sulphuric acid (that is, by a method of double saline decomposition), whilst 

 nitric acid is not displaced by sulphuric acid from true nitro-compounds ; for instance, 

 nitrobenzene, CtfHs'NO.j. As nitro-compounds are formed exclusively from hydrocarbons, 

 they are described with them in organic chemistry. 



The group NO 2 of nitro-compounds in many cases (like all the oxidised compounds of 

 nitrogen) passes into the ammonia group or into the ammonia radicle NH 2 . It is evident 

 that this requires the action of reducing substances evolving hydrogen : RNO 2 + 6H 

 = KNH 5 + 2H. 2 O. Thus Zinin converted nitrobenzene, C 6 H 5 'NO.>, into aniline, C 6 H 5 'NHo, 

 by the action of hydrogen sulphide. 



Admitting the existence of the group NO 2 , replacing hydrogen in various compounds, 

 then nitric acid may be considered as water in v/hich half the hydrogen is replaced by 

 the radicle of nitric acid. In this sense nitric acid is nitro-water, NCyOH, its anhydride 

 dinitro- water, (NO 2 ) 2 O, and nitrous acid nitre-hydrogen, XOoH. In nitric acid the radicle 

 of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the 

 radicle of benzene. 



It should here be remarked that the group NO 3 may be recognised in the salts of 

 nitric acid, because the salts have the composition M^NOs),,, just as the metallic chlorides 

 have the composition MCI,,. But the group NOs does not form any other compounds 

 beyond the salts, and therefore it should be considered as hydroxyl, HO, in which H is 

 replaced by NO 2 . 



38 The nitro-compounds play a very important part in mining and artillery. Detailed 

 accounts of them must be looked for in special works. The most important and histori- 

 ciil work in this connection is due to Berthelot, who elucidated much in connection with 

 explosive compounds by a series of both experimental and theoretical researches. 



59 Nitric acid may be entirely decomposed by passing its vapour over highly incan- 

 descent copper, because the oxides of nitrogen first formed give up their oxygen to the 



