272 PRINCIPLES OF CHEMISTRY 



sometimes it leads to the formation of ammonia itself. The formation 

 of one or other nitrogenous substance from nitric acid is determined 

 not only by the nature of the reacting substances, but also by the 

 relative mass of water and nitric acid, and also by the temperature and 

 pressure, or the sum total of the conditions of reaction ; and as in a 

 given mixture these conditions even vary (the temperature and the 

 relative mass vary), therefore a mixture of different products of the 

 deoxidation of nitric acid is not unfrequently formed. 



Thus the action of nitric acid on metals consists in their being 

 oxidised, whilst it is itself converted, according to the temperature, 

 concentration in which it is taken, and the nature of the metal, <fec., 

 into either lower oxides, or even into ammonia. 42 Many compounds 

 are oxidised by nitric acid like metals and other elements ; for instance, 

 lower oxides are converted into higher oxides. Thus, arsenious acid is 

 converted into arsenic acid, suboxide of iron into oxide, sulphurous 

 acid into sulphuric acid, the sulphides of the metals, M 2 S, into sulphates, 

 M. 2 SO 4 , &c. ; iii a word, nitric acid brings about oxidation, its oxygen 

 is taken up and transferred to many other substances. Certain sub- 

 stances are oxidised by strong nitric acid so rapidly and with so great 

 an evolution of heat that they deflagrate and burst into flame. Thus 

 turpentine, C 10 H 16 , bursts into flame when poured into fuming nitric 

 acid. In virtue of its oxidising property, nitric acid removes the 

 hydrogen from many substances. Thus it decomposes hydriodic acid, 

 separating the iodine and forming water ; and if fuming nitric acid be 

 poured into a flask containing gaseous hydriodic acid, then a rapid 

 reaction takes place, accompanied by flame and the separation of 

 violet vapours of iodine and brown fumes of oxides of nitrogen. 43 



42 It is observed that normal nitric acid oxidises many metals with much greater 

 difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by 

 dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid 

 or of the pure hydrate NHO-,. Nitric acid diluted with a large quantity of water does 

 not oxidise copper, but it oxidises tin ; dilute nitric acid also does not oxidise either silver 

 or mercury ; but, on the addition of nitrous acid, even dilute acid acts on the above metals. 

 This naturally depends on the smaller stability of nitrous acid, and on the fact that after 

 the commencement of the action the nitric acid is itself converted into nitrous acid, which 

 continues to act on the silver and mercury. 



43 When nitric acid acts on many organic substances it often happens that not only 

 is hydrogen removed, but also oxygen is combined ; thus, for example, nitric acid con- 

 verts toluene, C 7 H 8 , into benzoic acid, C 7 H 6 O 2 . In certain cases, also, a portion of 

 the carbon contained in an organic substance burns at the expense of the oxygen of the 

 nitric acid. So, for instance, phthalic acid, C 8 H 6 O 4 , is obtained from naphthalene, C 10 H 8 . 

 Thus the action of nitric acid on the hydrocarbons is often most complex ; there takes 

 place (besides nitrification) the separation of carbon, the displacement of hydrogen, 

 and the combination of oxygen. There are few organic substances which can with- 

 stand the action of nitric acid. Hence nitric acid acts in a powerfully transforming 

 manner on a number of organic substances. It leaves a yellow stain on the skin, and in 



