282 PRINCIPLES OF CHEMISTRY 



oxidation and the converse reaction is employed in practice as a 

 means for transferring the oxygen of the air to substances capable of 

 being oxidised. Having nitric oxide, it may easily be converted, with 

 the aid of the oxygen of the atmosphere and water, into nitric acid, 

 nitrous anhydride, and nitric peroxide, and by their means employed to- 

 oxidise other substances. In this oxidising action nitric oxide is again 

 formed, and it may again be converted into nitric acid, and so on with- 

 out end, if only there be oxygen and water. Hence the fact, which 

 at first appears to be a paradox, that by means of a small quantity of 

 nitric oxide in the presence of oxygen and water it is possible to oxidise 

 an indefinitely large quantity of substances which cannot be directly 

 oxidised either by the action of the atmospheric oxygen or by the 

 action of nitric oxide itself. The sulphurous anhydride, SO 2 , which 

 is obtained in the combustion of sulphur and in roasting many metallic 

 sulphides in the air, is an example of this kind. In practice this 

 gas is obtained by burning sulphur or iron pyrites, the latter being 

 thereby converted into oxide of iron and sulphurous anhydride. In 

 contact with the oxygen of the atmosphere this gas does not pass into- 

 the higher degree of oxidation sulphuric anhydride, SO 3 , and if it does 

 form sulphuric acid with water and the oxygen of the atmosphere, 

 SO 2 +H 2 O + O=H 2 SO 4 , it does so very slowly. With nitric acid (and 

 especially with nitrous acid, but not with nitrogen peroxide) and water, 

 sulphurous anhydride, on the contrary, very easily forms sulphuric acid, 

 and especially so when slightly heated (about 40), the nitric acid (or, 

 better still, nitrous acid) being converted into nitric oxide 



3SO 2 + 2NHO 3 -f 2H 2 O=2H 2 S0 4 + 2 



The presence of water is absolutely indispensable here, otherwise 

 sulphuric anhydride is formed, which combines with the oxides of 

 nitrogen (nitrous anhydride), forming a crystalline substance contain- 

 ing oxides of nitrogen (chamber crystals, which will be described in the 

 chapter on sulphur). Water destroys this compound, forming sulphuric 

 acid and separating the oxides of nitrogen. The water must be taken 

 in a greater quantity than that required for the formation of the hydrate 

 H 2 SO 4 , because the latter absorbs oxides of nitrogen. With an excess of 

 water, however, solution does not take place. If, in the above reaction, 

 only water, sulphurous anhydride, and nitric or nitrous acid be taken in 



place spontaneously ; substances even burn with difficulty in nitric oxide, probably because 

 a certain portion of the nitric oxide in decomposing gives oxygen, which combine* with 

 another portion of nitric oxide and forms nitric peroxide, a somewhat more stable com- 

 pound of nitrogen and oxygen. The further combinations of nitric oxide with oxygen all pro- 

 ceed with the evolution of heat, and take place spontaneously by contact with air alone. 

 From these examples it is seen how the use of thermochemical data is limited by facts. 



