OF NITH<X;KN WITH HYDROGEN AND OXYGEN 277 



which nitrogen dioxide in many other instances passes, and from which 

 it is easily formed. 48 



Xifrinix anhydride, N 2 O 3 , corresponds 49 with nitrous acid, NHO 2 , 

 and with the latter corresponds a series of salts, the nitrites for ex- 

 ample, the sodium salt NaNO 2 , the potassium salt KNO. 2 , the ammonium 

 salt (NH ,)X( ),, :i " the silver salt AgNO 2 , M <kc. Neither the anhydride 

 nor the hydrate of the acid is known in a perfectly pure state. The 

 anhydride has only been obtained as a very unstable substance, and has 

 not yet been investigated with proper fulness ; and when efforts are 

 made to obtain the acid NHO 2 from its salts, it always gives water and 

 the anhydride, whilst the latter, as an intermediate oxide, easily splits 

 up into NO + NO 2 . But the salts of nitrous acid are distinguished for 

 their great stability. Potassium nitrate, KNO 3 , may be converted into 

 potassium nitrite by depriving it of a portion of its oxygen ; for 

 instance, by fusing it (at a not too great heat) with metals, such as 

 lead, KNO 3 + Pb=KNO 2 + PbO. The resultant salt is soluble in 

 water, whilst the oxide of lead is insoluble. With sulphuric and other 

 acids the solution of potassium nitrite 52 immediately evolves a brown 

 gas, nitrous anhydride : 2KN0 2 + H 2 SO 4 =K 2 SO 4 + N 2 O 3 + H 2 O. The 

 same gas (N 2 O 3 ) is obtained by passing nitric oxide at through 

 liquid peroxide of nitrogen, 53 or by heating starch with nitric acid of sp. 



48 Nitric acid of sp. gr. 1'51 in dissolving nitrogen peroxide becomes brown, whilst 

 nitric acid of sp. gr. T32 is coloured greenish blue, and acid of sp. gr. below 1'15 remains 

 colourless on absorbing nitrogen peroxide. 



49 Nitrogen peroxide as a mixed substance has no corresponding independent salts. 



50 Ammonium nitrite may be easily obtained in solution by a similar method of double 

 decomposition (for instance, of the barium salt with ammonium sulphate) to the other 

 salts of nitrous acid, but it decomposes with great ease when evaporated, with the evo- 

 lution of gaseous nitrogen, as has been already mentioned (Chap. V.). If the solution, 

 however, be evaporated at the ordinary temperature under the receiver of an air-pump, 

 a solid saline mass is obtained, which is easily decomposed when heated. The dry salt 

 even decomposes with an explosion when struck, or when heated to about 70 NH 4 NO 2 = 

 iiHoO + N 2 . It is also formed by the action of aqueous ammonia on a mixture of nitric 

 oxide and oxygen, or by the action of ozone on ammonia, and in many other instances. 



' Silver nitrite, AgNO 2 , is obtained as a very slightly soluble substance, as a preci- 

 pitate, 011 mixing solutions of silver nitrate, AgNO 5 , and potassium nitrite, KNO 2 . It 

 is soluble in a large volume of water, and this is taken advantage of to free it from 

 silver oxide, which is also present in the precipitate, owing to the fact that potassium 

 nitrite always contains a certain amount of oxide, which with water gives the hydroxide, 

 forming oxide of silver with silver nitrate. The solution of silver nitrite gives, by double 

 decomposition with metallic chlorides (for instance, barium chloride), insoluble silver 

 chloride and the nitrite of the metal taken (for instance, barium nitrite, Ba(NOo)._>). 



'- Probably potassium nitrite, KNO 2 , when strongly heated, especially with metallic 

 oxides, evolves N and O, and gives potassium oxide, K 2 O, because nitre is liable to such 

 a decomposition, but it has, as yet, been but little investigated. 



53 It is evident that the reaction NoO 5 = NO 2 + NO is reversible, and that it resembles 

 the conversion of N 2 O 4 into NO 2 , but as yet this reaction has not been thoroughly 



