NITROUS ACID. 289 



vapour. Nitrous acid prepared in this way is decomposed 

 at once when thrown into water ; an effervescence occurring 

 from the escape of nitric oxide, and nitric acid being produced 

 which gives stability to a portion of the nitrous acid. Nitrous 

 acid cannot be made to unite directly with alkalies and earths, 

 probably owing to the action of water first described. But 

 when oxygen gas is mixed with a large excess of nitric oxide, 

 in contact with a solution of caustic potash, the gases were 

 found by Gay-Lussac always to disappear in the proportions 

 of nitrous acid, which was produced and entered into combi- 

 nation with the potash, forming a nitrite of potash. Similar 

 nitrites may also be produced by calcining the nitrate of potash 

 till the fused salt becomes alkaline ; or by boiling the 

 nitrate of lead with metallic lead. The nitrite of potash may 

 be dissolved and filtered, and the solution precipitated by ni- 

 trate of silver ; a process which gives the nitrite of silver, a salt 

 possessing a sparing degree of solubility like that of cream of 

 tartar, but which may be purified by solution and crystal- 

 lization, and then affords a ready means of obtaining the other 

 nitrites by double decomposition (Mitscherlich.) When free 

 sulphuric acid is added to a solution of nitrite of silver, the li- 

 berated nitrous acid is immediately resolved into nitric acid and 

 nitric oxide. The subnitrite of lead, on the other hand, may 

 be decomposed by the bisulphate of potash or soda to obtain a 

 neutral nitrite of one of these bases (Berzelius.) 



Nitrous acid is also capable of combining with several acids, 

 in particular with iodic, nitric, and sulphuric acids. Its combi- 

 nation with the last is a crystalline solid of specific gravity 1.831, 

 which is of considerable interest from its occurrence in the ma- 

 nufacture of sulphuric acid. According to the analysis of 

 Gaultier de Claubry, its constituents are 5 eq. of sulphuric acid, 

 2 of nitrous acid and 4 of water. When moist sulphurous acid 

 gas and peroxide of nitrogen are in contact, this crystalline com- 

 pound is formed, the sulphurous acid gaining the oxygen which 

 the other loses. A little nitrogen appears at the same time, so 

 that a portion of the peroxide of nitrogen must be completely 

 decomposed. If the crystalline compound comes in contact 

 with steam or a small quantity of water, the sulphuric acid com- 

 bines exclusively with the water, and the liberated nitrous acid 

 is resolved into nitric oxide and peroxide of nitrogen, both of 

 which escape as gas. But with a large quantity of water a por- 



