HYPOSULPHURIC ACID. 335 



Solutions of the nitrosulphates are not stable above the 

 freezing point, but their stability is much increased by an excess 

 of alkali. They are resolved into sulphate and nitrous oxide, 

 by the mere contact of certain substances, which do not them- 

 selves undergo any change, such as spongy platinum, silver and 

 its oxide, charcoal powder and peroxide of manganese, by acids, 

 even carbonic acid, and by metallic salts. 



HYPOSULPHURIC ACID. 

 Eg. 902.3 or 72.24. ; S 2 O 5 ; not isolable. 



Preparation. This acid of sulphur was discovered by Gay- 

 Lussac and Welter, in 1819. To prepare it, a quantity of 

 peroxide of manganese, which must not be hydrated, is reduced 

 to an extremely fine powder, suspended by agitation in water, 

 and sulphurous acid gas is transmitted through the water. The 

 temperature is apt to rise during the absorption of the gas, but 

 must be repressed, otherwise much sulphuric acid is produced, 

 the formation of which, indeed, it is impossible to prevent en- 

 tirely, but of which the quantity is reduced almost to nothing, 

 when the liquor is kept cold during the operation. The perox- 

 ide of manganese disappears, and a solution of hyposulphate of 

 the protoxide of manganese is formed ; 2 equivalents of sulphu- 

 rous acid, and 1 of peroxide of manganese, forming one of hy- 

 posulphuric acid and one of protoxide of manganese, or 



2SO 2 and MnOj = MnO + S 2 O 5 . 



The solution is filtered, and then mixed with a solution of 

 sulphuret of barium, which occasions the precipitation of the 

 insoluble sulphuret of manganese, with the transference of the 

 hyposulphuric acid to barytes. From this hyposulphate of 

 barytes, the hyposulphates of other metallic oxides may be pre- 

 pared, by adding their sulphates to that salt, when the insoluble 

 sulphate of barytes will precipitate, and the hyposulphate of 

 the metallic oxide added remain in solution. But to procure 

 the hyposulphuric acid itself, the solution of hyposulphate of 

 barytes may be evaporated to dryness, and being perfectly pure, 

 it is reduced to a fine powder, weighed and dissolved in water ; 

 for 100 parts of it 18.78 parts of oil of vitriol are taken, which 

 after dilution with 3 or 4 times as much water, are employed to 

 decompose the salt of barytes. The liberated hyposulphuric 



