HYPOSULPHUROUS ACID. 381 



always avoided, as the hyposulphites are all partially decom- 

 posed at 212 ;) and affords large crystals of the hyposulphite 

 of soda. When solution of caustic soda is digested upon 

 sulphur, the latter is likewise dissolved, and a mixture of 1 eq. 

 of hyposulphite of soda with 2 eq. of sulphuret of sodium results, 

 of which the last may dissolve an excess of sulphur : 



3NaO and 4S == NaO + S 9 O 9 and 2NaS. 



2^2 



Exposed to the air, this solution slowly absorbs oxygen, and if 

 it contains a certain excess of sulphur, passes entirely into hypo- 

 sulphite of soda. 



The hyposulphite of lime is also formed, by digesting together 

 1 part of sulphur and 3 of hydrate of lime at a high temperature, 

 when changes of the same nature occur as with sulphur and 

 caustic soda, and the solution becomes red ; a stream of sul- 

 phurous acid gas is conducted through the solution after it has 

 cooled, and converts the whole salt into hyposulphite, occasion- 

 ing at the same time a considerable deposition of sulphur. The 

 reaction here is rather complicated, the sulphurous acid uniting 

 with one portion of sulphur, to form hyposulphurous acid, and 

 also liberating another portion of the same element from the 

 sulphuret of calcium. It is expressed in the following for- 

 mula: 



2CaS and 3SO 2 = 2CaO-f 2S 2 O 2 , and S. 



Zinc and iron also dissolve in the solution of sulphurous acid in 

 water, with little or no effervescence, deriving the oxygen neces- 

 sary to convert them into oxides, not from water, but from the 

 sulphurous acid, two-thirds of which are thereby converted into 

 hyposulphurous acid, which combines with half of the oxide 

 produced ; while the other third, remaining as sulphurous acid, 

 unites with the other moiety of the same oxide : 



3SO 2 and 2Zn = Zn O, S 2 O 2 and Zn O, SO 2 . 



The hyposulphite obtained by this process is, therefore, mixed 

 with a sulphite. 



Properties. The acid of these salts undergoes decomposition 

 when they are strongly heated, or treated with an acid. It 

 forms soluble salts with lime and strontian, in which respect it 

 differs from sulphurous and sulphuric acids ; the hyposulphite 

 of barytes is insoluble. It also forms a remarkable salt with 



