AMMONIUM SULPHITES, THIOSULPHATE AND TRITHIONATE. 203 



Äinmonium pyrosulphite, (NH4)2S205. — When, in the process 

 just given for preparing the normal sulphite, the passage of 

 sulphur dioxide is not stopped when the solution is full of crystals, 

 these gradually dissolve up and the solution becomes greenish- 

 yellow. Then, as it gets charged with sulphur dioxide, in the 

 cooling mixture, the pyrosulphite crystallises out from it, in quan- 

 tity equivalent to a little over one-fifth of the ammonia taken, 

 being thrown out of solution by the sulphur dioxide. The salt 

 can be obtained dry and pure in the same way as the normal 

 sulphite, except that sulphuric acid, to which a little solid alkali 

 sulphite has been added, is used in the desiccator, though it is 

 very deliquescent and changeable when not carefully preserved 

 from moisture. This salt is also easily obtainable by evapora- 

 ting its aqueous solution, but hardly free from sulphate, and not 

 without some decomposition, through loss of sulphur dioxide and 

 through oxidation. It is much more soluble than the normal 

 sulphite. 



Ammonium thiosulphate. — An old solution of calcium thio- 

 sulphate, obtained by boiling lime and sulphur together in water 

 and leaving the solution until much of the pentasulphide had 

 been oxidised by the air, was decanted from insoluble matters, 

 mixed with ammonium carbonate in some excess, filtered, and 

 then freely exposed to the air for some time at 50-60°. In this 

 way a very concentrated solution of ammonium thiosulphate was 

 obtained, free from sulphate and other salts. The solution of 

 this very soluble salt was then dried up to a crystalline mass in 

 the desiccator. The well-dried crystals have been found by Lock 

 and Kluess {Ber., 1889, 22, 3099) to be anhydrous. 



A?nvioiiium trithionate. — This salt has apparently not hitherto 

 been prepared by any one. Being exceedingly soluble in water. 



