302 E. DIVERS AND T. HAGA : 



There remains to be explained the great variability in the 

 commencement of precipitation of the acid carbonate. This takes 

 j)lace the sooner the faster tlie sulphur dioxide is blown into the 

 solution. When it occurs in the earlier stages of the process, it 

 is, therefore, accompanied by greater predominance than usual of 

 production of hydroximidosulphate over production of sulphite. 

 It does not however depend upon this, ibr while sulphur dioxide 

 liberates a molecule of carbon dioxide in changing carbonate into 

 sulphite, four mois, of it are needed to liberate one mol. of 

 carbon dioxide in changing carbonate and nitrite into hydrox- 

 imidosulphate. 



An explanation is suggested by a consideration of the fact 

 that wdieu working the process at a moderate rate, the first 

 crystallisation of acid carbonate takes place long after the point 

 at which the solution must contain the maximum of the salt, at 

 least potentially, the point, that is, when half the carbonate has 

 become either sulphonate or sulphite. When it does occur the 

 quantity of it in solution has become much less. Only where 

 crystallisation is started early by a very rapid addition of sulphur 

 dioxide, does the acid carbonate continue to separate out in much 

 such quantity as it could do at the stage of the process reached. 

 The cause in one word is supersaturation. The acid carbonate, 

 it would seem, is slow to begin to precipitate from the solution 

 while that is not charged with carbon dioxide. At a medium 

 rate of working this only happens in the later stages, any normal 

 carbonate and even much normal sulphite present keeping down 

 tlie quantity of carbon dioxide, but by a rapid rate of working 

 local saturation occurs and the acidified portion of the solution 

 then crystallises. Once crystallisation has been started, it proceeds 

 unchecked. In slower working when crystallisation only begins 



