98 



4. Sulphide of iron held in solution by the sulphide of 

 sodium. 



5. Chloride of sodium. 



6. Several cyanogen compounds of sodium, and especially 

 ferrocyanide of sodium. 



These liquors are evaporated down in large cast iron pots 

 and in order to destroy or oxidise the sulphides of sodium 

 and iron, as also the cyanogen compounds, an equivalent 

 quantity of soda-saltpetre is added. All the oxidisable 

 sulphur compounds, together with the small quantity of 

 sulphide of iron, are changed to sulphate of soda and peroxide 

 of iron by the nitrate of soda in the boiling liquor, at a 

 temperature not below 260° to 270° F. The cyanogen com- 

 pounds, on the other hand, are only decomposed by the nitre 

 as soon as the liquor begins to pass from the watery into the 

 dry fusion and the uncombined water of the hydrate of soda has 

 been driven off. When the whole mass of alkali (generally about 

 four tons) reaches a low red heat, a regular evolution of gas 

 is observed : this is evidently owing to the oxygen produced 

 by the decomposition of the nitrate, and to the nitrogen from 

 the decomposition of the cyanides ; at the same time a plen- 

 tiful liberation of graphite is observed, covering the whole 

 surface of the liquor with a bright layer of graphite. This 

 liberation of graphite is still more plainly seen if no nitre be 

 added to the liquor at first, or only so much as is sufficient to 

 oxidise the sulphur compounds ; but if a few pounds of nitrate 

 of soda be added when the water has been driven off, and 

 the mass is allowed to become red hot, a violent reaction takes 

 place, and a large quantity of graphite is set free. This 

 sudden liberation of graphite proves that this substance cannot 

 be derived from the cast iron of the pot in which the fusion is 

 made. So violent is the evolution of gas, that a complete 

 cloud of fine particles of caustic soda is carried up into the air, 

 rendering it almost impossible to remain in the neighbourhood 

 of the operation. In this way all the cyanogen compounds 



