98 
4. Sulphide of iron held in solution by the sulphide of 
sodium. 
5. Chloride of sodium. 
G. 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 
