366 J. M. Ordway on making Caustic Soda. 
dations, till all the free water is expelled. Soon after the com- 
mencement of the evaporation, saline matter begins to fall 
consisting of sulphate, sulphite, and carbonate of soda, and this 
deposit—which should be dipped out as it accumulates at the 
bottom—continues to form as long as the solution remains af 
seldom takes more than one per cent of nitrate to destroy any 
remaining sulphid. When the melted soda has become quiet 
and a little of it poured out on a cold iron plate, shows by imme- 
diate solidification and its white color that all is right, the whole 
is laded out into iron molds, from which it is transferred as soon 
as cold to air-tight casks. When the operations have been con- 
ducted with ordinary care, the product is white or a little grey- 
ish, and is pure enough for all common uses. Bee 
The main feature of this process is an oxydizing operation, 12 
principle not very unlike the roasting of pyritous ores. But the 
sulphurets of the heavy metals can be roasted, per sé, while sul- 
phuretted hydrate of soda, it being by itself fusible at the requi- 
site heat, has to be mixed with an inert substance to reduce it to 
the dry and porous state, and so render its particles entirely and 
speedily accessible to atmospheric oxygen. And this is why the 
u 
separating its particles; and as the same oxyd will serve over and 
over again indefinitely, we need only —— to last till the 
lots used are exhausted of alkali and drain 3 f 
used again. ere is no need of drying it. Venetian red * a 
_ god form of the oxyd, but I have always used a pure earthy 
hematite that had first been calcined and ground. It is quite 
ese 
well, and probably a mixture of the two oxyds would do better 
___ It may not be amiss to mention that hemispherical castarom 
aaa Scr feet four inches in diameter and half an inch thick 
. at the sides and three quarters at the bottom, have been found 
trial to be very suitable both for roasting and for melting. 
a kettle 600 po of caustic soda may 1 finished off 
1 cause a loss of time. 
