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veniently have another furnace of the same kind for the 
conversion of massicot into minium or red lead. 
The yellow sediment is now carried to the hearth of 
the furnace : the fires on each side are lighted, and kept 
up as before. As the coaks or cinders form, they are raked 
out and fresh coals put in. The yellow oxyd is now stir- 
red only often enough to keep it in a separate and pow- 
dery state, and very gradually to expose fresh surfaces to 
the air. In from 40 to 48 hours, the yellow colour gra- 
dually disappears and is converted into a deep brownish 
red. (This is in my opinion the puce coloured per-oxyd 
of lead.) But when cold, this brownish red, passes into 
the bright red of the minium or red lead of commerce. 
When taken out of the furnace, it is spread over the surface 
of a large box to cool. It is then passed through a sieve 
in a close vessel, to prevent loss, and collected for sale. 
The coal used should be but moderately bituminous, 
evidently because the carbonaceous smoke retards the 
oxydation. The coaking, Newcastle coal, is usually pre- 
ferred in England. They are persuaded that red lead 
cannot be made with wood. It may be more difficult, 
partly on account of the smoke, and partly on account of 
the acid vapour of wood unless it be very dry. Certainly 
red lead cannot be made by applying the heat externally : 
it must be applied to the surface of the lead. 
I shall give a plate of the red lead furnace in my next. 
The litharge of commerce is the refuse of the refining 
furnaces, although it can be made also in the red lead fur- 
naces* The lumps of lead that cannot easily be convert- 
ed into oxyd, might as I think be advantageously thrown 
aside, for refining ; the silver, if they contained it, would 
doubtless be an obstacle to the oxydation. Litharge, seems 
that state of lead, best adapted to the decomposition of 
alkaline neutrals ; but I do not know that Dr. Thomp- 
son’s opinion has been established, that it is in any degree 
