METALLURGY. 137 
that the gold sand carried by the water from one mill may be conveyed to 
the other. Two such machines are seen together in the drawing. a@ is the 
mill-basin secured to the frame, A, and having a hollow tube in its centre 
through which the shaft, d, driven by the wheel, g, of the runner, turns. 
Upon the top of the shaft, d, is a cross-bar, 6, which is connected with the 
runner by two arms; in the centre of the runner is a funnel-formed opening 
which receives the stream of water and sand, which in its passage beneath 
the runner is brought into contact with the quicksilver at the bottom of 
the basin a, which is also kept in motion by iron plates upon the bottom of 
the runner, and which amalgamates with a portion of the gold, the sand 
and water passing on to a similar mill where the process is continued. 
5. Worxine Iron. 
No metal is of so great importance to man as iron; on this account we 
have selected it to give a rather detailed description of its manufacture. As 
it leaves the furnace after the operation of smelting, it is known as raw-iron 
or pig-iron, of which two kinds are distinguished, white-iron and grey; the 
former has a silver white color, and is used in the manufacture of steel ; the 
latter is of every shade, from black to light grey. Wrought-iron is of a 
light grey fracture, running partly into white, partly into gray. Steel hasa 
greyish white fracture, is harder than iron, and is worked with more difficulty. 
We have already spoken of the furnaces made use of in extracting iron 
from the ore, and will now say a few words upon the process itself. The ore 
having been broken into small pieces and roasted, is ready for the smelting 
process, which reduces the oxygen and separates the compounds of silicic 
acid in the form of slag. Charcoal, stone coal, or turf, may be used accord- 
ing as they are to be obtained to advantage. 
The furnace is first slowly heated, to prevent it from cracking, and is then 
charged lightly with coal and ore alternately. When the first traces of 
metal show themselves the crucible is cleaned, the tap hole closed, and the 
blast is let on, lightly at first, and gradually increased for five or six days, 
when it has its full power. The labors of the blast-furnace then consist in 
renewing the supply of ore and coal from time to time, and keeping the fur- 
nace free of slag. 
When sufficient metal has collected in the crucible, the tap hole is 
opened and it is suffered to run off; in blast-furnaces with open breast this 
takes place every twelve, eighteen, or twenty-four hours; the tap hole is 
then cleaned out and again closed. 
Within the last twenty years many experiments have been made with 
hot-blast-furnaces ; in these the blast is heated before it is supplied to the 
furnace. Many methods have been adopted for effecting this purpose ; 
sometimes the air is heated in separate furnaces, at others the waste heat 
from the furnace itself is employed; generally it is made to pass through 
heated cast-iron pipes, the convolutions of which are surrounded by fire ; at 
others it is accomplished in air-tight chambers (pl. 28, jig. 11). 
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