212 
Iron , 
sumes a similar form ; but has a black ground colour i 
mixed with browns, or is entirely black. When the latter 
colour prevails, the texture of the cinder becomes porous ; 
the quantity of iron left, is now very considerable, and 
such as will be easily extracted in the assay-furnace, with 
proper fluxes. In cases of total derangement in the furnace, 
the scoria will still retain this black colour, although the 
quantity ol metal may amount to 25 per cent : the frac- 
ture, however, becomes dense, and its specific gravity in- 
creases in proportion to the quantity of metal it holds in- 
corporated. 
The next source of information, as to the quality of the 
iron in the furnace, is to be got from die colour of the 
scoria upon the working bars, which are from time to time ; 
inserted to keep the furnace free from lumps, and to bring 
forward the scoria. When super-carbonated crude iron is 
in the hearth, the vitrid crust upon the bars will be of a 
black colour and smooth surface, fully covered with large 
plates and brilliant of plumbago. 
As the quality of the metal approaches to No. 2. (carbo- 
nated) the carburet upon the scoria decreases both in point 
of quantity and size. 
When carbo-oxygenated iron (No. 3.) is in the furnace, 
the working bars are always coated with a lighter co- 
loured scoria than when the former varieties exist ; a 
speck of plumbago is now only found here and there, and 
that of the smallest size. When the quality of the metal 
is oxygenated (No. 4.) not only have the plates of carbu- 
ret disappeared, but also the coally colour on the external 
surface of the scoria ; what now attaches to the bars, is 
nearly of the same nature and colour as the lava emitted 
at the notch of the dam. 
These criteria are infallible ; for, as the fusibility or 
earbonation of the metal is promoted in a direct ratio to 
the comparative quantity of the coally principle present 
