194 
Iron . 
From the foregoing particulars upon coal may be learn- 
ed how much is dependent upon the native construction 
of coal and its constituent parts ; I shall next advert to 
the effects produced by its improper preparation. 
When coals intended for the blast furnace are suffi- 
ciently charred, they ought, in point of colour, to be of a 
silver grey ; their fracture will appear lamellated and po- 
rous if splint coals have been used ; softer coals form 
themselves into branches slightly curved* and, when pro- 
perly prepared, are always very porous. 1 have frequent- 
ly found that the better the cokes were charred, the more 
water they will absorb. Coals half burnt do not take up 
half so much water, because their fracture continues in 
part to be smooth and less porous than when thoroughly 
burnt. 
When half prepared cokes are introduced into the fur- 
nace, the metal formerly carbonated will lose its grey frac- 
ture, and approach to the quality of oxygenated iron. 
Their presence is easily detected by the unusual quantity 
of thick vapour arising along with the flame. Besides, 
the water and sulphur, which raw coals introduce into 
the furnace, and which always impair the quantity of car- 
bon by the various solutions effected by the presence of 
o , ygen, hydrogen, &c. the fitness of the coal for com- 
bustion, and the support of the ore, is much diminished by 
this second course of ignition and disengagement of bitu- 
men. The pressure of the incumbent ores also, fracture 
and reduce the cokes into small pieces, which produce a 
considerable portion of coke-dust ; this is partly carried 
to the top of the furnace before the blast ; sometimes be- 
low it appears in immense quantities, ignited to whiteness, 
and liquid as sand. Coal thus detached from the mass, 
exposed to the action of a compressed current of air, is unfit 
for conveying the carbonic principle to the metal ; and as 
it frequently belongs to the just proportion of charcoal 
