Iron . 
199 
Besides the proportion of mixtures, which chiefly contri- 
bute to the fusibility of iron-stones, a second degree of fu- 
sibility is dependent upon the richness of the ore in iron : 
this is so obvious in the use of the Cumberland and Lan- 
I cashire ores, that the consequences of their introduction 
will be perceived, by the change of the scoria and metal, 
in half the time that change would be effected by ordinary 
iron-stones. It has been frequently noticed, that crude 
iron contained pure carbon in proportion to its fusibility ; 
then the more fusible, or supercarbonated qualities, must 
take up, comparatively, a considerable portion of the car- 
bonaceous principle from the fueh From this results a 
striking consequence, that the quantity of fuel should, 
over and above its relation to the mixtures, bear a just 
|| proportion to the quantity of iron in the stone : for ex- 
ample, let the weight per charge of fuel at a blast-furnace 
be 400 lb., and let this be supposed sufficiently to fuse 
and carbonate the iron contained in 360 lb. of iron-stone ; 
let the quantity of metal be supposed 35 per cent, then 
the produce will be 126 lb. Should a change take place, 
and iron-stone richer in iron be applied, though the same 
i by weight, and should this iron-stone yield of torrefied 
I stone 45 per cent, its produce will be 162 lb. or 40 lb. 
more than the former. As there exists no greater propor- 
tion of carbon in the furnace, it is evident that the existing 
quantity, being distributed over nearly l-3d of more metal, 
j! must therefore be in more sparing quantity in the whole, 
and the value of the metal consequently reduced. 
3. The weight of oxygen contained in iron-stones is 
the next object of serious consideration. I have already 
shewn, from experiment, that our iron-stones naturally 
contain from 9 to 14 per cent, of oxygen, which remains 
after torrefaction : it has also been shewn, that this quan- 
tity of hurtul mixture may easily be doubled by over- 
roasting or under-roasting the stone ; and that the bad ef- 
