IRON MANUFACTURE. 553 
the reduction of the iron ore by the carbonic oxide of the gas, and the 
preventing of the use of solid carbon in the upper part of the furnace, 
instead of at the tuyeres; but as the sensible heat of the furnace is almost 
entirely derived from the combustion of carbon to carbonic oxide at the 
tuyeres, it is obvious that what causes in all cases the greatest use of 
fuel is the necessity of a large heat production there from having in 
the hearth an excessive amount of material to be heated ; in other words, 
every addition to the amount of slag produced in the furnace calls for 
a large addition to the amount of fuel; carbon burned to carbonic acid 
furnishes 8,000 heat units; whereas, burned to carbonic oxide, as at the 
tuyeres, it merely furnishes 2,400 heat units, or less than one-third the 
former amount. 
The increase of the amount of slag from one to two tons to the ton 
of iron would require on that account alone the increase of the fuel 
from 1.2 to 1.4 tons to the ton of iron, for this extra ton of slag with- 
draws 500 to 600 units of heat. The fuel necessary to furnish this heat 
at the tuyeres would be 600-2,400, or 3 ton; if this be calculated as raw 
coal it means about halfaton. The real increase is even greater than 
this, as the added fuel means increased blast and gas also. 
It seems as though this fact would largely determine the economy 
of every endeavor to enrich the mean composition of the ore charges. 
That this theoretical deduction is not without its practical value, is 
shown by the fact that most of the large furnaces are giving up the use 
of lean ores where rich mixtures even at higher prices can be obtained. 
When a furnace, by using Lake ore and coke, can make a ton of iron 
with but little more than a ton of coke, it becomes a simple matter of 
calculation as to whether it will pay to replace the rich ore by a low 
grade cheap one. The increased amount of coal is not the only evil, 
but as the capacity of a furnace is not really measured in the iron it 
makes but in the fuel it burns; the increase in fuel used is really a 
diminution in the production of the furnace, and the consequent in- 
crease in cost of the iron by the proportionate amount of the total fixed 
expense of the works. It is the want of consideration of these matters 
which has led to the erection and subsequent abandonment of more 
than one large and exy-ensive furnace after treating poor ores. 
It is the same fact that makes all methods by which the mean rich- 
ness of the ore can be increased so valuable as an adjunct to iron smelt- 
