Iron . 
sary additional proportions of fuel, capable of correcting 
the quality of silicious iron-stones. Not so, however, to 
those who may at some future period succeed him ; ne- 
cessity * at all times ingenious, assisted by the increasing 
light which science daily sheds over our manufactures, 
will devise the means of calling into profitable existence 
the metal contained in all those ores which mav have fal- 
len into disrepute in the present day, or from which at this 
time it is thought impracticable to extract metal in the 
large way. 
The usual criterions by which iron-stone is judged, 
whether it be sufficiently rich in iron for the purpose of 
smelting, are the following : 
1. The degree of tenacity with which it adheres to the 
tongue after torrefaction ; 2. Its colour : 3. The obedi- 
ence to the magnet when pulverised : 4. By depriving of 
its iron a given weight of the ore, in contact with charcoal 
and fusible earths in the assay-furnace. 
The first and third of these methods are liable to great 
error. The adhesion to the tongue will be more in pro- 
portion to the quantity of its clay and its kind contained in 
the stone, than to its real contents in iron. Iron-stone 
may also be torrefied in such a manner as to deprive its 
internal surface of this property ; as it is only peculiar to 
the stone at a certain stage of torrefaction. 
Those iron-stones with which Britain so much abounds, 
and which are now universally used for the production of 
cast-iron with pit-coal, are commonly found in horizontal 
strata, subject to the same acclivity and declivity with 
the other stratified substances under the surface ; their 
inclination from the horizon varying according to the na- 
ture of the ground, and the disposition of the imbedding 
and incumbent strata. Such variety is exhibited, that 
strata of iron-stone are found descending 1 yard in 24, 1 
m 12, 1 in 8, and sometimes 1 in 4, Where the accib 
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