208 Iron . 
pie is absent, the surface of the iron acquires roughness 
and asperity* 
It may perhaps be proper here to mention, once for all, 
that although, for convenience, the manufacturer has, from 
a just estimation of the value of the metal in a subsequent 
manufacture, affixed certain numbers for determinate qua* 
lities of iron, yet it is difficult to say at what degree of sa- 
turation of carbon each respective term commences 2 suf- 
fice it then to say, that the two alterative principles, oxy- 
gen and carbon, form two distinct classes, that in which 
oxygen predominates— and that in which carbon predomi- 
nates ; the latter comprehends No* 1 and 2 of the manu- 
facturers, the former includes oxygenated, white and mot- 
tled ; and the equalization of these mixtures form, as has s 
already been noticed, the variety of carbo- oxygenated 1 
crude iron. 
I shall now observe some things relative to the various 
faces which crude iron assumes. No. 1 and 2, with their 
intermediate qualities, possess surfaces more or less con- 
vex, and frequently with thin blisters: this we attribute 
to the presence of carbon, which being plentifully inter- 
spersed betwixt and throughout the particles of the metal, 
the tendency which the iron has to shrink in cooling is en- 
tirely done away ; it tends to distend the aggregate of the 
mass, and to give a round face, by gradually elevating the 
central parts of the surface, which are always last to lose 
their fluidity. 
Again, that quality of iron known by the name of No. 3, 
or carbo- oxygenated, or with about half its full dose of 
carbon, is most commonly found with a flat surface. If 
we still farther trace the appearance of the surface of pig 
iron, when run from the furnace, we shall find No* 4 either 
with a white or mottled fracture, possessed of concave fa- 
ces, rough and deeply pitted. Beyond this it may be ima- 
gined that every degree of further oxygenation would be 
