and the Operations of the Blaft-Furnace. ‘131 
is the confequence of the extreme abfence of carbon; and 
that, in proportion as this’principle is abfent, the furface of 
_ the iron acquires roughnefs and afperity. 
It may perhaps be proper here to mention, once for all: 
that although, for convenience, the manufacturer has, from 
a juft eftimation of the value of the metal in a fubfequent 
manufacture, afixed certain numbers for determinate quali- 
ties of iron, yet it is dificult to fay at what degree of fatu- 
‘ration of carbon each refpective term commences: fuffice 
it then to fay, that the two alterative principles, oxygen and 
earbon, form two diftinct clafles, that in which oxygen pre- 
dominates, and that in which carbon predominates ; the 
latter comprehends No. 1 and 2 of the manufacturers, the 
former includes oxygenated, white and mottled; and the 
equalifation of thefe mixtures form, as has already been no- 
ticed, the variety of carbo-oxygenated crude iron. 
I fhall now obferve fome things relative to the various 
faces whicly crude iron aflaumes. .No,1 and 2, with their 
intermediate qualities, poffefs furfaces more or ‘Jefs convex, 
and frequently with thin blifters: this we attribute to the. 
prefence of carbon, which being plentifully interfperfed bes 
‘twixt and throughout the particles of the metal, the ten- 
dency which the iron has to fhrink in cooling is entirely 
done away; it tends to diftend the aggregate of the mafs, 
_ and to give around face, by gradually elevating the central 
"parts oF: the furface, which are always lalt to lofe their flat 
__ idity. 
Again, that quality of iron known by the name of No. 3, 
or carbo-oxygenated, is mo{t commonly found with a flat 
furface. If we fiill farther trace the appearance of the fur- 
face of pig-iron, when run from the furnace, we {hall find 
No. 4, either with a white or mottled fracture, poflefied of 
concave faces rough end deeply pitted. Beyond this it may 
_ be imagined that every degree of further oxygenation would 
be productive of a furface deeper in the curve, and rougher, 
with additional afperities. The contrary is the cafe: when 
» crude iron is fo far debafed as to be run from the furnace in 
clotted Jumps highly oxygenated, the furface of the’ pigs is 
found to be more convex than that of No.1 iron 3) but then 
HedKi: a the 
