408 
Cast Steel 1 
by slow cooling while it is still fluid, a part of this excess 
of carbon will separate from the rest of the mass and rise 
to the surface, forming a flasky crust of plumbago or car- 
buret of iron. Some however of the plumbago will still 
remain enveloped by and disperseel through the iron, giv- 
ing it an uniform dark grey or black colour if the propor- 
tion is considerable, or only mottling it if the excess of 
this substance is very slight. 
The actual proportions of carbon contained in the dif- 
ferent kinds of iron have not yet been ascertained with any 
accuracy either by analysis or synthesis ; but thus much 
appears certain, that bar iron in general contains a smaller 
quantity than the softer varieties of steel, and these again 
always contain less carbon than the common and finer 
cast steel ; that in the white, the mottled, the grey, and 
the black varieties of cast iron, the dose of carbon is con- 
stantly augmenting, in the last of which the proportion of 
carbon is probably about ^ t of the whole. 
Oxygen is also contained in most of the varieties 
of iron, and the effects occasioned by it in the different pro- 
cesses to which this metal is subjected, require more atten- 
tion than has hitherto been paid to them. Cast iron ap- 
pears to be highly charged with oxygen, and on this ac* 
count requires to be supersaturated with carbon in order 
to be converted with any economy into bar iron. It may 
seem at first a paradox to maintain the co-existence of oxy- 
gen and carbon in the same metallic mass, especially con- 
sidering the great heat to which it is exposed in the process 
of reduction, since it is an universal and uncontroverted 
fact, that metallic oxyds are decomposed by carbon at a 
high temperature, the oxygen and carbon uniting together 
and being dissipated in the form of gas, the metallic regu- 
lus remaining behind. But when the roughness of the 
smelting process in blast furnaces, and the large quanti- 
ty ox materials operated on at once, as well as the great 
