CARBURETS.
On the supposition that metals combine with charcoal, the appropriate
names for the compounds would be _carburets_ of the respective metals.
This combination, if it exist at all, seems very rare, that with iron
being the only one generally acknowledged. No combinations of carbone
with the earths and alkalies, have, as far as I know, been noticed;
and those with the elements oxygen, hydrogen, sulphur and phosphorus
have been described in the former volume. Since that was printed an
ingenious experimental essay on the “_Sulphuret of carbon_ or alcohol
of sulphur,” has been published by Berzelius and Dr. Marcet. Some
account of this compound, under the name _carburetted sulphur_, has
been given (VOL. 1. page 462); but the additional information is of
sufficient importance to require notice here. The pure liquid is of sp.
gr. 1.272; and the elasticity of its vapour at 66° is equal to 10.76
inches. It burns with a blue flame and sulphureous odour, without
sensibly depositing water on cold glass exposed to the fumes. It has an
acrid, pungent taste, and a nauseous smell, differing from sulphuretted
hydrogen. By various experiments it was found to consist of sulphur and
carbone in the ratio of 85 to 15 nearly; that is, 2 atoms of sulphur to
1 of carbone. From other experiments it did not appear to contain any
hydrogen.
From some experiments I made in June 1818, on the combustion of the
vapour of carburet of sulphur in oxygen gas, I was led to suspect at
least, that an atom of hydrogen attaches to the two of sulphur and one
of carbone in its constitution. But not having an opportunity to pursue
the subject, I merely make the observation for future experience to
determine upon the question.
1. _Carburets of iron._
There are three distinct substances which are now commonly believed to
be constituted of carbone and iron, known by the names of _Plumbago_,
or black lead, _Cast Iron_ and _Steel_.
Plumbago is a natural production, found in greatest perfection in
the Borrowdale mine, near Keswick, Cumberland. It is chiefly used in
making pencils.--It seems to be constituted of carbone and iron by the
concurrent experience of all who have examined it: but the proportions
are not uniform, some having found 10 and others only 5 per cent. of
iron in it. From this circumstance it would seem doubtful whether iron
is an essential element. As carbone is known to be exhibited in various
forms of aggregation, it is not improbable that plumbago may be one of
those forms; it is evidently not a mere mixture of common charcoal and
iron, or its oxide.
_Cast iron_ or crude iron is the metal when first extracted from the
ore; it usually contains carbone, oxygen, phosphorus and silica, in
small proportions, with perhaps other earths occasionally. It cannot
be considered as having these elements united in definite proportions;
for they vary much, and probably give to crude iron its several
modifications. Cast iron contains about 80 per cent. of its weight
of iron in a state capable of solution in dilute sulphuric acid, and
yielding a corresponding quantity of hydrogen gas. The residue, in
a specimen I examined, was nearly as magnetic as iron itself. When
treated with boiling muriatic acid, the insoluble part was reduced to
2½ per cent. upon the original weight of the iron, and some hydrogen
gas given out. It was then about as magnetic as the common black oxide
of iron; when heated it assumed a glowing red and lost nearly ½ grain;
it was still magnetic, and boiling muriatic acid extracted more iron
from it.
The hydrogen gas from dilute sulphuric acid and cast iron contains no
carbonic acid in my experience; neither does it yield any when exploded
with pure oxygen gas.
The small residuum after treating cast iron with acids was found by
Bergman and others to resemble plumbago, being constituted chiefly of
carbone and iron.
From the above it would seem that cast iron consists chiefly of pure
iron, with the addition of very small proportions of oxygen and
carbone; the oxygen may be about 1 per cent. and the carbone about 2.
These proportions, though sufficient to modify the properties of iron
to a certain extent, can scarcely be considered as constituting cast
iron a homogeneous mass.
_Steel._ This most important modification of iron has engaged the
attention of many chemists and metallurgists. It may be procured, but
not equally pure, by different methods. One is to keep the cast iron
for a considerable time in fusion and in a very high degree of heat;
whilst its surface is covered with melted scoriæ, so as to preclude the
contact of the atmosphere with the iron. This, it is conceived, gives
time for the carbone and oxygen to combine and escape in the form of
carbonic acid. This steel is of inferior purity.
Steel of cementation is made by stratifying bars of pure iron with
charcoal powder in large earthen crucibles, carefully closed up with
clay. These are exposed to a high degree of heat in a furnace for 8
or 10 days. This is called _blistered_ steel, from the appearance of
blisters on its surface.
Cast steel is made from blistered steel by breaking the bars and
putting them into a large crucible with pounded glass and charcoal. The
crucible is closed with a lid of the same ware and placed in an air
furnace. When the fusion is complete the metal is cast into ingots.
This is the most valuable and probably the purest steel.
When steel is heated red and plunged into cold water, it is _hardened_;
that is, it becomes much harder than iron or than steel without this
operation. Hardened steel is brittle, and cannot be extended by the
hammer or corroded by a file till it is again softened by being heated
and then gradually cooled.
One of the most remarkable properties of hardened steel is that of
being _tempered_, as it is called; by which it is adapted to the
different purposes of the manufacturing artists. Tempering consists
in heating the hardened steel till it acquires a straw colour for
edge tools, a blue colour for watch springs, and elastic articles in
general; &c. &c.
Hardened steel is qualified to acquire magnetism, and to retain it so
as to become a permanent magnet. This power of retaining magnetism
distinguishes steel from pure iron.
From the above account of steel, it is evident there is an essential
difference between it and pure iron. That difference consists,
according to the common opinion, in steel being a _carburet_ of iron,
or carbone and iron united. The fact of the union of carbone and iron
in the formation of steel does not seem to me satisfactorily proved.
Mr. Collier asserts that iron gains about ¹/₁₈₀th of its weight by
being converted into steel.[18] But Mr. Mushet found that though steel
gains weight upon the iron when copiously imbedded in charcoal, yet it
loses weight if the charcoal is only ¹/₉₀ or ¹/₁₀₀ of the weight of the
iron.[19] The same ingenious gentleman seems to estimate the carbone in
cast steel, from synthetic experiments, to be ¹/₁₀₀th of its weight.
[Footnote 18: Manchester Memoirs, Vol. v. page 120.]
[Footnote 19: Philos. Mag. Vol. xiii.]
From analytic experiments, however, there does not appear reason to
believe that steel contains so much, if any charcoal. Pure steel
dissolved in dilute sulphuric acid gives hydrogen gas containing no
carbonic acid nor oxide, neither is there any appreciable residuum of
any kind in general.
On considering all the circumstances, I am inclined to believe, that
the properties which distinguish steel from iron are rather owing to a
peculiar crystallization or arrangement of the ultimate particles of
iron, than to their combination with carbone or any other substance.
In all cases where steel is formed, the mass is brought into a liquid
form, or nearly approaching to it, a circumstance which allows the
particles to be subject to the law of crystallization. We see that
great change is made in steel by the mere _tempering_ of it, which
cannot be ascribed to the loss or gain of any substance, but to some
modification of the internal arrangement of its particles. Why then
may not its differences from iron be ascribed to the same cause? It is
allowed that steel, by being repeatedly heated and hammered, becomes
iron: that is, it should seem, the change of figure disturbs the
regular arrangement of the particles. And it may be further observed,
in corroboration of the opinion that cast iron is capable of being made
permanently magnetic, from its having been in fusion more probably than
from its near approximation to steel in its component parts. The most
powerful artificial magnets, after being forged of steel, are said to
undergo the operation of steelifying again, before they are hardened
finally to receive the magnetic virtue.