_Of_ IRON.
Iron is lighter and less ductile than Copper; but it is much harder,
and of more difficult fusion.
It is the only body that has the property of being attracted by the
magnet, which therefore serves to discover it wherever it is. But it
must be observed, that it hath this property only when in its metalline
state, and loses it when converted to an earth or calx. Hence very few
Iron-ores are attracted by the load-stone: because, for the most part,
they are only sorts of earths, which require a phlogiston to be added
before they can be brought to the form of true Iron.
When Iron hath undergone no other preparation but the fusion which is
necessary to smelt it from its ore, it is usually quite brittle, and
flies to pieces under the hammer: which arises in some measure from its
containing a certain portion of unmetallic earth interposed between its
parts. This we call _Pig Iron_.
By melting this a second time it is rendered purer, and more free from
heterogeneous matters: but still, as its proper parts are probably not
brought sufficiently near, or closely enough united, till the Iron hath
undergone some further preparation besides that of fusion, it seldom
hath any degree of malleability.
The way to give it this property is to make it just red-hot, and then
hammer it for some time in all directions; to the end that its parts
may be properly united, incorporated, and welded together, and that
the heterogeneous matters which keep them asunder may be separated.
Iron made by this means as malleable as possible we call _Bar Iron_, or
_Forged Iron_.
Bar Iron is still harder to fuse than Pig Iron: to make it flow
requires the utmost force of fire.
Iron has the property of imbibing a greater quantity of phlogiston
than is necessary to give it the metalline form. It may be made to
take in this super-abundant phlogiston two ways: the first is by fusing
it again with matters that contain the phlogiston; the second is, by
encompassing it with a quantity of such matters, charcoal-dust, for
instance, and then exposing it so encompassed, for a certain time, to
a degree of fire barely sufficient to keep it red-hot. This second
method, whereby one substance is incorporated with another by means
of fire, but without fusing either of them, is in general called
_Cementation_.
Iron thus impregnated with an additional quantity of phlogiston is
called _Steel_. The hardness of Steel may be considerably augmented by
_tempering_ it; that is, by making it red-hot, and suddenly quenching
it in some cold liquor. The hotter the metal, and the colder the
liquor in which it is quenched, the harder will the Steel be. By this
means tools are made, such as files and sheers, capable of cutting
and dividing the hardest bodies, as glass, pebbles, and Iron itself.
The colour of Steel is darker than that of Iron, and the facets which
appear on breaking it are smaller. It is also less ductile and more
brittle, especially when tempered.
As Iron may be impregnated with an additional quantity of phlogiston,
and thereby converted into Steel, so may Steel be again deprived
of that super-abundant phlogiston, and brought back to the condition
of Iron. This is effected by cementing it with poor earths, such
as calcined bones and chalk. By the same operation Steel may be
_untempered_; nay, it will lose the hardness it had acquired by
tempering, if it be but made red-hot, and left to cool gradually. As
Iron and Steel differ only in the respects we have here taken notice
of, their properties being in all other respects the same, what follows
is equally applicable to both.
Iron being exposed to the action of fire for some time, especially when
divided into small particles, such as filings, is calcined and loses
its phlogiston. By this means it turns to a kind of reddish yellow
earth, which, on account of its colour, is called _Crocus Martis_, or
_Saffron of Mars_.
This calx of Iron has the singular property of flowing in the fire
with somewhat less difficulty than Iron itself; whereas every other
metalline calx flows with less ease than the metal that produced it. It
has moreover the remarkable property of uniting with the phlogiston,
and of being reduced to Iron without fusion; requiring for that purpose
only to be made red-hot.
Iron may be incorporated with Silver, and even with Gold, by means of
certain operations. Under the article of Lead we shall see how it may
be separated from these metals.
The acids produce on it much the same effects as on Copper; every one
of them acts upon it. Certain neutral salts, alkalis, and even water
itself, are capable of dissolving it; and hence it is also very subject
to rust. The vitriolic acid dissolves it with the greater ease: but
the circumstances which attend the solution thereof are different from
those with which the same Acid dissolves Copper: for, 1. whereas the
vitriolic acid must be concentrated to dissolve Copper, it must on
the contrary be diluted with water to dissolve Iron, which it will
not touch when well dephlegmated. 2. The vapours which rise in this
dissolution are inflammable; so that if it be made in a small-necked
bottle, and the flame of a candle be applied to the mouth thereof, the
vapours in the bottle take fire with such rapidity as to produce a
considerable explosion.
This solution is of a beautiful green colour; and from this union of
the vitriolic acid with Iron there results a neutral metalline salt,
which has the property of shooting into crystals of a rhomboidal
figure, and a green colour. These crystals are called _Green Vitriol_,
_Vitriol of Mars_, and _Copperas_.
Green Vitriol hath a saltish and astringent taste. As it retains a
great deal of water in crystallizing, it quickly flows by the action
of fire: but this fluidity is owing to its water only, and is not a
real fusion; for, as soon as its moisture is evaporated, it resumes a
solid form. Its green transparent colour is now changed into an opaque
white: and, if the calcination be continued, its acid also exhales and
is dissipated in vapours; and as it loses that, it turns gradually to
a yellow colour, which comes so much the nearer to a red the longer
the calcination is continued, or the higher the force of the fire is
raised; which being driven to the utmost, what remains is of a very
deep red. This remainder is nothing but the body of the Iron, which
having lost its phlogiston is now no more than an earth, nearly of the
same nature with that which is left after calcining the metal itself.
Green Vitriol dissolved in water spontaneously lets fall a yellowish
earthy sediment. If this solution be defecated by filtration, it still
continues to deposite some of the same substance, till the vitriol be
wholly decomposed. This sediment is nothing but the earth of Iron,
which is then called _Ochre_.
The nitrous acid dissolves Iron with great ease. This solution is of a
yellow colour, inclining more or less to a russet, or dark-brown, as it
is more or less saturated with Iron. Iron dissolved by this acid, also,
falls spontaneously in a kind of calx, which is incapable of being
dissolved a second time; for the nitrous acid will not act upon Iron
that has lost its phlogiston. This solution does not crystallize, and
if evaporated to dryness attracts the moisture of the air.
Spirit of salt likewise dissolves Iron, and this solution is green. The
vapours which rise during the dissolution are inflammable, like those
which ascend when this metal is attacked by the vitriolic acid. _Aqua
regis_ makes a solution of Iron, which is of a yellow colour.
Iron hath a greater affinity than either Silver or Copper with the
nitrous and vitriolic acids: so that if iron be presented to a solution
of either in one of these two acids, the dissolved metal will be
precipitated; because the acid quits it for the Iron, with which it has
a greater affinity.
On this occasion it must be observed, that if a solution of Copper in
the vitriolic acid be precipitated by means of Iron, the precipitate
has the form and splendour of a metal, and does not require the
addition of a phlogiston to reduce it to true Copper; which is not the
case, as has been shewn, when the precipitation is effected by earths
or alkaline salts.
The colour of this metalline precipitate hath deceived several persons,
who being unacquainted with such phenomena, and with the nature of blue
vitriol, imagined that Iron was transmuted into Copper, when they saw
a bit of Iron laid in a solution of that vitriol become, in form and
external appearance, exactly like Copper: whereas the surface only of
the Iron was crusted over with the particles of Copper contained in the
vitriol, which had gradually fallen upon and adhered to the Iron, as
they were precipitated out of the solution.
Among the solvents of Iron we mentioned fixed alkalis; and that they
have such a power is proved by the following phenomenon. If a large
proportion of alkaline salts be suddenly mixed with a solution of Iron
in an acid, no precipitation ensues, and the liquor remains clear and
pellucid; or if at first it look a little turbid, that appearance lasts
but a moment, and the liquor presently recovers its transparency.
The reason is, that the quantity of alkali is more than sufficient
to saturate all the acid of the solution, and the super-abundant
portion thereof, meeting with the Iron already finely divided by the
acid, dissolves it with ease as fast as it falls, and so prevents
its muddying the liquor. To evince that this is so in fact, let the
alkali be applied in a quantity that is not sufficient, or but barely
sufficient, to saturate the acid, and the Iron will then precipitate
like any other metal.
Water also acts upon Iron; and therefore Iron exposed to moisture grows
rusty. If iron-filings be exposed to the dew, they turn wholly to a
rust, which is called _Crocus Martis Aperiens_.
Iron exposed to the fire together with nitre makes it detonate pretty
briskly, sets it in a flame, and decomposes it with rapidity.
This metal hath a greater affinity than any other metalline substance
with sulphur; on which account it is successfully used to precipitate
and separate all metalline substances combined with sulphur.
Sulphur uniting with Iron communicates to it such a degree of
fusibility, that if a mass of this metal heated red-hot be rubbed with
a bit of sulphur, it incessantly runs into as perfect a fusion as a
metal exposed to the focus of a large burning-glass.