_Of Iron._
Iron is a metal of a bluish colour, of the greatest hardness, the most
variable in its properties, and the most useful of all the metals; so
that without it it is hardly possible for any people to make great
advances in arts and civilization.
This metal readily parts with its phlogiston, so as to be very subject
to calcine, or rust, by exposure to the air. The same is evident by the
colours which appear on its surface when exposed to heat, and also when
it is struck with flint; the particles that fly from it being iron
partially calcined. In consequence of its readily parting with its
phlogiston, it is capable of burning, like wood or other fuel, in pure
air.
Iron and platina have the property of _welding_ when very hot, so that
two pieces may be joined without any solder.
When iron is heated in contact with steam, part of the water takes the
place of the phlogiston, while the rest unites with it, and makes
inflammable air. By this means the iron acquires one third more weight,
and becomes what is called _finery cinder_. This substance, heated in
inflammable air, imbibes it, parts with its water, and becomes perfect
iron again. If the iron be heated in pure air, it also imbibes the
water, of which that air chiefly consists, and also a portion of the
peculiar element of the pure air.
The solution of iron in spirit of vitriol produces _green copperas_;
which being calcined, becomes a red substance, called _colcothar_.
The precipitate of iron, by an infusion of galls, is the colouring
matter in _ink_, which is kept suspended by means of gum. The
precipitate from the same solution by phlogisticated alkali, is
_Prussian blue_.
Water saturated with fixed air dissolves iron, and makes a pleasant
chalybeat.
The calx of iron gives a green colour to glass.
Iron readily combines with sulphur. When they are found combined by
nature, the substance is called _pyrites_.
The union of phosphoric acid with iron makes it brittle when cold,
commonly called _cold short_; and the union of arsenic makes it brittle
when hot, thence called _red short_.
Iron unites with gold, silver, and platina, and plunged in a white heat
into mercury, it becomes coated with it; and if the process be
frequently repeated, it will become brittle, which shews that there is
some mutual action between them.
Iron readily unites with tin; and by dipping thin plates of iron into
melted tin, they get a complete coating of it, and make the _tinned
plates_ in common use.
When crude iron comes from the smelting furnace it is brittle; and when
it is white within, it is extremely hard; but when it has a black grain,
owing to its having more phlogiston, it is softer, and may be filed and
bored.
Cast iron becomes _malleable_ by being exposed to a blast of air when
nearly melting; the consequence of which is a discharge of inflammable
air, and the separation of a liquid substance, which, when concreted, is
called _finery cinder_. The iron generally loses one fourth of its
weight in the process. Crude iron contains much _plumbago_, and the
access of pure air probably assists in discharging it, by converting it
into air, chiefly inflammable.
Malleable iron, exposed to a red heat in contact with charcoal, called
_cementation_, converts it into _steel_, which has the properties of
becoming much harder than iron, and very elastic, by being first made
very hot, and then suddenly cooled, by plunging it in cold water. By
first making it very hard, and then giving different degrees of heat,
and cooling it in those different degrees, it is capable of a great
variety of _tempers_, proper for different uses. Of the degrees of heat
workmen judge by the change of colour on its surface. Steel, like crude
iron, is capable of being melted without losing its properties. It is
then called _cast steel_, and is of a more uniform texture. Iron
acquires some little weight by being converted into steel; and when
dissolved in acid, it yields more plumbago. Steel has something less
specific gravity than iron. If the cementation be continued too long,
the steel acquires a darkish fracture, it is more fusible, and incapable
of welding. Steel heated in contact with earthy matters, is reduced to
iron.
Iron is the only substance capable of _magnetism_; and hardened steel
alone is capable of retaining magnetism. The loadstone is an ore of
iron.