_Of Ores_.
Metallizable earths, commonly called _ores_, when united to phlogiston,
make the metals, distinguishable for their specific gravity, their
opacity, shining appearance, and fusibility.
All the proper metals are _malleable_, and those which are not so are
called _semi-metals_.
The metals again are subdivided into the _perfect_ and _imperfect_. The
former, which are _gold_, _silver_, and _platina_, suffer no change by
fusion, or the longest continued heat: whereas heat calcines or
dissipates the phlogiston of the imperfect metals, which are _mercury_,
_lead_, _copper_, _iron_, and _tin_, so that they return to the state of
earth; and this earth is always heavier than the metal, though of less
specific gravity, having received an addition of weight from water or
air: but these earths, or ores, being exposed to heat in contact with
substances containing phlogiston, again become metals, and are then said
to be _revived_.
The semi-metals are _bismuth_, _zinc_, _nickel_, regulus of _arsenic_,
of _cobalt_, of _antimony_, of _manganese_, of _wolfram_, and of
_molybdena_.
All metallic substances are crystallizable, and each in a peculiar form,
which is discovered by leaving a hole in the bottom of the crucible in
which they are melted, and drawing out the stopper, when the mass is
beginning to lose its fluidity.
Some of the metals will not unite to others when hot, and others of them
will; and such as will unite with others are called _solders_. Thus tin
is a solder for lead, and brass, gold, or silver, for iron.
Ores are never found in regular strata, like the different kinds of
earth; but in places which have formerly been cavities, running in all
directions, with respect to the regular strata, and commonly called
_veins_.
Many of the ores in their natural state are said to be _mineralized_
with arsenic or sulphur, those substances being intimately united with
the metallic earths.
In order to convert the ores into metals, some of them are first reduced
to powder, to wash out the earthy or saline particles. They are then
kept in a red heat, which the workmen call _roasting_, in order to drive
away the arsenic, or sulphur, which are volatile; and in the last place
they are fused in contact with charcoal, or other substances containing
phlogiston; and to promote the fusion, lime-stone is frequently mixed
with them. When the operation is completed, the unmetallic parts are
converted into glass, or _scoria_, which lies on the surface, whereas
the metal is found at the bottom.
To discover the quantity of metal in a small piece of ore is called
_assaying_.
When metals are fused together, the specific gravity, fusibility, and
other properties are changed, and in such a manner as could not be
discovered from the properties of the constituent parts.
_Of Gold._
Gold is the heaviest of all metallic bodies except platina. It appears
yellow or reddish by reflected light, but green or blue by transmitted
light, when it is reduced to thin plates.
Though gold undergoes no change in a common furnace, or burning lens, it
may, in part, at least, be calcined by the electric shock.
Gold has the greatest _ductility_, and in wires of equal diameters, it
has the greatest _tenacity_, of all the metals. One grain of it may be
made to cover 56 square inches; some gold leaf being less than a
200,000th part of an inch thick; and when it is made to cover a silver
wire, the gold upon it may not be more than one twelfth part of the
thickness of the gold leaf.
This metal is soluble in aqua regia; and being precipitated by a
volatile alkali, makes a powder called _aurum fulminans_, which is one
fourth heavier than the gold, and explodes with great violence in a heat
something greater than that of boiling water.
Tin precipitates gold in the form of a purple powder, called the _powder
of Cassius_, from the inventor of it, and is used in enamels, or the
glassy coating which is given to metals by heat.
Gold unites with most of the metals, especially with mercury, and these
mixtures are called _amalgams_. In gilding, the amalgam is applied to
the surface of the metal to be gilded, and the mercury is driven off by
heat, leaving the gold attached to the surface.
Gold mixed with iron, makes it harder, for the purpose of cutting
instruments.
To separate gold from the imperfect metals, such as copper, &c. it is
mixed with lead, and then exposed to a strong heat, which calcines the
lead, and with it the imperfect metals, leaving the gold pure. This
process is called _cupellation_, from being performed in a small
crucible called a _cupell_. When the gold is mixed with silver, three
parts more of silver are put to it, and then the silver is dissolved by
nitrous acid, leaving the gold pure. This process is called
_quartation_, from the gold being one fourth part of the mass.
The fineness of gold is generally estimated by dividing the gold into
twenty-four parts, called _carats_. The phrase twenty-three carats fine
means that the mass contains twenty-three parts out of twenty-four of
pure gold, the remainder being _alloy_, of some baser metal. The
fineness of gold may in some measure be discovered by the colour it
leaves upon a _touch-stone_, or fine-grained basaltes.
Gold is generally found nearly pure, but mixed with earth, or diffused
in fine grains through stones.