_Of_ COPPER.
Of all the imperfect metals Copper comes the nearest to Gold and
Silver. Its natural colour is a deep-red yellow. It resists a very
violent degree of fire for a considerable time; but losing its
phlogiston at last, it changes its metalline form for that of a calx,
or a pure reddish earth. This calx is hardly, if at all, reducible to
glass, without the addition of something to promote its fusion; all
that the fiercest heat can do being only to render it soft. Copper,
even while it retains its metalline form, and is very pure, requires a
considerable degree of fire to melt it, and does not begin to flow till
long after it is red-hot. When in fusion, it communicates a greenish
colour to the flame of the coals.
This metal is inferior to Silver in point of gravity; nor is its
ductility so great, though it be pretty considerable: but, on the
other hand, it exceeds that metal in hardness. It unites readily with
Gold and Silver; nor does it greatly lessen their beauty when added to
them in a small quantity: nay, it even procures them some advantages;
such as making them harder, and less subject to lose their ductility,
of which those metals are often liable to be deprived, by the mixture
of the smallest heterogeneous particle. This may probably arise from
hence, that the ductility of Copper has the peculiarity of resisting
most of those causes which rob the perfect metals of theirs.
The property, which other metalline substances have in common with
Copper, of losing the phlogiston by calcining and then vitrifying,
furnishes us with a method of separating them from Gold and Silver,
when they are combined therewith. Nothing more is required than to
expose the mass compounded of the perfect metals and other metalline
substances to a degree of heat sufficient to calcine whatever is not
either Gold or Silver. It is evident, that, by this means, these two
metals will be obtained as pure as is possible; for, as hath already
been said, no metalline calx or glass is capable of uniting with metals
possessed of their phlogiston. On this principle is formed the whole
business of refining Gold and Silver.
When the perfect metals have no other alloy but Copper, as this metal
is not to be calcined or vitrified without great difficulty, which is
increased by its union with the unvitrifiable metals, it is easy to see
that it is almost impossible to separate them without adding something
to facilitate the vitrification of the Copper. Such metals as have the
property of turning easily to glass are very fit for this purpose; and
it is necessary to add a certain quantity thereof, when Gold or Silver
is to be purified from the alloy of Copper. We shall have occasion to
be more particular on this subject when we come to treat of Lead.
Copper is soluble in all the acids, to which it communicates a green
colour, and sometimes a blue. Even the neutral salts, and water itself,
act upon this metal. With regard to water indeed, as the procuring
it absolutely pure and free from any saline mixture is next to an
impossibility, it remains a question whether the effect it produces on
Copper be not owing to certain saline particles contained in it. It is
this great facility of being dissolved that renders Copper so subject
to rust; which is nothing else but some parts of its surface corroded
by saline particles contained in the surrounding air and water.
The rust of Copper is always green or blue, or of a colour between
these two. Internally used it is very noxious, being a real poison, as
are all the solutions of this metal made by any acid whatever. The blue
colour which Copper constantly assumes, when corroded by any saline
substance, is a sure sign by which it may be discovered wherever it
exists, even in a very small quantity.
Copper dissolved in the vitriolic acid forms a kind of metalline salt,
which shoots into rhomboidal crystals of a most beautiful blue colour.
These crystals are called _Blue Vitriol_, or _Vitriol of Copper_. They
are sometimes found ready formed in the bowels of the earth; and may
be artificially made by dissolving Copper in the vitriolic acid; but
the solution will not succeed unless the acid be well dephlegmated.
The taste of this vitriol is saltish and astringent. It retains a
considerable quantity of water in crystallizing, on which account it is
easily rendered fluid by fire.
It must be observed, that, when it is exposed to a certain degree of
heat, in order to free it of its humidity, a great part of its acid
flies off at the same time: and hence it is that, after calcination,
there remains only a kind of earth, or metalline calx, of a red colour,
which contains but very little acid. This earth cannot be brought to
flow but with the greatest difficulty.
A solution of Copper in the nitrous acid forms a salt which does not
crystallize, but, when dried, powerfully attracts the moisture of the
air. The same thing happens when it is dissolved in the spirit of salt,
or in _aqua regis_.
If the Copper thus dissolved by any of these acids be precipitated
by an earth or an alkali, it retains nearly the colour it had in the
solution: but these precipitates are scarce any thing more than the
earth of Copper, or Copper deprived of most of its phlogiston; so that
if they were exposed to a violent fire, without any additament, a great
part of them would be converted into an earth that could never be
reduced to a metalline form. Therefore, when we intend to reduce these
precipitates to Copper, it is necessary to add a certain quantity of a
substance capable of restoring to them the phlogiston they have lost.
The substance which hath been found fittest for such reductions is
charcoal-dust; because charcoal is nothing but a phlogiston closely
combined with an earth, which renders it exceedingly fixed, and capable
of resisting a violent force of fire. But as charcoal will not melt,
and consequently is capable of preventing rather than forwarding the
flux of a metalline calx or glass, which nevertheless is essentially
necessary to complete the reduction, it hath been contrived to mix
it, or any other substance containing the phlogiston, with such fixed
alkalis as easily flow, and are fit to promote the flux of other
bodies. These mixtures are called _Reducing Fluxes_; because the
general name of _Fluxes_ is given to all salts or mixtures of salts,
which facilitate fusion.
If Sulphur be applied to Copper made perfectly red-hot, the metal
immediately runs; and these two substances uniting form a new compound
much more fusible than pure Copper.
This compound is destroyed by the sole force of fire, for two reasons:
the first is, that, sulphur being volatile, the fire is capable
of subliming a great part of it, especially when it is in a great
proportion to the Copper with which it is joined; the second is, that
the portion of sulphur which remains, being more intimately united with
the Copper, though it be rendered less combustible by that union, is
nevertheless burnt and consumed in time. Copper being combined with
sulphur, and together with it exposed to the force of fire, is found
to be partly changed into a blue vitriol; because the vitriolic acid,
being disengaged by burning the sulphur, is by that means qualified to
dissolve the Copper. The affinity of Copper with sulphur is greater
than that of Silver.
This metal, as well as the other imperfect metals and the semi-metals,
being mingled with nitre and exposed to the fire, is decomposed and
calcined much sooner than by itself; because the phlogiston which it
contains occasions the deflagration of the nitre, and consequently
the two substances mutually decompose each other. There are certain
metalline substances whose phlogiston is so abundant, and so weakly
connected with their earth, that when they are thus treated with nitre,
there arises immediately a detonation, accompanied with flame, and as
violent as if sulphur or charcoal-dust had been employed; so that in a
moment the metalline substance loses its phlogiston, and is calcined.
The nitre, after these detonations, always assumes an alkaline
character.