_Of Lead._
Lead is a metal of a bluish tinge, of no great tenacity, but very
considerable specific gravity, being heavier than silver. It melts long
before it is red hot, and is then calcined, if it be in contact with
respirable air. When boiling it emits fumes, and calcines very rapidly.
It may be granulated by being poured into a wooden box, and agitated.
During congelation it is brittle, so that the parts will separate by the
stroke of a hammer; and by this means the form of its crystals may be
discovered.
In the progress of calcination it first becomes a dusky grey powder,
then yellow, when it is called _massicot_; then, by imbibing pure air,
it becomes red, and is called _minium_, or _red lead_. In a greater
degree of heat it becomes massicot again, having parted with its pure
air. If the heat be too great, and applied rapidly, it becomes a flaky
substance, called _litharge_; and with more heat it becomes a _glass_,
which readily unites with metallic calces and earths, and is a principal
ingredient in the manufacture of _flint glass_, giving it its peculiar
density and refractive power.
Though lead soon tarnishes, the imperfect calx thus made does not
separate from the rest of the metal, and therefore protects it from any
farther action of the air, by which means it is very useful for the
covering of houses, and other similar purposes. All acids act upon lead,
and form with it different saline substances. _White-lead_ consists of
its union with vinegar and pure air. Also dissolved in vinegar, and
crystallized, it becomes _sugar of lead_, which, like all the other
preparations of this metal, is a deadly poison.
Oils dissolve the calces of lead, which, by this means, is the basis of
paints, plaisters, &c.
By means of heat litharge decomposes common salt, the lead uniting with
the marine acid, and forming a yellow substance, used in painting, and
by this means the fossil alkali is separated.
Lead unites with most metals, though not with iron. Two parts of lead
and one of tin make a _solder_, which melts with less heat than either
of the metals separately; but a composition of eight parts of bismuth,
five of lead, and three of tin, makes a metal which melts in boiling
water.
This metal will be dissolved by water if it contain any saline matter,
and the drinking of it occasions a peculiar kind of cholic.
Lead is sometimes found native, but generally minerally mineralized with
sulphur or arsenic, and often mixed with a small quantity of silver.
_Of Copper._
Copper is a metal of a reddish or brownish colour, considerably
sonorous, and very malleable.
At a heat far below ignition, the surface, of copper becomes covered
with a range of prismatic colours, the commencement of its calcination;
and with more heat a black scale is formed, which easily separates from
the metal, and in a strong heat it melts, and burns with a bluish green
flame.
Copper rusts by exposure to the air; but the partially-calcined surface
adheres to the metal, as in the case of lead, and thus preserves it from
farther corrosion.
Copper dissolved in the vitriolic acid forms crystals of a blot colour,
called _blue copperas_. From this solution it is precipitated by iron,
which by this means becomes coated with copper. The nitrous acid
dissolves copper with most rapidity, producing nitrous air. If the
solution be distilled, almost all the acid will be retained in the
residuum, which is white; but more heat will expel the acid, chiefly in
the form of dephlogisticated air, and the remainder will be a black
substance, consisting of the pure calx of copper. The vegetable acids
dissolve copper as well as the mineral ones, which makes the use of this
metal for culinary purposes in some cases dangerous. To prevent this
they give it a coat of tin. The solution of copper in the vegetable acid
is called _verdigris_.
Alkalies dissolve copper as well as acids. With the volatile alkali a
blue liquor is formed, but in some cases it becomes colourless. All the
circumstances of this change of colour have not yet been examined. Both
oil and sulphur will dissolve copper, and with the latter it forms a
blackish grey compound, used by dyers.
Copper readily unites with melted tin, at a temperature much lower than
that which is necessary to melt the copper; by which means copper
vessels are easily covered with a coating of tin. A mixture of copper
and tin, called _bronze_, the specific gravity of which is greater than
that of the medium of the two metals, is used in casting statues,
cannon, and bells; and in a certain proportion this mixture is excellent
for the purpose of mirrors of reflecting telescopes, receiving a fine
polish, and not being apt to tarnish. Copper and arsenic make a brittle
compound called _tombach_; and with zinc it makes the useful compound
commonly called _brass_, in which zinc is about one third of its weight.
Copper is sometimes found native; but commonly mixed with sulphur, in
ores of a red, green, or blue colour.
Copper being an earlier discovery than that of iron, was formerly used
for weapons and the shoeing of horses; and the ancients had a method,
with which we are not well acquainted, of giving it a considerable
degree of hardness, so that a sword made of it might have a pretty good
edge.