amber. These are generally in a very perfect state, and consist of
flies, small moths, &c. Grains of sand, pieces of iron pyrites, and the
leaves of plants, are also sometimes found in it. Insects, sand, and
other substances, are likewise remarked in a species of gum, called _gum
animè_, which, in colour, appearance, and qualities, so nearly resembles
amber, that it is almost impossible to distinguish the two substances
from each other. Large productions, which were formerly supposed to have
been made of amber, such as a column ten feet high in the Florentine
Museum, are now usually considered to have been formed of this gum; and
many of the large beads of what are sold as amber necklaces are made of
it.
If a piece of amber be fixed on the point of a knife and lighted, it
will burn entirely away, emitting at the same time a white smoke, and a
somewhat agreeable though sickly odour. When rubbed it has the property
of attracting light bodies; hence one of the ancient Greek philosophers
attributed to it a certain kind of life. From the name of _electron_,
which was given to it by them, in consequence of this property, we
derive our word electricity.
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CLASS IV.–METALLIC SUBSTANCES.
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OF METALS IN GENERAL.
225. METALS, in a perfect state, are easily distinguished from other
minerals, by a peculiar brilliancy which pervades their whole substance,
and which has the name of _metallic lustre_; by their complete opacity,
and their great weight in proportion to that of other mineral
substances.
When taken from the earth they are found in one or other of the four
following states: 1. In a native or metallic state, 2. Combined with
sulphur, 3. In a state of oxide (21) 4. Combined with acid.
Metals, when found in a state of combination with other substances, have
the name of _ores_. They are in general deposited in veins (4), of
various thickness, and at various depths in the earth. The mode of
obtaining them is to penetrate from the surface of the earth to the
vein, and there to follow it, in whatever direction it may lie. The
hollow places thus formed are called _mines_, and the men employed in
them are denominated _miners_. When the veins are at a great depth, or
extend to any considerable distance beneath the surface of the earth, it
is necessary, at intervals, to make openings, or _shafts_, to the
surface, for the admission and circulation of the air; and also to draw
off the water which collects at the bottom, by drains, pumps, or
steam─engines, as the situation or circumstances require.
After the metallic ores are drawn from the mine, they, in general, go
through several processes before they are in a state fit for use. Some
of them are first washed in running water, to clear them from earthy
particles. They are then piled with combustible substances, and burnt or
roasted, for the purpose of ridding them of the sulphur or arsenic with
which they may happen to be combined, and which rises from them in a
state of fume or smoke. Thus, having been freed from impurities, they
undergo the operation of melting, in furnaces constructed according to
the nature of the respective metals, or the uses to which they are to be
subsequently applied.
The knowledge of metals is a subject of great importance to mankind.
Their use in trade is so frequent, and in the arts so various and so
interesting, that few objects can be more worthy of attention than
these.
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ORDER I.—MALLEABLE METALS;
OR, SUCH AS ARE CAPABLE OF BEING FLATTENED OR ELONGATED BY THE HAMMER,
WITHOUT TEARING OR BREAKING.
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226. _PLATINA, the most ponderous of all the metals with which we are
acquainted, is, when purified, about twenty times heavier than water. It
is also one of the hardest and most difficult to be melted, is of white
colour, but darker and not so bright as silver, and is found only in
small blunted and angular grains or scales in the sands of some of the
rivers in South America._
If platina could be obtained in sufficient quantity, it would perhaps be
the most valuable of all metals. The important uses to which it is
applicable may easily be imagined when we state that it is nearly as
hard as iron, and that the most intense fire and most powerful acids
have scarcely any effect upon it. Platina is not fusible by the heat of
a forge, but requires either the concentrated rays of the sun in a
burning mirror, the galvanic electricity, or a flame produced by the
agency of oxygen gas.
It is admirably adapted for the uses of the philosophical chemist:
although vessels made of it must always be found expensive, from its
being necessary to solder them with gold; and although it has the
disadvantage of being subject to corrosion by the application or use of
caustic alkalies. Vessels made of it are not liable to be broken, and
are as indestructible as those made of gold. When properly refined, its
colour is somewhat betwixt that of silver and iron. Not being liable to
tarnish like silver, platina is manufactured into several kinds of
trinkets.
Its ductility is so great that it may be rolled into plates, or drawn
into wire; and platina wire, for strength and tenacity, is considered
much preferable to that either of gold or silver of equal thickness.
Platina is also made into mirrors for reflecting telescopes, into
mathematical instruments, pendulums, and clock─work; particularly where
it is requisite that the construction of these should be more than
usually correct, as platina is not only free from liability to rust, but
is likewise subject to very little dilatation by heat. It is sometimes
beaten into leaves and applied to porcelain, in the same manner as leaf
gold; and its oxide (21) is used in enamel painting, and might be used,
with great advantage, in the painting and ornamenting of porcelain. The
platina employed for all these purposes is repeatedly melted with
arsenic, as without the aid of this it could only be obtained in very
small masses, owing to the intense heat that is required for its fusion.
This extraordinary metal was unknown in Europe until about the year
1735, when it was first brought from South America by Don Antonio Ulloa.
227. _GOLD is a metal distinguished by its yellow colour; by its being
next in weight to platina, softer than silver, but considerably more
hard than tin; and being more easily melted than copper._
_It is found in various states, massive, in grains, small_ _scales, and
capillary, or in small branches. It cannot be dissolved in any acid
except that called aqua regia (207), and is more than nineteen times
heavier than water._
The countries of hot climates are those chiefly in which gold is
discovered. It abounds in the sands of many African rivers, and is very
common in several districts both of South America and India. The gold
mines of Lima and Peru have had great celebrity; but, since the late
commotions in the Spanish colonies, the working of them has been much
neglected. It is from Brazil that the greatest part of the gold which is
seen in commerce is brought. The annual produce of the various gold
mines in America has been estimated at nearly 9,500,000_l._ sterling.
The principal gold mines in Europe are those of Hungary, and next to
them those of Saltzburg. Spain is probably very rich in gold.
Considerable mines were worked there in former times, particularly in
the province of Asturia; but, after the discovery of America, these were
given up or lost. Gold has been found in Sweden and Norway, and also in
several parts of Ireland, but particularly in the county of
Wicklow.—Among the sands of a mountain stream in that county, and among
the sand of the valley on each side, lumps of gold are occasionally
found. Pieces have been discovered which weighed twenty─two ounces, but
they are generally much smaller, from two or three ounces to a few
grains. It is said that lumps of gold, of large size, have been used as
weights in some of the common shops, and that others have been placed to
keep open the doors of cottages and houses in some parts of Ireland, the
owners not knowing what they were. Gold is also occasionally found in
Cornwall, and some other counties of England. Wherever it occurs it is
commonly observed in a state of alloy with copper or silver, and in the
form of grains, plates, or small crystals.
Gold was formerly obtained in Scotland. It is asserted that, at the
marriage of James V. there were covered dishes filled with coins made of
Scottish gold, and that a portion of these was presented to each of the
guests by way of dessert. Very extensive operations for the discovery of
gold were carried on during the reign of Queen Elizabeth, at Leadhills,
in Lanarkshire, under the direction of an Englishman whose name was
Bulmer. The trenches, the heaps of soil that were turned up, and other
marks of these operations, are yet visible near the road between
Leadhills and Elvanfoot. It is said that 300 men were then employed; and
that, in the course of a few years, a quantity of gold was collected,
equal in value to 100,000_l._ sterling. Not many years ago similar
operations were commenced under the superintendence of a celebrated
manager of the Scottish lead mines. The gold was found immediately under
the vegetable soil; and the method of obtaining it was to direct a small
stream of water, so as to carry the soil along with it, to basins or
hollow places, where the water might deposit the matters carried down by
the force of its current. The matter thus deposited was repeatedly
washed, till the whole of the earthy substances were carried off. The
gold, being heaviest, sunk to the bottom, and remained behind. The soil
still furnishes gold; but the produce would by no means be equal to the
expense of collecting it. Searching for gold, therefore, is now regarded
only as an amusement, and not as a source of profit. Grains of this
metal are sometimes found, after great floods, among the sand of brooks
in different parts of Scotland.
The mode of extracting gold from its ore is by reducing it into a fine
powder, and mixing this powder with quicksilver (228). The latter having
the quality of uniting with itself every particle of the precious metal,
but being incapable of union with the other substances, extracts it even
from the largest portions of earth. The quicksilver, which has absorbed
the gold, is then separated by means of heat; it flies off in vapour,
and leaves the other metal in the vessel used for the operation.
Gold has been known, and in request, from the very earliest ages of the
world. By the assent of civilized nations, it has become the
representative of wealth under the form of money; and it is now an
universal circulating medium for the purchase of all kinds of
commodities. It has been chosen to occupy this important place on
account of its scarcity, its weight, and other valuable properties.
As gold is not liable to tarnish or rust, it is frequently employed for
ornaments of dress. But, beyond its use in the coinage, its most
important uses are for goldsmith’s work, in jewellery, and for gilding.
In each of these its standard or purity is different. That denominated
_coinage_, or _sterling gold_, consists of an alloy of about twenty─two
parts of gold with two parts of copper; whilst gold of the _new
standard_, of which gold plate, watch─cases, and many other articles are
made, consists of only eighteen parts of gold, and six parts of copper.
Each of these is stamped at Goldsmiths’ Hall; the former with a lion, a
leopard’s head (the mark of the goldsmith’s company), a letter denoting
the year, the king’s head, and the manufacturer’s initials; the latter
is stamped with the king’s head, letter for the year, a crown, the
number 18 to designate its quality, and the manufacturers initials. The
coinage gold of Portugal and America is of the same standard as our own;
that of France is somewhat inferior; and Spanish gold is inferior to the
French. The Dutch ducats and some of the Moorish coins are of gold
unalloyed. _Trinket gold_, which is unstamped, is in general much less
pure than any of the above; and the _pale gold_ which is used by
jewellers is an alloy of gold with silver.
The ductility and tenacity of this metal, particularly when alloyed with
copper, are extremely remarkable, and are fully proved by the great
extent to which a very small quantity of it may be beaten into leaves,
or drawn into wire. Leaves of gold may be beaten so thin, that a single
grain may be made into fifty─six leaves, each an inch square. These
leaves are only 1/282000 of an inch thick; and the gold leaf which is
used to cover silver wire is but the twelfth part of that thickness. An
ounce of gold upon silver wire is capable of being extended more than
1,300 miles in length: and sixteen ounces of gold, which, in the form of
a cube, would not measure more than an inch and a quarter on each side,
will completely gild a silver wire in length sufficient to compass the
whole earth like a hoop.
Gold is beaten into leaves upon a smooth block of marble, fitted into
the middle of a wooden frame about two feet square, in such manner that
the surfaces of the marble and of the frame are exactly level. On three
of the sides there is a high ledge; and the front, which is open, has a
flap of leather attached to it, which the man who beats the gold uses as
an apron for preserving the fragments that fall off. In this process
there are three kinds of animal membranes used, some of which are laid
between the leaves to prevent their uniting together, and others over
them to defend them from being injured by the hammer. The exterior cover
is of parchment. For interlaying with the gold, the smoothest and
closest vellum that can be procured is first used; and, when the gold
becomes thinner, this is exchanged for much finer skin, made of the
entrails of oxen, prepared for this express purpose, and hence called
_gold beater’s skin_. After the leaf has been beaten to a sufficient
degree of thinness, it is taken up by a cane instrument, and thrown flat
upon a leathern cushion, where it is cut to a proper size with a square
frame of cane, or wood edged with cane. These pieces are then fitted
into books of twenty─five leaves each, the paper of which has been well
smoothed, and rubbed with red bole (127), to prevent them from sticking.
The leaves are about three inches square, and the gold of each book
weighs somewhat more than four grains and a half.
It was anciently the custom to beat gold into thin plates, and to gild
the walls of apartments, the surfaces of dishes, drinking utensils, and
other articles, by covering them with such. But this was not only an
expensive, but it must have been a most clumsy mode of ornament. The
present modes of gilding are very different. When wood is to be gilded,
the surface is first smeared with an adhesive kind of oil, or with a
kind of glue called size; and the gold leaf, above mentioned, is then
spread upon it by a tuft of cotton or other soft substance.
The _gilding of iron or copper_ is performed by cleaning and polishing
its surface, and then heating it till it has a blue colour. When this
has been done, a layer of gold leaf is put on, slightly burnished down,
and exposed to a gentle fire. It is usual, in common work, to place
three such layers, or four at the most, each consisting of a single
leaf. The heating is repeated at each layer, and last of all the work is
burnished. For gilding in _or moulu_, as it is denominated by the
French, an amalgam consisting of ten parts of mercury and one part of
gold is used. This is spread upon the metal, and is afterwards exposed
to the action of a fire sufficiently strong to evaporate the mercury and
leave the gold behind. The gilding in _or moulu_ is much more solid and
permanent than that by the former method.
When gilding is pale and dirty, it may be revived by means of what is
called _gilding wax_, a composition of yellow wax, bole (127), verdigris
(230), and alum.
A very beautiful gilding upon metals, and particularly upon silver, is
effected by soaking clean linen rags in a solution of gold made by aqua
regia (207). The rags are dried and burnt; and the ashes are carefully
preserved. These ashes are used by taking a sound cork, moistening it
with a little water, dipping it into the ashes, and then rubbing
strongly a portion of them on the surface of the silver, which should be
perfectly clean and bright. By this simple and economical process, it
will be covered with an extremely thin coating of gold, the colour and
brilliancy of which may be heightened by burnishing. The ornaments upon
snuff─boxes, fans, and various kinds of trinkets, are merely thin plates
of silver, gilded in this manner.
The _edges of tea─cups_, and other similar articles, may be gilded,
though not in a very durable manner, by applying a thin coat of amber
varnish (224), and then placing leaf─gold upon it. When the varnish is
dry, the gold is to be burnished.
Gold, in a state of solution, is sometimes used for staining marble,
ivory, ornamental feathers, and other articles, a purple─red colour,
which cannot be effaced. By chemical processes an oxide (21) is obtained
from this metal, which is employed for giving those beautiful shades of
lilac, rose colour, red and purple, which we observe in glass and
porcelain.
A _gold powder for painting_ may be made by uniting one part of gold
with eight parts of mercury (228), and afterwards evaporating the latter
by heat.
The article denominated _gold wire_ is generally silver wire gilded,
very little wire being made entirely of gold. Its uses are chiefly for
embroidery and filagree work. _Gold thread_ consists of flatted silver
gilt wire, laid over a thread of yellow silk, by twisting it in a
machine with iron bobbins. It is of this, and not of gold, that the
article called _gold lace_ is made. The Chinese, instead of flatted
wire, use slips of gilt paper, which they interweave in their stuffs,
and twist upon silk threads.
228. _MERCURY, in its native state, is called quicksilver, and is found
in small globules of shining, silvery appearance, scattered through
different kinds of stones, clay, and ores. It is nearly fourteen times
heavier than water._
_The principal ore of mercury, and that from which the metal is chiefly
obtained, is_ cinnabar. _This is of red colour, and consists of mercury
mineralized with sulphur. It is sometimes found in a massive state,
sometimes in grains, and sometimes crystallized; and chiefly among rocks
of the coal formation._
The most productive mines of cinnabar are in the palatinate of Germany,
at Idria in Carniola, and at Almaden in Spain. Those of Idria are
supposed to be more valuable than any of the others. Their first
discovery, which was somewhat more than three hundred years ago, was
made in a very extraordinary manner. This part of the country was then
much inhabited by coopers; and one of the men, on retiring from work in
the evening, placed a new tub under a dropping spring, to try if it
would hold water; and, when he came in the morning, he found it so heavy
that he could scarcely move it. Examining into the cause of this
extraordinary circumstance, the man observed that it was owing to a
shining and ponderous fluid which was at the bottom. The affair was
noised abroad, and a society of persons was formed to search further,
and discover the mine from which this quicksilver had flowed. Such was
their success that the reigning Duke of Austria paid them a compensation
for the discovery, and took the mine into his own possession. The
greatest perpendicular depth of this mine is now more than 830 feet. It
is descended by buckets, or by ladders placed obliquely in a zigzag
direction. In some parts of the mine the pure metal flows in small
streams, so that in six hours a man has been known to collect more than
thirty─six pounds weight of it. In other parts it is found in a
multitude of little drops, either in ores or in clay. The whole produce
of the mine is said to exceed a hundred tons weight of mercury per
annum.
It has been asserted that, several years ago, in digging out clay for
the foundation of a house opposite to the King’s Arms inn, in the street
called Hyde─hill, in Berwick─upon─Tweed, a quantity of native mercury
was discovered. The clay, when dug out, lay for some time in the place
to which it was conveyed; and the mercury was observed to exude from the
small fissures or cracks that were formed as it dried. It is said that,
several years afterwards, in making some alteration in the yard of the
same house, the workmen penetrated into the same bed of clay; and that
it then appeared to be impregnated with native mercury, which ran out in
small globules.
Mercury is sometimes imported into Europe from Peru, and from the East
Indies.
The mode of extracting it from _cinnabar_ is said to be by mixing this
ore either with pounded chalk, or with half its weight of iron filings,
and distilling it in a stoneware retort. By this process the sulphur
combines with the iron, and the mercury, in a state of purity, passes
into the receiver.
When pure or native mercury occurs in mixture with other substances,
these are stamped or ground into a coarse powder. Water is poured upon
them; they are briskly stirred until the water becomes thick and turbid,
and then are left to settle. This operation is repeated till the water
runs off perfectly clear. The substance at the bottom, which is
principally mercury, is then put into large iron retorts and the metal
is obtained, free from all extraneous matters, by distillation.
It is the singular property of this metal, which has no other alliance
whatever with silver than its appearance, to be capable of division, by
the least effort, into an indefinite number of particles, each of which
assumes a spherical form; and to be always in a fluid state in the
common temperature of our atmosphere. Even during the most intense
frost, it still retains its fluidity. By the effect, however, of extreme
cold artificially produced, mercury becomes a solid metal, and in this
state may be beaten with a hammer and extended without breaking; but
care must be taken that it does not touch the fingers, as it would
blister them and cause unpleasant sores, in the same manner as any
burning substance.
Mercury has been known from the remotest ages; and it was employed by
the ancients in gilding, and in the operations of separating gold and
silver from their ores, in the same manner as at present. Being the
heaviest of all fluids of which we have any knowledge, and not
congealing in the temperature of our climate, it has been preferred,
before all others, for barometers, as a measure of the weight of the
atmosphere. And, as heat dilates mercury similarly to other fluids, it
is likewise made into thermometers. Mercury is sometimes used in
medicine in its pure metallic state.
The combinations of mercury with other metals are termed _amalgams_.
That of mercury and gold is formed so readily, that if gold be dipped
into mercury, its surface immediately becomes as white as silver. An
amalgam of mercury and gold is employed for the gilding, and of mercury
and silver for the silvering of metals.
Mercury and tin combined together form the substance that is used for
the _silvering of looking─glasses_. The process is as follows: A
quantity of tin─foil, equal in size to the glass, is evenly placed on a
flat stone or table; and mercury, in which some tin has been dissolved,
is poured upon it, and spread with a feather, or bunch of cloth, until
its union has covered every part. A plate of glass is then cautiously
slided upon it, from one end to the other, in such manner that part of
the redundant mercury is driven off, or swept away before its edge. The
remainder is now united to the tin. The glass is then loaded with
weights all over, so as to press out still more of the mercury. By
inclining the table, this remaining mercury becomes discharged; and, in
a few hours, the rest of the tin─foil and mercury adhere so firmly to
the glass, that the weight may be removed without any danger of its
falling. About two ounces of mercury are requisite for covering, in this
manner, three square feet of glass.