Chapter VII: Preface: ─────── (7)

Amber, when wrought into ornaments, is first split on a leaden plate, and then turned on a particular kind of whetstone. The polishing of it is performed with chalk and water, or chalk and oil; and the work is finished by rubbing the whole with clean flannel. Without great attention it becomes very hot, and either flies into pieces, or takes fire during the operation.

After having been roasted or melted, amber is readily soluble in oil, and, in this state, constitutes the basis of several kinds of varnish. It was formerly much used in medicine, but, in this respect, it is now almost wholly neglected. Some persons, however, have still an absurd notion that a collar or necklace of amber, tied round an infant’s neck, will enable it to cut its teeth in safety. Oil of amber combined with liquid ammonia constitutes a white soapy liquor called _eau─de─luce_.

It has already been mentioned that insects are occasionally found in 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.

────────────────────────────────────────────────────────────────────────

CLASS IV.–METALLIC SUBSTANCES.

───────

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.

────────────────────────────────────────────────────────────────────────

ORDER I.—MALLEABLE METALS;

OR, SUCH AS ARE CAPABLE OF BEING FLATTENED OR ELONGATED BY THE HAMMER, WITHOUT TEARING OR BREAKING.

───────

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.

By means of mercury a _fulminating powder_ is made, which, when struck with a hammer on an anvil or flat iron, such as is used by laundresses, explodes with a stunning and disagreeable report, and with such force as to indent both the anvil and the hammer. Four or five grains are as much of this powder as ought to be used for such experiments. Its force is much greater than that of gunpowder, but does not extend so far. Hence it is a substance which might be rendered of great use in the blasting of rocks.

_Corrosive sublimate_ is an extremely poisonous preparation from mercury. Among other uses, it is employed by dyers as a mordant to fix their colours. From certain proportions of corrosive sublimate rubbed together, until they are perfectly incorporated, is formed _calomel_; a salt which, of late years, has been extensively and most usefully employed in medicine.

A valuable red colour or pigment called _vermilion_, or _artificial cinnabar_, which was as well known to the ancients as it is to the moderns, is usually formed of three parts of mercury and one of sulphur, melted together, heated to redness, and then sublimated out of contact of the air. The manufacture of vermilion was long kept a secret by the Dutch; and it is stated that, before the late war, nearly 50,000 pounds weight of it were annually made, in three furnaces, by four workmen, near Amsterdam. Native cinnabar is sometimes used for the same purpose; but the artificial kind is preferred on account of the purity and brightness of its colour.

229. _SILVER is a white, brilliant, sonorous, and ductile metal, somewhat more than ten times heavier than water._

_It is found in different states. Of these the principal is denominated_ native silver, _from its being nearly in a state of purity. Native silver sometimes occurs in small lumps, sometimes in a crystallized form, and sometimes in leaves, threads, or wire. In many instances the latter are so connected with each other as to resemble the branches of trees, in which case the ore is called_ dendritic. _There are also several_ ores of silver, _in which this metal is combined with lead, antimony, arsenic, sulphur, and other substances._

The silver that is produced from the mines of Potosi, in South America, is of the dendritic kind; and is considered by the Spaniards as the purest that is known. A range of mountains near Potosi, about twenty miles in circumference, is said to be perforated by more than 300 shafts, or openings of mines, and to produce, in the whole, from 30,000 to 40,000 dollars’ worth of ore per week. The annual produce of the silver mines in America has been estimated at near 2,400,000_l._ sterling.

Silver is also found in several parts of Europe; and, some years ago, there were mines of this metal, worked to a great extent, at Konigsberg in Norway. These were discovered in 1623, and they were found so profitable, that in 1751 forty─one shafts and twelve veins were wrought there; and 3,500 officers, artificers, and labourers, were employed. The perpendicular depth of the principal shaft was more than 750 feet. Specimens of native silver are not uncommon from some of the copper─mines of Cornwall; and, many years ago, a vein of silver ore was, for a short time, wrought with considerable advantage in the parish of Alva, Stirlingshire, Scotland. It is said that from 40,000_l._ to 50,000_l._ worth of silver was obtained from it before the repository was exhausted. We are informed that a mass of capillary native silver was found, in veins traversing the blue─coloured limestone of Isla, one of the Western Islands of Scotland. Great quantities of silver are extracted from lead. There was lately melted in one refining house in London 50,000_l._ worth of this metal, from lead of the Beralston mines in Devonshire.

Different methods are employed, in different countries, to extract silver from its ore. In Mexico and Peru the mineral is pounded, roasted, washed, and then mixed with mercury in vessels filled with water; a mill being employed for the more perfectly agitating and mingling them. By this process the silver combines with the mercury. The alloy thus obtained, after undergoing some further processes, is submitted to the action of heat, by which the mercury passes off in a state of vapour, leaving the silver behind. The silver is then melted and cast into bars or ingots. In other countries, after the earthy matters are cleared from the silver ore by pounding and washing, the remainder is melted with lead: which, by a subsequent process, is separated, and leaves the silver alone and pure.

This metal ranks next in value to gold. Like gold, it is coined into money, and is manufactured into various kinds of utensils, such as goblets, vases, spoons, and dishes, which have the general appellation of _silver plate_. For all these purposes it is alloyed with copper, which does not affect its whiteness, and is not easily detected, unless it be in too great proportion: the intention of this is to render it harder than it would otherwise be, and thereby the better to adapt it to receive fine and sharp impressions on being cast. Our _standard silver_ is composed of somewhat more than 12¼ parts of pure metal and one part of copper; and the metal of this standard is used, both for silver plate, and in the coinage. The mark or stamp which is given to it at Goldsmiths’ Hall is similar to that which has been explained for sterling gold.

After platina (226) and gold (227), silver is considered the most unchangeable of all metals. The air does not easily act upon its surface in such manner as to injure it; but, when long exposed to the atmosphere, especially in frequented or smoky places, it acquires a covering or rust of dark brown colour, which, on examination, is found to be what chemists denominate _sulphuret of silver_. The fumes of sulphur and other inflammable substances blacken silver. Various powders have been contrived with a view to restore to plate its original lustre; but these should be used with caution, as some of them are very injurious.

Silver is nearly as ductile as gold. It may be beaten into leaves so thin that a single grain in weight will cover a space of more than fifty─one inches; and it may be drawn into wire much finer than a human hair, indeed so fine that a single grain of silver has, in this form, been extended nearly to the length of 400 feet. It is this wire gilded that has the name of gold wire; and what is denominated _gold lace_ (227) is but flatted silver thread gilt, twisted round silk, and woven.

The _plating_ of copper with silver is a very useful operation, and is thus performed. Plates of silver are bound with iron wire, upon small ingots of copper. The quantity generally allowed is one ounce of silver to twelve ounces of copper. The surface of the plate of silver is made not quite so large as that of the copper; and upon the edges of the copper, which are not covered by the silver, a little borax (204) is put. By exposing the whole to a strong heat, the borax melts; and, in melting, contributes to fuse that part of the silver to which it is contiguous, and to attach it, in that state, to the copper. The ingot, with its silver plate, is then rolled between steel rollers moved by machinery, till it is of proper thickness. It is afterwards cut into such sizes and to such shapes as may be required for use. An ounce of silver is thus often rolled out into a surface of three square feet, having its thickness, upon the copper, not more than the three─thousandth part of an inch. Hence we ought not to be surprised at the silver being soon worn from the sharp edges of plated goods. To prevent this, it is customary, with the best articles, to have all the edges, and the parts liable to be worn, formed, to a considerable thickness, of silver.

What is called _French plate_ is made by heating copper, or more frequently, brass, to a certain degree, then applying leaf─silver to the surface, and strongly rubbing it with a burnisher. The durability of this plating depends of course on the number of leaves which are applied on a given surface. For ornaments that are not much used ten leaves may be sufficient; but a hundred will not last long, if the metal be exposed to frequent handling or washing.

Besides the above, there are various modes of _silvering_ metal articles, or, as it is called, _washing_ them with silver. All these are performed by different chemical preparations of this metal.

The article denominated _shell─silver_, used by painters, is prepared, by carefully grinding silver─leaf, with a little honey or gum water upon a slab, or in a mortar, and separating the honey or gum by means of water. When this is washed away, the silver may be put on paper, or kept in shells, for use. When it is to be used, it must again be diluted with gum water.

The application of silver─leaf for the silvering of paper or wood is similar to that of gold─leaf (227).

Silver, dissolved in aqua fortis (nitric acid, 30), yields crystals, which, afterwards melted in crucibles, form that grey mass usually called _lunar caustic_, and by chemists _nitrat of silver_. This preparation is of considerable use in surgical cases, being employed to keep down fungous or proud flesh, in wounds and ulcers, and also for the consuming of warts, small wens, and other excrescences upon the skin. It is likewise, though a most violent medicine, sometimes given internally, but in very small doses, to persons subject to epileptic fits. The liquid in which the silver is dissolved becomes excessively caustic. It gives to the skin, the hair, and almost all animal substances, an indelible black colour. Hence it is often used as a specific for dyeing the human hair. No person, however, would employ it for this purpose, who was acquainted with its injurious qualities, not only to the hair itself, but also to the skin, if permitted to come in contact with it.

The article called _indelible_, or _permanent marking ink_, for marking linen, and other wearing apparel, is formed by dissolving, in a glass mortar, two drachms of nitrat of silver, in six drachms of pure water, and then adding to them two drachms, by measure, of thick gum water. This is the ink for writing on the linen.—In another vessel dissolve half an ounce of salt of tartar, or of the subcarbonat of soda, of commerce, in four ounces of water; and add to the solution half an ounce, by measure, of thick gum water. This forms the preparatory liquor. With this the linen is to be thoroughly wetted at the part intended to be marked. The linen is to be dried, and then to be written upon by a clean pen dipped in the marking ink. The letters will at first be pale, but by exposure to light and heat, they will soon become black; and be so permanently fixed, that no washing nor bleaching can efface them.

The attention of the curious has of late been turned to a very extraordinary compound called _fulminating silver_, which explodes without heat, and with even the slightest degree of friction. Of this compound little _fulminating balls_ have been made. These are globules of thin glass, each somewhat larger than a pea, and containing a grain or two of fulminating silver. After the silver is put in, it is secured by a piece of soft paper, pasted over the ball, so as completely to cover it. These balls explode by merely crushing them under the heel of the shoe. What are called _fulminating bombs_ are similar balls, but of the size of hazel nuts. No one should attempt to explode these by crushing them with the shoe, as their explosive effect is so violent as sometimes to prove injurious.

Fulminating silver requires the utmost care. It should never be put into phials, nor should it be in any way handled so as to produce much friction. It is the most dangerous preparation that is known. The mere touch of a hard substance will sometimes explode it; and its very preparation is so hazardous that this ought never to be attempted without a mask upon the face with strong glass eyes.

The following are three pleasing experiments with preparations of silver:

1. Mix or amalgamate together four parts of silver leaf with two parts of mercury (228) and dissolve this in diluted aqua fortis. To the solution add as much water as will be equal to thirty times the weight of the metals employed. Pour a portion of the above mixture into a phial, and place at the bottom a small piece of silver. After it has stood awhile, little filaments of silver will be seen to shoot up from it somewhat in the form of a shrub. This apparent vegetation is popularly called the _tree of Diana_.

2. A production nearly similar may be obtained by adding a little quicksilver to a solution of nitrat of silver in water.

3. Drop upon a clean plate of copper a small quantity of solution of lunar caustic, or nitrat of silver. In a short time a metallic vegetation will be perceptible, branching out in pleasing forms, and in various directions.

230. _COPPER is a red or orange─coloured metal, about nine times heavier than water. It is the most sonorous of all metals, and, except iron, the most elastic._

_It is found under a great variety of forms, sometimes in masses of pure metal, but, more frequently, in combination with other substances, particularly sulphur._

There are valuable copper mines in every quarter of the world; and the use of copper is probably of greater antiquity than that of any other metal. It is mentioned in the Old Testament; and, at a very early period, domestic utensils and instruments of war were made of bronze, or a compound of copper and tin. Even during the Trojan war, as we learn from Homer, the combatants had no other armour than what was made of bronze. The Greek and Roman sculptors are said to have executed fine works of art in porphyry, granite, and other hard minerals, by means of copper instruments; whence historians have been induced to believe that the ancients possessed the secret of rendering this metal as hard as steel: some of them even imagined that they had the means of converting it into steel.

Copper is very abundant in several parts of Great Britain, particularly in the island of Anglesea. The copper mines of Anglesea are situated on the top of a mountain, and form an enormous cavity more than five hundred yards long, a hundred yards broad, and a hundred yards deep. The ore is got from the mine by pickaxes, and blasting with gunpowder. It is then broken with hammers into small pieces, an operation which is chiefly performed by women and children. After this, it is piled into kilns of great length, and each about six feet high; from the upper parts of which flues are attached that communicate with what are called sulphur chambers. The kilns are closely covered; and fires are lighted in different parts, that the ore may undergo the process of roasting. The whole mass gradually kindles, and the sulphur, which is combined with the ore, is expelled in fumes, by the heat, and is conveyed, through the flues, to the sulphur chamber. This process occupies from three to ten months, according to the size of the kilns; and, during that period, the sulphur chamber is cleared four or five times. When the operation is complete, or the ore is freed from the sulphur, it is taken to places denominated slacking pits. It is subsequently conveyed to the smelting houses, where, by intense heat, the pure metal is drawn off in a fluid state.

As the water, which passes through several parts of the Paris mine, is strongly impregnated with sulphat of copper (209), or copper held in solution by sulphuric acid (24), the proprietors turn the course of this water through certain large and shallow pits, which they have formed for the purpose, and in each of which they place a quantity of iron. A decomposition here takes place: the iron is corroded, and, at length, entirely dissolved, and the copper, in the form of a brown mud, falls to the bottom. One ton weight of iron, thus immersed, will produce nearly two tons of copper mud, each of which, when melted, will yield sixteen hundred weight of metal. This mode of obtaining copper is said to have been an accidental discovery from one of the workmen, several years ago, having left a shovel in the water, which, when afterwards taken out, appeared changed into copper.

The magnitude of the above mentioned copper works may readily be conceived, when it is stated that the beds of ore are, in some places, more than sixty feet in depth: that the proprietors employ more than 1000 workmen; and that they ship, from the adjacent port of Amlwch, upwards of 20,000 tons of copper, annually.

There is at Ecton, in Staffordshire, a copper mine which is now worked at the depth of 1416 feet below the surface of the ground. This is the deepest mine in England.

The uses of copper are numerous and important. When rolled into sheets, betwixt large iron cylinders, it is employed for the covering of houses, sheathing the bottoms of ships, and other purposes. As a covering for houses, copper is lighter than slate, but whether it be more durable has not been yet ascertained. The coppering of ships tends to facilitate their progress through the water, by presenting a smoother surface than that of wood, and not permitting shell animals to fasten to it as they do to wood. It likewise preserves the bottoms of the ships from being punctured by marine worms; and consequently secures to them a longer duration than they would otherwise have. Plates, or flat pieces of copper, are used by artists for engraving pictures upon, either by cutting them with a sharp steel instrument, or corroding them with aqua fortis (206), in lines drawn by a needle through a thin coat of wax spread upon their surface.

Copper is manufactured into various kinds of cooking utensils. Great care, however, ought to be taken that acid liquors, or even water intended for drinking, or to be mixed with food, be not suffered to stand long in such vessels, otherwise they will dissolve so much of the metal as to give them disagreeable and even poisonous qualities. Yet, it is remarkable that, while acid liquors are kept boiling, they do not seem to dissolve any of the metal. Hence it is that confectioners, by skilful management, prepare the most acid syrups in copper vessels, without their receiving any unpleasant taste or injurious quality from the metal. All vessels formed of this metal which are employed in cookery, ought to have their inner surface covered with a coat of tin (238).

As copper does not, like iron, strike fire by collision, it has on this, as well as on some other accounts, been substituted for iron in the machinery which is employed in gunpowder mills. It is also made into water pipes, and sometimes into sash frames. Under the hammer it is capable of being beaten into thin leaves like gold. Copper wire is much employed by bell─hangers and other artisans. The filings of this metal are used for giving a green colour to some kinds of artificial fire─works.

Several preparations of copper are employed in medicine, some of them internally, and others externally; but most of the former are violently emetic.

_Verdigris_ is a rust or oxide (21) of copper, usually prepared from that metal by corroding it with vinegar. There is a large manufactory of verdigris at Montpelier in France. The workmen place alternate strata of copper plates and husks of grapes, the latter of which speedily become acid and corrode the metal. The verdigris, thus formed, is scraped off as it collects on the surface; it is afterwards dried, and put in bags or casks for sale. A manufactory of verdigris has lately been established at Deptford, near London.

A solution of this substance in distilled vinegar affords permanent crystals, which are improperly called _distilled verdigris_, and are made into a green paint. Verdigris is principally consumed by dyers in combination with logwood, for striking a black colour. It is a virulent poison.

Oxide of copper is employed for giving a beautiful green colour to porcelain. It also imparts the same colour to glass, and hence is frequently employed for the formation of artificial emeralds.

_Alloys of Copper._

Of all metals that are known, copper is the most susceptible of alloy. The most frequent and useful of these alloys are made with copper and zinc, in different proportions.

_Brass_ is an alloy composed of three parts of copper, and about a fourth part of zinc (241). It is a beautiful, useful, and well─known yellow metal. Not being so apt to tarnish and rust as copper, and being, in other respects, better adapted for the purpose than that metal, it is much used for clock─work, and for mathematical and astronomical instruments. It is more ductile than either copper or iron, and hence is peculiarly fitted to be made into wire, for the strings of musical instruments, and other purposes. Sieves are woven with brass wire, after the manner of cambric weaving, and of such extreme fineness that similar ones could not possibly be made with copper wire. Brass wire, flatted and gilded, is sometimes made into lace. The finest brass is manufactured at Geneva. It unites great beauty of colour to a high degree of ductility; and is used chiefly for escapement wheels, and other nicer parts of watch─making. For work in which there is no friction it is necessary to cover brass with a kind of varnish or _laquer_, to improve its colour, and prevent it from being tarnished by exposure to the atmosphere.

_Prince’s Metal_, or _Pinchbeck_, is an alloy containing three parts of zinc (241), and four of copper. This metal has nearly the same colour as gold, and was formerly much in use for the manufacture of ornamental articles of different kinds.

_Dutch Gold_ is formed by the cementation of copper─plates with calamine (241), hammered out into leaves. This article is chiefly manufactured in Holland and Germany, and has about five times the thickness of gold leaf.

_Bronze_, and _the metal of which cannons are made_, consist of from six to twelve parts of tin (238) combined with 100 parts of copper. This alloy is brittle, heavier than copper, and of a yellow colour. Before the method of working iron was brought to perfection, it was used by the ancients for the manufacture of sharp─pointed instruments; and it is supposed to have been the _æs_ or brass of the Romans.

_Bell Metal_, or the metal of which bells are formed, is usually composed of three parts of copper and one of tin. Its colour is greyish white; and it is very hard, sonorous, and elastic.

Bronze and bell metal are not, however, always made of copper and tin only. They frequently have other admixtures, consisting of lead, zinc, or arsenic. Bell─makers sometimes abuse the vulgar credulity by pretending that they add a certain quantity of silver to the alloy, for the purpose of rendering the bells more melodious: but they are better acquainted with their business than to employ so valuable a metal in the operation.

_White Copper_ is an alloy composed of equal parts of copper and arsenic (242). The metal produced by this mixture is of a whitish colour, but with a coppery tinge. It is freed from the latter by being melted several times; and, by this process, is at last rendered as white as silver. White copper is very brittle; but, if the arsenic be evaporated by heat, it resumes its ductility, and still preserves its white colour. When the operation is well performed, it is easy, at the first glance, to mistake white copper for silver; but the difference may immediately be ascertained from the properties inherent to the two metals.

White copper is employed in the manufacture of many kinds of trinkets: and of a great number of domestic utensils; such as tea─pots, coffee─pots, and candle─sticks.

231. _MALACHITE is a solid green copper ore, the surface of which has frequently a bubbled appearance, and the interior is marked with numerous irregular zones, and layers of different shades of green. It is somewhat more than three times as heavy as water, and is so soft as to be easily scratched by a knife._

In its appearance, malachite somewhat resembles green jasper; but it is by no means so hard. It is, however, capable of being cut and polished as a gem, and is manufactured into various kinds of trinkets, which of late years have been much in request for necklaces, brooches, and bracelets. It is also cut into slabs, and mounted into snuff─boxes. Such is the size of which it is sometimes found, that M. Patrin saw, at Petersburgh, a plate of malachite thirty─two inches long and seventeen inches broad, which was valued at 20,000 livres; but the finest specimens in Europe are some slabs that are adapted as the tops of tables, sideboards, &c. at Trianon, in the Park of Versailles: the largest of these are nearly four feet in length and two feet wide. They may indeed have been formed by various pieces joined together; but, if so, the joints are so completely concealed as not to be discoverable even by the closest examination. Malachite is sometimes employed for the engraving of cameos, but is seldom cut in intaglio. Smaller pieces of this substance, that are used for trinkets, are about the same value as carnelian. Independently of its use, in the above respects, and also as an ore of copper, malachite, when pure, is ground into powder, and employed as a green pigment.

The Vosges Mountains in Lorraine, and certain copper mines of Saxony, are celebrated for producing very fine specimens of malachite. This beautiful mineral is also found in our own country, in the copper mines of Cornwall and Wales.

_232. TURQUOISE. The beautiful light blue substances that are called turquoises have usually been considered as the bones or teeth of animals, impregnated with blue oxide (21) of copper; but they are sometimes found in nodules which are certainly not of an osseous nature._