Part 19

It is not of much use in the arts; but its fusibility renders the working of it very simple and easy. It is employed in the composition of some of the soft kinds of solder; and is also used for giving hardness to tin and other metals. Amalgamated with mercury it renders that metal less fluid; and the addition of it to mercury and tin is found useful in the foliating or silvering of looking─glasses. Some manufacturers use it in the composition of pewter; but it is said that this ought not to be done, particularly for the formation of vessels intended to contain food, as bismuth partakes of the noxious properties of lead, and sometimes contains even arsenic. It is also occasionally employed in the fabrication of printers’ types.

A very singular metal is formed by melting together eight parts of bismuth, five of lead, and three of tin. Tea─spoons formed of this metal surprise all who are unacquainted with their nature: they have somewhat the appearance of common spoons, but they melt as soon as they are put into boiling water.

Bismuth reduced to powder, mixed with the white of eggs and applied to wood, gives it, when gradually dried and rubbed with a polisher, the appearance of being silvered. If this metal be dissolved in aquafortis (30), and water be poured into the solution, a white powder precipitates, which is an oxide of bismuth, and which, after being well washed, is used as a pigment, under the name of _pearl─white_. From its beautiful appearance, this powder is sometimes employed by ladies for painting their skin; a practice which cannot be too much condemned, both on account of the danger with which it is attended, and from its soon injuring both the texture and natural colour of the skin. It has the further disadvantage of turning black when touched by the fumes of fetid and other substances; and ladies, who have used this cosmetic, and have afterwards bathed in the Harrowgate waters, have come from the bath a perfectly tawny colour. It was probably the oxide of bismuth which the Roman ladies used for whitening their skin; for Martial, in speaking of a lady, who made too free an use of cosmetics, describes her as afraid even of the sun. The oxide of bismuth is used in the composition of most of the pomades employed in France for painting the face.

A preparation of bismuth has lately been employed in medicine, as a remedy against spasmodic affections of the stomach.

The following is a pleasing experiment, illustrative of metallic crystallization. Melt a ladleful of bismuth, and allow it to cool slowly and quietly till a thin crust is formed on the surface: then, with a pointed iron, make two small opposite apertures through the crust: and, through one of these, quickly pour out the fluid portion, as carefully and with as little motion of the mass as possible. The air having entered by the other aperture, there will appear, on removing the upper crust by means of a chisel, when the vessel has become cold, a cup─shaped concavity, studded with very brilliant crystals, and more or less regular according to the quantity of metal employed, the tranquillity and slowness with which it has cooled, and the dexterity with which the fluid portion of the mass was poured off before it became solid. The same effect may be produced by melting bismuth in a crucible which has a hole in the bottom, lightly closed by an iron rod or stopper; this is to be drawn out when the mass begins to congeal. By so doing, the upper portion, which is fluid, is made to run off, and a cake studded with crystals will be left.

247. _COBALT is a semi─metal of grey colour with a shade of red, brittle, somewhat harder than silver, nearly eight times as heavy as water, is attracted by the magnet, and is itself capable of being rendered permanently magnetical._

_The ores of cobalt are not numerous, and are, for the most part, combinations of this substance with other metals, or of its oxides (24) with arsenic, or with sulphuric acid (21)._

The name of this metal implies an evil being, (_Kobold_, German, goblin) and is said to have been given on account of the vapour of arsenic, which issues from it, tormenting the miners, and making them believe that they are afflicted by wicked spirits. Hence it was once customary in Germany to introduce into the church service a prayer that God would preserve miners and their works from _cobalts_ and _spirits_.

Cobalt is found in several parts of Europe, but most plentifully in the southern borders of France, and in Saxony; and the cobalt ores of Hesse, although they were formerly used for no other purpose than the mending of roads, are said now to yield a clear profit of nearly 15,000_l._ a year. Some parts of our own country yield this substance in considerable abundance, particularly the Mendip Hills in Somersetshire, and a mine near Penzance in Cornwall.

After the ore is taken from the earth, it is broken into pieces about the size of a hen’s egg, and the stony parts are picked out. The sorted mineral is then pounded in mills, and sifted through brass─wire sieves. The lighter particles are next carried off by water. After undergoing some other preparations, to rid it of the impurities and foreign matters with which it is connected, it appears in the form of a dark grey oxide. The working of the cobalt ores in Germany is considered so injurious, on account of the arsenic with which they are combined, that much of the labour is performed by criminals who are condemned to it for the commission of crimes which, by the laws of the country, have deserved the punishment of death.

As a metal, cobalt was unknown till the year 1733, when it was discovered by a celebrated Swedish chemist whose name was Brant. In its metallic state it is not employed in the useful arts; but in a state of oxide it is found extremely valuable in the colouring of porcelain, in painting, enamelling, and for other purposes. Cobalt and ultramarine form the most permanent blue colours with which we are acquainted. The old painters generally used them for the representation of the sky and of blue drapery, and this is the reason why these parts in some old pictures have been found so much more durable than any others.

_Zaffre_ is an oxide of cobalt mixed with about three times its own weight of calcined and pounded flint. It has been chiefly imported into this country from Saxony and Bohemia, but it is now also manufactured from cobalt dug from mines in the Mendip Hills and in Cornwall. In Staffordshire there are several persons who carry on a considerable trade in preparing this colour for the earthenware manufacturers of that county.

This substance is extremely valuable for the colouring of porcelain and glass; as it resists without change, the effects of the most intense heat. Hence also it is advantageously used for giving various shades of blue to enamels, and to glass manufactured in imitation of lapis lazuli, turquoise, sapphire, and various precious stones. So intense is the colour imparted by it that a single grain of zaffre will give a full blue tint to 240 grains of glass.

_Smalt_ is a kind of glass, of dark blue colour, formed by melting zaffre with three parts of sand and one of potash; when this substance is ground to a coarse powder, it has the name of _strewing─smalt_, and is much used by sign painters, as an ornamental filling up of the vacant space betwixt the letters of signs. In Germany it is frequently employed instead of sand for the purpose of drying ink after writing. The same substance reduced to a perfectly fine or impalpable powder, is the article which is sold under the name of _powder─blue_, and which is not only used by laundresses and others in the getting up of linen, but also as the basis of several kinds of paint; and by the manufacturers of writing and printing papers, to give a blue tinge to those articles.

A solution of the oxide of cobalt in spirit of salt (muriatic acid, 29) and afterwards diluted till nearly the whole of its colour disappears, forms one of the most beautiful _sympathetic inks_ with which we are acquainted. If a landscape be drawn with Indian ink, and, afterwards, the foliage be washed over with this solution, it will have no peculiar appearance; but, on holding the paper near the fire, the part representing the vegetation will gradually assume a green tint, which will subside on removing the paper into a cool situation.

248. _MANGANESE, in the state that we usually see it, is a black oxide of a metal which is of a silvery grey colour, of leafy or foliated texture, and somewhat more than six times as heavy as water._

Mines of manganese have long been worked in several parts of Great Britain, but particularly in the counties of Devon and Somerset. Near Exeter and in the Mendip Hills this mineral is found in great abundance.

It is employed for various useful purposes. In the manufacture of the finer kinds of glass it is used in a double capacity, both as a colouring material and as a destroyer of colour. As a colouring ingredient, the imitators of several precious stones are indebted to it for the red and purple tints which they give to the oriental ruby, the balais ruby, and the amethyst.

The violet colour given to porcelain is obtained from manganese. This substance is also used for the glazing of black earthen ware, as a paint, and an ingredient in printers’ ink. As a discharger of colour it is applied in small quantities, and, by the oxygen which it gives out, it is said completely to destroy any tinge left in the glass, by the presence of iron, and some other colouring matters. This property has obtained for it the appellation of the _soap of glass_.

It is from manganese that all the oxygen gas (21) used by chemists is obtained. By the application of a red heat this is yielded in such abundance that an ounce of the oxide of this metal will yield about two quarts of gas. The consumption of manganese has, of late years, become very considerable by the discovery of the oxygenated muriatic acid, which is now extensively used in the bleaching of linen and cotton; that liquor being made by the distillation of the oxide of manganese with spirit of salt (muriatic acid, 29).

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COMPOUND MINERALS,

OR

ROCKS.

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249. There exist considerable masses of minerals in a state of combination, or aggregation with each other. These constitute the rocks and soil of which the globe of the earth is composed; and the study of them is called GEOLOGY. The opinions of learned men relative to their structure, and original formation, have produced various systems denominated _theories of the earth_; but, when we consider that the greatest depth beneath the surface to which the art and industry of man have been able to penetrate, does not exceed 1/35000 part of the earth’s diameter, we must confess that this is very insufficient to allow of any correct opinion being thereby formed concerning the structure of the whole.

Modern geologists, for the more convenient arrangement of the compound minerals, have divided them into four classes, which they denominate _primitive rocks_, _secondary rocks_, _alluvial depositions_, and _volcanic rocks_.

I. PRIMITIVE ROCKS.

250. _These are so called from their being considered by geologists, to belong to the first formed parts of the globe._

Rocks of this description are of a nature extremely hard. They contain no vestiges whatever of animal or organic remains; and the substances of which they are composed are crystallized. They rise through other rocks at various elevations, in every quarter of the globe; and never either alternate with, or rest upon rocks that enclose organic remains, though they are themselves frequently covered by such.

The following are the principal kinds.

251. _GRANITE or moonstone is a compound rock composed of felspar (110), quartz (76), and mica (123), each in crystalline grains of various size, and promiscuously arranged; sometimes one and sometimes the other of these ingredients predominates, but generally the felspar._

This is one of the most common and most widely extended rocks that are known; and is considered as the foundation on which the secondary rocks are deposited. In Cornwall it is very abundant, and veins both of copper and tin are found in it. Granite forms the summits of the highest mountains in Scotland, of the highest of the Grampian Hills, the Alps, and the Pyrenees; and indeed the loftiest parts of most of the countries of the world. The Logan or rocking stones, in Cornwall, are immense blocks of granite.

The uses of this stone are numerous and important. Millstones, steps, troughs for stamping mills, and innumerable other articles, are made of it. The streets of London are chiefly paved with granite, and its hardness and durability render it peculiarly eligible for this use. Weather has little effect upon it. Consequently, when applied to architectural purposes it is found infinitely preferable to Portland stone, of which nearly all the public buildings of modern date in London have been constructed, and many of which are fast going to decay. This circumstance induced the proprietors of the Waterloo Bridge to adopt granite in the construction of that edifice. Mr. Smeaton also chose it for the outer walls of the Eddystone Lighthouse.

252. SCOTTISH GRANITE.—Scotland is remarkable for many kinds of granite, some of which are susceptible of an excellent polish. The greatest part of the mountain of Ben Nevis, near Fort William, is composed of a reddish granite, one of the best and most beautiful that is known. This mountain is nearly a mile in perpendicular height, and is said to contain granite enough for all the kingdoms of the earth, although they should be as partial to this stone as the ancient Egyptians were. Columns and obelisks of any size and height might be cut from it: for the rock is one uniform mass, without appearance of strata, division, or fissure of any kind. A convincing proof has been given of the strength and hardness of this granite, in a fragment of several tons’ weight, which fell from nearly the top of a precipice five hundred yards in height, upon a hard and solid rock below, and yet continued entire.

253. GRANITE OF INGRIA.—A beautiful red granite is found in some parts of Russia, remarkable on account of the felspar (110) that it contains, appearing in round or oval pieces, from half an inch to two inches in diameter. This granite, when polished, exhibits shining spots of round or oval shape, which give to it somewhat the appearance of being studded with precious stones.

The royal summer garden at Petersburg is decorated with a superb colonnade of Ingrian granite. The columns are sixty in number, and each of a single piece twenty feet high, and three feet in diameter. Many of the public buildings in Petersburg are of this granite. An immense block of it thirty─two feet long, twenty─one feet broad, and seventeen feet high, forms the pedestal of the celebrated equestrian statue of Peter the Great, in that city.

254. GRAPHIC GRANITE.—A singular kind of granite has been discovered in the island of Corsica, and lately near Portsoy in the north of Scotland. The ground of this granite is a whitish or reddish yellow felspar, in which are embedded crystals of quartz each from an inch to an inch and half long, and several lines in diameter. The name of graphic granite was given to it in consequence of an imaginary resemblance which the sections of these crystals have to Hebrew, or Arabic, and sometimes to musical characters.

255. _GNEISS is a primitive rock, consisting, like granite, of felspar (110), quartz (76), and mica (123), but differing from that rock in its structure, being slaty._

Mountains of gneiss are not so steep as those of granite, and their summits are usually rounded. Ben Lomond and others in Scotland, and mount Rosa in Italy, are almost wholly of gneiss, as well as the middle part of the Pyrenees. It is not an uncommon rock, but in Britain is of less frequent occurrence than granite.

Many valuable metallic ores are found in veins of gneiss. This rock also sometimes contains crystals of garnet (70), and tourmaline (69).

256. _MICA SLATE, or MICACEOUS SCHISTUS, is a primitive rock of slaty structure, consisting principally of quartz (76) and mica (123)._

Like gneiss, it is rich in ores. It often contains beds of magnetic ironstone (235), galena (239), copper, blende (241), cinnabar (228), and sometimes even gold. It frequently has garnets, and sometimes tourmalines (69), interspersed in different parts of it.

Mica slate occurs in many parts of Scotland; the mountain of Schehallien, and the rocky adjacent country, are in a great degree composed of it.

257. _CLAY SLATE is a primitive rock generally of dull blue colour, more or less compact, always slaty, and always stratified._

Under the appellation of clay slate are included _roofing slate_ (120), _whet slate_ (122), _drawing slate_ (121), and some other kinds already described.

Few rocks abound more in veins and beds of valuable metals than slate. In different countries it contains ores of tin, lead, cobalt (247), silver, and copper; and gold, and mercury (228) sometimes occur in it. The celebrated quicksilver mines of Idria (228), and the immense mass of copper at Parys mountain in the island of Anglesea (230), are in clay slate. Crystals of pyrites (236), and sometimes garnets (70), and thin layers of quartz (76), and felspar (110), are all occasionally found embedded in it.

This is a widely─extended rock; it sometimes forms whole mountains, and even chains of mountains; but these usually have a gentle acclivity. The summit of the celebrated mountain called Skiddaw in Cumberland is of clay slate.

258. _PRIMITIVE LIMESTONE is a simple mountain rock of crystalline or granular structure; and generally of white, yellowish, greenish, or reddish colour._

To this species of rock belong many of the rich and beautiful kinds of marble already described (143, &c.). Carrara, or statuary marble (146), is a familiar instance of it. Whole mountains in Stiria, Carinthia, Carniola, and the Pyrenees, and three mountains in Switzerland, 10,000 feet in height, are of primitive limestone. The mountain of Filabres in Spain, is said to consist of one block of white granular marble, 2,000 feet high, and three miles in circuit; without intermixture of other earths or stones, and almost without a fissure.

Various mineral ores, in beds and veins, as lead, zinc (241), and iron, are occasionally found in this kind of rock.

259. _PRIMITIVE TRAP is a mountain rock composed of a black mineral called hornblende, mixed, in some varieties, with felspar (110), and, in others, with mica (123)._

The word _trap_ is of German origin, signifying a _stair_; and rocks of this formation are called trap rocks, because their strata, when exposed, usually jut out, one beneath the other, somewhat like a stair. Under this term is comprehended a series of rocks, distinguished chiefly by the hornblende, which they all contain.

Rocks belonging to this formation are numerous. They occur in Scotland; and abundantly in Derbyshire and some other parts of England. In many countries they constitute considerable hills. They abound in ores.

260. _SERPENTINE is a primitive rock, usually consisting of quartz (76), magnesia (198), alumine (197), with a portion of oxide (21) of iron._

This rock and its various uses, have been already described (132). It generally occurs in shapeless masses and beds; and seldom in distinct strata. It is found in Cornwall, the island of Anglesea, and several parts of Scotland; but it rarely forms mountains.

Ores of lead, silver, and copper, are sometimes found in serpentine.

261. _PORPHYRY is a primitive rock, consisting of quartz (76) or felspar (110), or both, embedded in a solid and compact cement or ground._

_The ground or basis of porphyry varies in the different kinds. In some it is claystone, in others pitchstone, hornstone, or compact felspar._

When not covered by other formations, porphyry sometimes forms single rocks; but it never constitutes elevated mountains. It occurs in beds of considerable magnitude, but never appears in distinct and well─defined strata.

There are many beautiful and splendid works in porphyry. Obelisks, statues, and columns, wrought in it, have had great celebrity. It is susceptible of a polish as high as that of marble, but is so hard, that the expense of working it has caused it to be much neglected by the moderns. This hardness, however, renders it very durable, and also constitutes it a material of great utility for mortars, slabs for grinding colours upon, and for several other purposes.

Porphyry was much esteemed by the ancient Egyptians; and Pliny informs us that the procurator─general in Egypt, under Claudius Cæsar, brought thence, for that Emperor, certain statues of porphyry, which he conceived to be very valuable: this act, however, was not much approved, and the example was not followed by any other Roman.

The principal quarries of porphyry are in Egypt; but this stone is also found in Italy, Germany, and various parts of the European continent. It may be traced from Norway to the borders of the Black Sea, and it has been discovered in some of the western and northern parts of Great Britain.

262. _SIENITE is a rock composed chiefly of felspar (110) and hornblende. Its colours are usually reddish and black._

_Some varieties of it contain quartz (76) and mica (123), with but little hornblende. In these the colours are various._

Although this is a less abundant rock than any of those that have yet been mentioned, it occurs, in great abundance, at Mount Mado, in the island of Jersey. There are extensive quarries of it in that mountain, not only for the use of the island, but for exportation to England, and other distant countries. The cliffs, for a long space, and an elevation of a hundred feet or more, consist entirely of sienite, in large masses, which are apparently uninterrupted by a single fissure. Shafts for columns of considerable length have been taken from these quarries; and, were the demand sufficient to call for new openings, it is imagined that columns of twenty feet and upwards in length might be raised. The felspar is of a flesh colour, and the stone is capable of a beautiful polish.

A somewhat similar kind of sienite is found at Grande Roque, in the island of Guernsey, in large masses, which are quarried for building stones. Sienite also occurs in some parts of Scotland and Derbyshire; in Saxony, Hungary, the island of Cyprus, and Egypt. Its name has been derived from that of the city of Syene, in Upper Egypt, where it is found in great abundance.

Sienite was much used by the ancients in ornamental architecture. What was called the red granite of Egypt (for this rock has usually been considered a granite) furnished numerous magnificent obelisks and columns, of a single piece, which have been much admired in Rome and other places. The ancient artists sometimes cut this kind of stone into statues, vases, monumental and other works. The celebrated column in Egypt, upwards of ninety feet high, and known by the name of Pompey’s Pillar, is formed of sienite.

In veins of this rock are found, in different countries, many kinds of metallic ores: among others, silver, iron, tin, copper, and lead.

263. _QUARTZ ROCK is a simple mountain rock, usually of granular texture, and whitish colour._

_It sometimes contains mica, in which case it has a slaty form._

In certain mountains of Scotland, and the Scottish islands, quartz rock is very abundant. On the Continent it appears in Saxony, Bohemia, Silesia, and several other countries. We are informed that a mountain, 350 feet high, and near 5000 feet broad and long, one of the Altaisch chain, in Siberia, consists entirely of a milk─white quartz.