Chapter III: Preface: ─────── (3)

_Glass_ is made by mixing sand, or prepared flint, with a certain proportion of soda (200) or potash (205); and exposing these substances, in a furnace, to a violent heat. When they are in a perfectly fluid state, part of the melted matter is taken out at the end of a long hollow tube. This is done by dipping the tube into it, and turning it about until a sufficient quantity is taken up; the workman, at each turn, rolling it gently upon a piece of iron, to unite it more intimately. He then blows through the tube till the melted mass, at the extremity, swells like a bubble; after which he rolls it again on a smooth surface to polish it, and repeats the blowing until the glass is brought as nearly to the size and form of the vessel required, as he thinks necessary.

If he be forming a common bottle, the melted matter at the end of the tube is put into a mould of the exact size and shape of the body of a bottle; and the neck is formed by drawing out the ductile glass at the upper extremity.

If he be making a vessel with a large or wide orifice, the glass, in its melted state, is opened and widened with an iron tool; after which, being again heated, it is whirled about with a circular motion, and, by the centrifugal force thus produced, is extended to the size required. Should a handle, foot, or any thing of similar kind be required, that is made separately, and stuck on in its melted state.

_Window glass_ is made in a similar manner, except that the mass at the end of a tube is formed into a cylindrical shape. This being cut longitudinally by scissars or shears, is gradually bent back until it becomes a flat plate.

Large _plate glass_ for looking─glasses is made by suffering the mass, in a state of complete fusion, to flow upon a casting table, with iron ledges. These confine the melted matter, and, as it cools, a metallic roller is passed over it, to reduce it to an uniform thickness.

Glass utensils, unless very small and thin, require to be gradually cooled in an oven. This operation is called _annealing_, and is necessary in order to prevent them from cracking by change of temperature, wiping, or slight accidental scratches.

It appears that the manufacture of glass was known very early; but glass perfectly transparent was esteemed of extremely high value. It is stated that the Emperor Nero purchased two glass cups with handles for a sum which was equivalent to 50,000_l._ of our money. The windows of some of the houses of the ancient city of Pompeii, which was buried by an eruption of Mount Vesuvius, in the year 79, were glazed, but the glass was thick, and not transparent.

By many persons flint is used as a test for ascertaining the purity of silver coins. This is done by rubbing them upon the flint; and if the mark which they leave be not perfectly white, they are rejected as counterfeit.

91. _CALCEDONY is a species of quartz, generally of whitish, bluish, or smoke─grey colour; and, when broken, it appears internally dull, and somewhat splintery._

_It is generally found in a massive state, is harder than flint, generally semi─transparent, and 2½ times heavier than water._

The name of this stone is derived from Chalcedon, in Upper Asia, whence it appears to have been originally obtained, and where it is still found in considerable abundance. Several superb specimens of calcedony have been found in Britain, and particularly in some of the tin and copper mines of Cornwall. It occurs in several parts of Scotland; and in many of the countries of the Continent. In the Leverian Museum there was a specimen of calcedony, which weighed more than 200 pounds. Its whole surface appeared such that, at first sight, one might imagine it to have formerly been in a liquid state: it had much the appearance that thick oil has while boiling.

Few stones are susceptible of a higher or more beautiful polish than calcedony. Hence the different varieties of it are cut into ring and seal stones, necklaces, ear─pendants, small vases, cups, and snuff─boxes.

92. _ONYX is a kind of calcedony, generally marked alternately with stripes of white and black, or white and brown._

Its name is derived from the Greek language, and has been given on account of its resemblance in colour to the whitish band at the base of the human nail. The distinction which appears to be made betwixt onyx and _sardonyx_, arises from the colours of the former being arranged either concentrically, or in a somewhat confused manner, and those of the latter in regular stripes or bands.

Both these kinds are highly esteemed by lapidaries, for the formation of vases, snuff boxes, and trinkets of various kinds. Of sardonyx the ancients made those beautiful cameos, many of which still ornament our cabinets. The ingenuity they have shown, in the accommodation of the natural veins and marks of the stone to the figures engraven upon them, is such as to excite, in many instances, the greatest admiration.

It is said that we are entirely ignorant of the country whence the ancient artists obtained the large specimens of sardonyx which are now found in some cabinets.

Onyx is imported from the East Indies, Siberia, Germany, and Portugal.

93. _CARNELIAN is another kind of calcedony usually of a red or flesh colour, though sometimes white, orange, or yellow._

On several of the British shores carnelians are found with other pebbles: but the most beautiful and valuable kinds are imported from the East Indies. These are sometimes so large as to measure nearly three inches in diameter. The kinds principally in request are those of pure white, and bright red colour; and jewellers have the art of changing the colour of the yellow varieties to red, by heat.

No stone is so much in request for seals as carnelian. It is likewise cut into beads for necklaces, and stones for ear─rings; into crosses, bracelets, and other trinkets, which, in India, form a considerable branch of traffic. The amount of the sale value of different kinds of carnelian goods vended by the East India Company in 1807, was 11,187_l._: but, in other years, it has not usually been so much as half that sum.

Formerly carnelians were exported from Japan to Holland; and thence were carried to Oberstein, in France, to be exchanged for the agates of that country, which were exported to China.

The carnelian was much esteemed by the ancients; and many fine engraved carnelians are preserved in different collections.

94. _CHRYSOPRASE, an extremely hard kind of stone, of clear and delicate apple─green colour, is considered to be a kind of calcedony._

This beautiful mineral has hitherto been found only in the vicinity of Kosemitz, and in a few other parts of Lower Silesia. It is susceptible of a high polish, and is much prized by jewellers when its colour is deep and pure. Its colour, however, is so fugitive, that, if kept in a warm and dry situation, it loses the greatest part of it; and if exposed to moisture it becomes much altered. Lapidaries assert, that great care ought to be taken in the polishing of it;—pretending that if, from want of sufficient moisture, or by the too rapid motion of the wheel, it be over─heated, it will become whitish or turbid.

Chrysoprase is generally cut into a convex form, or what jewellers call _en cabochon_; and is set with green taffeta beneath it, as foil. It is used for ring stones, brooches, and other ornaments; and is found to harmonize well with diamonds and pearls. The larger and more impure masses are cut into snuff─boxes, seal stones, and similar articles. Some of the finest specimens of chrysoprase that are known, are to be seen in the cathedral church of Prague, where a small closet is inlaid with them.

Imitations of chrysoprase are sometimes imposed upon the public; but these are easily known by persons who are acquainted with the nature of precious stones.

95. _BLOODSTONE, or HELIOTROPE, is an opaque stone of the quartz family, generally of dark green colour, with a somewhat bluish cast, and marked with blood─red spots or stripes._

_It usually occurs in masses of irregular form; and, when cut thin, is sometimes translucent at the edges._

The most valuable kinds of bloodstone are imported from the East. They are not so opaque as those which are found in Germany, and are marked with more vivid spots. As bloodstone is capable of a high polish, and is even better calculated for engraving upon than carnelian (93), it is in great request for seal stones, for the tops and bottoms of snuff─boxes, and other articles on which costly gold mountings are frequently bestowed. Its dark colour and opaque appearance prevent its being much used for beads. Great quantities of it are consumed in China as ornaments to the girdle clasps of the superior ranks of people. Absurd as it may appear, many persons entertain a notion that this stone worn in the dress will prevent bleeding at the nose. Good bloodstone and carnelian are considered to be about the same value.

There are many cameos and intaglios, both by ancients and moderns, executed in bloodstone. In the National Library at Paris, there is a fine engraved stone of this kind, representing the head of Christ whilst undergoing the punishment of scourging, and so cut that the red spots are made to represent drops of blood.

The ancients procured bloodstones chiefly from Ethiopia; but, at present, the most highly esteemed varieties are brought from Bucharia, Great Tartary, and Siberia. A kind of mineral nearly resembling this is found in Rum, one of the western isles of Scotland.

The spots in bloodstone are particles of red jasper.

96. _JASPER is a species of quartz, and one of the hardest stones with which we are acquainted. It varies much in colour, being red, green, yellow, blue, olive, violet, black, and often variegated, spotted, or veined with several other colours. It is usually opaque, but is capable of receiving a beautiful polish._

_This stone is found in large and shapeless masses, and constitutes an ingredient in mountains of various parts of the world._

Such is the hardness of jasper, that the savages of Canada avail themselves of it for the fabrication of the heads of javelins, and sometimes also of arrows. It is used by artists for the formation of vases, snuff─boxes, seals, and trinkets of various kinds; and formerly cups and saucers were sometimes made of it. Many beautiful antique engravings have been made upon jasper.

In the province of Andalusia, in Spain, there are four fine quarries of jasper. One of these is celebrated for a blood─red stone, streaked with white, exceedingly hard and very handsome, of which the beautiful columns of the tabernacle in the Escurial are made. This quarry is in the territory of Cogullus, in the archbishopric of Seville, and was purchased by the Crown in 1581; but was afterwards so far neglected that even the place where it lay was not remembered. It was, however, again discovered about the end of the reign of Charles the Third, after a very expensive search made by order of the government.

Jasper occurs in the Pentland hills, near Edinburgh, and in several other parts of Scotland; in the Shetland Islands, and Hebrides. It has been observed in most of the countries of the Continent; and is found, in great abundance, in Siberia.

97. RED JASPER _is an opaque red stone which is found embedded in red clay─ironstone in Baden_; and is cut and polished for various ornamental purposes. There are extant many fine antique engravings on red jasper.

98. EGYPTIAN PEBBLE _is a kind of jasper, that is found in globular or rounded pieces, and is distinguishable when cut or broken, by its numerous colours, arranged in concentric stripes or layers_. It is chiefly brought from Egypt; and, as it is capable of receiving a fine polish, and when polished is very beautiful, it is manufactured into several kinds of ornamental articles. From the great abundance in which it is supplied, it is, however, much less valuable than carnelian (93). The colours of the Egyptian pebble frequently assume very singular forms. There was one in the Leverian Museum which exhibited, in the centre, the resemblance of a pantaloon, or a man wearing a fool’s cap.

99. STRIPED, or RIBBON JASPER, _is marked with alternate stripes of different colours_; and is found in Siberia, Saxony, and even in the Pentland hills, near Edinburgh. It receives an excellent polish, and is frequently cut into the tops and bottoms of snuff─boxes. The red and green layers of jasper, being well defined and regular, this kind is used for several purposes of ornament, particularly for cameos.

100. _AGATE, or AGATE JASPER, as some mineralogists denominate it, is a semi─transparent stone of the quartz family, which is capable of receiving a high and very beautiful polish._

_These stones are always found in a shapeless or massive form, and nearly of all colours, except bright red and green._

The name of agate is derived from the river Achates, in Sicily, in the vicinity of which these stones were obtained by the ancients in great abundance. They are now found in several parts of Scotland; in Iceland, Saxony, and Hungary; and they are occasionally brought into Europe from China and the East Indies.

Agates are used in several kinds of ornamental work, and particularly for necklaces and seals. They are occasionally made into cups, the handles of knives and forks, hilts of swords and hangers, and the tops and bottoms of snuff─boxes. The less ornamental kinds are manufactured into small mortars, which are employed by enamellers and others, for pounding such substances as are too hard to be reduced in any other way. They are also made into instruments for grinding colours, and into polishers for the glazing of linen. In the Electoral Cabinet at Dresden, and the Ducal Cabinet in Brunswick, there are several elegant vases formed of agate.

The most beautiful agates which our island produces are known by the name of _Scots Pebbles_. These are found in various parts of Scotland, but principally on the sea─shore, in the neighbourhood of Dunbar. _Agate pebbles_ are found on several of the English shores, as those of Suffolk, Dorset, Scotland, Wales, and Ireland; and sometimes even in gravel pits. Many of them will bear cutting and polishing as well as the best agates of foreign countries.

Agates are occasionally seen to be figured in very singular manner; but this, in some instances at least, is suspected to be the work of art. One is mentioned in the church of St. Mark, at Venice, which had the representation of a king’s head surmounted by a diadem. On another, was represented a man in the attitude of running. But the most remarkable of all seems to have been one which contained a representation of the nine Muses, with Apollo in the midst of them!

It must be remarked that agate is not, as some mineralogists imagine, a simple mineral, but that it is composed of various species of the quartz family, intimately blended together. It consists chiefly of calcedony (91), with flint, hornstone, carnelian (93), jasper (96), cacholong (105), amethyst (79), and quartz (76). Of these minerals sometimes only two, and sometimes three or more, occur in the same agate. Its varieties, consequently, are extremely numerous.

101. MOCHOA STONE _is a kind of agate, which has on its surface the resemblance of moss_; and this so nearly approaching a natural appearance, that some persons have actually supposed it to be occasioned by a condensation of moss into stone. Its name is derived from _mocks_, the German word for moss.

These stones are used for several ornamental purposes; and are not unfrequently imitated, by spreading a solution of copper in nitric acid or aqua fortis (30) over the surface of a plain agate, and then setting a small iron nail on its head in the middle. The acid unites with the iron, and deposits the copper in beautiful ramifications from the centre. The nail must then be removed, and the surface carefully washed by dipping the stone into warm water. Afterwards, on the application of a moderate heat, the copper becomes black. As, however, the deposition is merely superficial, it requires to be covered with glass, to preserve it from injury.

102. _OPALS are a semi─transparent kind of stones, which have a milky cast, and, when held betwixt the eye and the light, exhibit a changeable appearance of colour._

_They are always found in a shapeless or massive state, are brittle, and considerably less hard than most other precious stones._

The only opal mines in the world are those of Hungary. About four centuries ago, opals were obtained, in such abundance, from these mines, that upwards of three hundred persons were employed in them. They still produce opals, some of which are so valuable as to pass, in commerce, under the appellation of _oriental opals_, whilst others are so poor as to be of no value whatever to the jeweller. Opals are also found in other parts of Europe; and in the island of Sumatra and several parts of the East Indies.

Few precious stones are more beautiful than opals. Their elegant play of colours, brilliant blue, green, red, and yellow, variously modified, has procured for them a distinguished rank among gems. Notwithstanding this, they are but ill suited to the purposes of jewellery, on account of their softness, their great frangibility, and their sometimes splitting on a sudden change of temperature. They are usually set without bottoms; but sometimes with a black bottom, and sometimes with a foil of red, blue, or gold colour. Their value is such that a fine oriental opal is considered worth about twice as much as an oriental sapphire of the same size. By the Turks they are so peculiarly esteemed, that a fine opal of moderate size has sometimes been sold at the price of a diamond. The esteem in which they were held among the ancient Romans was such, that Nonius, the Roman senator, is stated to have preferred banishment to parting with a favourite opal which Mark Antony was anxious to possess.

In the abbey of St. Denys, near Paris, there was formerly a curious ancient opal which was green on the outside, and, when viewed against the light, exhibited a fine ruby colour: and in the Imperial Cabinet at Vienna, there are two pieces of opal, from the mines in Hungary, one of which is about five inches long, and 2½ inches broad; and the other the size and shape of a hen’s egg. Both these stones exhibit a very rich and splendid play of colours.

In the purchasing of opals great caution is requisite, as fine glass pastes have not unfrequently been substituted for them, and sold at enormous prices.

103. HYDROPHANOUS OPAL, _or_ OCULUS MUNDI, _is a kind of opal, the distinguishing characteristic of which is, that it gradually becomes transparent, and exhibits a beautiful play of colour after being immersed in water_. It is either of a whitish brown, yellowish green, milky grey, or yellow colour, and opaque; and, when touched by the tongue, adheres to it.

The name of oculus mundi has been given to these stones from an internal luminous spot, which changes its position according to the direction in which they are held to the light. The countries in which they are chiefly found are Hungary and Iceland.

They are sometimes set in rings; and the prices at which they were formerly valued were, in the highest degree, unaccountable and absurd. At present their value is considerably lower, though they are still in great request as objects of curiosity. The phenomenon of their becoming transparent in water is supposed to be occasioned by that fluid soaking through their whole substance, in the same manner as the transparency of paper is occasioned by immersing it in oil. An hydrophanous opal weighing 27½ grains was kept four minutes in water, and, on being taken out, weighed 32½ grains, having received in this short period an augmentation of five grains, or more than one sixth part of its whole weight. When taken from the water, these stones as they dry become again opaque.

To preserve them in beauty and perfection, care should be taken not to immerse them in any but pure water, and to take them out as soon as they have acquired their full transparency. If these precautions be neglected, the pores will soon become filled with earthy particles: the stones will cease to exhibit their peculiar property, and will ever afterwards continue opaque.

104. COMMON OPAL _is a semi─transparent kind of opal, which does not exhibit any changeable refraction of colour_. It is found in Germany, France, Italy, and other countries of the Continent, and is employed for brooches and other ornaments. A green─coloured Saxon variety is sometimes cut into ring─stones.

105. MOTHER─OF─PEARL OPAL, _or_ CACHOLONG, _is a milk─white, yellowish, or greyish─white kind of opal_, which occurs in Iceland, Greenland, Spain, and the island of Elba. It is sometimes cut into a concave form, for brooches, and other female ornaments. Italian artists also use it for mosaic work.

106. WOOD OPAL _appears to be wood that, by some extraordinary operation of nature, has been converted into opal_. Some specimens exhibit, very beautifully, the ligneous texture. This kind of opal is chiefly cut into plates for the tops and bottoms of snuff─boxes. It is found in alluvial land in some parts of Germany and Hungary. Several years ago the trunk of a tree, penetrated with opal, and so heavy that eight oxen were requisite to draw it, was found in Hungary.

PITCHSTONE FAMILY.

107. _OBSIDIAN is a kind of glass, generally of blackish colour, formed in volcanoes, from which it issues in thick streams._

This substance has been used for various purposes. It is possible to cut and polish it; but its brittleness and frangibility are so great, that, without much care, it will fly into pieces during the working. The reflectors of telescopes are sometimes formed of it. In Mexico and Peru obsidian is cut into mirrors; and the inhabitants of those countries used formerly to manufacture it into knives and other cutting instruments. Hernandez says that he saw more than a hundred of these knives made in an hour. Cortez, in a letter to the Emperor Charles the Fifth, relates that he saw razors that had been formed of obsidian. The natives of Easter and Ascension Islands use this substance for cutting instruments; and also for pointing their lances and spears, and, in place of flint, for striking fire with. According to the account that has been given by Pliny, the ancients sometimes formed obsidian into mirrors, and ornamental articles of different kinds. The Danish lapidaries, who obtain considerable quantities of it from Iceland, cut it into snuff─boxes, ring─stones, and ear─pendants.

Obsidian is found near Mount Hecla, and in other parts of Iceland. Sir George Mackenzie, during his journey through that island, observed an immense mass of this substance, which appeared to him to have been part of a stream that had flowed from a volcano. It is also found in Sicily, and several other islands of the Mediterranean; and in nearly all parts of the world where there are volcanoes.

108. _PUMICE is an extremely light and porous mineral, of somewhat fibrous texture, and of white, grey, reddish, brown, or black colour._

From the texture of this mineral, which is chiefly brought from the neighbourhood of volcanoes, some persons have imagined it to be asbestos decomposed by the action of fire. Its lightness is such that, placed in water, it will float.

To mechanics and other artists pumice stone is a very useful mineral. It is employed for cleansing and smoothing the surface of wood, leather, metal, stones, glass, and other substances; and it is used by parchment─makers, curriers, and hat─makers. Hence it forms a considerable article of trade: and is exported from the Lipari Islands, in great quantities, to the different countries of Europe. Sailors in the Mediterranean rub their beards off with pumice, instead of shaving. On account of its porosity, it is used in Teneriffe as a filtering stone. It forms a pernicious ingredient in some kinds of tooth powder; and in Italy is ground and used instead of sand, in the making of mortar. Pumice occurs in Ireland, along with obsidian (107); and it abounds in several islands of the Grecian Archipelago.

AZURE STONE FAMILY.

109. _LAPIS LAZULI, AZURE STONE, or LAZULITE, is a mineral of azure─blue colour in various shades, and generally accompanied with white or clouded spots, and also with pyrites (236), which have the appearance of golden veins or spots. Its texture is earthy, and fracture uneven. It is opaque, or nearly so, and, in some parts, is sufficiently hard to strike fire with steel. We are not informed that lapis lazuli is otherwise found than in shapeless masses or lumps._

About fifty years ago this stone was an article much in fashion for various ornamental parts of dress. Being capable of very high polish, it was cut into beads, stones for rings, bracelets, and necklaces. It was also cut into ornamental vases, small statues, and the tops and bottoms of snuff─boxes; but of late it has been almost wholly out of use for these purposes. Before the French Revolution it was imported, to considerable extent, into that country from the Persian Gulf for the inlaying of richly─decorated altars; and its value was appreciated according to the proportion of its yellow spots or veins: these, by many persons, were erroneously considered to be of gold.

The most important purpose to which lapis lazuli is now applied is in the manufacturing of the beautiful and brilliant blue colour so much esteemed by painters, called _ultramarine_. For the making of this, such pieces are selected as contain the greatest proportion of blue substance, and consequently the least yellow or white. These are burned or calcined, reduced to a fine powder, made into a paste with wax, linseed oil, and resinous matters of different kinds, and afterwards separated by washing. The powder that is left in this operation, which requires much time and great attention to perform, is ultramarine.

There are few colours so little susceptible of change from the effects of time as ultramarine: the consequence of this has been that, as several of the ancient painters introduced it for the representation of blue drapery, their pictures, in many instances, are now devoid of harmony, as this colour alone has stood, whilst all the others have changed.

Lapis lazuli is principally brought from Persia, Natolia, and China; but it is also found in Siberia and Tartary. In Europe it has been discovered only in Germany, and among the ruins of Rome.

A coarse imitation of it is sometimes made by throwing copper filings into blue enamel whilst in a melted state.

FELSPAR FAMILY.

110. _COMMON FELSPAR is a hard kind of stone which varies much in colour, being flesh─red, bluish grey, yellowish white, milk─white, or brownish yellow._

_It is found in a massive state, disseminated or crystallized in four, six, and ten─sided prisms; will strike fire with steel, and is sometimes opaque and coloured, sometimes transparent and whitish._

The name of felspar is derived from the German language, and signifies _spar of the fields_. It is a very common substance, and constitutes a principal part of many of the highest mountains of the world. When exposed to weather, it gradually acquires an earthy appearance, and at length passes into porcelain clay (118).

Felspar is of great use in the manufacture of the finer kinds of earthenware. Of the two substances which chiefly compose the porcelain of China, one called _petunzé_ is a whitish laminar kind of felspar. This mineral is used in the celebrated porcelain that is manufactured at Sevres, near Paris, for the purpose of giving to it a white and transparent appearance. Previously to being used, it is pulverized, made into a paste, and suffered to dry. It is sometimes applied to the surface of ornamental vases in the form of enamel.

111. AMAZON STONE _is a green variety of felspar_, which is found in small rolled pieces on the bank of the river of Amazons, in South America, whence it has its name. It is susceptible of a beautiful polish, and is often cut into ring─stones, brooches, and the tops of snuff─boxes. Lapidaries consider it to be most estimable when accompanied by mica, which gives it a kind of speckled perlaceous appearance.

112. _LABRADOR FELSPAR is a very beautiful stone, of smoky grey colour, intermingled with veins and shades of blue, green, and golden yellow, exhibiting a brilliant play of_ _colours, according to the position with respect to the light in which it is held._

The original discovery of this singular mineral was by the Moravian missionaries, on the island of St. Paul, near the coast of Labrador; but it has since been found in various parts of Norway and Siberia. Persons who have passed in boats along the rivers of Labrador, have described the extremely brilliant and beautiful appearance which the rocks of this substance frequently exhibit in shallow places, at the bottom of the water. The visitors of the late Leverian Museum will, no doubt, recollect a remarkably fine mass of Labrador felspar, the surface of which was polished, and exhibited some of the most splendid and beautiful colours that can be imagined. It was considered to have been the most capital specimen that was ever brought to England.

This mineral, on account of its hardness, its brilliancy, and its capability of receiving a high polish, is in considerable estimation among lapidaries for different kinds of ornamental work, particularly for the tops and bottoms of snuff─boxes, for brooches, and necklaces.

113. _MOONSTONE, or ADULARIA, is the purest kind of felspar that is known; and is considered to have the same relation to common felspar that rock crystal has to common quartz. Its colour is white, sometimes with a shade of yellow, red, or green._

The translucent varieties of this stone, when viewed in a certain direction, sometimes exhibit a pearly and silvery play of colour. These are valued by jewellers, who cut them into a semi─globular form, and sell them under the name of moonstone. Those specimens are considered most estimable which, when cut in a very low oval, present the silvery spot in the centre of the stone. They are generally used for rings and brooches; and when set round with diamonds, their pearly lustre exhibits a striking and agreeable contrast with the brilliancy of that gem.

Adularia is said to have been first discovered by an Italian mineralogist, near Mount St. Gothard, in Switzerland. He named it Adularia felspar, in the belief that the mountain on which he had found it was named Adula. This, however, was not the case; for Mount Adula is at some distance from St. Gothard, in the Grisons. This mineral has since been found in the granite of the island of Arran, in France, and Germany. The finest specimens are brought from Ceylon.

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ORDER II.–SOFT STONES.

(Those which will not scratch Glass.)

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CLAY FAMILY.

114. Clay is a mixture of alumine (33) and silex (38), and is too well known to require much description.

It is opaque, has an earthy texture, is about twice as heavy as water, when moistened is very ductile, adheres slightly to the tongue; and with its peculiar smell (called clayey) every one is acquainted.

115. _COMMON CLAY, or POTTER’s CLAY, which is found in nearly every country of the world, is sometimes white, has a blue or yellowish tinge, or is brown or reddish._

It is the peculiar quality of this substance to become so hard by heat that it will even strike fire with steel. The ductility of clay, and its property of thus hardening in the fire, have rendered it an article of indispensable utility to mankind in all civilized countries. It is formed into eating vessels of almost every description; plates, dishes, cups, basins, bowls, and pans for keeping provisions in. For these almost any kind of clay may be advantageously used; but it is necessary to mix it with sand, for the purpose of rendering the vessels that are made of it more firm and strong. Those that are applied to culinary, and other uses in which it is requisite for them not to be penetrable by water, are covered with a glazing. This glazing, for coarse ware, is sometimes made with lead, and sometimes by throwing a certain portion of salt into the furnace. In the formation of the better kinds of earthenware, the clay is made into a paste with water, moulded into the requisite shape upon an horizontal wheel, the inside being formed by one hand of the potter, and the outside by the other, as the wheel turns round. When the pieces have been baked, they are dipped into a glazing mixture, consisting of white lead, ground flints, and water, and are exposed a second time to the fire. The different colours of earthenware are obtained by means of various kinds of metallic oxides (21).

The coarser kinds of clay are manufactured into _bricks_ for the building of houses, and _tiles_ for the covering and paving of them. These are formed in moulds of the requisite shape, afterwards dried for some time in the sun, and finally piled in kilns, and there baked to a proper degree of hardness. The earth for bricks ought to be sufficiently fine, free from pebbles, and not too sandy, which would render them heavy and brittle; nor ought it to be entirely free from sand, as this would make them crack in drying.

Clay is a substance of inestimable value for forming the bottoms of ponds, and the bottoms and sides of canals and reservoirs, to prevent the water from draining away. It also composes, in a great measure, those tenacious earths called arable soils. What is peculiarly denominated clay land is known by its holding water, and not soon drying when wetted. Such land requires much labour from the husbandman, before it can be sufficiently pulverized, or brought to a fit state for being productive of corn or grass.

116. PIPE CLAY _is a fine and yellowish white variety of common clay_. It is very plastic, adheres strongly to the tongue; and, in a strong heat, is hardened, and rendered perfectly white.

It is of this clay that tobacco pipes are made, by the simple process of casting them in moulds, forming a hole through the stems by means of a wire, generally dipping the small end into some glazing material, and then baking them. Pipe clay is also formed into oblong pieces, dried, and employed for cleaning white woollen cloths, and for various purposes of domestic utility. It is likewise the basis of the yellow, of what is called _Queen’s ware_ pottery. This is glazed in a manner somewhat different from that of common pottery. The glazing mixture consists of a certain proportion of carbonat of lead (239), ground flint, and flint glass, worked with water to the thickness of cream. The ware, before it is glazed, is baked, and thus acquires the property of strongly imbibing moisture. It is then dipped into the above composition; exposed a second time to the fire, by which the glaze it has imbibed is melted. A thin glossy coat is thus formed upon its surface, which is more or less yellow, according to the greater or less proportion of lead that has been used.

117. _LOAM is a yellowish or brownish kind of clay; sometimes containing a considerable proportion of sand. It occurs in immense beds, and is found in almost every part of the world._

This substance, when mixed with straw or hair, to prevent it from cracking, is extensively used for the building of what are called mud cottages or houses. These are generally reared on a foundation of stone, or brickwork, to secure them from injury by the moisture of the earth. It is said to be the most advantageous practice to form the loam into bricks, and to dry these in the shade, and afterwards in the sun. The use of such bricks is of great antiquity. We are informed that the ancient city of Damascus, and even the walls of Babylon, were constructed of bricks made of loam.

118. _PORCELAIN CLAY is generally of white or reddish white colour, sometimes inclining to yellowish or grey. When dry, it absorbs moisture rapidly; and it becomes very tenacious when kneaded._

_It is known from the other clays by the fineness of its particles, its soiling the fingers much when handled, and its fine but meagre feel._

The usual distinction betwixt earthen ware and porcelain is, that the former is opaque, and the latter semi─transparent. In the manufacture of porcelain the clay is sometimes used alone, and sometimes intermixed with other earths, or with felspar (110). The earliest manufacture of porcelain is supposed to have been that in China and Japan. The quantity produced in China must formerly have been extremely great; as not only a considerable portion of the eastern parts of the world, but almost the whole of Europe, was supplied with it. In a single province it is said that nearly a million of persons were at one time employed in this manufacture.

The manufactory at Sevres, in France, has long been celebrated both for the excellence and elegance of its porcelain. There are well─known manufactories of porcelain at Meissen in Saxony, at Berlin, and in Austria; but none of these are at present superior to our own, in Worcestershire and Staffordshire.

Porcelain clay occurs chiefly in countries which abound with granite (251) and gneiss (255). It is found in small quantity in Cornwall, and other granite districts of England, as well as in those of Scotland and Ireland. But the most valuable kinds of this clay are found in China and Japan.

The mineral is not used in the state in which it is found in the earth; but is previously washed several times to free it from impurities. After the process of washing, only about fifteen parts of pure clay remain: this is the _kaolin_ of the Chinese. To form the composition of the porcelain, this clay is mixed, in certain proportions, with quartz (76), flint, gypsum (192), steatite (124), or other substances; and the mixture is sifted several times through hair sieves. It is afterwards moistened with rain water, and, in the form of a paste, is put into covered casks. Here a fermentation soon takes place, which changes its smell, colour, and consistence. Its colour passes from white into dark grey; and the matter becomes both tougher and more soft than before. The peculiar mode of preparing this mixture, and the art of rightly managing it, are secrets in most porcelain manufactories.

The next operation consists in giving to the paste thus formed the requisite shape of the vessels. This is done first by kneading it with the hands; and then by taking up certain portions of it, and turning it on a lathe, in the manner of common pottery (115), but with more care.

The third operation is the baking or firing. This is done in furnaces of a particular construction, and generally lasts from thirty─six to forty─eight hours. The state of the baking is shown by proof pieces, as they are called, which are placed in convenient situations, and can be drawn out, from time to time, for examination. The porcelain in this state, is named _biscuit porcelain_; and figures, and such other porcelain articles as are neither to be painted nor exposed to water are in the state of biscuit.

A fourth operation is covering the surface of the biscuit with a varnish or enamel. This is composed of pure white quartz (76), white porcelain, and calcined crystals of gypsum (192); and sometimes principally of felspar (110). These substances are carefully ground, then diffused through water, and formed into a paste. When used, the paste is diluted in water, so as to give it considerable fluidity; and the pieces of biscuit porcelain are separately plunged into it, in such manner as to cover their whole surface. These are then exposed to a heat sufficient to melt the enamel or covering: and in this state they constitute white porcelain.

If the porcelain is to be painted, it must again be exposed to heat in the furnace. The colours used for the painting of it are all derived from metals; and many of them, though dull when applied, acquire considerable lustre by the action of the fire. The colours are always mixed with some kind of flux, such as a mixture of glass (204), borax (208), and nitre, melted together, and afterwards ground.

Gum or oil of lavender is used for mixing up the colours. When the painting is finished, the pieces are exposed to a heat sufficient to melt the flux, and thus fix the colour.

119. _TRIPOLI is a kind of clay of yellowish grey, brown, or white colour, sometimes striped or spotted, and of an earthy texture._

_It feels harsh and dry to the touch; is soft, scarcely adheres to the tongue, and will not take a polish from the nail._

This substance obtained its name from having formerly been imported into Europe from Tripoli, on the north coast of Africa. It is, however, now found in several parts of Germany; and a granulated kind has been discovered in England.

Tripoli is used for the polishing of metals and stones. For this purpose, it is mixed with sulphur, in the proportion of two parts of tripoli to one of sulphur. These are well rubbed together on a marble slab, and are applied to the stone or metal with a piece of leather.

When tripoli is combined with red ironstone, it is used for the polishing of optical glasses. It is sometimes made into moulds, in which small metallic or glass figures and, medallions are cast; and a kind of tripoli is found near Burgos, in Spain, which is used as an ingredient in the manufacture of porcelain.

In Derbyshire,, and several parts of Staffordshire, is dug a kind of tripoli which has the name of _rotten stone_. This is considered to be a produce of limestone shale, which has undergone a decomposition by exposure to the air and moisture. It is used for most of the same purposes as tripoli.

CLAY SLATE FAMILY.

120. _CLAY SLATE, or ROOFING SLATE, is a kind of stone of foliated texture, and greyish, black, brown, green, or bluish colour._

_It breaks into splinters, does not adhere to the tongue, yields generally a clear sound when struck, and is nearly thrice as heavy as water._

Vast and extensive beds of slate occur in different parts of the world; and this mineral sometimes constitutes even a principal portion of mountains. In our own country there are many important quarries of it, particularly in Westmoreland, Yorkshire, Wales, and Derbyshire.

The uses of slate are numerous and important; but its principal use is for the roofing of houses. For this purpose it is split into thin plates or laminæ. These are fastened to the rafters by pegs driven through them; and are made to lap over each other at the edges, in such manner as to exclude the rain and other moisture. The kinds which are preferred for this purpose are such as have the smoothest surface, and split into the thinnest plates. It is requisite that slates should be damp when they are split, otherwise this cannot be done without difficulty. Hence it is generally customary to split the masses as soon as possible after they have been separated from the rock.

Slate should not be porous. If it be so, rain and snow water will pass through it, and destroy the wood─work of the house on which it is placed. Porous slate is also liable to have moss and lichens grow upon and cover it. These plants retain moisture long, and keep the surface, and even the interior of the slate, moist; so that, during the winter season, by the freezing of the moisture, the slate is apt to split and fall into pieces. To ascertain whether the slate be of requisite compactness, it should be completely dried, then weighed, and afterwards soaked for some time in water. When taken out it is to be wiped with a cloth, and again weighed. If it have not acquired any considerable increase of weight, it is a proof of its being sufficiently compact. If, on the contrary, it have absorbed much of the water, and have become considerably heavier by the immersion, it is shown to be of a porous texture. Slates that are brittle are bad. If they emit a tolerably clear sound, when struck with a hammer, it is considered a proof that they are not too brittle: if, on the contrary, the sound be dull, they are soft and shattery. A good slate ought also to resist the action of a considerable degree of heat.

The slates that are principally used in London are brought from North Wales, from quarries that are worked near Bangor. There are also extensive slate quarries near Kendal, in Westmoreland; and the Kendal slates, which are of a bluish green colour, are more highly esteemed than those from Wales. They are not of large size, but they possess great durability, and give a peculiarly neat appearance to the roofs on which they are placed. The slate quarries near Easdale, in Scotland, are so extensive as to furnish annually more than 5,000,000 in number, and to give employment to upwards of 300 men.

_French slates_ were much used in London about seventy years ago; but they have been found too small, thin, and light, to resist the winds and storms of this changeable climate.

Dark─coloured, compact, and solid slates are manufactured into _writing slates_, or _table slates_, as they are sometimes called. In the preparation of these, the slate, after it is split of proper thickness, is smoothed with an iron instrument. It is then ground with sandstone, and slightly polished with tripoli (119), and, lastly, rubbed with charcoal powder. It is cut into the requisite shape, set in a wooden frame, and is then ready for use.

For writing on these slates, pencils are used which are also made of slate. These, which are called _slate pencils_, are made of a particular kind of slate, that, on splitting, falls into long splintery fragments. It is necessary that the pencils should be considerably softer than the slate to be written upon, so that they may leave a whitish streak on its surface, without scratching it. Such is the shivery nature of the slate of which they are made, that, if it be exposed for some time to the action of the sun or frost, it is rendered useless. Hence, workmen are careful to cover it up and sprinkle it with water, as soon as it is taken from the quarry, and to preserve it in damp cellars. The pieces are afterwards split by a particular instrument, and then wrought into the requisite shape.

In some of the quarries in Derbyshire and Wales the slate is so thick as to admit of being split into large and tabular pieces. These are used for gravestones, and for slabs for dairies and cellars. Paving stones and mile─stones are also formed of them; and vessels for the salting of meat, and setting of milk in dairies. For the latter use slate is peculiarly well adapted, on account of its resistance of greasy or oily substances. But this property renders it unfit for any purpose for which it is requisite to be painted; as, the oil not entering the stone, the paint soon peels off, and leaves the stone as black as it was at first. Cut into narrow strips, slate has also been applied, in the neighbourhood of Bangor, North Wales, for the formation of fences.

When sufficiently solid for the purpose, slate is cut into inkstands, and turned into vases, and fancy articles of various kinds. And a singular circumstance has been remarked, that, if a window or door be suddenly opened, in an apartment where the workmen are turning these, they will sometimes fly in pieces; though, after the work is finished, they may be exposed to the usual changes of temperature without injury.

Pounded slate is advantageously used for cleaning iron and other works in metal. When well ground, and mixed with a certain proportion of loam, slate is made into moulds for the casting of metals in; and, when burned and coarsely ground, is used instead of sand in the making of a solid and impermeable mortar or cement, for the parts of buildings that are covered with water.

121. _BLACK CHALK, or DRAWING SLATE, is an earthy substance, of slaty texture; generally of a greyish, sometimes a bluish black colour._

_It is soft and smooth to the touch, and, in handling, stains the fingers._

To crayon painters, and other artists, black chalk is a very useful article. Considerable quantities of it are imported from France, Spain, and Italy. The best is brought from Italy. This is more free from gritty particles, more firm and compact in its texture, and in its touch much smoother than the chalk of any other country. It contains somewhat more than one─tenth part of its weight of charcoal. When prepared for use, it is cut into square pieces, which are sometimes enclosed in wooden cases, like black lead pencils. These pencils are said to become dry, hard, and unfit for use, by long keeping. To preserve them in greatest perfection, they should be kept in a moist place. Some artists prefer pencils that are made of the chalk finely ground, mixed with a certain proportion of gum water, and cast in moulds. Care should be taken not to put too much gum, as the pencils will not, in such case, leave any mark on the paper.

Drawing slate is sometimes used as a black colour for painting. For this purpose it is pounded or ground, and then mixed with oil or size, according to the kind of work for which it is required. When black chalk is strongly heated, it loses its colour, and assumes that of a reddish grey.

122. _HONE, or WHET SLATE, is a well─known kind of stone, of somewhat slaty texture, and generally of dull white, or greenish grey colour. Its surface is smooth, and feels unctuous to the touch._

These stones, when properly cut and smoothed, are of indispensable utility to carpenters, cutlers, and others, for sharpening their cutting instruments upon. Those of the finest grain are used for lancets, penknives, and razors. For this purpose their surface, when used, is covered with a small quantity of oil; by which, after a while, they are rendered considerably harder than they were at first. They ought to be kept in damp and cool places; for, if much exposed to the sun, they become too hard and dry for many purposes to which they are applied.

There is a vulgar and erroneous notion that hones are holly wood, which by lying in petrifying water, have been thereby converted into stone. The greater number of them have a fine and a coarse side. From the circumstance of their having been originally brought into this country from Turkey and the Levant, they are sometimes called _Turkey stones_. They are now found in Saxony and Bohemia, in North Wales, and near Drogheda, in Ireland.

The powder of whet slate is sometimes used, instead of emery, for the cutting and polishing of metals.

MICA FAMILY.

123. _COMMON MICA, GLIMMER, or MUSCOVY GLASS, is a mineral substance of foliated texture, which is capable of being divided into extremely thin leaves that have a sensible elasticity, and are transparent._

_The colour of mica is greenish, sometimes nearly black, reddish, brown, yellow, or silvery white, with, occasionally, a metallic lustre on the surface. Mica is so soft as easily to be scratched; and, when divided across the plates, seems rather to tear than break._

This is one of the most abundant mineral substances that is known. It not only occurs in a massive and crystallized state, but it enters into the composition of many rocks; is found filling up their fissures, or crystallized in the cavities of the veins which traverse them. In some countries, as in Siberia, it is an article of commerce, and is obtained from mines like other minerals. From these it is extracted by hammers and chisels. It is then washed, to free it from the impurities which adhere to it; split into thin leaves or pieces; and assorted into different kinds, according to their goodness, purity, and size. We are informed by the Abbé Haüy, that plates of mica a yard or more in width have been obtained from the mines in some parts of Russia.