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polishing, and, above all, the rapidity with which many of them become weathered and tarnished.
Sculptors and architects generally arrange the marbles of a country into some such divisions as the following:-One-coloured, as the black and white; variegated, when marked with irregular spots and veins; madreporic, when studded with encrinal or coral markings; shell, when only a few shells are interspersed through the mass; lumachelli, entirely composed of shells ; cipolin, containing veins of greenish talc; breccia, marbles formed of angular fragments of different composition and colour; and puddingstone, when the fragments are round instead of angular. The celebrated marbles of Greece and Rome, such as the Parian, the Pentelic, the Carrara, - &c. were of one uniform colour, and only occasionally marked with grayish or greenish veins. Besides these, which were chiefly employed in sculpture, and in the decoration of their public edifices, the ancients indulged in a variety of fancy marbles for minor ornamental purposes--such as black, red, green, yellow, spotted, and veined. The localities of some of these ancient marbles are lost, but inexhaustible supplies of first-rate statuary and architectural marbles can still be obtained from the Archipelago, from Carrara, Genoa, Corsica, Sicily, and other parts of Italy. At Carrara alone, about 1200 men are employed at the different quarries, and at the mills for sawing the marble. The annual rental is calculated at about £28,000, and the value of the yearly exportations of the raw material at not less than half a million. So accessible are these quarries, and so free from flaws is the rock in some portions, that blocks of more than 200 cubic feet can be detached by means rude and primitive compared with quarrying in Britain. The value of the material differs according to the quality and size of the block, large blocks being from £2 to £3 per cubic foot; a price scarcely half of what was sometimes paid during the usurpation of Italy by Napoleon.
Many marbles of excellent quality are found in France; in England they are abundant in the counties of Derby, Devon, and Anglesea, the last being of a green colour; in Scotland, at Assynt, Ballachulish, and in the islands of syree, Skye, and Jura; and in Ireland, at Kilkenny and other places. The Kilkenny marble is black, and encloses shells of a whitish colour, which, when cut across and polished, present various circular markings, which add to the beauty of the slab. The United States also furnishes some excellent architectural marbles, principally of primary formation. One range, which passes unbroken through several of the States, is perhaps one of the most extensive and valuable primary limestones in the world. It is of a pure white colour, and of a highly crystalline texture, affording blocks of more than fifty feet long and eight feet thick. It is employed in several of the States' public buildings—as, for example, the City Hall of New York, and Girard College, Philadelphia.
Magnesian Limestone--Magnesia. Magnesian limestone, which appears extensively in England, Germany, and other continental countries, occurs often in beds of great thickness, immediately above the coal-measures, just as the mountain or carboniferous limestone lies immediately beneath. It is usually of a cream-yellow colour, and of very variable consistency, some layers being soft and powdery, others irregularly crystalline and concretionary, and some compact'and homogeneous. The compact granular variety is generally known by the name of Dolomite, after Dolomieu, a French geologist. Magnesian limestone is, for the most part, used as the ordinary carbonates of lime; that is, for agricultural and building purposes--some of the English quarries furnishing an exceedingly durable material. The new houses of parliament, for example, are built of a magnesian limestone; that of Bolsover Moor, in Derbyshire, having been selected after the most rigid scientific tests of a commission of inquiry. Besides these uses, some of the more compact and homogeneous schists are employed for lithographic blocks, the chief supply for that purpose being derived from Germany, though lithographic schists are also obtained from the white lias limestone in England.
Magnesian limestone is so called from its containing a notable per centage of magnesia--a well-known medicinal earth, com: monly obtained by burning the carbonate of magnesia. The calcined magnesia of the druggist is procured either from this source, or from the bittern of sea-salt, or from the waters of certain springs impregnated with the sulphate of magnesia. Natural carbonate of magnesia is found in Piedmont, in Moravia, in the United States, and in the East Indies. It exists as a component part of many mineral substances, making them feel soft and soapy to the touch.-Meerschaum (German, foam of the sea), a substance in great repute among tobacco-pipe fanciers, is an earthy carbonate of magnesia, extremely light, and of a yellowish-brown colour. It is found in various parts of southern Europe, particularly in Greece and Turkey, where, besides being fashioned into, pipe-bowls, it serves also the purposes of a fulling-earth. Germany, however, is the great seat of the meerschaum pipe manufacture, whence France and England obtain their supplies.
Chalk, another well-known mineral, is a carbonate of lime of a white or whitish-gray colour, having a soft meagre feel and earthy fracture. It is the last or youngest of the secondary rocks, and constitutes an important geological feature of England—thé chalk-hills which form the white cliffs of our southern shores having conferred the ancient name of Albion (alba, white) upon our island. Calcined like common lime, it is used for manure and cement, in polishing metals and glass, as a marking material, and in painting and whitewashing: For the last purpose it is purified by trituration and elutriation, and sold under the name of whiting, or. Spanish white. The chalk-formation yields also the flint of commerce; but this more properly falls to be considered under the class Siliceous substances.
Marl is one of the most recent calcareous deposits, being in many places still in the course of formation. Though essentially a mixture of carbonate of lime and clay, it occurs in various states of purity, from a marly clay, which will scarcely effervesce under acids, to shell-marl, containing from 80 to 90 per cent. of lime. Marl-clay, for instance, occurs as a whitish friable clay, with an admixture of lime, and sometimes also of magnesian earth; the term cláy-marl is used when the calcareous matter prevails over the clay. Shell-marl is almost wholly composed of lime and freshwater shells, with a trace of clay and other earthy matter, and where solidified by chemical aggregation, is known as rock-marl. Marl uniformly occurs in valleys formerly the sites of lakes, or in existing lakes, and seems to be partly derived from the waters of calcareous springs which enter such lakes, and partly from the shells and secretions of the fresh-water molluscs which inhabit them. It is dug from open excavations or pits, and applied to certain soils as a manure, or as a top-dressing for pasture.
Calcareous sand, which consists almost entirely of comminuted shells, is another recent product occasionally employed as a fertiliser. It is found in layers in ancient or raised beaches, and in masses by the sea-shore, where, thrown up by the waves, it often consolidates into beds of considerable thickness. As an instance of its value, Sir H. de la Beche mentions that between five and six millions of cubic feet are annually conveyed from the Cornish coasts, to be spread over the land in the interior as a mineral manure.
Gypsum- Alabaster. Gypsum, also known as sulphate of lime and plaster of Paris, is found in England, and in many other countries. It occurs in various states of crystallisation and purity: thus the ordinary gypsum of commerce is soft, and imperfectly crystalline ; selenite is a transparent, highly crystalline mass; satin gypsum is fibrous and crystalline; and alabaster is pure white, and translucent. Gypsum occurs both in old and new formations, but principally in the new red sandstone, and in the tertiary beds, or those above the chalk. It is mined in various localities in England, and extensively quarried at Montmartre near Paris-whence it has derived its ordinary name of Plaster of Paris. Calcined, pulverised, and mixed with water, it is run into moulds, forming stucco images, mouldings, and ornamental fronts for buildings. It is
also used for stereotype and pottery moulds, and for medals and casts of various kinds. Mingled with a certain per centage of quicklime, it makes an excellent mortar; its virtues as a fertiliser have been also greatly extolled.
Some of the English gypseous alabasters, such as those of Derbyshire and Staffordshire, stand the turning-lathe well, and are accordingly formed into jars, vases, and other mantelpiece ornaments. The finest specimens, however, are found near Volterra, in Tuscany. These are of a pure white colour, and granular texture, and when cut and polished, outrival the finest Carrara marble, from which they are, however, readily distinguished by their softness and liability to tarnish. A large trade in alabaster-work is carried on in Florence, Leghorn, and Milan, where the material is fashioned, partly by the chisel and partly by the turning-lathe, into statues, vases, lamps, boxes, stands for time-pieces, and other ornamental objects. All sculptures of alabaster should invariably be kept under a glass shade, as a few months' exposure destroys at once their purity of colour and marble translucency.
Coral, or coral-stone, is another calcareous material of commerce which deserves to be noticed. Being entirely the secretion of certain marine animalcules, it is pretty nearly a pure carbonate of lime, and occurs in the warmer "latitudes of the Pacific in vast barriers and reefs, often from fifty to one hundred feet in thickness, and from a few miles to hundreds of leagues in linear extent. Selecting for their residence some submarine ledge of rock, the animalcules begin to ply their vocation, increase, and spread, ever adding to their calcareous secretions, which by and by come to the surface, when they stop and carry on their operations laterally—thus in time elaborating masses which may well compete with any of the ancient rock-formations.
There are numerous varieties of the coral animalcule, each variety forming a coral of different shape, but still of the same substance; and ultimately, when indurated by ages, of the same solid and rockylike consistence. Coral-rock is occasionally employed in the South Sea Islands as a building stone; but the recent branching corals are solely in request for ornamental purposes—their value depending upon the size, solidity, and colour of the specimen. Black and red varieties are the most highly-prized--portions of Sicilian coral having been known to bring as much as eight or ten guineas per ounce. The price, however, is extremely variable, other portions of the same mass selling for less than a shilling a pound. Regular coral fisheries are established in the Straits of Messina, on the shores of Majorca and Ivica, the coast of Provence, and in other parts of the Mediterranean. Abundant supplies are also obtained from the Red Sea, the Persian Gulf, the coast of Sumatra, &c.
ARGILLACEOUS SUBSTANCES. Under this section we include all those substances in which clay (argilla) is a prevailing ingredient—as the common clay of the brick and tile-maker, the prepared clay of the potter, fullers' earth, and the slate now so generally used for rooting. Argillaceous compounds occur in every formation, from the lowest slate, through the shales and fire-clays of the coal, up to the plastic elays of the tertiary and superficial deposits.
Clay. The common superficial clay, which is so liberally spread over our island, must be familiar to every one. It is of various colours -yellow, red, or bluish ; more or less mixed up with sand and fragments of rock; and when softened with water, becomes plastic and tenacious. It is this variety that is ordinarily used for the manufacture of bricks, roofing and drain-tiles, chimney-pots, and the coarser sorts of earthenware. For these purposes it is broken down, kneaded with water, and freed from the grosser impurities, after which it is beat up into the desired consistency, passed through moulds, dried so far in the atmosphere, and then burned in clamps or in kilns. Though enormous quantities of bricks and tiles are consumed in Great Britain, most of the manufactories are rude and primitive affairs, conducted in the open fields, or in simple sheds, which scarcely yield a shelter. Of late years, several ingenious brick and tile-making machines have been constructed, which press and fashion the prepared material into form with astonishing rapidity. For bricks, slabs, crucibles, &c. which have to resist the action of fire, some of the coal-measure clays are generally had recourse to; these, from their greater purity, and a certain per centage of silica, being susceptible of a more thorough burning. In England, the Windsor, Stourbridge, and Welsh fire-clays are esteemed the best—the latter yielding those large square slabs employed in the construction of drying-kilns, brewers' coppers, sugar-boilers, furnaces, &c. Tiles and bricks were at one time subject to a duty; but now only the latter are charged, producing a revenue of about £450,000. Recently, this sort of manufacture has increased prodigiously in England and Scotland, their joint produce being upwards of 1,540,000,000 bricks annually, independent of Irish manufacture, upon which there is no duty.
Pipe-clay, potters’-clay, and porcelain-clay, are but technical names for pure varieties of well-prepared specimens of the same substance. We have seen that common brown ware can be made from ordinary clay ; but when the finer varieties of white ware or china are attempted, not only finer clays must be sought, but even these must be mixed with a certain proportion of calcined Aint or silex. One of the finest varieties of aluminous earth is the China-clay of Devon and Cornwall, or the kaolin of the Chinese.