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supply of fresh air, however, must be regularly and unceasingly maintained in every part of the workings; and not only so, but care must be taken to prevent the accumulation of two gases most destructive to human life; namely, carburetted hydrogen and carbonic gas--the fire-damp and choke-damp of the miners. For this purpose the various underground workings are so arranged and boarded off, that while one set receives the descending current, another carries it forward again to the pit bottom, where, by means of rarefaction, produced by a huge fire, it is carried up the shift to the atmosphere. By these means not only is fresh air supplied to the miners, but the deleterious gases are carried off, and the whole subterranean recesses rendered safe and healthy. The most ingenious of human inventions are, however, imperfect; and choke-damp and fire-damp will exude from the coal seam, and lurk in recesses, there either to suffocate the first comer, or to explode the instant that a lamp is brought in contact. To prevent these casualities as much as possible, various air-tight trap-doors and boardings are employed, and the miner is furnished with safety-lamps of various constructions, which, while they afford sufficient light, prevent the carburetted hydrogen from coming in contact with the flame within. These remarks apply in particular to the Newcastle coal-field, where, in consequence of such difficulties, coal-mining is conducted with greater care and skill than in any other district; but it must be remembered that there are many tields where fire-damp is unknown, and where the most ordinary ventilation is sufficient to prevent the accumulation of carbonic acid or any obnoxious effluvia. Indeed we know of an excellent coal-field which returns its thousands annually, and where no precaution either as to lamps or ventilation is necessary-all that is requisite being occasional wooden props to prevent falls of loose material from the roof of the compartment in which the miner may be working. In some of the largest Pennsylvanian mines even this precaution is unnecessary, the anthracite being of great thickness, and so exposed and level, that it is hewn out either in open quarry or in huge drifts, precisely after the fashion of our railway tunnels.
Important and varied as are the uses, and vast as must be the consumption, of this mineral in Britain, yet so abundant is it, that in many localities the best household coal never exceeds 7s. a ton, while in Edinburgh it averages about 12s.; and in London, to which it is all sea-borne, it ranges between 18s. and 22s. "Notwithstanding the cheapness of the produce of this kind,” says Mr Ansted, “the value of the coal actually brought to the surface in Britain amounts annually to nearly ten millions of pounds sterling, and almost the whole of this is derived, although in unequal proportions, from the Newcastle, the South Welsh, the Staffordshire, and Scotch coal-fields. With regard to the first of these--the Newcastle coal-field-it is said that upwards of six millions of tons are there annually
raised up out of the bowels of the earth; that 60,000 persons are employed in the mining operations; that 1400 vessels are constantly engaged in conveying the coal (amounting to three millions of tons) required for the consumption of the metropolis alone; and that the capital employed in simply conducting this trade amounts to several millions of pounds sterling". From this single instance some idea may be formed of the magnitude of the entire trade in coal, which is doubtless one of the most important props of our country's commerce.
As to the origin of coal, no matter what the variety, there can be no doubt that it is essentially vegetable. Not only are fossil trunks, branches, leaves, and fruits found in the mass, but scarcely a portion of it, when submitted to the microscope, but shows the ducts and fibres of a true vegetable structure. We know, moreover, that vegetable matter, when subjected to moisture and pressure, and excluded from the action of the air, will in a short period pass into a bituminous or carbonaceous mass, which time and greater pressure and heat would by and by convert into true mineral coal. Peat, were it excluded from atmospheric influence, would soon pass into a species of coal: brown coal and lignite, in which the trunks and branches of the trees are still perceptible, are only varieties less perfect than the true coal; and even in the old coal-formation itself, various beds present various degrees of perfection, according as the vegetable mass seems to have been more quickly and perfectly removed from the action of the atmosphere." How the masses of vegetable matter were accumulated, is still a subject of speculation with geologists—some contending that the trees, grasses, ferns, &c. which compose it, must have grown and accumulated just as peat-mosses do at the present day, and that the land was then submerged, and the mass covered over by layers of sand and mud, which, hardening, formed strata of stone and shale; others reject this theory as untenable, and consider the whole strata (sandstone, shale, &c.) of the coal-measures to have been deposited in estuaries liable to periodic inundations, like those of the Niger and Ganges, but only on a more gigantic scale. According to this notion, which is more in accordance with the phenomena presented, coal is partly composed of vegetables which grew in situ in the form of jungle, and partly of masses drifted down from the interior by the waters of the river.
Though the chief use of coal be doubtless that of producing heat, there are certain minor purposes to which some of the varieties are applied. Thus we have occasionally seen very pretty vases, and other ornaments, made from cannei coal when it is sufficiently compact and lustrous. It is easily turned, and takes a polish which is not readily tarnished; the only objection to it being its brittleness, and liability to be injured by fire.
Jet, of which necklaces, ear-rings, and other ornaments are made, is but a variety of coal, as common in its origin and nature as that which we pile on our fires. It is occasionally found in the lignite beds of England, but principally in Germany and Prussia, where it occurs associated with amber, which is regarded as a fossil gum, while jet seems to be the trunk and branches of trees more completely bituminised and freer from earthy impurities than cannel or other coals.-Amber, a wellknown yellow resin-like substance, is believed, as stated, to be a fossil gum or resin ; and its connexion with deposits of lignite seems to confirm that opinion. It is solid, brittle, commonly transparent, and when rubbed, becomes electrical. It is found in various countries, more particularly on the Adriatic and Sicilian shores; on the Baltic, between Memel and Dantzic, where there are regular mines of it; and in Japan, Madagascar, and the Philippine Islands. It is used chiefly in the manufacture of beads and necklaces, and in the preparation of varnishes. The largest known specimen of amber was found near the surface of the ground in Lithuania, about twelve miles from the Baltic: it weighs eighteen pounds, and is in the royal cabinet at Berlin. Other curious specimens have been detected enclosing insects, and even drops of water—these apparently having been enclosed when the gum was exuding in a fluid state from the living tree,
Naphtha-Petroleum-Asphalte. Naphtha, petroleum, mineral pitch, and asphalte, may in a great measure be regarded as one and the same substance in different degrees of concentration and purity. Thus naphtha, on exposure to the air, soon loses its limpid appearance, and passes into petroleum; and petroleum, under similar treatment, shrinks into a viscous slaggy state, undistinguishable from mineral pitch.
Natural naphtha is a limpid, or but slightly-coloured bitumen, highly inflammable, and of a strong bituminous, but not disagreeable odour. It is found at Baku on the Caspian, at Hit on the Euphrates, and at other places in Mesopotamia ; it occurs abundantly in the lower districts of the Birman empire; is found at various places in the north of Italy, as Piacenza, Modena, &c.; and in some districts of North America. It generally, exudes from fissures in the rocky strata, or is collected in shallow wells, dug in the clays and shales where it occurs. A similar liquid can be obtained by distilling petroleum, coal-tar, and other bitumens; but the artificial product has a more penetrating and unpleasant odour. Naphtha has the property of dissolving most of the essential oils and resins, and is at present largely used as a solvent of caoutchouc. It is also used for lamps; and the cities of Parma and Genoa are said to be lighted with the produce of the wells in the duchies of Modena and Parma.
Petroleum, or rock-oil, is another liquid bitumen, of a brownish colour and variable consistency, and yielding a strong disagreeable odour. It is found exuding from various secondary strata, but chiefly in coal districts, where it is evidently a product of that formation. It occurs in small quantities in various localities of Britain, but abundantly in other countries of Europe, in Persia, the Birman empire, in Texas, and in the islands of Trinidad and Barbadoes. On exposure to the air, petroleum thickens, and assumes a darker hue, in which state it is generally known by the name of mineral pitch, or Barbadoes tar. On further exposure, and especially when mingled with earthy impurities, it passes into a solid state, then becoming the common asphalte or bitumen of commerce. In its ordinary liquid state it is burned for light; worked into balls with earth and gravel, it is used in eastern countries as fuel; and mingled with grease, it is occasionally employed as a substitute for tar in coating vessels.
Asphalte, so called from its adhesive nature, differs from mineral pitch in being solid and brittle at the ordinary state of the atmosphere. It melts easily, and is highly inflammable, leaving, when pure, little or no ash after combustion. It is found in most of the localities where petroleum springs occur, being nothing more than their accumulated produce. The chief supplies are obtained from the shores of the Dead Sea, from Barbadoes, from Trinidad, where it occupies a basin or lake about three miles in circumference, and from Clermont, Seyssel, and Bourg in France, where it occurs in limestone and calcareous shales. Asphalte was employed by the ancients in some of their cements, and also in the process of embalming. It is now extensively used in the formation of pavement, roofing, and other economical purposes. Melted and mingled with properly sifted gravel, or iron slag, it forms a very durable and unexpensive pavement, being liable to be softened, however, during intense heats.
Under this head we include such economic minerals as contain a notable proportion of calx or lime in their composition. Common limestone, magnesian and lithographic limestones, marble, chalk, marl, gypsum, and alabaster, are familiar examples. Some of these have evidently been deposited from calcareous waters ; others are as evidently the production of animalcules, like the coral insect; and some are almost wholly composed of the shells of molluscs, and of other calcareous exuviæ.
Whatever may have been their several origins, they have all undergone certain chemical and structural changes since their formation—thus rendering them less or more compact and crystalline, producing a dull massive rock or a brilliant marble, an opaque gypsum or a translucent alabaster.
Limestones fit for building and agricultural purposes are found in every formation, from the oldest to those of the most recent origin. The rock is generally dug in open quarries, but occasionally, when it dips rapidly, and is worth the expense, it is followed downward by mining—the greater part of the stratum being excavated, and only portions left at intervals to support the superincumbent material. It is then broken into fragments of moderate size, and conveyed to a kiln, where, being placed in alternate layers with coal or turf, it is roasted, thereby expelling its water and carbonic acid. In this state it is known as shell or unslaked lime, and requires to be drenched with water to convert it into a powdery quicklime. As quicklime, it is used by the farmer ; but it requires to be further slaked and mingled with a certain proportion of good sharp sand to render it suitable for mortar. Besides building and agricultural purposes, a large quantity of lime is used as a iux in metallurgic processes, such strata being sought for this purpose as contain but a small per centage of impurities. Considerable quantities are also used in the purification of gas, in soap-making, leather-dressing, dyeing, medicine, and in many other economical processes. The supply of limestone in our own country is inexhaustible; it is worked in beds from one foot to one hundred feet in thickness; the mountain or carboniferous limestone which underlies the coal-formation often exceeding that thickness, and ranging unbroken for many miles in extent.
Marble is but a technical term for any species of limestone sufficiently pure and compact to be susceptible of a fine polished surface. No matter what the colour, whether white or black, whether studded with the strange forms of fossils, or streaked with the most fantastic veinings, marble is but a carbonate of lime, containing only a few subordinate impurities, which do no more than affect its colours and markings. The best varieties are obtained from the primary and transition formations, in which they occur compact, crystalline, and not unfrequently replete with party-coloured veinings. Pretty enough marbles for slabs and other architectural purposes are sometimes obtained from the secondary formations, these being, in general, curiously marked with the shells, encrinites, and other corals which are imbedded in the mass. None of these, however, are susceptible of the same degree of polish as the primary marbles, some of which, like that of Carrara, seems almost translucent. Most countries of any extent have varieties of native marbles, which, though inferior to those of Italy and the Archipelago, might still be more extensively used than they are, were it not for the expense in cutting and