Part 10

175. TIRIE MARBLE.—Few of the British kinds of marble have been more admired than that obtained from Tirie, one of the Western Islands of Scotland. _It is of a reddish, sometimes a delicate rose─coloured tint, and sometimes white; and is always intermixed with other minerals which add to its beauty._ The most common of these is of black colour, and called hornblende; the others are pale green sahlite, blackish brown mica (123), and green chlorite. In some varieties the hornblende is more abundant than the marble.

176. ASSYNT MARBLE.—At Assynt, in Sutherland, _a white marble_ has been discovered, which is perfectly solid and pure, and entirely free from blemishes or stains. Blocks or slabs of it may be cut of almost any size that can be required. This marble acquires a smooth surface, but remains of a dead hue; whence, of course, its uses as an ornamental marble are much circumscribed.

177. ISLE OF SKY MARBLE.—There is found in the Isle of Sky a marble of pure white colour, which appears capable of yielding large and valuable blocks. Its fracture is granular and splintery, and its texture fine. It is harder, heavier, and more compact than the marble of Carrara (146); and is apparently well fitted for all the purposes of sculpture. But it has the defect of being very unequally hard. While some parts of the stone are nearly as easy to work as that of Carrara, other parts are so hard as to add a charge of near fifty per cent. to the cost of the working.

178. SUTHERLAND MARBLE.—Some beautiful specimens of marble of dark brown colour, veined with whitish, light red, or light brown, have lately been brought from the county of Sutherland. These appear of close texture, are susceptible of a beautiful polish, and are capable of being wrought into extremely beautiful slabs for chimney─pieces and other ornamental purposes.

179. GLEN TILT MARBLE _is of white or grey colour, and veined or spotted with yellow or green; some specimens are nearly white_. The granulations are peculiarly large; and, in its aspect and composition, the Glen Tilt has great general resemblance to the Pentelic marble (143). This marble has of late attracted the notice of the Duke of Athol, through the suggestion of Dr. Macculloch; and chimney─pieces of it have since been made. It is obtained from a valley of the same name in the county of Perth.

180. BLAIRGOWRIE MARBLE.—A few miles from Blairgowrie, in Perthshire, there is an excellent granulated broad─bedded marble, _of sugar─loaf texture, and as white as the finest statuary marble_. It may be easily raised in blocks and in slabs of great size, perfectly free from blemishes. This marble is supposed to be well adapted for ornamental architecture, but its large sparry texture renders it unfit for the sculptor.

181. GLENAVON MARBLE _is of white colour, with large granular concretions_, somewhat like spangles, and as large as the scales of fishes. This is a valuable kind; but its situation in the forest of Glenavon, on the property of the Duke of Gordon, is remote and difficult of access.

182. BALLICHULISH MARBLE.—On the north side of the ferry of Ballichulish, in Lochaber, there is a rock of marble, of beautiful _ash─grey colour_, and of a fine, regular, and uniform grain, which is capable of being wrought into blocks or slabs of any size, and is susceptible of a fine polish. This marble is finely sprinkled throughout with grains and specks of pyrites (236), and with grains and specks of a beautiful lead ore, which to the eye appears to be rich in silver. If used for ornamental purposes, it would be a bright and beautiful metallic marble.

183. BLAIRMACHYLDACH MARBLE.—In the bed of a river, at the farm of Blairmachyldach, about three miles south of Fort William, is a singular marble, consisting of _a black ground, flowered with white_. It is of fine close grain, but not very hard. The flowering in it is light, and beautiful, like fine needle─work, or rather resembling the frosty fret─work upon glass windows, in a winter morning.

The cutting and polishing of marble appear to have been performed by the ancients nearly in the same manner as it is with us. In polishing, the first substance employed is a sharp, coarse─grained sand. Afterwards a finer sand is used, then emery (58) in different degrees of fineness. These are followed by a red powder called tripoli (119): and the last polish is given with putty.

184. _BLACK MARBLE is a species of limestone, of uniform black colour, and easily distinguishable, by an excessively disagreeable smell, which is emitted on rubbing two pieces of it together, or striking it with a hammer._

Few minerals are susceptible of a more beautiful polish than this. It is consequently much used for chimney─pieces, small columns, vases, and other ornamental work. There are two quarries of black marble near Bakewell, in Derbyshire: and it is manufactured to a considerable extent by Messrs. Brown and Co. at Derby, who have fixed up in their ware─rooms a large slab of it as a looking─glass.

By the ancients it was much prized. Marcus Scaurus is said to have ornamented his palace with columns of black marble, each thirty─eight feet high; and many of the monuments of ancient Persepolis were executed in it. M. D’Avejan, Bishop of Alais, used a kind of black marble for paving the apartments of his palace; but the friction and heat rendered it so fetid that his successors were compelled to substitute another species of stone in its place.—The pavements, however, of many churches, and of the porticos of several galleries, on the Continent, are of black marble.

185. _CALCAREOUS ALABASTER is a species of limestone of somewhat whitish or yellowish colour, translucent, and internally splendent or shining._

_It is nearly a pure carbonat of lime; and occurs in masses, hanging, like immense icicles, from the roofs of lime─stone caverns, and also coating the sides of such caverns._

The formation of this substance is deserving of notice. The water which oozes through the crevices of limestone rocks, becomes strongly impregnated with minute particles of lime. This water, when it has reached the roof or side of a cavern, is generally suspended, for a considerable time, before a drop of sufficient size to fall by its own weight is formed. In the interval which thus elapses, some of the particles of lime are separated from the water, owing to the escape of the carbonic acid (26), and adhere to the roof. In this manner successive particles are separated, and are attached to each other, until what is called a _stalactite_, having somewhat the appearance of an icicle, is formed. These stalactites are sometimes solid, having a lamellar structure; sometimes of a fibrous texture, radiating from the centre to the circumference, as may be observed when they are broken; and sometimes hollow. If the water collects and drops too rapidly to allow time for the formation of a stalactite, it falls upon the floor, and there forms an irregular lump of alabaster, which has the name of _stalagmite_. In some caverns, the separation of the calcareous matter takes place both at the roof and on the floor; and, in course of time, the substance upon each increasing, they meet, and form pillars, sometimes of great magnitude.

Caverns of this kind occur in almost every country. Those of Derbyshire are well known; but the most celebrated stalactitic cave in the world is that of Antiparos, in the Grecian Archipelago.

The kind of limestone formed in the above manner is what the ancients generally denominated _alabaster_. It was employed by them for the same purposes as marble, was cut into tables, columns, vases, and sometimes even into statues. They also used it in the manufacture of vases or boxes for containing unguents. It is supposed to have been a vessel formed of this stone that is mentioned in the Gospel of St. Matthew, where it is said there came unto our Saviour “a woman having an alabaster box of precious ointment.” In the National Museum at Paris there is a colossal figure of an Egyptian deity, which is cut in a kind of alabaster brought from the mountains between the Nile and the Red Sea.

186. _TUFA, or INCRUSTING CARBONAT OF LIME, is a calcareous substance deposited by such water as is impregnated with lime._

_It clothes, with a stony coat, the smaller branches of trees, leaves, moss, plants, and other substances; and thus preserves them from decay, by protecting them from the action of the atmosphere._

Most of the substances termed by the common people _petrifactions_ belong to this kind of lime. They are, however, merely covered with, and by no means converted into stone.

The dropping well at Knaresborough, in Yorkshire, is particularly celebrated for them. An overhanging rock, several yards in depth, has been gradually formed of the calcareous matter which the water holds in solution; and, from this rock, it incessantly drops into the basin below. The persons who have the care of the place constantly keep these petrified articles for sale. Even old wigs and hair brooms are subjected to the powers of the water, to furnish subjects for attraction to the visitors. There are other springs of this description in Oxfordshire and Somersetshire, and particularly at Matlock, in Derbyshire. We are informed that at Dalton, on the south side of Mendip, the workmen not unfrequently discover large pieces of oak enveloped in blocks of stone which are four or five tons in weight.

Blocks of tufa are, in some countries, cut and used for building stones; and this substance, when burned, becomes an excellent lime. Pieces of it are sometimes hollowed, and used as filtering stones.

In the British Museum there is a human skull completely incrusted with stone, which was found in the river Tiber.

The warm baths of Hungary are often so thickly coated at the sides and bottom with tufa, that, during certain intervals, it actually fills up the tubes and canals through which they are supplied. The _fur in teakettles_ is a somewhat similar deposit from water in boiling.

187. _PORTLAND STONE, BATH STONE, KETTON STONE, are different kinds of limestone; and, of a texture so hard and compact as to be used in building._

_They have their names from the places where they are respectively found, in Portland Island, near Bath, and at Ketton, in the county of Rutland._

Of Ketton stone several of the colleges in Cambridge are built. Its grain has a singular resemblance to the petrified roe of a fish, whence also it is sometimes called _roestone_. The bridges, St. Paul’s Cathedral, the Monument, and nearly all the buildings of late date in London, are constructed of Portland stone.

Some of these kinds of stone, when first dug out of the quarry, are so soft that they are readily worked into any form which use or ornament may require. This is owing to the moisture with which they are naturally impregnated; but when they once become hardened, by exposure to the sun and air, they are extremely firm and solid. On the contrary, other kinds of limestone that are used for buildings imbibe and retain the moisture of the atmosphere, in consequence of which they burst or are crumbled by frost.

We are informed that Portland stone was first used in London in the reign of James the First, that monarch, by the advice of his architects, having employed it in the construction of the banquetting house at Whitehall. After the great fire in London, it was brought into general use by Sir Christopher Wren.

188. _MARL is a combination of clay, silex (76), and lime: and is denominated calcareous, argillaceous, or siliceous, as the lime, clay, or silex, is most abundant._

The calcareous part of marl is frequently composed of shells, whence it frequently has the name of _shell marl_; and where these are predominant, it affords an excellent manure for sandy, dry, gravelly, or light lands. Marl likewise produces very beneficial effects on mossy and clayey soils; and these effects, where it has been properly applied, have been observable for twelve or fourteen years. Some kinds of marl that contain but a small portion of lime have been successfully used in the manufacture of earthenware.

This mineral is usually found at the depth of from five to nine feet beneath the surface of the ground, and deposited between beds of clay and sand. It is dug out with spades; and, in the digging of it, in Ireland, the workmen not unfrequently meet with the horns of deer and other curious fossils.

The usual mode by which persons, generally unacquainted with minerals, distinguish this from other clayey substances, is, to break a small piece of dry marl into a glass of vinegar. If it be marl it will immediately dissolve with considerable effervescence; and the briskness of the effervescence will be in proportion to the quantity of lime which it contains.

189. _FLORENCE MARBLE is a kind of indurated or hardened marl, and is remarkable for presenting, when polished, the appearance of ruined edifices or rocks._

This kind of marble is never used in architecture. Little slabs of it are cut for Mosaic work, and to be framed like pictures; and the latter, when of considerable dimensions, are sometimes purchased at a high price. If held at a distance from the eye, an inexperienced observer might mistake a slab of Florence marble for a drawing in bistre. Here, observes a French writer, we remark a shattered Gothic castle, there the mouldering fragments of a cathedral; in one part ruined walls, and in another shattered bastions and towers. But, when we approach the picture, the illusion vanishes, and those imaginary figures which, at a distance, appeared to be so correctly drawn, become changed into irregular spots, lines, and shades, which present nothing distinct to the view.

190. COTTAM MARBLE, which, when cut and polished, also exhibits the appearance of a landscape, is a kind of compact marl. It has its name from being found at Cottam, near Bristol.

191. _LIAS, or CALP is a kind of limestone of bluish black, or greyish blue colour, and composed chiefly of lime, silex (76), clay, and oxide of iron (21)._

This stone, when burned, forms a cement which has the property of setting very strongly under water. It has also, of late years, been employed in a manner which merits particular notice, for the multiplying of copies of drawings and penmanship. A drawing is made on prepared paper with a peculiar kind of ink. A slab of lias, about an inch thick, is then heated; the drawing is placed upon it, and both are passed through a rolling press. The paper is afterwards wetted, and washed from the stone; but the ink, being of a gummy or glutinous quality, becomes in part absorbed by the stone, and remains. The stone is then ready for the printer. Previously to taking off each impression, the stone is wetted with a sponge; fresh ink (which is said somewhat to resemble printers’ ink, and is put on with a ball similar to that used by letter─press printers) is then applied. This is prevented, by the water, from adhering to any part except to the ink that had been absorbed, by the stone, from the paper on which the drawing was originally made. Paper is then placed on the stone, both are passed through a rolling press as before, and a perfect impression of the drawing is made upon the paper.

This art has been practised in Germany with great success; and with the difference only of the original drawing being made upon the stone instead of paper. Many beautiful specimens of drawings, taken from slabs of lias, may be seen in this country. It is said that copies of military drawings and orders were, to a very large amount, multiplied by this means at the headquarters of the armies lately employed on the Continent.

An artificial composition is sometimes used instead of lias.

Considerable quarries of this stone are wrought in Germany. It is also found at Leixlip, near Dublin; in beds at Aberthaw, in Glamorganshire; in Dorsetshire, and near Bath.

SULPHAT OF LIME.

192. _ALABASTER, or GYPSUM, is a kind of sulphat of lime, or of lime in combination with sulphuric acid (24), which has a shivery and glittering texture; and is of white colour tinged with grey or red, and sometimes striped, veined, or spotted. When crystallized, the primitive form of its crystals is a regular four─sided prism_ (Pl. II, Fig. 14.)

Being considerably softer than marble, this mineral is not capable of receiving a good polish. From this circumstance it is, however, the more easily worked. It is manufactured into chimney─pieces, columns, busts, ornamental vases, and lamps; the latter of which transmit a soft and pleasing light. Such is sometimes the transparency of alabaster, that it has been employed for windows; and, at Florence, there is now a church which receives its light through the medium of this substance.

The ancients, though acquainted with the art of making glass, had not attained the knowledge of reducing it into thin transparent plates; and frequently employed alabaster for windows. Of this stone the Temple of Fortune, which was built by order of the Emperor Nero, was erected. It had no windows whatever, and received only a soft kind of light through its walls; appearing rather as if the light issued from the interior, than that it was admitted from without.

The hot springs of St. Philip, which supply the baths of Tuscany, are so strongly impregnated with alabaster, that artists take advantage of this to obtain impressions of bas─reliefs, by merely exposing their moulds to a current of the water until they become filled with the earthy deposit. These impressions, when taken out, are found to be as hard as marble, and are very beautiful. There are, in the British Museum, some casts of medals formed from the water of these springs.

When alabaster is heated, it falls into a soft white powder, which, on being mixed with water, absorbs it so rapidly, that if it be formed into a paste, it dries and becomes hard in a few minutes. In this state it is called _plaster of Paris_; and is employed for the making of statues, casts, and other ornamental work, which, though of a beautiful white colour, are very brittle. When mixed with coloured gummy or glutinous substances, it yields plasters of different hues, and has the name of _stucco_; and, in this state, is used for lining the walls and ceilings of rooms. This plaster is much in request in the northern counties of England, for the floors of dairies, store─rooms, granaries, and other apartments; and, when properly formed, it constitutes a very smooth and durable flooring.

The fine white varieties of gypsum are used as an ingredient in the composition of earthenware and porcelain; and the glaze, or enamel, with which porcelain is covered, has the purest gypsum for one of its ingredients. Of late years this mineral has been advantageously employed as a manure for fertilizing the soil.

Gypsum is found in Cheshire and Derbyshire, as well as in several parts of the Continent. That which is imported into this country from Italy and Spain is considered the best.

193. FIBROUS GYPSUM.—There is a variety of gypsum which has a somewhat fibrous appearance, and which, when cut in a convex form, and polished, reflects a light not much unlike that of the cats─eye (86). Hence it is sometimes sold to ignorant persons for that stone. It has also been imposed upon purchasers for the gem called moonstone (113). Fibrous gypsum is cut into ear─pendants, crosses, beads for necklaces, and other female ornaments; but its softness is such as to allow of its being easily injured both by dirt and friction.

FLUAT OF LIME.

194. _FLUOR SPAR, or DERBYSHIRE SPAR, is a mineral formed by the combination of lime with fluoric acid (27)._

_It sometimes occurs in a massive, and sometimes in a crystallized state; the primitive form of its crystals being a regular octohedron_ (Pl. II, Fig. 5). _Its colour is usually bluish, green, yellow, whitish, or a mixture of some of these._

When heated, this substance cracks, and shines brightly in the dark. But if kept hot for some time, it ceases to be luminous, and this property cannot be restored to it. If also two pieces be rubbed strongly together, they become luminous in the dark.

From this spar are made several kinds of ornamental vases of considerable size, columns, and toys, which, from being extremely varied in their colours and appearance, and admitting of a high polish, are very beautiful. When a piece of fluor spar is to be wrought into a vase, or any similar article, it is first carved with a mallet and chisel into a somewhat spherical form. It is then fixed to a turner’s lathe, and, with great care, is formed into the shape that is required. When this is complete, it has to be polished, which is done first with gritstone and pumice (108), and lastly with emery (58) and putty. The lathes formerly in use were worked by the foot; but those now adopted are worked by machinery, the advantage of the more steady motion of which has been that ornaments of much more delicate structure can now be formed than before. The manufacture of articles from fluor spar gives employment to a great number of industrious families in Derbyshire. This mineral occurs in several parts of that county, where it has the name of _Blue John_, and where it is obtained from caverns at a considerable depth beneath the surface of the earth. It is also found in various countries both of the European and American continents.

The acid produced from fluor spar is called _fluoric acid_ (27), and has the peculiar property of corroding glass and flint, and consequently cannot be kept in glass bottles. Artists, by means of fluoric acid, are enabled to etch on glass, in the same manner as, with aqua fortis (nitric acid), they do on copper. The process is sufficiently simple. The glass is first a little heated, for the purpose of covering it thinly over with wax; then, with a needle or other fine point the drawing is to be made, by cutting through the wax to the surface of the glass. The edges are next to have a little wall of wax raised upon them. This done, the glass must be placed in an horizontal position, and sifted over with fluor finely pounded; and lastly, a mixture of one part of spirit of vitriol or sulphuric acid (24) with two or three parts of water is to be poured gently upon it. The acid will be prevented from running off by the wax; and, in the course of a little while, if these be cleared away, the glass will be found corroded in all the lines along which the needle passed.

The mode of obtaining fluoric acid for chemical purposes is, by pouring sulphuric acid upon powdered spar in a leaden retort, and applying to it a gentle heat. This acid should be used with great caution; for, when applied to the skin, it instantly disorganizes it, and produces very painful sores.

BARYTES FAMILY.

195. These minerals are sometimes called _ponderous earths_, and have their name from a Greek word signifying _heavy_. They comprehend all the combinations of barytes with acids.

When purified, they form a greyish white, porous substance, which is easily reducible to powder; has no perceptible smell, but has a harsh and more burning taste than lime, and changes the blues of vegetable colours to green.

Although barytes is one of the most useful chemical tests that we are acquainted with, it is not much employed in the arts, because, when purified, it is found too expensive. It is capable of being made into a very tenacious cement; and painters use a preparation that is made from it as a white colour which will not change. This is sold in the shops under the name of “Hume’s permanent white.” Barytes taken into the stomach proves a virulent poison; yet a preparation of it is used in medicine, and particularly for the removal of scrophulous complaints. When finely pounded and mixed with oatmeal, _carbonat of barytes_ has been found an efficacious poison for rats.