of 56 fire-clay, 34 carbon, 8 iron oxide, and 2 magnesia; the Hessian, of fire-clay containing a little iron oxide and silicious sand; the Cornish, of the best Poole or Stourbridge clay, ground pots, and sand; and the Birmingham glass-pots, of the best Stourbridge or Monmouth clay. Fire-clay suitable for crucibles" pot-clay" as it is termed is by no means abundant, and brings a high price, that in the Stourbridge seam selling at five times the price of the ordinary material.

Alum, &c.

Among fire-resisting substances may be noticed the alum of commerce, the nature and preparation of which is described in Chapter XIV. Steeped in a solution of this salt, cloth, paper, and wood become almost incombustible—at least do not ignite so readily, and when ignited, burn away slowly and with difficulty. "If alum and common salt," says Jackson ('Minerals and their Uses'), "are reduced to an impalpable powder, and mixed with spirits of wine, and several coats of this be laid upon the skin of the hand, a red-hot iron may be held without inconvenience. This is, in fact, the secret of the human salamanders, or incombustible jugglers, fire-eaters, &c." As a check to the inflammability of light dresses worn by ladies, the solution of alum might be more extensively and advantageously employed.

Other solutions have been recommended by chemists, such as magnesium sulphate and borax, ammonium sulphate and gypsum, and sodium tungstate-all of which lessen the inflammability of fabrics, by enveloping their fibres with a thin coating, which prevents the escape of inflammable gases.

II. NATURAL SUBSTANCES.

Firestones.

Any stone that stands heat for a considerable time without perceptible injury is entitled to the designation of a Firestone. The term, however, is usually applied to certain sandstones of the greensand, oolitic, and coal formations employed in the construction of ovens, glass-furnaces, and similar erections subjected to high and oftentimes to intermittent temperatures. The upper greensand of Kent and Surrey (Reigate) yields a stone of this description which was at one time much prized; some of the soft yellow sandstones of the Tyne (Walker and Bellingham) have also been employed in furnace-structures; and the sandstone of Craigenbank, near Borrowstounness, has been

shipped to St Petersburg for furnaces, ovens, and similar purposes. Such sandstones, however, are now all but superseded by fire-clay fabrics. The firestone of Nevada, U.S., is described as a light, porous, silicious rock, having a specific gravity of 1.49, capable of being sawn into blocks of any form, and able to resist intense and intermittent temperatures.

Leckstones.

Another stone occasionally employed in oven and furnace structures, but especially as oven-soles, is obtained from the traps of the coal-formation. The Leckstones, as they are called, are open, granular varieties of trap (old ash-beds), and when carefully selected and dressed, stand well the alternations of heating and cooling. We have seen them quarried in Fife and Linlithgow, but the cheaper and handier slabs of fire-clay have driven them, we believe, entirely out of the market.

Potstone.

Potstone, the lapis ollaris of the ancients, and so called from its use, is a soft magnesian or talcose rock, of a greenish-grey or leek-green colour, sectile, and capable of being fashioned into pots, vases, and other articles. Mineralogically, it is an uncertain admixture of talc, chlorite, asbestos, and the like, occurring in beds among the serpentines and crystalline rocks of various countries: in other words, coarse granular varieties of steatite or soapstone. Like most magnesian minerals, it is little affected by heat, and was early fashioned into pots and pipkins-pipkins, which had the property not only of being unaffected by changes of heat and cold, but of communicating no bad taste or quality to the food, and of being thoroughly cleaned by being heated red-hot. It is now seldom used for this purpose save in northern Italy and the Grisons; but slabs of steatite are employed in Norway, Sweden, and America for furnaces, stove-linings, and ovens. It is also fashioned into gas-burners, which possess the property of not corroding, nor becoming clogged up, as is frequently the case with those made of metal. The Corsicans, it is said, use a fibrous variety, or asbestos (for both are nearly allied silicates of magnesia), in the fabrication of pots or pipkins, by mixing it with clay, and thereby obtaining a lighter and more durable vessel than could be obtained from the use of clay alone.

Some talc-slates and gneisses also offer great resistance to high temperatures, and are occasionally employed in the con

struction of reverberatory and other furnaces. Indeed, most silicio-magnesian rocks are endowed with this property, besides being readily cut and fashioned into any required form.

Asbestos.

Asbestos and Amianthus are the names applied by mineralogists to fine fibrous varieties of tremolite, actinolite, and other members of the hornblende family. They are all essentially silicates of magnesia, with a little lime, and traces of alumina, iron, manganese, and water. The fibres-often readily separable, elastic, and flexible-were used by the ancients in the manufacture of an incombustible cloth for the funeral pile; hence the name asbestos, inconsumable; and hence also amianthus, unsoilable, because the fabric when passed through the fire came out clean and unaltered. There are many varieties, and these receive their names from their appearance and quality-as rock-wood, rock-cork, mountain-leather, fossil-paper, fossil-flax, &c. In rock-wood, the fibres are long, parallel, curved, and compact; in rock-cork they have a felted texture, and so light as to swim on water; in mountain-leather, they form flat flexible pieces; and in fossil-flax they are so loose and silky that Dolomieu, when in Corsica, used it for packing his other minerals. Asbestos thus passes from the silky flexibility of amianthus to a degree of rigidity and compactness which admits of receiving a polish. It occurs among the metamorphic rocks of many countries, and especially in connection with serpentine, which it traverses in irregular veins, varying from half an inch to two or even three feet in thickness-the fibres being transverse to the cheeks of the vein.

As a refractory or fire-resisting substance, it has been put to many uses: by the ancients, in the manufacture of incombustible fabrics, for funeral-pyre sheets, and for table-napkins; and in modern times, for incombustible lamp-wicks, for filling gas-grates the fibres remaining red-hot without being consumed for making fire-proof safes, and occasionally for manufacturing indestructible paper. Recently it has been employed on a large and important scale in the manufacture of "packing" for steam-pistons, and of "paper-board" for the junction of steam-pipes, and the like. For these purposes it is obtained in large supplies from Italy, Corsica, the Tyrol, &c., in Europe, and from Massachusetts in America. The variety employed at the Asbestos Steam-packing Company's factory in Glasgow is obtained, we believe, from the north of Italy and Corsica, and is of a lamellar-fibrous texture, varying from three to

twenty or more inches in length. Being exceedingly tough, it is first broken into pieces by sledge-hammer, then passed through corrugated rollers to soften it, next put through a teasing-machine and reduced to flossy fibres, and ultimately passed through a mill which slightly twists it into strands, enclosing these at the same time in a sheathing of pack-thread to give them greater consistency. In this state it is ready for the mechanical engineer, and is reported to stand longer than any other material yet employed as steam-packing. The paper-board is felted into sheets of various dimensions, whose thickness varies from that of a sheet of writing-paper to that of the heaviest pasteboard, and makes one of the handiest substitutes for lutings that has ever been adopted.

The substances noticed in the preceding pages are of vital importance to the Arts and Manufactures. They are essential in all our smelting and refining furnaces; in our grates, hearths, flues, and chimneys; in our gas and oil distilleries; in our baking, pottery, and glass ovens; and, in fact, wherever intense or long-continued heat has to be resisted. One of their main recommendations is that they are all abundant, easily obtained, and readily manipulated. Their supply is equal to any demand, and there is practically no limit to the forms or modes in which they can be applied. Considering their cheapness and abundance, and the frequency of destructive fires in factories and warehouses, it seems culpable that they are not more extensively employed as floorings, partitions, linings, and even roofings-their lightness, and the exactitude with which they can be shaped and fitted together, rendering them nearly as convenient as wood, without its liability to combustion.

XIII.

PIGMENTS, DYES, AND DETERGENTS.

THE substances noticed under the present head are intimately connected with the Arts and Manufactures, both in the way of utility and ornament. The pigments when well prepared and carefully applied, are the best preservations of wooden and metallic structures; the dyes give beauty and colouring to fabrics otherwise tame and uninviting; and the detergents are essential to cleanliness and health. It is true that pigments, dyes, and detergents are obtained from the vegetable and animal as well as from the mineral kingdom; but those from the latter have a brilliancy, permanence, and abundance which do not belong to those of the former. These substances early attracted the attention of mankind-the savage smearing his body with ochre, daubing his robe with reddle, or washing himself with magnesian clay, seeking, only in a ruder way, the same effect as the man of civilisation and refinement by his most costly colours and detergents. Few of these substances can be applied in their natural state; most of them have to undergo treatment and preparation; and some of them can only be obtained in perfection by intricate chemical processes and manipulation. It is more, however, with the raw materials that we have here to deal — their mineral characters and geological positions-the processes by which they are brought to their finished state being altogether technological.

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I. PIGMENTS.

By paints," says Wagner, in his 'Handbook of Chemical Technology, "we understand substances which, as a rule, are insoluble in water, and are mixed with either weak glue solution, being then termed water-colours, or with linseed-oil, called oil - paints. To these pigments belong white-lead, red-lead,