For larger holes, metal tubes such as fig. 71, fed with diamond powder, are used; they grind out an annular recess, and remove a solid core; copper and other tools fed with emery or sand may be thus used for glass, marble, and various other substances. The same mode has been adopted for cutting out stone waterpipes from within one another by the aid of steam machinery.

Fig. 72, represents the conical diamond used by engravers for the purpose of etching, either by hand, or with the various machines for laying etching grounds, for ruling medals, and other works. These diamonds are turned to the cone by a fragment of another diamond, the outside skin or an angle being used, but the tool suffers almost as much abrasion as the conical point, from their nearly equal hardness; therefore the process is expensive, although when properly managed entirely successful.

To conclude the notice of the diamond tools, figs. 73 and 74 show the side and end views of a splinter suitable for cutting fine lines and divisions upon mathematical instruments, &c. The similitude between it and the glazier's diamond will be remarked, but in the present case the splinter is selected with a fine acute edge, as the natural angle would be too obtuse for the purpose.

Mr. Ross, the inventor of the ingenious Dividing Engine rewarded by the Society of Arts in 1831, informs me that with a diamond point of this kind, presented to him by Mr. Turrell, he was enabled to graduate ten circles upon platinum, each degree subdivided into four parts; at the end of which time the diamond, although not apparently the worse, was accidentally broken. A steel point would have suffered in the graduation of one-third of a single circle upon platinum, so as to have called for additional pressure with the progress of the work, which in so delicate an operation is of course highly objectionable*.

nary drill-bow and breast-plate, for drilling out the hardened steel nipple of a gun, which had been broken short off in the barrel; no material difficulty was experienced, although the stone appeared to be slenderly held.

* In collecting the materials for this chapter, I have gratefully to acknowledge the assistance of J. Tennant, Esq., Professor of Mineralogy, King's College, London; Joseph Hall, Esq., Marble and Spar Works, Derby, and some others.

The workshops of the following manufacturers, &c., have also been kindly laid open to my inspection: Messrs. Corotti, Cox, Dallaway, Dumenil, Ellis, Inderwick, Lund, Magnus, &c. By these means I have been able to obtain the most practical information upon the several subjects, the principal difficulty having been to keep the matter within the limits required by the nature of the work.

CHAPTER X.

THE METALS.

SECT. I.-ARRANGEMENT OF THE SUBJECT.

THE numerous materials from the three kingdoms hitherto described, with but three exceptions, namely, clay, horn and tortoiseshell, are used in the simple natural states in which they are found, without any change of form beyond that of reduction, as the finished works are in almost every case produced by cutting or chipping away the portions in excess; the works are consequently smaller than the rough masses from within which they are obtained, and the fragments cannot be re-united without the aid of some artificial means, such as screws, rivets, cements, &c.

Several of the substances now to be considered, differ greatly from all the foregoing materials, by possessing in various degrees of excellence, the properties of fusibility, malleability, &c. These assist in various ways in the first separation of the metals from the earthy bases in which they are found; also in the perfect combination of the smallest particles, either of one single or of different kinds, into solid masses; and in the conversion of these by the agency of the crucible, the hammer, and various attendant means, to that endless variety of forms in which the metals are used, many of which are merely a state of part-manufacture or what may be called a "transition" stage.

The chemist enumerates forty-one different metals; of these many are entirely confined to the laboratory, part are only used in the chemical arts, and those to which I propose to refer as connected with our subject, are but fourteen, namely, Antimony, Bismuth, Copper, Gold, Iron, Lead, Mercury, Nickel, Palladium, Platinum, Rhodium, Silver, Tin, and Zinc.

Of these, mercury is always fluid in our latitudes, and antimony and bismuth are brittle metals, consequently they are not used alone in construction, and nickel although malleable is rarely so employed; subtracting these reduces the number to ten.

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THE MANUFACTURE OF CAST-IRON.

Palladium, platinum, and rhodium are principally used on account of their infusibility and resistance to acids, for a few purposes connected with science; their abstraction reduces the number to seven. Gold and silver are mostly reserved for coin, and articles of luxury, so that taking away these also, the majority of the works of mechanical art fall exclusively on five, namely, copper, iron, lead, tin, and zinc. These five practical metals, so to speak, are again virtually extended by an infinitude of combinations, or alloys, principally amongst themselves, although with the occasional introduction of the metals before named, and some few others.

Of all the metals however, iron is the one to which from its manifold changes and adaptions, and from its abundance, the most importance is to be attached; so much so, that were we compelled to the choice, it would be doubtless politic to sacrifice all the others for its possession. It is subjected to several of the methods of treatment, common to the other ordinary metals, and to a variety of changes no less important peculiarly its own.

On these several grounds, therefore, I shall devote the principal share of attention to iron and its several modifications, and it is intended as a general illustration, to commence with a slight sketch of the manufacture of iron from the ore into castiron; wrought-iron; blistered, shear, and cast steel; and its part manufacture for the purpose of commerce, into ingots, bars, sheets, &c. This will be followed by its further preparation, by forging, into some of the elements of machinery and tools; the various changes of hardening and tempering, applied under a variety of circumstances, will be then described.

I propose subsequently to consider the thirteen other metals, both in their simple and alloyed states, which will lead me to give a general outline of the methods of casting objects of various forms, and also the practice of soldering, which is dependent on the fusible property of some of the metals.

SECT. II.-THE MANUFACTURE OF CAST-IRON.

The ore having been raised, the first process to which it is subjected is called calcining or roasting; the iron-stone or rawmine is intermixed with coal and thrown into heaps, commonly from thirty to sixty feet in length, ten to sixteen feet wide, and about five to ten feet high; the heaps are ignited and allowed to

burn themselves out, which takes place in three or four weeks. This calcines the ore, and drives off a portion of the water, sulphur, and other volatile matters, after which the ore is said to be torrified; this process is also performed in kilns.

The smelting is generally performed in England and Wales with coke, and therefore another distinct part of the manufacture of iron is the preparation of the coke, which like torrifying the ore is also performed upon an enormous scale either in open heaps or in kilns, more generally the former.

The smelting furnace used in South Wales is represented in fig.75*; its height is about fortyfive feet, its diameter at the largest part, or boshes, from twelve to eighteen feet, and it terminates at the bottom in the hearth, which is originally a cube of about a yard on each side, but soon becomes of an irregular form from the intensity of the heat.

In mountainous countries such as Wales, the furnace is usually built by the side of a hill; upon the summit of which the coke and mine are prepared,

Fig. 75.

so that they, along with the due proportion of limestone, may be wheeled in barrows along the bridge represented, into the mouth of the furnace. In level countries, the charge has to be dragged to the filling place up an inclined plane, by means of the steam engine; a full barrow proceeds along the upper surface of the rail, arrived at the top it turns over, discharges its contents into the furnace and returns on the lower side, much the same as the buckets of a dredging machine; there are two such barrows, and from their action they are called tipplers: the most general plan however, and the best, is to fill the furnace by hand, a man being stationed at the top, on the plane provided for the purpose. The furnace requires in addition to the solid materials, an

* This wood-cut is reduced from fig. 1, plate 6, of Mr. Mushet's "Papers on Iron and Steel;" the exterior sectional lines are alone copied, to give a general notion of the form of the internal cavity and the thickness of the walls.

184

OPERATION OF THE FURNACE

enormous supply of air, which is driven in by blowing engines of various constructions, either at the ordinary temperature or in a heated state, and at one, two, or three sides of the cubical hearth through appropriate pipes or tuyeres; the fourth or front side of the hearth, being reserved for the dam stone, over which the cinder or scoria flows in a fluid state, and for the aperture through which the charge of melted iron is removed.

When the charge arrives at the hottest part of the furnace, the carbon of the fuel is considered to unite with the oxygen of the ore, and to escape in the form of carbonic acid gas, and carbonic oxide. The lime serves as a flux to fuse the clay and silex of the iron-stone into an imperfect glass or scoria; and the particles of the metal now released, ooze out from the iron-stones, mix with some of the carbon of the fuel, fall in drops through the fiery mass, and collect on the bed or hearth of the furnace ; whilst the scoria floats on the surface of the fluid metal, and defends it from the air.

When the scoria has accumulated in sufficient quantity to reach a proper aperture in the front of the furnace, it flows away as a constant stream of liquid lava; and the furnace is tapped at intervals, to allow the charge of metal to run out into channels formed in a bed of sand for its reception: it now assumes the name of crude-iron, cast-iron, or the pig-iron of commerce; which is a compound more or less pure, of iron and carbon in different proportions.

The choice of the flux depends on the nature of the ore or mine. For argillaceous ores, which are the most common in England, lime is required; and frequently the cinders or slag from the fineries and forge are mixed with the lime. For calcareous ores, like those of the Forest of Dean in Gloucestershire, clay is added, in order to establish a similar train of affinities as regards the earthy matters of the ore; one of the main objects being to fuse all the earths into a glass, so fluid as not to detain the globules of metal, from descending through it to the general mass beneath.

When the iron ores are very pure, as the rich hæmatite ores of Cumberland, clay is introduced into the furnace; but these are more commonly mixed in small quantities with the poorer ores of other districts, and are used without being calcined. It is fortunate that in this country nature has generally supplied the