sued in dressing the ores of lead and tin; and the same method is to a certain extent adopted with the copper ores :—

The ore having been brought to the surface, if a lead or copper ore, is first sorted by hand: the purest portions, or prills, as the Cornish miners term them, are set aside, and are ready for smelting without further preparation; but the bulk of the ore is broken by hammers into lumps of about the size of a walnut, and the best pieces are again picked out by hand. The rougher portions go to the crushing mill, which consists of a pair of horizontal cylinders placed parallel to each other at a little distance apart: the cylinders may be either grooved or plain. The ore is supplied to them by a hopper from above. After passing through the mill, the crushed ore is sifted through coarse sieves; the coarser parts are set aside for the stampers, and the finer portion is subjected to the operation of jigging. This consists in plunging the ore contained in a sieve into a pit, through which water is constantly flowing: the workman keeps the ore in continual agitation, alternately raising and lowering the sieve, to which he also gives an alternate rotatory motion, taking care always to keep it beneath the surface of the water. By this means, the contents of the sieve are separated into layers of different quality. If it be a lead ore which is undergoing treatment, the galena, from its friable character, is easily reduced to small fragments: most of the galena, therefore, passes through the sieve and subsides to the bottom of the pit, whilst what is left upon the sieve consists chiefly of the less friable fluorspar and quartz. This residue is mixed with the inferior qualities of ore, and is transferred to the stamping mill, whilst the richer part is set aside for smelting.

Tin ore is usually disseminated through a compact hard matrix, and passes at once to the stampers.

The stamping mill consists of five or six upright wooden beams, the lower ends of which are shod with iron, each beam weighing about 24 cwt. These are placed in a wooden frame, and are alternately lifted up and allowed to fall back upon the ore by the action of arms projecting from a horizontal axle, which is turned by water or steam power. The ore is placed on an inclined plane behind the stampers, and slides down under them, and is crushed. The crushed particles, when reduced to a sufficient degree of fineness, are washed out through a grating in front, by the action of a current of water which is constantly flowing through the mill; the washed ore is carried into a channel in which two pits are formed; in the one nearest the mill the purer and heavier part of the ore, or crop, is deposited; whilst the more finely divided

WASHING THE ORES-THE BUDDLE.

333

portion, technically termed slime, or schlich, accumulates in the second.

The crushed ore now undergoes a series of washings, the object of which is to separate the impurities from the valuable part of the ore.

FIG. 327.

The crop is first subjected to washing in the buddle; this is a wooden trough, fig. 327, about 8 feet long, 3 wide, and 2 deep, fixed in the ground, with one end somewhat elevated. At the upper end, a small stream of water enters, and is reduced to a uniform thin sheet by means of a distributing board, a, on which a number of small pieces of wood are fastened to break the stream. The ore to be washed is placed in small quantities at a time upon a board, B, somewhat more inclined than the body of the buddle, and it is spread out into a thin layer; the water carries it forward: the richer portions subside near the head of of the trough, and the lighter ones are carried further down. "The heads' are then tossed in the kieve, or tub, shown at c, which is filled with water, and ore added by a workman, who keeps the contents of the kieve in continual agitation by turning the paddle or agitator, the handle of which is seen projecting at the top. When the vessel is nearly full, the agitation is stopped - the kieve is struck smartly upon the side several times, and

its contents are allowed to subside; the upper half of the sediment is again passed through the buddle. Various modifications of the washing process are resorted to, but they are all the same in principle.

A rough estimate of the value of any sample of dressed ore is obtained by the process called vanning :-A small quantity of the ore is placed on a shovel, and agitated gently with a peculiar circular movement in water, then, by giving it a dexterous lateral shake, the different constituents arrange themselves according to their density-the galena, or the tin-stone, at the bottom; above this are iron pyrites and blende; and at the top are the fluor-spar

334

CHEMICAL OPERATIONS-ROASTING, OR OXIDATION.

and quartz. The eye then at a glance roughly estimates the quantity of each.

The water employed in the various washings is not allowed at once to run to waste, but is made to pass through a long shallow channel, in which the slime and mud which have been carried away in the different operations may subside. This slime still retains some portion of ore; and in order to recover this as far as possible, it is again subjected to the action of a fine stream of water, either upon an inclined table, which acts in a manner similar to the buddle, or it is washed upon a swinging table, the bed of which is also inclined, but moveable, and is suspended by chains from supports at the four corners; the bed is alternately thrust forward two or three inches by the revolution of a cam-wheel, and is then allowed to fall back against solid wooden bearings with a sudden jar. The ore is spread upon a board which overhangs the upper part of this table, and carried forward by a gentle stream of water; the heavier particles of the ore, owing to the superior momentum which their density gives them, are by this jarring movement of the table carried back to the upper part of it, whilst the lighter impurities are washed away.

(530) Roasting, or Oxidation.-The chemical operations are divisible into two main branches, one dependent on the addition, the other on the removal, of oxygen. If the mineral contain volatile ingredients, such as sulphur or arsenic, the process of roasting, or oxidation, is first resorted to. In principle it is very simple; the mode of effecting it varies, however, in different cases. In the most common method, a furnace of particular construction, termed a reverberatory, is employed. Fig. 328 shows a section of a reverberatory furnace, such as is employed for roasting copper

ores; t is the platform, from which the hoppers, H, H, are charged with the ore, which at proper intervals is allowed to fall upon the

REDUCTION, OR SMELTING.

335

bed, c c the fuel is consumed upon a distinct hearth, a, and does not come into contact with the ore, from which it is separated by the bridge, b: the heated gases, as they arise from the burning mass, are, from the construction of the arched roof, reverberated, or driven down upon the ore to be roasted, and then pass off through the flue f: when sufficiently. roasted, the ore is allowed to fall into the arched recess, E, beneath the bed of the furnace through openings, d, d, which are kept closed by sliding plates till the roasting is complete. After the fire has been lighted, a constant supply of air to the mineral is maintained, and care is taken to prevent the heat from rising so high as to melt the ore, which is stirred at intervals to expose fresh surfaces to the action of the air: the sulphur burns off in the shape of sulphurous anhydride, which escapes into the atmosphere; whilst the arsenicum forms arsenious anhydride, which, though volatile, speedily becomes condensed, and is collected on the sides of the chimney, or else in chambers constructed for its reception, whence it is removed at intervals, and subsequently purified. In metallurgic operations where sulphides of metals of different degrees of oxidability are present, it may happen that the sulphide of the more oxidizable metal is completely converted into a metallic oxide, whilst sulphurous anhydride escapes, and that the sulphide of the less oxidizable metal is reduced to the metallic state. For example, in roasting copper pyrites (the mixed sulphides of copper and iron), the iron is wholly converted into oxide, whilst the copper is extracted at once in the metallic state, by a series of careful roastings (870). In the case of sulphide of lead, where the metal possesses but a moderate degree of oxidability, it is also the practice so to regulate the supply of air in the furnace that the sulphur is wholly expelled in the oxidized condition, whilst the greater part of the lead is extracted in the form of metal during a single roasting in the reverberatory (890). Where the metal possesses a high degree of oxidability, as is the case with zinc, it is not practicable to limit the degree of oxidation in this manner during the roasting the metal itself passes into a state of oxide, simultaneously with the expulsion of the sulphur as sulphurous anhydride (702).

(531) Reduction, or Smelting.-The second chemical process for the extraction of the metals, that of reduction, is applicable to most metallic oxides, whether of natural or of artificial origin. The object in this case is to remove the oxygen, by presenting to the mineral some body which, at a high temperature, has a stronger attraction for oxygen than the metal itself possesses. The furnaces

336

REDUCTION OF THE METALS.

employed in this operation are often of great size, and vary in form with the nature of the metal: in them the ore is heated intensely, in contact with carbon; carbonic oxide and carbonic anhydride are thus produced, and from their gaseous nature are quickly removed from the sphere of action. It becomes necessary at this stage to get rid completely of the earthy and other impurities of the ore, which the mechanical operations never succeed in removing entirely, and which often form a large proportion of the ore. In order to effect this, certain fluxes, or substances which are capable of forming fusible compounds with the earthy matters, are added at the same time with the carbon; these melt and form a kind of glass, through which the reduced metal, from superior density, sinks, and is thus completely defended from contact with the air: the metal is at suitable intervals drawn off from the bottom of the furnace, while the melted glass-or slag, as it is termed-runs off at an aperture left in the side for the purLimestone is in some cases added to the ore with the view of aiding the fusion of the siliceous impurities: in other instances fluor-spar or some other readily fusible material is added, for the purpose of increasing the fluidity of the slag. Much judgment is required in the selection of the flux, and in deciding upon the proper proportion to be added: frequently this object is economically effected by a judicious mixture of different ores of the same metal, each of which aids the other by supplying some compound which was wanting to render the slag sufficiently fusible.

pose.

The various modifications of these processes will be described as they present themselves in connexion with the different metals which require these modifications. Other modes of separating individual metals are employed, which will be alluded to in their respective places. For details upon metallurgic processes, Percy's Metallurgy, or the fourth volume of Dumas' valuable work, Traité de Chimie appliquée aux Arts, may be consulted; and the second and third volumes of the same work contain many excellent descriptions of processes in which metallic chemistry is applied to the purposes of industry and commerce. Phillips's Metallurgy is a smaller and more compendious treatise on this subject.

(532) Classification of the Metals.-The metals may be divided into eight groups (page 10), regard being principally had in this arrangement to the convenience of indicating the method of testing for the presence of the metal, in the ordinary processes of analysis; in consequence of which it is sometimes necessary to depart from the strictly natural order.