This inconvenience is much felt in manufactories where they continue to use the plates for too long a time, as they become exceedingly thin after frequent repairs. One of Messrs. Mouchel's large plates reduces 1,400 kilogrammes (3,080 lbs. avoirdupois) from the largest size of wire to N° 6, which is of the thickness of a knitting-needle; 400 kilogrammes (880 lbs.) of this number are afterwards reduced in one single small plate to N° 24, which is carding-wire; and to finish them, they are passed through twelve times successively.

Wires are frequently drawn so fine as to be wrought along with other threads of silk, wool, or hemp; and thus they become a considerable article in the manufactures.

Dr. Wollaston, in 1813, communicated to the Royal Society the result of his experiments in drawing wire. Having required some fine wire for telescopes, and remembering that Muschenbrock mentioned wire 500 feet of which weighed only a single grain, he determined to try the experiment, although no method of making such fine wire had ever yet been published. With this view, he took a rod of silver, drilled a hole through it only one-tenth its diameter, filled this hole with gold, and succeeded in drawing it into wire till it did not exceed the three or four thousandth part of an inch, and could have thus drawn it to the greatest fineness perceptible by the senses. Drilling the silver he found very troublesome, and determined to try to draw platina-wire, as that metal would bear the silver to be cast round it. In this he succeeded with greater ease, drew the platina to any fineness, and plunged the silver in heated nitric-acid, which dissolved it, and left the gold or platina wire perfect.

LEAD MANUFACTURE.

LEAD ore is found in most parts of the world. In Britain the principal lead-mines are situated in Cornwall, Devonshire, and Somersetshire; in Derbyshire, Durham, Lancashire, Cumberland, and Westmoreland; in Shropshire, Flintshire, Denbighshire, Merionethshire, and Montgomeryshire; at the lead-hills in Scotland, on the borders of Dumfrieshire and Lanarkshire, in Ayrshire, and at Strontian in Argyleshire.

The smelting of the ore is performed by either a blastfurnace, called an ore-hearth, or a reverberatory-furnace. In the former method, the ore and fuel are mixed together,

and exposed to the action of the blast, which quickly fuses the metal, and causes it to fall into the lower part of the hearth, where it is protected from the oxygen of the blast by the scoria that floats upon its surface.

When the fluid lead is tapped, or drawn off, a sufficient quantity of it is left in the furnace to float the liquid scoria; but when the whole of the lead is to be drawn off, the blast is stopped, and some lime is thrown into the furnace to concrete the scoria, while the lead is run out.

In smelting by the reverberatory-furnace, which is uudoubtedly the best in places where there is an ample supply of coal, the fire is made at one end, and the flame passes over the hearth, and enters into an oblique chimney, which terminates in a perpendicular one, called a stack, of considerable height. The length of the hearth, from the place where the fire enters to the chimney, is about eleven feet, two of which constitute the throat of the furnace; the remainder forms a concave surface, four and a half feet wide at the throat of the furnace, seven feet four inches at the distance of two feet from the throat, seven feet two inches in the middle of the hearth, five feet eleven inches at two feet distance from the chimney, and two feet ten inches where the flame enters the chimney at two apertures, each ten inches square; the throat of the furnace is two feet long, four feet wide, and six inches deep; the length of the fire-place four feet, equal to the width of the throat; its width two feet, and depth three feet, from the grate up to the throat of the furnace; the section of the oblique chimney is sixteen inches square, and of the perpendicular twenty inches, supposing a straight horizontal line drawn from the lower plane of the throat of the chimney to the opposite side of the furnace; the lower part of the concave hearth, which is in the middle of this cavity, is nineteen inches below this line, the roof of the furnace being seventeen inches above the same line; the rest of the hearth is conformably concave.

The furnace on one side has three openings, about ten inches square, at equal distances from each other, and provided with iron doors, which can be removed as occasion may require. Besides these apertures, which are for the purpose of raking and stirring the ore, &c. and consequently, upon a level with the horizontal line above alluded to, there are two others of smaller dimensions, the one to tap the liquid lead, the other the scoria. The ore is introduced by a vessel in the shape of a hopper placed in the roof of the furnace.

The ores of lead, similarly to those of iron and most other

metals, are combined with various kinds of earthy matter, which require them to be well pounded before they are introduced into the reverberatory or smelting furnace. The pounding is sometimes performed by women using hammers, and sometimes the ores are pounded or crushed by causing them to pass through iron rollers loaded with great weights. After the ores have been pounded or crushed, the earthy matter is separated by washing.

The powder to be washed is put into a riddle or sieve, and placed in a large tub full of water; when, by a certain motion, the lighter or earthy parts are separated and thrown over the edge of the riddle, while the metal, which, as we have before stated, is always considerably heavier than its accompanying ingredients, is retained. There are some impurities, however, which cannot be separated by this process, consisting principally of blind, or black-jack, called mock ore, and pyrites, or sulphuret of iron, named Brazil.

In the process of smelting, the ore is spread upon the con cave hearth, so that the flame may act upon it, and release the sulphur. When the sulphur has escaped, the lead com bines with oxygen, and the oxyd of lead, thus formed, combines with and reduces the earthy matter to a liquid, which floats upon the surface of the metal, and for the remainder of the operation, protects it from the action of the oxygen. The temperature of the furnace is now considerably raised, to separate as quickly as possible the lead from the liquid scoria; after which a considerable portion of the scoria is tapped off, leaving only so much behind as is necessary to protect the metal from the action of the oxygen. The fire is now slackened, and a quantity of slack, or refuse pit-coal, thrown into the furnace, which serves to diminish the heat, and to concrete the melted scoria; though this last part of the process is not well done unless powdered lime be also added. The scoria being now hardened, is broken to pieces by a rake, and thrust to the opposite side of the furnace, where it is taken out through the apertures already mentioned,

The lead is now tapped, in a manner similar to that described in the manufacture of iron, and is allowed to run into a large iron pan, from whence it is laded into moulds to cast into pigs. When the ores abound with blind, or black-jack, or sulphate of iron, it becomes necessary to add the fluat of lime, as a flux.

The scoria is still found to contain some lead, independent of that in the state of oxyd, and chemically combined with it, and is consequently exposed to the heat of another furnace,

being a species of blast, and called a slag-hearth, which fuses the scoria, and causes the metal to penetrate through it, and fall into a cavity, where it is protected from the agency of the blast, and from whence it is taken and cast into pigs.

As all lead ores contain more or less of silver, we shall extract from Dr. Rees's Cyclopædia the method by which the silver, by the oxydation of the lead, is extracted.

"A shallow vessel, or cupel, is filled with prepared fern-ashes well rammed down, and a concavity cut out for the reception of the lead, with an opening on one side for the mouth of the bellows, through which the air is forcibly driven during the process. The French smelters cover the surface of the ashes with hay, and arrange symmetrically the pieces of lead upon it. When the fire is lighted, and the lead is in a state of fusion from the reverberation of the flame, the blast from the bellows is made to play forcibly on the surface, and in a short time a crust of yellow oxyd of lead, or litharge, is formed, and driven to the side of the cupel opposite to the mouth of the bellows, where a shallow side or aperture is made for it to pass over; another crust of litharge is formed and driven off, and this is repeated in succession till nearly all the lead has been converted into litharge and driven off. The operation continues about forty hours, when the complete separation of the lead is indicated by a brilliant lustre on the convex surface of the melted mass in the cupel, which is occasioned by the removal of the last erust of litharge that covered the silver. The French introduce water through a tube into the cupel, to cool the silver rapidly, and prevent its spirting out, which it does when the refrigeration is gradual, owing probably to its tendency to crystallize. In England the silver is left to cool in the cupel, and some inconvenience is caused by the spirting, which might be avoided by the former mode.

"The silver thus extracted is not sufficiently pure; it is again refined in a reverberatory-furnace, being placed in a cupel lined with bone ashes, and exposed to greater heat; the lead which has escaped oxydation by the first process, is converted into litharge, and absorbed by the ashes of the cupel.

The last portions of litharge in the first process are again refined for silver, of which it contains a part which was driven off with it. The litharge is converted into lead again by heating it with charcoal; part is sometimes sold for pigment, or converted into red-lead. The loss of lead by this process differs considerably, according to the quality of the lead.

The litharge commonly obtained from three tons of lead amounts to 58 hundred weight; but when it is again reduced to a metallic state, it seldom contains more than 52 hundred weight of lead, the loss on three tons being eight hundred weight. The Dutch are said to extract the silver from the same quantity of lead with only the loss of six hundred weight."

Having explained the process by which pig-lead is extracted from the ores, it now remains for us to show the methods by which pig-lead is manufactured into sheet-lead, or into the tubes called lead-pipes.

In the manufacture of sheet-lead, the ingots or pigs are put into a large caldron or furnace built with free-stone and earth. Near this furnace is the table or mould on which the sheet is to be cast; it is made of large pieces of wood, well jointed, and bound at the ends with bars of iron, and has a ledge or border of wood, about two or three inches thick, and one or two high, called the sharps. The tables are usually about three or four feet wide, and from eighteen to twenty feet long. The table is covered with very fine sand, which is prepared for the casting by moistening it with clear water, working it together with a stick, beating it flat with a mallet, and smoothing it with a piece of brass or wood.

A long narrow piece of wood, with notches cut in each end so as to fit the ledges, is placed over the table, and is so arranged, that the space between it and the sand shall be proportionate to the intended thickness of the plate. The workman gradually slides the strike from one end of the table to the other, by which means he obtains a sheet of the requisite, and in all parts of equal, thickness.

At the top of the table is a large triangular iron peel or shovel, with its fore part bearing upon the edge of the table, and the hinder part on a tressel, somewhat lower than the table; the design of which is, to prevent the liquid metal running off at the fore side, where there is no ledge. The metal being sufficiently fused, is taken out of the furnace or caldron with a large iron ladle, and is put into the peel, where it is cleansed of its impurities by using another large iron ladle pierced like a scummer. The handle of the peel is now raised, which causes the liquid metal to run into the mould, while the workman, with the strike, regulates the thickness. When the sheet is of the required thickness, the handle of the peel is lowered, and the sheet is allowed to cool. When set, the edges on both sides are planished in order to render them smooth and straight.