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of that and the outer brickwork is filled with a paste of very refractory clay, each layer being well beaten down. When the space is filled, the case is withdrawn, and the crust of clay dried with much precaution, every crack that may be caused by unequal dessication being filled up as fast as formed. This method of manufacture is very applicable to circular furnaces. In every case, however, it is necessary to border the edge with a band of iron to prevent injuries from tongs or pots. By using a mixture of 1 part of refractory clay, and 3 to 4 parts of sifted quartz sand, no cracks are formed during desiccation. This mixture is used on the Continent for the interior fittings of Sefström's blast furnace, as well as for larger blast furnaces for manufacturing purposes. It is said to stand a high temperature exceedingly well.

Makins* recommends for small furnaces the second kind of bricks, known as Windsor, or in the trade P. P. bricks. 'These are of a red colour, very siliceous, but soft, easily cut and shaped, and yet standing heat very well. The best method of cutting them is by a piece of zinc roughly notched out as a saw, and then the more accurate figure required may readily be given them by grinding upon a rough flat stone. In this way the small circular furnace formerly made by Newman, and sold by him as his universal furnace,' is lined by cutting the bricks with care to the radii of the circle they are to form, when they key in, like an arch, and so need no lining whatever.'

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THE ASH-PIT. On the one hand, it is well to have the power of cutting off access of air into the body of the furnace by the lower part, either to put out the fire entirely, or to deaden it whilst putting in a pot; and, on the other, to attain the maximum of temperature, we must have the means of allowing the air to pass with the greatest possible facility into the furnace. In order to do this it is necessary to furnish the ash-pit with doors, or valves, whereby the quantity admitted may be regulated as desired. It is advantageous to lead the air to the ash-pit from a deep and cold place, by

Makins's Metallurgy, p. 88.

means of a wide pipe. A chimney of less height will then be required.

The BARS are made in one piece, or are made up of moveable pieces of metal; the latter arrangement is the most convenient. Wherever a wind furnace is in use, the superior opening is closed by a cover made of a fire-tile, encircled with iron.

The CHIMNEY is a very essential part of a wind furnace : it is on its height and size that the draught depends, and, in consequence, the degree of heat produced within the furnace. In general, the higher and larger the chimney the stronger is the draught; so that, by giving it a great elevation, exceedingly high temperatures may be obtained.

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But there is a limit which it is useless to pass in a furnace destined for operations by the dry way; and besides this, the building a very high chimney presents many difficulties, and much expense, so that in laboratory operations, where a very strong current of air is required, recourse is had to a pair of double bellows. A temperature can be produced in

a wind furnace sufficiently strong to soften the most refractory crucibles by means of a chimney from thirty-six to forty feet high.

Chimneys are generally made square or rectangular, and have interiorly the same dimensions as the body of the furnace. About two feet above the upper part of the furnace they are furnished with a register or damper, by means of which the current of air may be regulated, or entirely stopped at will. The damper is a plate of iron sliding into a small opening across the chimney.

A wind furnace of the kind above described is represented by fig. 19.

The left hand figure in 19 is the plan, the middle an elevation, and the right is a sectional view. A the body of the furnace in which the crucibles to be heated are placed, G the bars, and P the ash-pit; the cover is formed of a thick fire-tile of the requisite size firmly encircled by a stout iron band, and furnished with a handle for convenience in moving it; B the flue, C the chimney, R the damper; Ha hood over the furnace, supported by iron bands h h h; M the handle of a ventilator T, which serves to carry off hot air and fumes from furnace when open; and, finally, S, a small sand bath, in which to set the red hot crucibles when taken from the fire: one foot square inside the fire-place of the furnace is a very good and convenient size; the remainder will then be in proportion.

BLAST FURNACES.-In this species of furnace, the air necessary to keep up the combustion is forced through the fuel by means of a blowing apparatus, instead of being introduced by the draught of a chimney as in the wind furnace.

The most convenient apparatus for forcing air into a furnace is a double bellows; a fan may be used, but it is not so powerful.

The quantity of air passing into a furnace varies with the length of the assay, and ought to increase gradually as the temperature becomes higher.

The following is the description of a most excellent blast furnace which has been in use for some years in the labora

tory of the Royal Institution :-The temperature produced by it is extraordinary, considering the small amount of time and fuel employed. It is sufficiently powerful to melt pure iron in a crucible in ten to fifteen minutes, the fire having been previously lighted. It will effect the fusion of rhodium, and even pieces of pure platinum have sunk together into one button in a crucible subjected to its heat.* All kinds of crucibles, including the Cornish and Hessian, soften, fuse, and become frothy in it; and it is the want of vessels which has hitherto put a limit to its application. The exterior (fig. 20) consists of a black-lead pot, eighteen inches in height, and thirteen inches in external diameter at the top; a small blue pot of seven and a half inches external diameter at the top had the lower part cut off, so to as leave an aperture of five inches. This, when put into the larger part, rested upon its lower external edge, the tops of the two being level. The interval between them, which gradually increased from the lower to the upper part, was filled with

FIG. 20.

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pulverised glass-blowers' pots, to which enough water had been added to moisten the powder, which was pressed down by sticks, so as to make the whole a compact mass. A round grate was then dropped into the furnace, of such a size that it rested about an inch above the lower edge of the inner pot: the space beneath it, therefore, constituted the air-chamber, and the part above, the body of the furThe former was 7 inches from the grate to the bottom, and the latter 7 inches from the grate to the top. Finally, a longitudinal hole, conical in form, and 1 inches in diameter in the exterior, was cut through the outer pot, forming an opening in the air-chamber at the lower part, its use being to receive the nozzle of the bellows by which the draught was thrown in.

nace.

Sefström's Blast Furnace, obtainable at most chemical instrument makers, is also very powerful and convenient;

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it consists of a double furnace. It is made of stout sheet iron, lined with fire-clay, and is used with coke, or charcoal and coke, cut into pieces of about a cubic inch in size. The blast of air is supplied by a powerful blowing-machine It will readily produce a white heat. Indeed the limit to its power seems to be the difficulty of finding crucibles or interior furnace fittings which will stand the temperatures produced in it without softening. Kersten states that he increases the heat in Sefström's blast furnace by using a hot blast.

H. Ste-Claire Deville has employed for melting platinum a furnace of 30 centim. height, and 28 centim. wide, which rests upon a cast-iron plate full of holes. This is connected with a forge bellows. After blowing for a few minutes, the temperature of the furnace will have reached the highest possible degree, but this zone of maximum heat only extends to a small height above the bottom of the furnace. Above this point a considerable quantity of carbonic oxide gas is formed, which burns with a very long flame. The heat produced in this furnace is so high, that the best crucibles melt, and only crucibles made of good and well burned lime can be used.

The MUFFLE, or CUPEL FURNACE, is a furnace in the centre of which is placed a small semi-cylindrical oven, which is termed the muffle. These furnaces were in use as early as the thirteenth century. Their construction and dimensions depend :

1. On the temperature which the furnace is intended to produce;

2. On the number of cupellations required to be performed at one time; and

3. On the kind of fuel used.

The muffles are mostly made of refractory clay, generally in one piece, but it is best to make them in two pieces; the bottom being one, and the cover or vault the other.

Muffles are sometimes made of cast iron-cast in one piece. They are useful in small furnaces intended for cupellations only.

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