soften the most refractory crucibles by means of a chimney from thirty-six to forty feet high. They are generally made square or rectangular, and have interiorly the same dimensions as the crucible of the furnace. About two feet above the upper part of the furnace, these chimneys 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.

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, in the small way, is not so powerful.

The quantity of air passing into a furnace varies with the length of the assay, and ought to increase gradually, the stream at first being small, and as the temperature becomes higher, the bellows ought to be worked with more force.

The following is the description of a most excellent blast furnace which has been in use for some years in the laboratory of the Royal Institution. I have in my own laboratory one of these furnaces, and 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

C

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 consists of a blue pot, (blacklead 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 as to leave an aperture of five inches. This, when put into the larger pot, 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 pulverized 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 furnace. The 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 into the air chamber at the lower part; its use being to receive the nozzle of the bellows by which the draught was thrown in.

The Muffle, or Cupel Furnace is a species of rever

beratory furnace, in the centre of which is placed a small semi-cylindrical oven which is termed the muffle.

This muffle being completely surrounded by ignited fuel acquires a very high temperature, and in its interior, all operations requiring the presence of air, and which may not be carried on in contact with carbonaceous matters may be performed, such as roastings, scorifications and cupellations.

When from ten to twenty cupellations have to be effected at one time, large brick furnaces are employed; and, in consequence, much fuel is consumed to waste, where only a few cupellations are required. This has occasioned many persons to endeavour to form small furnaces, where one or two cupellations may be carried on with the smallest possible quantity of fuel. MM. Aufrye and d'Arcet have contrived a furnace which is capable of fulfilling all these conditions.

The furnace is elliptical and about 7 inches wide and 18 high; its ash-pit has but one circular opening, and its height is such, that when the furnace is placed upon it, and the whole upon a table, the assayer can, when seated, readily observe the course of the assay within the muffle. The hearth has five openings, in one of which the muffle is placed; in another, a brick to support it; a third for the purpose of introducing a poker to stir the ashes, and make them fall through the grateholes; this can be closed with a small earthen plug; and lastly, two round holes, placed in its largest diameter, to facilitate the introduction of air, either by draught or a pair of bellows, as the case may require. The support for the fuel is generally a plate of earthenware, pierced with holes, and bound round with iron

wire to keep it together in case it cracks by changes of temperature; but it is better to use an iron grating.

The dome of the furnace has a circular opening, which can be closed by a plug of earthenware; this opening serves for the introduction of the fuel. A chimney is necessary to increase the draught; it is made of sheet iron, and may be from 1 to 2 feet in height, and ought to fit the upper part of the dome very exactly. At its base there is a small gallery, also of sheet iron, in which is intended to be placed the new cupels, so that they may be strongly heated before introduction to the muffle. This saves many of them from fracture. MM. Aufryé and d'Arcet have estimated the quantity of charcoal necessary to heat this furnace. The following are comparative experiments.

Coke or charcoal may be used in this furnace, but the fire must be lighted by means of charcoal alone, because coke is very difficult to inflame in a cold furWhen it is red hot, it may be fed with coke alone, or better still, a mixture of coke and charcoal.

nace.

AUXILIARY APPARATUS.

Ordinary assay furnaces require very few instruments; they are: firstly, pokers, or stirring rods, made of stout bar-iron; these may be straight, as for stirring

the fuel from the top of the furnace, or curved for clearing the bars from clinkers or ashes; secondly, straight and curved tongs, of different sizes and forms, for placing crucibles, &c., in the fire; thirdly, sheet iron shovels, for the purpose of introducing coke and other fuel into the furnace; they are best provided with holes of small diameter, so that coke and charcoal may be separated from dust by sifting before it is burnt; and lastly, a wooden mask, furnished with glass openings, through which the state of the fire, &c., may be ascertained without injury to the face. The glass is best of a deep neutral tint; a pair of neutral tint spectacles are very serviceable, when the eyes alone are wished to be protected.

In cupel furnaces, pokers or stirring rods, both curved and straight, are required, as also a mask or spectacles. The tongs are varied in form; thus we have the ordinary straight tongs for the purpose of holding fuel, hot doors, &c., then light tongs, whose nose is slightly curved, for introducing and withdrawing the cupels; also very light elastic pincers to place the assay buttons in the cupel; and lastly, small iron spoons, for the introduction of granulated lead.

I will now describe the furnace I am in the habit of using. It may very aptly be termed the "universal furnace," as it is capable of performing all that is required of any furnace in an assay. It is to be much recommended for its durability and cheapness, and lastly, for its small size, compared with the heat it can produce. It is made, I believe, only by Mr. Newman of Regent Street, and is constructed externally of sheet iron, very stout, and is lined with fire brick, not cemented