Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/792

726 ash-pit doors run; c, the door and ash-pit ; d, the iron casing to the furnace; &amp;lt;?, the brick lining ; /, the ash-pit ; gg, the two bars inserted in the brick lining, one in front, one at the back, supporting the furnace bars, which can easily be remov ed at pleasure ; h, one of the bars on which the muffle plate rests; i, a movable tray on which the mouth coal is placed; k, a section of a muffle charged with its full complement of 50 cupels, showing also the rows of holes over each row of cu pels, through which a current of air passes; similar holes are placed at the back in three rows ; they are not pierced through horizontally, but slope towards the ceiling of the muffle at such an angle so as to exclude the ashes; represents the extra covering of fire-clay ; m, the an thracite coal, showing the level; n, the feeding and regulating door ; o, the ketch or inclined plane on which the sliding-bar travels ; p, the door, with running staples in which the bar slides; q, the mode in which the movable front is brought round and fixed by the thumb screws r ; sv, the hood; t, handle for removing the front; ivx, the damper and handle. 2.—Section of Furnace. Fig. 3 represents the upper interior view of the furnace bars with the muffle stand or plate, showing also the space in tended for the fuel. 3.—Section of Furnace. Fig. 4 is the mouth of the muffle door, representing the mode of regulating the current of air by cylinders of charcoal. Fig. 5 is the movable muffle door. Fig. 6 is a representation of a muffle, 14 inches long, 7½ inches wide, until it begins to taper at about 1½ inches from the front (see fig. 7), when it does not exceed 5 5/8 inches. The height is o inches, in the clear 5 jj- inches. Its sides are perforated with holes about a quarter of an inch in diameter. Fig. 4. Fiz. 5. Fis.. G. Fig. 7. Fig. 8 is an annealing-iron for softening the assays after they are flattened and rolled. It resembles a square bar of iron about ^th of an inch thick, having strengthening pieces rivetted at each end and two in the middle bllb, between which are receiving places for the assays aaa. Fia. 8. Annealing Iron. The apertures are made diagonally, as shown by ccc, that the assays may not fall completely to the bottom of the box, so that they may be conveniently removed. The under part of the box has a kind of double keel d rivetted on it, so that in taking it from the furnace there be no danger of upsetting it on the annealing trident. Fig. 9 represents the trident for removing the annealing- iron from the furnace. FIG. 9. Trident. Fig. 10 represents the cupel mould; fig. 11 being the section of the same, showing the four pieces of which it consists. The mould is circu lar, made of forged steel nicely turned. There are several substances of which cupels may be made, but the one in general use is the ash of burnt bones. This consists princi pally of phosphate of lime, with a little carbonate and some fluoride of calcium. The bones of sheep and horees are best for cupels. They should be burnt until they are quite white, care being taken not to heat the bone-earth too strong ly. It must then be finely ground, sifted, and washed Fia. several times with boiling distilled water till all soluble salts are removed. The finest particles of the powdered bone-earth will remain longest suspended in the washing waters. This must be allowed to settle separately, and should be reserved for giving a final coating to the surface of the cupels. For the body of the cupels the bone-ash should be about as fine as wheat flour. The bone-ash being moistened with a quantity of water, just sufficient to make the particles adhere, is put into the mould a, fig. 11, and pressed down level with the surface. The mould is then put together, as in fig. 1 0, and the pestle struck with a hammer so as to compress the bone earth into a solid cake. The surface of the cupel may then have sifted over it a little of the very fine levigated bone-ash, and the pestle again hammered on it. The pestle is to be turned lightly round so as to smooth the inner surface of the cupel, and then withdrawn. The cupel is removed from the mould by gentle pressure on the narrowest end. It must be dried gently by a stove, and lastly ignited in a muffle to expel all moisture. It is then ready for use.

Fig 12 shows the appearance of the finished regn cupel, which is generally 1 inch by ths of an inch. The lead used in cupellation should be of the lg- greatest purity, because, as most lead contains a small portion of silver, this silver would necessarily combine with the assay and vitiate the accuracy of the result. Another important consideration is the quantity of lead to be used with each assay. This information is generally obtained by an ex perimental assay, unless, as most frequently happens, the circumstances of the case enable the assayer to judge the 