the cooled slag, the central core being much poorer, and upon this fact is based the catch pot or "tap pot" system of Devereux still in very common use. Figs. 82 to 84 show one very good form of catch pot with roller bearings and the Keller patent

wheels, the axles being fastened by wedges instead of by setscrews; besides the ordinary splash guards, S, it has a projecting lip, L, to take the wear in tipping. At 6 inches above the bottom is a taphole 1 inches in diameter, surrounded by a ring and rib reaching the projecting lip in order to protect the pot against injury when the interior slag is tapped by driving a square pointed 3-inch steel bar through the cooled crust. In this way a crust of chilled slag is recovered for resmelting which contains most of the matte shots, while the cake of matte at the bottom of the pot remains undisturbed, even if still fluid, which it does not do when an attempt is made to pour out over the rim the interior fluid slag after the crust has solidified.

As regards the surface of the dump, some metallurgists recommend laying down tracks of narrow cast-iron plates for the

slag pots to run upon, in order to save labour in pulling the pots considerable distances. The ordinary way, however, to keep the surface of the dump smooth and hard is by levelling a series of squares about 3 feet across with pieces of sheet iron and filling with molten slag, this work being always kept up as the dump advances.

As the distance from the furnaces to the edge of the dump increases, handling in small pots becomes altogether too expensive, and it becomes necessary to use larger pots running on rails and drawn by horses or by locomotives. These larger pots are of two general patterns, single (v. Figs. 85 to 87) or

double (like Fig. 88), which shows the Nesmith pot truck largely used in Colorado. Though of large capacity, however, this double pot takes up a great deal of space about the furnaces,

and is, therefore, very awkward to introduce in many old works. The double pot truck of the Broken Hill Proprietary Company has a much smaller wheel base, which enables it to turn much sharper curves, and it is altogether the best device of the double pot style in use. Each pot holds about 18 cwts. of slag, and the trucks are handled by means of 5 feet turntables on steel rollers, one of which is placed in advance of each pair of furnaces.

Separation of Slag and Matte. With the increase in viscosity of the slag caused by higher lime, alumina, magnesia, and zinc contents, and with the greater care in checking losses caused by keen competition among smelters, it became necessary to secure a better separation of slag from matte than is afforded by any of the above slag pots, more especially as the percentage of matte produced shows a constant tendency to increase of late years. The methods adopted may be divided into two main classes-A, Those in which the separation takes place quite away from the furnace; and B, those in which separation is sought to be brought about in connection with the furnace itself. Taking those of class A first, we have:

1. The Overflow Pot.-This is usually more or less of the slag pot form, but much larger, and provided with a cast-iron spout through which the slag can escape after depositing the greater part of its matte. Sometimes a cast-iron cover is provided, but generally a simple crust of slag is allowed to form over the top to prevent rapid chilling of that below. The greater portion of the matte collects in the bottom of the pot, whence it may be tapped through a ring-protected aperture, like that shown in Fig. 82; or, if in insufficient quantity, by allowing the overflow pot to cool and breaking up its contents.

2. The Exterior Forehearth or "Settler."-This appliance, consisting in the main of a cast-iron box on wheels, is but seldom used with ordinary lead furnaces smelting ores, though commonly enough when concentrating matte. At the Globe (Denver) and several other progressive works, however, rectangular forehearths or settlers of very large size (up to 120 cubic feet capacity) have been installed with good results. For details of the appliances used in matte concentration, v. Part II.

3. Large Crane-handled Settling Pots.-The system in use at the Grant Works (Denver), and described by Austin,* is shown in Figs. 89 and 90. A series of five pits placed in position on the slag dump beneath an overhead track, which runs parallel with the front of the furnaces, contains each a large separating pot, about 5 feet diameter and 5 feet deep, fitted with trunnions and hooks, and holding 4 tons of slag. These pots are filled successively with slag produced in all the furnaces, which is conveyed to them in ordinary small pots, the shells of which are Eng. and Min. Journ., November 23, 1895.

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set aside for resmelting. When each settling pot is full, it is allowed to stand from ten to fifteen minutes in order to allow the shots of matte to settle out, and then the overhead traveller, c, being brought into position over it and the block let down, the links, X, are hooked over the crossbar, and the pot raised by a steam winch. One of the slag cars, S, is then brought into position on the transverse track beneath, and the slag is tapped into it through one of the holes, a or a1 (according to the amount of matte). The pot still containing the matte is then moved over another smaller slag truck (holding 1000 lbs. only), and the matte tapped out through another hole at the extreme bottom, together with any fluid slag left. The pot is returned to where the hook, h, can be inserted into the ring at the bottom by means of which it is inverted, and the shell of solidified slag turned out for remelting, after which it is again turned up and replaced in its pit for refilling. There are only two slag trucks to serve the five separating pots; they are hauled by a mule to the edge of the dump, and are easily tipped by means of a worm wheel, as shown in Figs. 85 to 87. When empty, a couple of shovelfuls of dry clay are thrown in, and they are again ready to take the charge of another separating pot.

4. The Reverberatory Settling Furnace.*-Messrs. Iles, of the Globe Works (Denver), and Rhodes, late of the Arkansas Valley Works (Leadville), have worked on the principle of an external source of heat for keeping the slag perfectly fluid, so as to give the shots of matte a better chance of settling out. In both plans the reverberatory (which should preferably in a large works be duplicated to allow of cleaning out and repairing from time to time) is water jacketed, is situated at a distance from the furnaces, and has a separate fireplace with separate tapholes for slag and matte, the latter being low down on one side near the firebox, while the other is either on the opposite side (Rhodes) or at the far end (Iles).

In the Rhodes separator (Arkansas Valley) the slag from the furnaces is brought in hand pots and poured in through a lip at the end, the shells (10 per cent.) being returned for resmelting. The capacity of the furnace is 100 tons of slag, which comes direct from the slag tap of the furnaces, it being found that the ore-matte is required in order to collect and settle that carried by the slag in suspension. The slag is tapped every half hour into a train of Nesmith double-pot trucks drawn by a locomotive; the matte is tapped into hand pots and granulated for roasting.

At the Globe Works, where the Iles separator is used, the slag from the furnaces is tapped at intervals into a very large and long settling box of 120 cubic feet capacity with a side taphole for matte, which at these works is 8 to 12 per cent. of the

* Private notes, 1897.