METALLURGY 59 and allow it to pass through uncmshcd. Sometimes two sets of cylinders are arranged one above the other, so that the grit from the upper falls into the jaws of the lower set to receive further com minution. The diameter of the cylinders is from a foot to a yard, their length from 9 inches to a yard, the velocity of a point on the periphery a foot to a yard per second. The quantity of ore reduced Eer hour per horse-power is about 5 cubic feet for quartz or other ard minerals, and about 14 cubic feet for minerals of moderate hardness. For the production of a relatively fine powder the pounding-mill is frequently used, which, in its action, is analogous to a mortar and pestle. The mortar is a rectangular trough, while the pestle is replaced by a parallel set of heavy metal or metal-shod beams, which (by means of a revolving cylinder with cogs catching pro jections on the beams) are lifted up in succession and then let fall by their own weight so as to pound up the ore in the trough. The ore is supplied from a prismatic reservoir with a sloping bottom leading into a canal through which the stones slide into the trough. A current of water, which constantly flows into the trough from below, lifts up the finer particles and carries them away over the edge of the trough into a settling tank. The object pursued in powdering an ore is to prepare it for being purified by washing. But the velocity with which a solid particle falls through water depends on its size as well as on its specific gravity an increase in either accelerating the fall ; hence, where the difference in specific gravity between the things to be separated is small, the washing must be preceded by a separation of the ore- powder into portions of approximately equal fineness. This is often effected by passing the ore through a system of sieves of different width of mesh superposed over one another, the coarser sieve always occupying the higher position. Sometimes the sieves are made to "go dry," sometimes they are aided in their action by a current of water which, more effectually than mere shaking, pre vents adherence of dust to coarser parts. Another contrivance is the " Drum " (fig. 2). A long perforated circular cylinder made of sheet-iron, open at both ends, is suspended, in a sloping position, by a revolving shaft passing through its axis. The size of the perforations is generally made to increase in passing from the upper to the lower belts of the cylinder. While the drum FIG. 2. Drum. is revolving, the ore, suspended in water, flows in at the upper end, and in travelling down it casts off first its finest and then its coarser parts, the coarsest only reaching the exit at the lower end. The several grades of powder produced fall each into a separate division of the collecting tank. The drum, of course, is subject to endless modifications. _ A very ingenious combination is H. E. Taylor s "Drum Dressing Machine" (fig. 3). It consists of three truncated cone-shaped drums D, fixed co-axially to the same horizontal revolving shaft, FIG. 3. Taylor s Drum Dressing Machine. so that the narrow end of No. 1 projects into the wider end of No. 2, and No. 2 similarly into No. 3. The drums are not perforated, but are armed inside with screw-threads formed of strips of sheet metal fixed edgeways to the drum. The ore grit to be dressed is placed in a hopper A, and from it, by a worm 15 fixed to the revolving shaft, is being screwed forward into a short fixed truncated cone C projecting into the revolving drum No. 1, into which it flows in a constant current. The rotary motion of the drum tends to convey the ore along the spiral path prescribed by the screw-thread towards the other end, and from it into drum No. 2, and so on. But the ore in each drum meets with a jet of water E impelling it the opposite way, and the effect is that, in each drum, the lighter parts follow the water, and with it run off over the entrance edge to be collected in a special tank, while the coarser parts roll down the spiral path toward the next drum to undergo further parting. The tank or pit for drum 1 receives the finest and lightest parts, that of drum 2 a heavier, that of drum 3 a still heavier portion, while only the very heaviest matter finds its way out of the exit end of No. 3 into a fourth receptacle. Of the large number of other ore-dressers, only two need be men tioned here. The "Clausthal Turn-Table" consists of a circular table, the sur face of which rises from the periphery towards the centre so as to form a very flat cone of about 170, which is fixed co-axially to a ver tical rotary shaft. At the apex of the table, surrounding the shaft, but independent of its motion, there is a circular trough of sheet zinc, divided into two compartments ; one receives a stream of water carrying the ore, the other a supply of pure water. A large annu lar trough of sheet zinc is placed below the periphery of the table, so as to receive whatever may fall over the edge. It also is divided into compartments, as shall be explained further on. Supposing the table to be at rest, a sector of about 60 of it would be constantly run over by the ore-mud out of the first compartment of the upper trough. This mud current would suffer partial separation into heavier and lighter parts, rich ore resting in the higher and poorer in the lower latitudes, and a still poorer ore falling over the periphery into the lower trough. The same happens with the moving table ; only each sector of such partially analysed ore under goes further purification by passing through about 90 of water- shower. After passing this, it meets with a perforated fixed water- pipe going up radially to about half the radius of the table. This pipe also carries sweeping brushes, so that the belt of ore from the lower latitudes of the table is swept off into the corresponding section of the receiving trough. What of ore remains on the higher latitudes subsequently meets with a similar arrangement which sweeps it off into its compartment. If the table turns from the left to the right, and we follow the process, beginning at the left edge of the ore-mud compartment, it will be seen that a first sector of the receiving trough gathers the light dross, a succeeding one an intermediate product, a third the most highly purified ore. The "intermediate" is generally run into the ore-mud trough of a second table to be further analysed. In the "Continuous Wash-Pumps" (Continuirliche Setzpumpe) of the Harz, three funnel-shaped vessels (one of which is shown in fig. 4) are set in a frame beside one another, but at different levels, so that any overflow from No. 1 runs into No. 2 and thence into No. 3. Each funnel connnuuicates below with its own compart ment of a common cistern. Into each funnel a riddle with narrow meshes is in serted somewhere near the upper end, while, beside the riddle, there is a pump of short range, which, by means of an excentric, is worked so that the piston alternately goes rapidly down and slowly up. The mode of working is best explained by an example. At Breinigerberg in Rhenish Prussia the apparatus serves to separate a complex ore into the following four parts, which we enumerate in the order of their specific gravities (1) galena (the heaviest), (2) pyrites, (3) blende, (4) dross. Sieve No. 1 is charged with granules of galena, just large enough not to slip through the meshes, No. 2 similarly with granules of Fio. 4. Continuous Wa.sli-1 ump. Similarly wm ^i.mun-a " . pyrites, No. 3 with those of blende. The crude ore-mud goes into sieve 1 the jerking action of the pump alternately tosses the particles up into the water and allows them to fall ; the heaviest naturally come down first, but what is most striking is notliin" will pass through the bed of galena but what is at least
as heavy as galena itself. In a similar manner No. 2 and *o. 3