out at the pipe f. To prevent the charcoal rising and clogging the pipe f, it is confined by a board cover d. There are six of these filters in a set, so arranged that the overflow from one will pass down the glazed pipe of the next, and so on until the solution is exhausted and passes to the sump. About 300 gallons of solution can pass through each filter hourly. 43. Cleaning the Filters.-The solution remaining in the tubs is poured off; then the charcoal containing the gold is removed and sent to the furnace room. The first tub, or the one nearest the fresh gold solution, is removed after three days and its contents sent to the furnace room. The second tub is moved up to take the first tub's place, the third to take the place of the second, and finally the former first tub is made the sixth of the series by filling it with fresh charcoal. A solution that contains gold and .004 per cent. of cyanide has the gold almost completely precipitated, rarely containing more than .22 gram per ton after leaving the filter. 44. Recovering the Bullion. The charcoal is next burned in a reverberatory furnace. The ash is then sifted with a 30-mesh trommel enclosed in a box, and whatever remains in the trommel is returned to the furnace for reburning. The ash in the box is fused in a graphite crucible with borax in about the following proportions: borax, 3 pounds; ash, 11⁄2 pounds. The cost of precipitation is said to be about 25 cents per ounce of bullion. PRECIPITATION WITH ZINC FUMES 45. Definition of Fume.-Zinc fume is a product obtained in zinc smelting. It is a blue powder containing about 90 per cent. of metallic zinc. The cyanide solution containing the gold and silver is placed in a precipitating tank having a capacity of about 30 tons. The tanks have a -inch iron pipe leading to the bottom, through which air is passed at a pressure of about 15 pounds, for the purpose of keeping the solution agitated during the time the zinc dust is added. It takes about 5 pounds of zinc dust to precipitate the gold and silver from 30 tons of cyanide solution. The zinc dust is sieved into the tank occasionally, from the time it is half full of the solution until it is full. The pulp is agitated for a few minutes after it is full of cyanide solution and all the zinc dust has been added. -The sus 46. Collecting Zinc-Fume Precipitates. pended matter, or precipitate, is allowed to settle for hour. The solution is then decanted from the settled precipitate through a pipe that enters the precipitating tank about 8 inches above the bottom. As this solution contains some gold slimes, it is passed through a filter press to collect them. The pressure tanks are located below the precipitating tanks, thus allowing the solution and precipitate to be drawn quickly into the pressure tank and passed through the filter press, usually without the aid of pressure. The precipitation of the gold and silver with zinc dust is almost instantaneous and very complete, not leaving over 20 cents in each ton of cyanide solution. The consumption of zinc is about 14 pounds for each ounce of gold precipitated. COMBINATION CYANIDE PLANTS 47. Advantages of Agitation.-It has been said that by agitating the pulp, the cyanide process may be hastened, and it may be added that agitation sometimes permits slimes to be treated directly. The plant illustrated in Fig. 4 was constructed by D. A. Schiedel for the purpose of treating slimes by cyanide and agitation. The plant, which was built of iron and steel, was composed of a tank a, a vacuum filter b, precipitating boxes c, sump tank d, and gold-solution tank f. The machinery was to be driven by a waterwheel e. 48. The Schiedel Tank.—The agitating tank is 5 feet in diameter and 5 feet high with a 4-inch steel-plate shell N. M. III-30 and a cast-iron bottom 2 inches thick. To the bottom is cast a cone m, through which passes a vertical shaft n, which carries four arms o that hang down into the vat and have four-inch steel paddles r 6 inches wide fastened to them. These paddles are twisted like the blades of a propeller, and the arms to which they are attached at right angles are strengthened by a collar s. The shaft with the paddles can be raised by the screw spindle t. The driving gear u is placed below the tank and agitator. 4 inches in diameter in the tank bottom discharges by means of a stop-cock the contents of the agitating tank through a pipe into a Schiedel patent vacuum filter b. A perfect separation of the gold-cyanide solution from the residues is here effected. An opening 49. Description of Filter Box.-The filter box which is shown in Fig. 5 has 4-inch steel plate for the sides and -inch steel plate for the bottom. It forms a rectangular box 3 feet 6 inches deep, 7 feet long by 5 feet wide. Two feet above the bottom, as shown in Fig. 5 (a), is a perforated steel filter bottom a of -inch boiler plate, made in three movable sections, supported by angle irons c attached to the sides, and by the vertical supports d. The perforations ƒ shown in the horizontal section, Fig. 5 (b), are inch in diameter and are arranged inch apart. The filter bottom fits closely to the sides of the apparatus; it is covered with a blanket g, which is kept in position by bars running along the four sides and fastened by thumbscrews h. A grating i of -inch round-iron bars, placed 3 inches apart and made in three sections, serves to protect the cloth. The space between the bars is filled with coarse sand. The filter partition divides the apparatus into two compartments, one above the other. The lower one / forms a closed box, which is connected with a duplex vacuum pump by the pipe j. By this means the air can be rarefied when the filter bottom is covered with pulp. The part above the filter receives the contents of the agitator. The bottom of the apparatus has a discharge pipe k, with a 3-inch stop-cock for |