the partition and baffle. A tray whose bottom is composed of iron screening of about 14 inch mesh rests on cleats six inches above the bottom of the box so as to allow space beneath for the collection of the gold and silver sludge as it becomes detached from the zinc. Should the boxes be made of steel it is better to keep the interior surface well covered with P. and B. paint as it has been found that when the solution contains much caustic soda, a couple is formed between the iron and the zinc, resulting in a marked increase in zinc consumption. The same applies in a less degree to the use of iron screening on the zinc trays and some metallurgists have substituted screens made of fibre or some non-metallic material. A The zinc filaments or shavings are packed in on top of the tray almost to the height of the compartment, care being taken that the whole space is uniformly filled especially in the corners. good method, devised by L. M. B. Bullock, is to wind the shavings off the lathe into wisps or hanks about three inches in diameter and an inch or two longer than the measurement of the compartment. When charging a box these hanks are doubled over at each end to the exact size and laid carefully side by side and close together forming a layer of one hank in thickness with all the threads parallel to one another. Another layer is then placed on this but transversely, and so on till the compartment is filled. The idea of making the hanks a little too long and then doubling the ends is to give increased compactness and density at the sides and corners where the greatest channelling occurs. A fine thread-like shaving is usually preferred to the ribbon form and the thickness should be such as to give the maximum surface per unit of weight without making it so slender as to disintegrate rapidly into short pieces under the action of the solution. A width of from 1500 to 800 of an inch is probably the most useful. The zinc, especially in the first three compartments should be dressed frequently. If the solution is strong in cyanide and rich in metal this may need to be done every day and the lower compartments every four or five days. In dressing and also in cleaning up the most approved practice is to move up the zinc from the lower compartments to replenish that which has been dissolved in the upper ones, filling the spaces thus left vacant with new zinc. By doing this the bulk of the precious metal is concentrated and maintained in the head compartments, minimizing the risk of precipitate being carried over the end of the box mechanically, and keeping a body of new clean zinc for stripping the last traces of metal from the solution. Moreover, it is only by this means that the partly consumed zinc can be completely used up. Even so there is always a certain amount that breaks into short pieces which tend to mat together and render the compartment impermeable to the solution. When the amount of the broken zinc is not excessive it can be disposed of by sprinkling a thin layer between each layer of long zinc in the upper compartments, but if, as sometimes happens, the quantity grows to unmanageable proportions, screen trays may be placed on top of the head compartments just below the level of the solution and the short zinc laid in a thin layer on the screens, being removed each day and washed to detach the adhering precipitate and expose fresh surfaces to the action of the solution. Another useful device for working up these zinc shorts is a small agitator tank about three feet diameter and two feet deep with a circular overflow launder. Between this launder and the rim of the tank is placed a circular screen of 30- to 60mesh wire cloth. The short zinc arising from each day's cleanup of the boxes is placed in the tank and a small stream of pregnant solution run in, the stirring gear keeping the whole charge in continuous agitation. As the rich solution dissolves the zinc and precipitates the gold and silver, the stream of solution continuously flows out through the circular screen into the annular launder, carrying with it the precipitate. This stream enters the head of one of the precipitation boxes and deposits its suspended material there, its soluble content being also completely precipitated as it flows on down through the zinc shavings. In dressing the boxes and especially in cleaning up it is most important not to allow the zinc to remain exposed to the air, as it gets hot and oxidizes rapidly, resulting in a considerable increase in zinc consumption and also favoring a formation of the obnoxious "white precipitate." Capacity of Zinc Boxes.-The total cubic capacity of zinc boxes necessary varies with the quantity of solution and its metal content and also with the method used for packing them with zinc shavings. The following were the conditions at Butters Copala Mines in 1906 where the boxes were packed by the Bullock method. There were two boxes of eight compartments each; both were 24 inches deep but one was two feet wide and the other, one foot six inches; the width of the compartments was in every case fifteen inches. Average value of solution before precipitation $1.01 in gold and 3.41 oz. in silver. Average value of solution after precipitation, at the rate of 15 tons per hour traces, at the rate of 25 tons per hour, $0.06 gold and 0.03 oz. silver. Daily tonnage of solution... Gross capacity of the two boxes, cu. ft. . . . Space actually occupied by zinc shavings, cu. ft...... 360 to 600 0.19 to 0.12 49.5 Solution per day for each cu. ft. of zinc, tons 7.2 to 12.1 When the boxes are packed by the usual method of throwing the loose shavings in, armful by armful, and pressing them down, the space necessary may be three or four times that allowed at Copala and for low-grade gold solution especially those from slime treatment a cubic capacity ten times as great as that quoted may be necessary. Zinc Dust Precipitation.-The method of applying the zinc to the solution in the form of dust or fume was first introduced by Sulman and Pickard but it did not attain to a wide commercial importance until C. W. Merrill devised his modification of it for the Homestake Mine. The special point about the latter process consisted in adding the dust to a flowing stream of solu tion instead of, as heretofore, to an agitating cone or tank. The dust is released from its receptacle by a feeder whose speed is adjustable and falls into a mixer or emulsifier where it is thoroughly wetted and then passes, usually, directly into the suction pipe of a triplex plunger pump, discharging into the precipitate press. The action is so rapid that it is practically complete by the time the emulsion reaches the chambers of the press but as a measure of precaution the delivery pipe is lengthened as much as possible, sometimes extending to 200 to 300 feet before reaching the press. If desired, the special triangular press designed for the purpose by the Merrill Metallurgical Company may be used but the writer has found that the ordinary square type of press answers the purpose equally well, the only thing necessary being to ensure that the filtering area is large enough to allow the necessary rate of flow per hour to pass through it, such area being for practical purposes about 1 sq. ft. per ton per day. In order to obtain a complete precipitation, that is, to produce an effluent assaying only 2 or 3 cents, a rather heavy feed of zinc dust is sometimes necessary resulting in a low-grade precipitate for melting. This is more especially noticeable in the treatment of low-grade gold ores and is less in evidence where the solution contains several ounces of silver per ton. At the Homestake it is often found advisable to run a drip of lead solution, either nitrate or acetate, at the point where the zinc dust enters the stream, in order to intensify the action. At this mill the precipitate is so low in grade that it is first acid-treated and then dried and fluxed with litharge preparatory to being finally cupelled. By this means a bullion 980 fine is produced. In order to reduce the quantity of zinc dust used while at the same time producing a barren solution for residue filter washes J. S. Colbath introduced a combination system at the mill of the El Rayo Mining Company. This consisted in carefully adjusting the supply of zinc dust and maintaining it at the lowest possible point so as to precipitate the greater part of the precious metals and allowing the effluent solution to assay as high |