struck with heavy mallets. It is highly important that the surface of the plate be perfectly smooth and set horizontally parallel to its width, so that the pulp will be evenly distributed over the entire surface. The table may be made of narrow widths of heavy planking, spiked firmly to the inclined framework; but a better design is made up of narrow strips of wood, set edgewise, lengthwise of the table, and bound together every two or three feet by light iron tierods. A table built in this manner will not warp and the even surface of the plate is more easily preserved. Tables made of sections of cast iron are sometimes used, but wood is cheaper and lighter and answers the purpose just as well. 24. California Milling Practice. In the California milling practice, a very peculiar and unpractical arrangement of the outside plates has been in vogue for years, and, though rapidly dying out, is still retained to a considerable extent. The length of the apron plate proper, which is ordinarily from 10 to 16 feet long, is cut down to from 10 to 48 inches, and from it the pulp is run into sluices from 14 inches to 2 feet wide and 10 to 16 feet long, paved with copper sluice plates. Besides being much. narrower than the apron plates, the sluice plates are given a slightly greater inclination, usually inch more to the foot. In spite of the increased speed and scouring action of the pulp stream after leaving the apron, as a result of the contracted channel and increased grade, the California millman of the past imagined that he was saving in the sluices gold and fine amalgam which could not be caught on the apron plates. Notwithstanding the fact that on its very face the whole idea is contrary to all reason, California millmen, being governed by precedent rather than by reason, have clung desperately to it for years, refusing to see what one would suppose impossible to overlook. 25. Preparing the Plates.—As has been stated before, quicksilver, which already contains some gold or silver, catches gold or silver more readily than mercury alone. If the surface of the plates be amalgamated with pure mercury, they will allow considerable gold to escape at first, but will steadily improve as the amalgam grows richer, until they are working normally. But if the surface be previously coated with a gold or silver amalgam well worked in, the plates will do full duty from the start. For this reason, new copper plates are coated with gold or silver amalgam before being used, or else plates electroplated with silver are used, the silver plating amalgamating with the mercury used in dressing the plates and giving the same effect as preparing with amalgam, with much less time and trouble. Gold amalgam works somewhat better on the plates than silver amalgam, but is seldom used, on account of the great expense. To prepare plates for amalgamation, they are first scoured with sand or emery paper until they are clean and bright. They are then washed with a strong soda or lye solution to remove all traces of grease. Dilute nitric acid-a 10-percent. solution-or a comparatively strong solution (about 24 per cent.) of potassium cyanide may be used instead of lye or soda. After washing with the chemicals, the plate is well washed with water and then a mixture of equal parts of sand and sal ammoniac and a little mercury is rubbed on with a scrubbing brush. The sand and sal ammoniac are used to keep the plate clean while the mercury is being rubbed in. Enough water is used to make a thick mud. More mercury is sprinkled upon the plate from a flask with a piece of cloth tied over the top, and the rubbing is kept up until the surface of the plate has absorbed all the mercury it will hold. The plate is allowed to stand for about an hour and is then washed clean with water or with cyanide of potassium and water, and more mercury is added if the plate will hold it. If the plates are plain copper, they are next given a coating of gold or silver amalgam and are then ready for use. The amalgam is rubbed in with a piece of rubber belting or cloth, the plate meanwhile being kept wet with sal ammoniac. When rubber belting is employed, a piece is fastened between two blocks of wood, so as to leave about 1 inch of the belting projecting from the wood. The entire block, belting and all, is sometimes called the rubber. If old mercury is used in preparing the plates, the use of special amalgam is unnecessary, as mercury strained, or even distilled, from amalgam retains enough gold or silver to start amalgamation at once. 26. Cleaning and Dressing Plates.-The plates are cleaned up at intervals varying with the richness of the ore being treated, the idea being to let as much amalgam accumulate as possible without loss from scouring. The amalgam does not accumulate evenly all over the plate, but in ridges, which grow steadily, and if left too long commence to scour off. If the mill is running on low-grade ores, the outside plates are usually wiped twice a day—morning and evening-or only once a day if the ore is very poor. As the richness of the ore increases, the interval between clean-ups shortens, and for very rich ores it is sometimes necessary to clean the apron plates every hour, or even oftener. The battery plates are not cleaned until the amalgam stands up in thick ridges; with very rich ores it may be necessary to clean them once or twice a week, or even every 48 hours, but they are usually cleaned every two weeks, when a general clean-up of the whole mill is made. 27. The plates are cleaned by rubbing the amalgam loose with a wiper made by fastening a piece of rubber belting between blocks of wood, with about half an inch of the rubber projecting, or with a whisk-broom, cut down to make it stiff. The apron plates are wiped from bottom to top, the men using bits of plank to kneel on while working. If necessary, fresh mercury is sprinkled on the plates after wiping and is rubbed in with a piece of rubber. In a general clean-up the stamps are hung up, two batteries at a time, and the screens, battery plates, and dies are removed and carefully cleaned of amalgam. The battery sands are removed and are either fed into one of the other batteries or are panned, bits of iron removed by a magnet, and the concentrates transferred to the clean-up pan; or, in some mills, they are saved and returned to the mortar on starting up again. All amalgam sticking to the stamps and to the inside of the mortar is carefully collected, and, with the rest of the amalgam from the clean-up, is first strained to remove the excess of mercury and then transferred to the clean-up pan, or-in small mills-to a hand mortar, where it is ground with fresh mercury and cleaned. The amalgam from the pan is again strained, and the balls of dry amalgam are then ready for retorting. 28. Scraping Plates.-Every three to six months, according to the richness of the ore, the plates are scraped with a piece of steel or a spatula, leaving only a very thin film of amalgam on the plates. In some mills the plates are "sweated," in order to get more gold out; but the plates frequently have to be resilvered or reamalgamated after sweating, and the operation involves much more labor than simply scraping; the thin film of amalgam left after scraping is, moreover, very advantageous in starting up again. Sweating is merely heating to loosen the amalgam. The plates are removed from the tables and heated over a wood fire, expelling most of the quicksilver, and the gold scale remaining behind is then scraped off. Practically the same result is accomplished without removing the plates from the tables by washing them with boiling water or playing a jet of steam on them to soften and loosen the amalgam, which is then scraped off. In some mills, chemicals are used in the sweating, the plate being first heated to expel the quicksilver and then rubbed with a solution of niter and sal ammoniac and again heated, when the gold rises in scales and blisters. The plate is sometimes plunged into a tank of boiling water on being removed from the fire, when the gold scales off. "Skinning" plates in this manner is not usually advisable during the life of the mill, as the plates are very apt to get buckled and be irreparably ruined, and there is then nothing left but to melt them up and get new plates. The gold recovered from the old plates is usually more than sufficient to pay for a new set, but it takes some time to get new plates to work properly, and they will soon absorb practically as much gold as would have remained in the old plates after scraping; so that there is really little or nothing gained financially, even if we do not consider the time lost. in changing the plates, and the time and amalgam lost in breaking in the new plates. The 29. Dressing Tarnished Plates.-New plates rapidly become tarnished from the action of the acids in the pulp, forming a thin film of copper salts, or "verdigris," over the surface of the plates, which prevents them from catching the gold and amalgam flowing over them. tarnish appears in spots-yellow, brown, or greenish-and spreads rapidly if not removed at once. To remove the stains, the battery is stopped and the spots scrubbed with a solution of sal ammoniac; this is left on for a few moments to dissolve the coating, then it is washed off and the plate. scrubbed with a potassium cyanide solution to brighten it; this is finally washed off, more mercury added if necessary, and the battery again started up. After the plates have once acquired a thick coating of amalgam they do not tarnish very readily; for this reason silver plates give less trouble from this source than plain copper. Plates dressed with nitric acid tarnish more rapidly than those dressed with soda and cyanide. In some mills, a little cyanide of potassium is fed into the mortar from time to time to prevent the plates from tarnishing, and soda or lye is frequently used in the mortar to counteract trouble from grease. 30. Mercury Feed and Loss.-The amount of mercury used depends upon the richness of the ore. The quicksilver does not wholly dissolve the gold scales, but only forms a coating of amalgam on the surface; consequently, weight for weight, the finer the gold the more mercury is necessary to amalgamate it, as the smaller particles present more surface in proportion to their weight than the larger ones. Generally speaking, with a clean gold ore, about 1 ounce of mercury should be charged into the mortar for each ounce of gold in the ore treated. Impurities alter the proportion greatly, however, and it is sometimes necessary to charge |