C.—s

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raw ore are also sometimes made with wooden shoes, this under certain conditions being a cheaper construction. In the Combination pan the number of the shoes is from six to twelve, and they weigh from 5001b. to 8001b., and are from 2in. to 3in. thick. The dies are similar in number, weight, and thickness. In some mills, as at Austin, when the shoes are worn off the mullcrs they are replaced by wooden ones, which answer the same purpose as the iron, the object being to give motion to the pulp, and not to grind. Method of Heating Pans. —The usual method of heating pans is by injecting live steam directly into the pulp by means of an iron pipe, which descends near the outside of the pan to within Gin. or Sin. of the bottom. This method is unquestionably the best, but it has its disadvantages, of which the diluting of the pulp and continual wearing-away of tho pipe are the greatest. This latter is particularly the case with roasted ores. In Virginia City, where the pulp is run thin, the slight dilution caused by steam is of little importance ; but in places where it is necessary to keep the pulp thick it is inconvenient. It is less objectionable in working free than in working roasted ores, for there are many kinds of the latter which., although they may be charged into the pan so thick that they check the motion of the inuller, yet in the course of half an hour or so, the different salts having dissolved, they become so thin that with the consequent rapid circulation they slop over, and this without the addition of steam. The effect of running roasted ore as a thin pulp is to a great extent counteracted by the fact that roasted ore, being easy of amalgamation, does not require the quicksilver to be so finely divided and so intimately mixed with it as is the case with raw ores. Another method of heating is by means of a steam-jet under the pan, and more rarely by means of a jacket, in which exhaust steam may be used. It is only with the freest ores that it is safe to heat pans in this way, and even with such ores the pan frequently leaks into the steam-chest. The reason for this is plain : the action between the salts (whether in the shape of chemicals added to the ore or formed in the roasting) and the iron of the pan will be strongest at the hottest point— namely, the bottom of the pan, and should there be a defective spot in the casting a hole would soon be eaten through. In Bodie this method is in use; but the conditions are favourable, as the ores are free. Number of llevolutions of Pans per Minute. —Pan-mullers generally make from sixty to ninety revolutions per minute. It is true that the faster the muller is run the more work is done in a given time ; but questions of wear and tear of machinery have thus far usually limited the speed to ninety revolutions. The pans at the California Mill are geared to run ninety-five revolutions. Weight of Charge. —A pan-charge weighs from 1,5001b. to 4,0001b., according to tho size of the pans. A 4,0001b. pan is sft. 6in. in diameter and 30in. deep. In working rich ore the charges are generally smaller than in working low-grade pulp. Temperature. —The temperature maintained in the pan has much influence upon the amalgamation of the ore. The effect of temperature on the amalgamation of the base metals, lead and copper, is not very well understood, and it can only be determined by careful and exhaustive experiments. It seems to be a well-established fact that in the amalgamation of chloride-ores, which carry a considerable percentage of carbonate of lead, a line bullion may be obtained, without an appreciable loss of silver, by keeping the temperature of the charge considerably below the boilingpoint, say at 120° Fahr. At exactly what temperature it will be necessary to keep a given kind of pulp experiment alone can show. Time, as before mentioned, has also an influence on the results. The reason for the earlier amalgamation of the silver at a temperature of 100° Fahr., or thereabouts, seems to be that the conversion of the carbonate into the chloride of lead (the form in which it is necessary for the lead to be in order to be precipitated by the iron and amalgamated by the quicksilver) takes place much less readily at a temperature of 100° Fahr. than it does at 200° Fahr. Were the conditions entirely uniform the metals would be amalgamated in the order of their electro-negative properties—that is to say, first silver, then copper, and last lead; but in practice this is not the case, as unquestionably lead is amalgamated at the same time as the silver. The reason is that the pan and the pulp do not furnish all the conditions of a true galvanic cell. For instance, it is possible to conceive of a particle of carbonate of lead not in conjunction with a particle of chloride of silver coming into contact with the iron of the pan in the solution of salt and bluestone, as the case may be. Under such circumstances the particle of carbonate would be converted into chloride and metallic lead, and precipitated and amalgamated. It is customary to keep roasted ore at a temperature of 200° Fahr. At the California Mill the pans are heated to 150° Fahr. Time of Working a Charge. —The time occupied for working a charge is from five to eight hours. It is largely governed by the value of the ore. With any given ore experience shows how far it is profitable to prolong the amalgamating process, which proceeds more and more slowly as the silver is taken up. In many cases, where a comparatively small portion of the silver is extracted by pan-amalgamation, this is not due to the imperfection of the process, but to the fact that under the local economic conditions it is not profitable to continue the process until a large percentage is saved. Continuous Amalgamation. —A system of handling ores in wet-crushing mills, known as the " Boss" process, has been introduced at Bodie, which dispenses with the laborious work of shovelling the pulp from the settling-tanks. The ore is, as usual, crushed in a battery, but with as little water as possible, and the pulp is then conducted to the first of a series of pans in which the mullers are revolving. From this pan it flows to the next, and so on through the series, until from the last pan it passes through a sieve, and finally from the last settler out of the mill. The pans are heated by steam in false bottoms, so that the pulp is not thinned—it becomes thicker as it progresses. The chemicals are added in the first pan, and the quicksilver is added at a stage when the pulp is thick enough to hold it. Grinding. —When ores are submitted to chloridizing roasting, grinding is scarcely necessary, because the silver-chloride passes into solution ; and to some extent this is also true of raw chlorideores : but with all other ores a finer comminution than can be attained in the battery is probably