198 MINING INDFSTEY. 



revolve. There are various appliances for raising or lowering the muller, so that 

 it may rest with its whole weight upon the pan-bottom in order to produce the 

 greatest grinding effect, or be maintained at any desired distance above it when 

 less friction or mere agitation is required. Various devices are also in use for 

 giving proper motion to the pulp, so that, when the muller is in revolution, the ma- 

 terial may be kept constantly in circulation, passing between the grinding sur- 

 faces and coming into contact with the quicksilver. Some pans are cast with 

 a hollow chamber, an inch or two deep in the bottom, for the admission of 

 steam in order to heat the pulp, while others employ only "live steam," which 

 is delivered directly into the puJp by a pipe for that purpose. 



The operation of the pan consists in the further reduction or grinding of 

 the stamped rock to a fine pulp and in the extraction of the precious metals by 

 amalgamation with quicksilver. The quantity of ore with which a pan is 

 charged for a single operation varies from 600 or 800 to 4,000 or 5,000 pounds, 

 according to the size of the pan. The ordinary charge of pans, most generally 

 in use at present, is 1,200 to 1,500 pounds. 



In charging the pan the muller is raised a little from the bottom, so as to 

 revolve freely at first, water is supplied by a hose pipe, and at the same time 

 the sand is thrown into the pan with a shovel. Steam is admitted, either to 

 the steam-chamber in the bottom of the pan or directly into the pulp. In the 

 former case the temperature can hardly be raised as high as in the latter ; but, 

 on the other hand, when steam is introduced directly, care is necessary to avoid 

 reducing too much the consistency of the pulp by the water of condensation. 

 The pulp should be sufficiently liquid to be kept in free circulation, but thick 

 enough to carry in suspension, throughout its entire mass, the finely divided 

 globules of quicksilver. In some mills both methods of heating are employed 

 in the same pans, the temperature being first raised with each charge by live 

 steam, and afterward sustained by admitting steam to the chamber only. 

 Some pans are covered with wooden covers to assist in retaining the heat. 

 When properly managed the temperature may be kept at or near 200° Fah- 

 renheit. When, in the use of live steam, the pulp becomes too thin the sup- 

 ply of steam is cut off, the covers removed, and the pulp allowed to thicken 

 by the evaporation of the water. The steam in the chamber may keep the 

 temperature up to the desired point in the meantime. Another advantage of 



