men. In retorting impure amalgam containing solid substances which volatilize and recondense in the condenser tube, clogging is very apt to occur, and the condenser should be so arranged that a rod may be slipped through the tube from time to time to keep it open. The heating should also be very slow at first, as a further precaution against explosions.

After the retort is charged the cover is put on. The cover and its seat are carefully faced, and in addition to this a luting of clay or an asbestos gasket is placed between the cover and the retort to prevent the escape of mercury fumes. The cover is held firmly on its seat by clamps, tightened either by wedges or by clamp screws. The pipe

to the condenser connects with the neck at the back of the cylindrical retort or screws into the cover of the pot retort. The condenser is merely a water-jacketed pipe; a constantly changing supply of cold water keeps the pipe cool and the volatilized mercury is recondensed and runs into a basin of cold water at the lower end of the condenser pipe. Thus there is very little chance of any mercury vapor escaping condensation. Care should be taken that no water is drawn back into the retort by sudden cooling, as the steam generated might cause an explosion. Some millmen use a rubber or canvas sack over the end of the condenser tube beneath the water, to avoid risk from this source, the condensed mercury running into this sack.

43. Heating the Retort.-The heat is gradually raised under the retort until the boiling point of mercury is reached and active distillation commences. It is kept at this point for one or two hours, according to the amount of amalgam, and is then again gradually raised to a bright red heat and held there for some time, to expel the last of the mercury. The fire is then drawn and the retort allowed to cool. After it is thoroughly cooled, the cover is removed and the metal withdrawn. The trays used in large retorts are divided into small compartments by partitions, so that the "retorts," as the masses of retorted metal are called,

will be of a convenient size and form for introducing into the melting crucible without breaking up. The retorted metal is porous and spongy, and usually contains a considerable proportion of impurities. It always retains a small amount

of mercury, which is only expelled in the final melting.

44. Melting the Bullion. The melting is done in clay or graphite crucibles, with borax and bicarbonate of soda; and if the "retort" contains much sulphur or base metals, a little niter is also used to oxidize these impurities. The fluxes aid in the fusion and slag off the impurities. fluxes are added a little at a time; as soon as their action has ceased and the slag becomes quiet, it is skimmed off and more flux added. This is continued till the surface of the melt remains perfectly clear and shiny, when the crucible is withdrawn and the bullion quickly poured into an iron ingot mold, previously warmed and greased on the inside with heavy mineral oil or beeswax. Most large mills do their own melting and refining, but many small mills sell their "retorts" to private refineries or directly to the mint.

45. There should be a "hood" above the melting furnace to carry away the fumes that arise when the crucible is uncovered for skimming and prevent their spreading through the room. This precaution is very frequently neglected, but the many cases of salivation among the melters are proof of the necessity of observing it.

GENERAL MILL ARRANGEMENT

MILL SITE

46. Gravity Assistance.-One of the first considerations in the erection of a gold, silver, or concentrating mill, next to the certainty of an ore supply to keep it running, is the selection of an advantageous location.

In order to avoid mechanical handling of the ore and to keep the expense of milling down as low as possible, the mill

designer takes advantage of the force of gravity and places the successive machines at successively lower levels, so that the material runs directly from each machine to the next one in order. To secure the necessary difference in elevation between the crusher and the final apparatus for this arrangement, without building the back of the mill very high, it is always desirable to place the mill on sloping ground. The slope should be chosen to correspond as closely as possible with the calculated slope of a line from the gates of the ore bins to the tailings discharge of the mill, in order to avoid all unnecessary building or excavation. When ground has to be cut away, strong retaining walls should be built at the back of the excavation and between the benches, as shown in Fig. 6, to prevent caving.

47. General Arrangement of Buildings and Apparatus.-Mills are usually arranged so that the mine cars or skips, or the ore wagons, if the ore has to be hauled to the mill, can run into or alongside the mill on an elevated track or staging (see Fig. 6). In most modern mills the ore is dumped on to grizzlies, and only the coarser lumps go to the rock breakers, the smaller stuff falling through the grizzlies into the ore bins below; this greatly lightens the duty of the crusher. Many mills are still found, however, where all the ore entering the mill is put through the rock breaker, regardless of its size. The coarse ore from the grizzlies passes on to the crushing floor, or, in most large modern mills, to a coarse storage bin, the gate of which opens upon the crushing floor. By keeping a supply of coarse ore in this way, the crusher may be kept steadily at work, and the power used by the mill kept more nearly constant. This is particularly desirable in concentrating mills where vanners are used, as these machines are very sensitive to change of power; a variable power makes their regulation much more difficult and renders constant attention necessary; and even if every possible precaution is observed, they will not do nearly so good work as when running under uniform power. In small mills the power consumed by the crusher is often

about one-fourth of the total power of the mill, so that throwing it in and out makes a decided difference in the speed of the other machinery. With large mills this is less important than in small mills, but, nevertheless, it is a notable factor in the working of a mill.

48.

Rock Breakers.-The mouth of the rock breaker is set level with the feed floor, so that the ore can be shoved into it and need not be raised, thus saving the feeder much work. Gyratory crushers are gradually displacing jaw crushers for large mills, both on account of their great capacity and the comparatively small jar and vibration. In modern milling practice, the rock breaker is frequently placed at the mine, and the ore comes to the mill bins already crushed. This relieves the mill of the strain and jar of the crusher and makes the consumption of power, and consequently the running of the mill, more uniform. The proper place for removing rock from ore is at the mine, and this can be better accomplished there when the ore is passed through a crusher.

49. Ore Bins.-The sills of the framework of the ore bins should all be on the same bench or terrace and should not be set on different levels along the slope. The bottom timbers of the bin proper are usually set sloping at an angle of about 45 degrees towards the gate, so that the ore will run down to the gate by its own weight. Bins are sometimes built flat-bottomed, but this necessitates shoveling the ore to empty the bin and thus offsets the increased capacity. The bins are double-boarded with heavy planks, usually with a layer of building paper in between to prevent the loss of fines. The inside bottom planks should be laid lengthwise down the slope, as they wear better this way and the ore slides more readily. Oak, beech, and birch make good ore-bin floors, the ore sliding over them making them smooth. The bin linings should be renewed as fast as they wear out. When large amounts of ore are handled through the bins, they are frequently lined with plates of iron. Owing to the fact that ore slides better on iron than on wood, it is