When the other ores containing gold are found in the same mine, they are carefully separated from the former, and are collected into heaps, arranged accord ing to their richness and hardness. The process fur ex tracting the gold from these, is much more complicat ed than that just described.
In extracting gold from these ores, the first part of the process consists in separating the metallic from the earthy substances. For this purpose the ore is reduced to powder, in a stamping-mill. The stamping-mill con sists of a large cistern, in which the ore is placed, and through which a stream of water passes, and of the stampers, which are heavy beams of wood, terminated below by iron, and which are moved upwards and downwards by a water-wheel.
Great attention is necessary in this part of the pro cess. that the whole of the foreign matter which can be removed by the hand, be separated before the ore is put into the cistern, and that the proper force be ap plied, according to the hardness of the ore. This is easily regulated, by increasing or diminishing the depth of the layer of ore in the cistern; hence the quantity of ore put in, is always inversely as its hardness ; ac cordingly, when the layer of ore is thin, the stampers have a greater fall. and the ore is thus subjected to greater force. It is necessary also, that the ore be placed in a particular way. The layer is so formed, that below the two outer stampers it is thickest, and gradually diminishes towards the centre. When the ore is reduced to powder, and sufficiently fine to float in the water, it is carried off, by the stream of this fluid into vessels, in which it is deposited ; the heal icst be ing deposited nearest the cistern. Particular care is also necessary, that the stamping be not done too rapid ly, otherwise the stampers are apt to throw up pieces of the ore, not sufficiently pulverised, which flow out with the fine powder, and prove detrimental in the sub sequent part of the process. The stamping :,hould therefore be performed slowly, and the fresh ore, as it is put in, must be placed below the middle stamper, where it is subjected to the greatest force, and from which it is thrown under the other stampers.
The vessels into which the water flows from the cis tern, are arm anged in a particular way, and a great deal depends on the performance of this part of the process ; for by altering the velocity of the current of water in them, the rapidity with which the powder is deposited also alters. The vessels are of different dimensions, that nearest the cistern being about 12 feet long, and 9 inches broad, and as many deep. The others gradu ally enlarge as they retire from the cistern. They
also vary in their inclination, the first having a slope of about 3 inches, the second about I inch, the third and fourth about 2 an inch, and the fifth and sixth being level. Each vessel is furnished with a groove at the extremity farthest from the cistern, into which pieces of wood are placed, varying in height, according to the quantity of ore that is collected in the vessel, and each vessel communicates with that beyond it.
When the water flows from the cistern into the first vessel, the heavy part of the ore is deposited and col lected where the wood is put into the groove ; as the powder reaches the top of this, another piece is put over the former, and so on till the vessel is nearly filled. When this is the case, the water is allowed to flow into the second vessel, and from this, when full, into the third, by which means the different parts of the ore are collected, according to their specific gravity in the dif ferent vessels. The parcels of ore thus procured, are still farther freed from impurities by washing. For this purpose, each parcel is put into a wicker basket, into which a stream of water slowly flows, and which, as it issues, is received on inclined tables, grooved in various directions. By this means, the ore is still farther separated, according to the specific gravity of the par ticles, the heaviest being deposited in the upper groove. Some of the ores of gold are subjected to another wash ing, in a vessel similar in shape to a fire-shovel, called a buddle. This is immersed in water, and a particular motion is given to it by the workman, by which the lighter particles are thrown out, and the heavy metallic matter is left in the huddle. If the gold procured by this process be mixed only with earthy matter, it is subjected to amalgamation, as already described ; or it is fused in crucibles with nitre. If, however, it be mixed with inflammable matter, as sulphur, it is roast ed, by which the sulphur is expelled. It. is then mixed with lime and galena, in quantity proport'onate to the gold contained in it, which is ascertained by assaying it, and kept at a red heat, in a reverberatory furnace, till part of the sulphur is expelled. The heat is then increased, till the whole becomes fluid, after which it is poured into moulds of sand The product of this operation consists of scoria above. and beneath of a metallic matter, composed of gold, silver, copper, lead, iron, and a little sulphur. By repeated fusions the gold is obtained, alloyed only with silver, copper, and lead, from which it is freed by the process of refining.