Page:The American Cyclopædia (1879) Volume VIII.djvu/92

84 cowhide; blasts of from 5 to 50 tons of powder at a time are fired, to prepare the ground for the action of water; nitro-glycerine and the diamond drill are used in running preparatory tunnels for drainage; chasms of 1,000 ft. in vertical depth are successfully crossed by huge iron pipes, to convey water to isolated points, thus obviating the ancient high, costly, and perishable flumes; and from ingeniously contrived and regulated nozzles streams as much as 6 in. in diameter are discharged under pressures sometimes exceeding 400 ft. of hydraulic head, with a velocity of 140 ft. and upward per second, delivering more than 1,600 lbs. of water in that unit of time. The water issuing from the nozzle seems to the touch as rigid as a bar of steel, and strikes the gravel bank in the same cylindrical, condensed shape, boring into it with immense power. The heavy bowlders are thrown about like pebbles; and the clay, earth, and gravel, disintegrated by the torrent, are swept along into the system of sluices. It has been estimated that, taking the miners' wages in California at $4 per day, the cost of handling a cubic yard of gravel would be nearly as follows: in the pan, $20; in the rocker, $5; with the long tom, $1; by hydraulic process and sluices, 5 cts. This method has rendered valuable many California placers that were esteemed worthless or exhausted; and its employment would doubtless revive the importance of abandoned gold fields in other parts of the world.—Quartz gold (that is, gold contained in veins, whether native in the quartzose or other gangue, or associated more or less intimately with metalliferous minerals) is extracted in most cases by first pulverizing the material, and then washing and amalgamating. Stamp mills, iron rollers, revolving plates, drums containing iron balls, Chilian mills, arrastras, and jaw crushers are among the machines employed in pulverizing rock. The arrastra consists of a circular pavement of stone, about 12 ft. in diameter, surrounded by a rough curb and forming a kind of tub about 2 ft. in depth. An upright shaft, working on a pivot in the centre of this circle, carries arms to which large stones or mullers are attached by chains or thongs. The arms, being revolved by horse or mule power, drag the mullers over the pavement, upon which the ore, previously broken into pieces of about the of pigeons' eggs, is distributed. Water is added from time to time, until the quartz has become reduced to a finely divided state, and the contents of the arrastra assume the consistency of thick cream. Quicksilver is then sprinkled over the surface, and the grinding is continued until amalgamation is complete. An ordinary twelve-foot arrastra will grind and amalgamate 450 lbs. of quartz in about seven or eight hours. The amalgam is obtained by diluting and agitating the mixture, and allowing the turbid liquid to run off. The arrastra is slow in operation and wasteful of power, but an excellent amalgamator. Hence the

principle has been very generally adopted in amalgamating, while the preliminary pulverization is effected by other machinery. The Chilian mill consists of a stone or iron basin, around which one or two vertical wheels or runners, frequently of granite, are made to travel. It is generally considered less efficient for amalgamation and scarcely more so for crushing, while it is more expensive to construct than the arrastra. Jaw crushers, of which Blake's well known stone breaker is the type, are widely employed for the preliminary reduction of rock to a size suitable for rollers or stamp mills. Stamping is usually regarded as the most economical and efficient means of pulverizing the ore. The mills constructed for this purpose are run by steam or water power, with the exception of occasional rude contrivances in which single stamps have been operated by horse power, and of the experiment now making, it is believed for the first time, in the island of Arruba, where wind is to be employed as a motive power. The best stamp mills in the world are believed to be those of California and Nevada. These are made up of batteries containing three, four, five, or six stamps each; five is the usual number. Each battery works in a cast-iron box or mortar, in the bottom of which are laid blocks of hardened iron, called dies, to receive the shock of the falling stamps. The broken rock is fed in suitable quantities into the mortar, and crushed between the dies and the stamps. Each stamp consists of a stem, a collar, a stamp head, and a shoe. The stem was formerly made of ash or other hard straight-grained wood, about 6 in. square, to the lower end of which a square iron stamp head was fastened. At present, in California, stems of 3 or 3½ inch round iron, some 12 ft. in length, are universally employed. The collar is secured upon the upper part of the stem, and forms a projection 3 or 4 in. wide, under which the cam of the horizontal driving shaft catches and lifts, and at the same time turns, the stamp. The stem fits below into the stamp head, a cylinder of tough cast iron, furnished on its lower face with a hard iron shoe, which can be replaced when worn out. The stamps are dropped 6 to 12 in., at the rate of from 25 to 90 drops per minute. Water flows into the mortar with the ore; and the finely divided product is splashed by the stamps through screens of wire cloth or perforated sheet iron, set in the walls of the mortar. Loose quicksilver and amalgamated copper plates are sometimes used inside the mortar. The mixture of crushed ore and water is differently treated in different places for the extraction of gold. Sometimes it is run over amalgamated copper plates; sometimes it is first concentrated by means of blankets; sometimes it is introduced into pans, somewhat on the principle of the arrastra, or into various other ingenious forms of apparatus, for the purpose of amalgamation. In the most