GOLD EXTRACTION PROCESSES. 487 



In combined efforts to treat larger quantities of sand, 

 the machines all consist essentially of a slightly inclined 

 trough, through which a stream of water is made to convey 

 the auriferous sand, mercury being usually sprinkled on at 

 short intervals of time. If the trough is long enough, and 

 the stream of water not too rapid, the gold and mercury 

 sink, and, uniting, are swept along together, until arrested 

 by some inequality of the bottom. 



Crevices and "riffles" or obstructions of various kinds 

 are arranged to catch the amalgam. The simplest con- 

 trivances are transverse slats of wood nailed to the bottom 

 of the trough or sluice. In Siberia square " pigeon-hole " 

 depressions have been continuously used for more that 

 fifty years. In California, the sluice is paved with square 

 blocks of wood placed an inch or more apart, or with large 

 rounded stones, or ordinary iron rails between which are 

 plenty of crevices where the amalgam can lodge. To catch 

 light spangles of gold, blankets are spread, the loose fibres 

 of which become charged with pyrites and gold, and in New 

 Zealand, plush is a favourite gold catcher. 



When a "clean-up" is desired, a stream of clear water is 

 run through the sluice, the riffles are taken up, the mercury 

 and amalgam washed down and allowed to accumulate at 

 some convenient spot, and then ladled out and squeezed in 

 bags of canvas or leather as in the days of Pliny, who 

 describes the process as follows : " ut et ipsum [i.e., 

 argentum vivum] ab auro discedat, in pellis subactas 

 effunditur, per quas sudoris vice defluens purum relinquit 

 aurum ". The excess of mercury being thus filtered off, 

 the pasty amalgam, containing about one-third of gold, is 

 retorted. 



The methods of conveying the auriferous material to 

 the sluices vary with the scale of the operations and the 

 other conditions. When rocking cradles or the smaller 

 sluices are used, the gravel is shovelled into them. In Siberia, 

 where the valleys are shallow and the inclination of the 

 ground small, the gravel is carried in carts up an inclined 

 plane to an elevated wooden platform whence the sluice 

 starts. In California, where the gulches are deep, the fall 



