MINING. 327 



pounds. It is so hung that the forward end is about an inch 

 above the bed, and the hind end drags on the bed and crushes 

 the quartz. 



250. Art\algamati<m. The pulverized auriferous quartz, as 

 it comes from the stamps, consists of fine particles of rock and 

 gold mixed together, and the objects of the miner are to sep- 

 arate them, save the metal, and let the other material escape. 

 Here again a small sluice, similar in principle to that used in 

 placer mining, is used ; but instead of riffle-bars, the bottom of 

 the sluice is copper, covered with quicksilver, or is a rough 

 blanket, in which the gold and heaviest sands are caught. In 

 many mills quicksilver is placed in the battery, two ounces of 

 quicksilver for one of gold ; and about two-thirds of the gold 

 is caught thus. Next the battery is the apron, a copper plate 

 covered with quicksilver, on which a good share of the gold 

 is caught. 



251. Co?icentration. Below the aprons different devices 

 for catching the gold are used in different mills. The blanket 

 is the most common. It is a coarse blanket, laid at the bot- 

 tom of a sluice through which the pulp from the battery runs, 

 and the gold, black sand, and sulphurets are caught in the 

 wool, while the lighter material runs off. The blanket is 

 washed out in a tub at intervals of half an hour or an hour. 



In some mines nearly half of the gold is mixed with pyrites, 

 and refuses to be caught by quicksilver. In such case a sluice 

 may be used to separate the sulphurets, which may form three 

 per cent, of the pulverized rock. This separation is called 

 concentration, and the material obtained is concentrated tail- 

 ings. The sulphurets are five times as heavy as water, and 

 twice as heavy as quartz, so the separation is not difficult 

 when the supply of water is abundant. 



252. Chlorination. In roasting for chlorination we have, 

 first, to oxydize the iron, and next, by introduction of salt, to 

 chloridize certain other substances which vary with the locality 

 from which the ore is obtained. When this is rightly done we 



