308 RESOURCES OF CALIFORNIA. 



lained. The amalgam is then heated, to drive off the mercury. 

 This may be done either in an open pan or in a close retort. In 

 the former, the quicksilver is lost ; in the latter, it is saved- 

 The pan is generally preferred. Often a shovel or plate of 

 iron is used. Three pounds of amalgam, from which the 

 liquid metal has been carefully pressed out, will yield one 

 pound of gold. The gold remaining after the quicksilver has 

 been driven off by heat from the amalgam, is a porous mass, 

 somewhat resembling sponge-cake in appearance. 



228. JRiffle-Jlars. The riffle-bars used as false bottoms 

 in sluices, are usually sawed longitudinally with the grain 

 of the wood, but "block riffle-bars" are considered prefer- 

 able ; the latter are cut across the tree, and the grain stands 

 upright in the sluice-box. The block riffle-bars are three times 

 more durable than the longitudinal ; and as the latter kind are 

 worn out in a week in some large sluices, there is a consider- 

 able saving in using the former. 



229. Double Sluices. Sluices are sometimes made 

 double that is, with a longitudinal division through the 

 middle, so that there are two distinct sluice-boxes side by 

 side. Two companies may be working side by side, so that it 

 will be cheaper for them to build their sluices jointly. An- 

 other device for saving gold in sluices is the " under-current 

 box." There is a grating of iron bars in the bottom of a box, 

 near the lower end of a sluice ; and under this grating is an- 

 other sluice, with an additional supply of clean water, and 

 with a lower grade. The grating allows only the fine mate- 

 rial to fall through ; and the current of water being moder- 

 ate, many particles of gold, that would otherwise be lost, are 

 saved. Sometimes the matter from the under-current box is 

 led back to the main sluice. 



230. Rock-Sluices. Large sluices are frequently paved 

 with stone, which makes a more durable false bottom than 

 wood, and catches fine gold better than riffle-bars. The stone 

 bottoms have another advantage that it is not so easy for 



