MIXING. 249 



80 warm that the hand can scarcely bear it, the amalgam is 

 softened and loosened, so that it can be scraped off readily. 

 The plate is then sprinkled anew with quicksilver, and is ready 

 for use again. Mercmy does not amalgamate with copper so 

 readily as with gold or silver. A copper plate, the sixteenth 

 of an inch thick, may be used for at least five years, and per- 

 haps for ten ; whereas a gold plate of equal thickness would, 

 if exposed to the action of quicksilver in the same manner, fall 

 to pieces in a few weeks. After a time the quicksilver per- 

 vades the copper, and gives it a silvery whiteness all through 

 on the under side. It is said that a solution of cyanuret or 

 prussiate of potash, is used instead of nitric acid m applying 

 mercury to copper plates, and that it is still better, there being 

 then no trouble with the green spots of nitrate of copper. 



A good amalgamated copper plate is considered as service- 

 able as a bed of quicksilver of equal size, and it is very much 

 cheaper and more convenient to manage. 



The dirt and water should be admitted to the copper plate, 

 by falling first through a sheet-iron plate, pierced with holes 

 half an inch long^ and a sixteenth of an inch wide. Some 

 miners place this sheet-iron plate immediately over the copper. 



Very soon after the water and dirt commence to run in the 

 sluice, all the sj^aces between the riffle-bars are filled with 

 sand, gravel, and dirt ; which, however, present many little 

 inequalities of surface, sufiicient to catch all the particles of 

 gold larger than a pin-head. The largest gold is caught near 

 the head of the sluice; and the farther down the sluice, the 

 finer the gold. In some sluices, where the pay-du't contains 

 much coarse gold, the quicksilver is introduced from thirty to 

 sixty yards below the head, so as to catch only the fine par- 

 ticles of metal. 



§ 181. Cleaning iij). — The separation of the gold, amalgam, 

 and quicksilver, from the dirt in the bottom of the sluice, is 

 called " cleaning up ;" and the period between one " clean- 

 ing" up and another is called a "run." A run in a common 

 board-sluice usually lasts from six to ten days. Ordinarily the 

 11* 



