Every sluice is an inclined channel through which flows a stream of water, carrying away all the lighter matter thrown into it, and separating it from the heavy. When the operations would not be permanent enough, or sometimes for other reasons, a ground sluice is preferred to the ordinary box sluice made of boards. Ground sluicing requires, however, six times as much water as does a box sluice to do the same amount of work. It is simply a gutter in the bed rock, and if the bottom is hard and uneven its inequalities will arrest the gold; if not, a number of boulders too heavy to be moved by the stream are put into the sluice to act as riffles. No mercury is used. The water is turned off and the collected coarse gold washed in the pan. Sluice boxes may be any length, from 30 to 5,000 feet. They vary in width from 1 to 5 feet, though generally 16 or 18 inches. The grade is proportioned to the fineness of the gold, varying from 8 inches to 2 feet to the 12-foot box or length. The bottom should be of 1-1/2 inch plank, and the sides of 1-inch boards. The boxes are made 4 inches wider at the upper end than at the lower, so as to telescope. The best method found yet for arresting fine gold is the copper plate amalgamated with mercury on its face. These plates are never used at the head of a sluice or other situation where there is much coarse gold, as they would be superfluous in such a situation, but are placed some distance down the sluice and are most efficacious in arresting the "flour," or excessively fine gold. Plates are always of copper above 1/16 inch thick, and may be 6 feet or more long, and of a width suited to the capacity of the sluice. When treated with quicksilver, they become as brittle as glass, and must be handled with care. The copper plate is first washed with a weak solution of nitric acid, and then mercury that has been treated with a weak nitric acid solution is rubbed on the plate. As this surface of quicksilver wears off, it may be replaced by a little fresh mercury. Any green slime on a plate is an evidence of copper salts in the water. It must be scraped off and the spot rubbed with fresh quicksilver. Gold attracts gold, therefore the plates should not be cleaned up too often.