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. The copper plate in quartz-mills is made in the same manner as in placer-sluices, under which head a description of the plate may be found. Some amalgamating bowls or basins are little Chilean mills and arastras, made of cast iron. One plan of amalgamation is to use a cast iron bowl about four feet in diameter and a foot deep. Near the bottom are horizontal iron arms, which revolve and stir the quicksilver and pulverized quartz together. Four or five of these bowls sit in a row but at different levels: the bottom of the first bowl being level with the top of the second, and so on. The pulverized quartz passes through them all. Under each bowl a fire is kept up, because heat forms the action of amalgamation. If there be any pyrites in the quartz, some common salt is thrown in to assist in releasing the gold from the embraces of the sulphurates, and preparing it to be seized by the mercury. Another amalgamating bowl revolves on an axis that stands at an angle of about seventy-five degrees to the horizon, so that the material in the bowl is continually moving; and the bottom is divided by little compartments, which make a constant riffle. In other bowls the pulverized quartz is forced with water through the mercury. The methods of amalgamation differ very much, and a book might be filled with a description and discussion of the processes used at different quartz-mills in California. _Sulphurets._--Many auriferous quartz veins contain considerable quantities of sulphurets or pyrites of iron, copper and lead, and their presence prevents amalgamation, and thus causes a great loss of gold. It is said that on some occasions in good mills, not more than twenty or thirty dollars have been obtained from a ton of vein-stone which had seven or eight hundred dollars of gold in every ton. The best method of treating the quartz containing pyrites, is to roast it, and thus drive off the sulphur, but this process is so expensive that it is seldom used; and the common practice is to crush and amalgamate the rock, and save the concentrated tailings for some future time, when there may be a sale for them, or when it will be cheaper to reduce them. The pulverized sulphurets are decomposed by exposure to the air, and after the tailings have been preserved for a time, they may pay better at the second amalgamation than at the first. A mixture of common salt assists the decomposition of the pyrites.