The coarseness of the mortar screens is subject to infinite variety, according to individual preference. The number of holes to the square inch ranges between 60 and 800 in Australia, and between 900 and 10,000 in the United States. The holes, when round, agree in numbers with those of sewing-machine needles, from 0 to 10. When slots are preferred to holes, they are generally 3/8-inch in length and No. 6 diameter. Russia sheet iron, or sheet steel 1/32-inch thick is the material of which they are made. It should weigh one pound to the square foot, be very soft and tough, have a clean, smooth surface, and show no rust or flaws. In Australia 1/16 sheet copper is preferred. The holes in any case must be punched in the sheet so that the rough edges are turned, and thus any pulp that finds its way into one of the holes is certain to get out again and not clog. A battery may require 13 sets of screens a year; each screen having a surface of about 1-1/2 square feet. Russia iron screens endure 15 to 40 days. As the work a stamp can do depends entirely upon how much pulp can escape through the screen in any given time, the latter is evidently a very important detail of a battery. Prospecting stamp batteries differ from ordinary batteries, chiefly in being of light build and weight. Amalgam coming from battery stamps is often mixed with all sorts of rubbish. After being gathered, it is dried with a sponge, foreign matter picked off the surface and clean quicksilver added. Soft unglazed paper thrust into the mercury removes the last vestiges of water, and then a card is drawn vertically or a piece of blanket horizontally across the mercury to clean it of iron. After squeezing, the amalgam is retorted. [Illustration: GOLD RETORT.] All the amalgam is placed in one large kettle and, if possible, the latter is put on a strong table having an inclined surface with a groove and hole at the lower end to catch any stray globules of quicksilver. Sodium amalgam, one ounce to each 75 pounds of mercury, is put in the amalgam kettle and the whole stirred. This sodium amalgam is not absolutely necessary, but is desirable. After some minutes, water is poured on the mercury and the whole stirred. All dirt rises to the surface and is removed with a sponge. The cleaning is continued until the mercury seems absolutely free from any impurity, when it is dried with a sponge. It is next turned into pointed bags of stout canvas and force applied until most of the quicksilver has squeezed through. The amalgam remains behind. The quicksilver still contains some gold, but it had better remain if the mercury is to be used again, as gold attracts gold; it can always be recovered by retorting.