Most of the Mexican silver is extracted from the ores by the process of amalgamation. For this purpose the ore is first crushed either by rollers, or, more generally, by stamps, called in Mexico _molinos_, which in principle resemble those used in the tin mines of Cornwall, but are not so powerful, and are worked either by water-power or by mules. [Illustration: STAMPING MILL.] The crushed ores are then conveyed to the _arrastres_, or grinding- mills, which are usually arranged in rows in a large gallery or shed. The ore, having been brought into a finely divided state, is next allowed to run out of the arrastre into shallow tanks or reservoirs, where it remains exposed to the sun until it has the appearance of thick mud, and in this state the process is proceeded with. The _lama_, as it is called, or slime, is now laid out on the _patio_, or amalgamation floor (which is in some places boarded, and in others paved with flat stones), in large masses called _tortas_, from forty to fifty feet in diameter, and about a foot thick; and so extensive are the floors that a large number of these tortas are seen in progress at the same time. When the ore has been laid out in masses on the patio, the operations necessary to produce the chemical changes commence. The first ingredient introduced is salt, in the proportion of fifty pounds to every ton of ore, and a number of mules are made to tread it, so that it may be dissolved in the water and intimately blended with the mass. On the following day another ingredient is introduced, called in Mexico _magistral_. It is common copper-pyrites, or sulphide of copper and iron, pulverised and calcined, which converts it into a sulphate. About twenty-five pounds of this magistral are added for every ton of ore in the torta, and the mules are again put in, and tread the mass for several hours. Chemical action now commences, and new combinations between the decomposed mineral substances are in progress. Quicksilver is then introduced, being spread over the torta in very small particles, which is effected by passing it through a coarse cloth. The quantity required is six times the estimated weight of the silver contained in the ore. [Illustration: GRINDING MILL.] The quicksilver being spread over the surface, the mules are once more put in, and tread the whole until it is well mixed. Great skill is now required to watch the progress of the amalgamation, and to decide whether any one of the ingredients that have been added to the ore is in deficiency or excess.