Potosi is among mountains generally of volcanic formation, and more than five thousand mines have been opened in its vicinity. The country is barren, and much of it is more than ten thousand feet above the level of the sea. [Sidenote: THE HIGHEST CITY IN THE WORLD.] The city of Potosi is more than two miles up in the air; that is to say, more than two miles above the sea level. It has a population at the present day of less than fifty thousand, though it contained at one period more than three times that number. It has been repeatedly shaken by earthquakes, and in some of these earthquakes a great many people have lost their lives. All supplies must be brought from a distance, as the country in the immediate neighborhood produces absolutely nothing. It is not a beautiful place of residence, and those who dwell there are almost entirely devoted to the production and preparation of silver. An idea of the richness of the mines may be formed, when it is known that between 1556 and 1800 the mines of Potosi alone yielded nine hundred and twenty-five million dollars’ worth of silver. [Sidenote: PROCESS OF AMALGAMATION.] Silver mines are pretty much alike in all parts of the world. They are also not much unlike mines of other metals. They are opened by shafts, tunnels, and the like, the same as other mines, though each locality has some processes of operations peculiar to itself. The ordinary methods of separating silver from its ores are based either upon forming an amalgam of the metal with the mercury, or in bringing it into combination with lead, and afterwards separating it. The ores of Mexico and Peru are treated by both processes. Some of the richest ores are picked out and thrown into the furnace. The amalgamating process, which was long used in the silver mines of Mexico and Peru, and is still generally practised there, was invented more than three hundred years ago. The ores are crushed by stamping machines, and then ground with water, in machines called arastras, a sort of circular mill, run by mule power. It is ground to as fine a condition as possible, and, after being allowed several days to dry, is spread out in circular heaps, about fifty feet in diameter and a foot in depth. To every ton of this substance three bushels of salt are added, and the whole is then carefully mixed. A chemical substance, of a coppery character, is then added, and a sort of fermentation takes place, in which great heat is thrown out. Quicksilver is then added, in small quantities at a time, and crudely mixed in, until the whole mass forms an amalgam. The whole process occupies from four to six weeks. The amalgam is then separated from the mass of ore by a system of washing similar to that practised in collecting gold. The mercury is then separated from the amalgam by the ordinary process of evaporation, and in the same manner as if removing it from gold.