Page:Popular Science Monthly Volume 73.djvu/277

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273
SILVER

1873, when the metal had a value of about $1.30 per ounce, a vast amount of study and experiment was devoted to the question of recovering it from its ores by milling processes, or methods not involving the fusion of the minerals. Preliminary roasting (sometimes in the presence of salt), followed by long-continued grinding with mercury, under water, was the system adopted in the majority of cases, the result being an amalgam of mercury and silver. This was then heated in retorts to volatilize the mercury, and the silver left behind was melted and cast into bars for shipment to market. Other processes involved the use of chemicals by the aid of which the metal was brought into a state of solution, and from which it was recovered by some method of precipitation. But as the west became opened by railroads so that ores could be cheaply transported to natural centers where coal or water power existed and labor was abundant and inexpensive, most of these processes were abandoned in favor of smelting, in which the ores, properly mixed to secure fusion, are melted in a blast furnace. The products are either copper or lead bars—according to the system used—and during the melt the precious metals unite with the baser ones, from which they are subsequently separated by electrolysis. In consequence of all this the production of silver may now be considered as a settled industry, and because the metal is now produced very largely as a by-product.

Since its discovery in the United States, and the application of business methods to the mining of its ores and their treatment, the world's output has amounted to nearly 170,000 tons. There has been nothing comparable to this enormous yield in any previous era of its history, and its fall in value may be considered as warranted, and perhaps permanent. The annual output of the world at the present time averages about 6,000 tons, and is not at all likely to seriously decline. The American and Mexican mines show no signs of exhaustion. On the contrary, new ones are continually being found. Asia and Europe are not likely to become large producers of the metal. The settled parts of the old world have been fairly well explored, and in the unsettled parts like Siberia, Turkey and Africa the mining laws are so burdensome that the prospector and individual miner (who are the advance guard in the exploration of the mineral resources of a land) will have nothing to do with those regions. In Australia somewhat similar conditions prevail. We may, however, confidently look to South America for many new and great silver mines, when the political situation becomes as stable as in Mexico, for Spanish mining law has always recognized the necessity of the prospector at the base of the industry. The extent to which the American crop of silver is a byproduct is shown by the following table worked out by the author for