8. Black s-ulphuret of silver, is blackish, brittle, cellular, affording globules of sil ver at the blowpipe. It is found abun dantly in the silver mines of Peru and Mexico. The Spaniards call it negrillo.
9. Chloride ofsilver, silver, or horn silver.— In consequence of its semi-transparent aspect, its yellowish or greenish color, and such softness that it may be cut with the nail, this ore has been compared to horn, and may be easily recognized. It melts at the flame of a candle, and may be reduced when heated along with iron or black flux, which are distinctive cha racters. It is seldom crystallized ; but occurs chiefly in irregular forms, some times covering the native silver as with a thick crust, as in Peru and Mexico. Its density is only 444.
General treatment of silver ores.—All ores which contain more than 7 lbs. of lead, or 1 lb. of copper, per cent., aro ex cluded from the reviving operation, or amalgamation ; because the lead would render the amalgam very impure, and the copper would be wasted. They are sorted for the amalgamation, in such a way, that the mixture of the poorer and richer ores may contain 4oz. of silver per 100 lbs. The most usual constituents of the ores are, sulphur, silver, antimonial silver, bismuth, sulphurets of arsenic, of copper, iron, lead 'nickel, cobalt), zinc, with several earthy minerals. It is essen tial that the ores to be amalgamated shall contain a certain proportion of sulphur, in order that they may decompose enough of sea salt in the roasting to disengage as much chlorine as to convert all the silver present into a chloride. With this view, ores poor in sulphur are mixed with those that are richer, to make up a determinate average. The ore-post is laid upon the bed-floor, in is rectangular heap, about 17 ells long, and 41 ells broad (13 yards and 3k); and upon that layer the requisite quantity of salt is let down from the floor above, through a wooden tunnel ; 40 cwts. of salt being allotted to 400 cwts. of ore. The heap being made up with alternate strata to the desired magnitude, must be then well mixed, and formed into small binge, called roast posts, weighing each from 31 to 41 cwts.
Roasting of the ores.—The furnaces appropriated to the roasting ore posts are of the reverberatory class, pro vided with soot chambers. The prepared ground ore is spread out upon the hearth, and dried with incessant turning over ; then the fire is raised so as to kindle the sulphur, and keep the ore redhot for one or two hours ; during which time dense white-gray vapors of arsenic, antimony, and water, are exhaled. The desulphnr ation next begins, with the appearance of a blue flame. This continues for three hours, during which the ignition is kept up. Whenever sulphurous acid ceases to be formed, the finishing calcination is to be commenced with increased firing ; the object being now to decompose the sea salt by means of the metallic sul phates that have been generated, to con vert them into chlorides, with the shnul taneous production of sulphate of soda. The stirring is to be continued till the proofs taken from the hearth no longer betray the smell of sulphurous, butonly of muriatie acid gas. Out of the nicht cham bers or soot vaults of the furnaces, from 96 to 100 cwts. of ore-dust are obtained, containing 16 lbs. of silver. This dust is to be treated like nnroasted ore. The fuel of the first fire is pitcoal ; of the fin ishing one, fir-wood. Of the former 1151 cubic feet, and of the latter, 294}, are, upon an average, consumed for every 100 cwts. of ore.
During the last roasting, the ore in creases in bulk by one-fourth, becomes in consequence a lighter powder, and of a brown color. When this process is completed, the ore is raked out upon the stone pavement, allowed to cool, then screened in close sieve-boxes, in order to separate the finer powder from the lumps. These are to be bruised, mixed with sea salt, and subjected to another calcination. The finer powder alone is taken to the millstones. The stones are of granite, and make from 100 to 120 revolutions per minute. The roasted ore, after it has passed through the bolter of the mill, must be as impalpable as the finest flour.