In :mother experiment, 800 lb. of alloy were melted in the furnace, and 25 lb. of oxide of manganese were added to them. The mixture was then well stirred, and in two hours afterwards 15 lb. more of manganese were thrown in, a similar quantity being added every two hours till the whole amounted to 100 lb. and the fusion and occa sional stirring were continued during ten hours. A. the expiry of this time, the copper was drawn off, and amounted to 520 lb. that is, 65 per cent. of the alloy em ployed. The scoria still retained a good deal of copper, but this was not extracted from it.
When the alloy contains silver, it may be assayed by dissolving it in nitric acid, precipitating the silver by muriate of soda, and the copper by a plate of iron.
Tin.
Different methods arc followed in reducing the ores of tin.
The ore which is procured from the mines of Corn wall, after being hand-dressed, is freed from impurities by stamping, as has been described under gold, after which, it is roasted in a reverberatory, to drive off the sulphur, part of which, however, is acidified, and unites with the copper and iron of the ore. The ore is again washed, by which it is nearly freed from all impurities; it is then mixed with one-fifth of its bulk of culm, and subjected to heat in a reverberatory for about six hours, during which the oxide of tin is reduced, and the metal collects at the bottom, covered with a black scoria. The tin is then drawn off into a shallow pit, in which it is freed from the scoria which collects on its surface. It is then taken out with ladles, and poured into moulds.
The metal thus obtained is afterwards exposed to a gentle heat in a small reverberatory furnace, by which the purest part of it melts first, and is drawn off. This forms grain tin ; what is left behind is common tin, which contains a small portion of iron, copper, and ar senic.
The water employed in the second washing of the ore contains a considerable quantity of sulphate of cop per, on which account it is kept and decomposed by iron. The scoria separated from the tin, when drawn from the reverberatory, retains a good deal of the metal—it is therefore stamped and melted along with the ore.
The stream tin stone of Cornwall is melted in a dif ferent way. As the ore is in a powdery state when pro cured, it is submitted to a stream of water, by which a great deal of the impurities are removed. It is after
wards bruised, and passed through wire sieves.
It is then thrown, with alternate quantities of char. coal, into a blast furnace, in which it is reduced, and escapes through a channel at the bottom into pits ; the scoria being removed as it collects, and thrown again into the furnace.
The metal is then put into a large iron pot, in which it is kept fused. When in this state, pieces of charcoal are plunged into it, which cause a fresh quantity of sco ria to be separated. The metal is then tried by remov ing a quantity in a ladle, and pouring it into the pot. If it appear bright like silver, and of uniform consistence, it is pure. After this it is poured into moulds, and forms good grain tin.
Assay.—In assaying an ore of tin, it is first reduced to coarse powder, and then washed, to free it from earthy matter. II' it contain arsenic, which is known by its emitting the odour of garlic when fused before the blow-pipe, it must be exposed to heat with charcoal, till the vapours of arsenic cease to be emitted. What re mains is then mixed with pitch and saw-dust, subjected to a strong heat in a crucible, lined with charcoal. The metallic button collected at the bottom is tin.
Lead.
The only ore of lead from which the metal is extract ed is galena, the smelting of which is very simple.
The ore after being brought from the mine is hand dressed, by which it is freed as much as possible of impurities. What remains is then washed, to remove still farther any extraneous matter, and is put into a re verberatory furnace, where it is speedily made red hot. When in this state it is frequently stirred, and when it begins to become soft, the heat is reduced till the whole of the sulphur is expelled. The fire is then made brisk, by which the lead is melted, and collects at the bottom. A little lime is then thrown in to thicken the scoria, and the lead is drawn off into oblong moulds ; a sufficient heat is again applied to the scoria, by which another por tion of lead is procured. The lead obtained in the first operation is considered the best, as it is more malleable than the other.
Assay —The assay of galena is very simple. It may be done either in the dry or the humid way.