It has been stated that silver is usually a component part of lead ore. To disengage this, where the quantity is sufficient to repay the expense, the lead, after it has been smelted, is subjected to the action of what is called a refining furnace. A continued blast of fresh air is thrown upon its surface by means of large bellows, while the lead is kept in a state as intensely hot as possible. This by degrees converts the lead into a yellow scaly oxide or dross. The oxide, thus formed, is driven off from the melted metal as it rises, and the silver is left alone at the bottom, in a metallic state. After the operation is complete, the oxide is fused with charcoal, and again reduced to metallic lead. * * * * * We must not omit to mention that, in some of the mines of Derbyshire, there is a singular variety of lead ore called _slickenside_. This is a kind of galena, which presents, to the eye, a smooth and bright surface, appearing as if it were plated. Sometimes it forms the sides of cavities; and it has the extraordinary property, when merely pierced with the miner’s tool, of rending with great violence, and exploding with a crackling noise. Some miners, fearless of danger, venture to scratch it with their tools; and, on coming again to the spot, they often find that, during their absence, the slickenside has exploded, and fallen off in considerable quantity. Sometimes, however, they suffer for their imprudence. Mr. Mawe, in his account of the Mineralogy of Derbyshire, says, that he has seen a man come out of a mine cut violently, as if he had been stabbed about the neck and in other parts of the body, in consequence of the explosion of slickenside which he had pierced. The cause of this extraordinary phenomenon has not been explained. 240. _NICKEL, when pure, is a fine white metal, somewhat resembling silver in appearance, but it is attracted by the magnet, and has itself the property of attracting iron._ _It is ductile and malleable, difficult of fusion, and about nine times heavier than water. This metal is always mixed with arsenic (242) and iron._ Nickel is found in Cornwall, and in some other counties of England; in Germany, Sweden, France, Spain, and several parts of Asia. The Chinese employ it in making white copper; and, in conjunction with copper and zinc, they manufacture it into various kinds of children’s toys. Nickel gives a certain degree of whiteness to iron. It is used, with advantage, by some of the Birmingham manufacturers, in combination with that metal, and by others in combination with brass. If it were possible to discover an easy method of working nickel, there can be little doubt but it would be found a very valuable metal for surgical instruments, for compass needles, and other articles, as it is not, like iron, liable to rust. When nickel is freely suspended, it points to the north and south, in precisely the same manner as the common magnetic needle.