Fresh air is constantly needed in these workings far underground. A supply is forced down in pipes, and then allowed to flow back to the surface. In this way a thorough circulation is kept up. Underground one loses all thought of the changes between night and day, for it is always dark there. Consequently we are surprised on coming up from the mine to find that night has settled over the town. Lights are twinkling everywhere, and miners with their pails of luncheon are coming for the night shift. Another interesting experience now awaits us in the form of a visit to the smelter. Here the bright copper is extracted from the rough-looking ores. How different the two substances appear! They look as if they had scarcely anything in common. The interior of the smelter seems like a bit of the infernal regions set upon the earth. While watching what goes on, we might imagine that we were far down in the earth, where Vulcan, the fire god, was at work. At night the scene is particularly weird and impressive, for the shadows and general indistinctness make everything appear strange. The glowing furnaces, the showers of sparks, the roar of the blast furnaces, the suffocating fumes of sulphur, and the half-naked figures of the Mexican workmen, passing to and fro with cloths over their mouths, form all together a bewildering scene. The ore is first pulverized, and then placed in large revolving cylinders, where it is roasted. A fire is started in the cylinder at first, but after the ore becomes so much heated that the sulphur in it begins to burn, no further artificial aid is necessary. Little by little the ore is added in quantities sufficient to keep the fire going. The object of the roasting is to drive off as much sulphur as possible. After being raked from the roasting furnace, the ore is wheeled in barrows to the huge upright furnaces and is thrown in. Here such materials as limestone and iron are also added to aid in the formation of a perfectly fused or molten mass. These substances are known as fluxes. With the melting of the ore the copper begins to separate from the impurities. The melted ore, in the form of a glowing liquid, gathers at the bottom of the furnace and runs out into a large kettle-like receptacle. When ore of these vessels is full it is tipped up and the molten copper which has collected at the bottom, because it is heavier than the slag, is allowed to run into another large kettle, supported by chains from a rolling truck above. [Illustration: FIG. 107.--SHIPPING COPPER MATTE] The slag is dumped into a car and is carried outside, while the huge dish containing the copper and some slag is swung to the opposite side of the building, where its contents are cast into another furnace. A very strong blast of air is forced up through the molten mass in this furnace, and the remaining portion of slag is blown out at the top in a shower of glowing particles.