[Illustration: FIG. 105.--COPPER SMELTER AND CITY OF BISBEE, ARIZONA The pipe leading up the hill carries away sulphur fumes from the smelter] The mine is opened by a shaft, that is, a square hole sunk in the ground. The shaft of this mine is a thousand feet deep, and is being continually extended downward. If we wish to go down into the mine, we must put on some old clothes and get the foreman to act as guide. The cage in which we are to descend stands at the mouth of the shaft, suspended by a steel rope. It looks much like the elevators found in city buildings. At different levels horizontal passages, called drifts, extend to the right and left upon the vein of copper ore. We step out of the car at one of these levels and with lighted candles start to walk through a portion of the mine. There are so many miles of tunnels that it would take us days to go through them all. Overhead, under our feet, and upon the sides of the drift, lies the vein of copper are, presenting a different appearance at different places. The various ores sparkle in the light and we gather specimens of each. The common are is chalcopyrite, a copper sulphide; that is, it is composed of copper and sulphur. It has a brass-yellow color, but is often stained with beautiful iridescent tints. In places the chalcopyrite has been changed to the delicate green carbonate of copper called malachite. In other places it has given place to the oxide of copper. The little crimson crystals of this mineral give bright metallic reflections. The deposit of copper ore is apparently inexhaustible, for in places the vein widens so that chambers one hundred feet wide and several hundred feet long and high have been made in taking it out. In going through the mine we have to be very careful not to step into openings in the floor of the passages, or drop rock fragments into them, for far below miners may be working. The places where the men are taking out the ore are called "stopes," and to reach them we have to crawl and creep through all sorts of winding passages, now through a "manhole," and now down a long ladder which descends into black depths. From the stopes the ore, as it is blasted out, is shovelled into chutes running down to some drift where there are men with cars. Each car holds about a ton of ore, and after being filled it is pushed along the drift and upon a cage which raises it to the surface. [Illustration: FIG. 106.--HOMES OF MINERS, BISBEE, ARIZONA] The mine is not wet, for there is so little rain in this region that there are few underground streams. In places, however, it is warm, for when the oxygen of the air reaches the fresh sulphide it begins to oxidize the ore; that is, it begins to burn it, and change it into a different compound, just as fire changes wood or coal. Wherever oxidation is going on, heat is produced.