Except during the melting down of the pig iron and other materials charged in the furnace, the flame and air take little part in the actual elimination of the metalloids. Their main function is to furnish the heat necessary. Being used so indirectly—mostly by radiation from the roof and walls—very great heat must be used and much would be wasted if special precautions were not taken to save it. The bath must be kept hot enough to remain molten after purification of the metal, which we were unable to accomplish in the wrought iron puddling furnace. Under each end of the rectangular furnace are two chambers built up with checker-work of fire brick. These sets are in duplicate and each has one chamber for air and one for gas. [Illustration: CHARGING MACHINE WITH BOX OF SCRAP HALF WAY INTO FURNACE ] Thus an open-hearth furnace will be seen to occupy a sort of hollow square, the furnace proper forming one side, the regenerative chambers two sides, with the chimney and flues the remaining side. “Reversing” valves force the incoming gas and air to travel each through its respective hot regenerating chamber up through the ports and into the furnace where they unite and burn with a very hot flame. The hot gases leave through similar ports in the other end of the furnace and on their way to the chimney heat the checker-work in the regenerative chamber. Every fifteen or twenty minutes the valves are reversed and the direction of flow is changed. In this way the incoming gas and air are preheated and in the furnace burn with a very much hotter flame than would cold gas with cold air. No blast is required, the draft caused by the chimney being sufficient. For protection of the roof from the great heat developed and the metal of the bath from too great oxidation, the air ports usually are located above the gas ports. The streams of air, while protecting the roof from the flame, at the same time are prevented from directly impinging upon and too strongly oxidizing the metal of the bath. The diagrammatic sketches given show roughly a furnace, regenerative chambers, ports, etc. [Illustration: CHARGING “HOT” METAL ] The original intention was to melt pig iron and reduce it; i. e., burn out the silicon, manganese, and carbon by action of the flame and addition of iron ore. This was the process worked out by Siemens in England. In France, P. and E. Martin altered the method by diluting molten pig iron in the Siemens furnace by melting and dissolving in it steel scrap. It was soon found that a combination of the two methods was better than either one alone and the open-hearth process acquired its name—the Siemens-Martin—in this way. In the United States about 20,000,000 tons of steel are made annually by the basic open-hearth process while only 1,100,000 tons are produced by the acid open-hearth process.