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42                    METALLURGY OF CAST IRON.
ranges from three to four inches in width, and in length from the bosh portion up to the top of the stack, as shown, the hearth being built solid, as seen in the sketch. A material now extensively used for filling this expansion space, K, is the slag of a furnace, after being granulated by the action of water. A loamy sand was at one time used, but it packs too firmly. Then, again, a coarse class of sharp sand has been used, but the slag as above prepared has been found the best. Experience has proven the necessity of such a system, as several furnaces have had their shells ruptured by the expansive force of fire-bricks when not permitted room to swell from the effects of the heat. Not only have furnaces provided for this lateral expansion, but also for longitudinal strains as well, as such action has been known to press the brick-work, bell, hopper, and charging platform upward from three to four inches above the top of the shell, or its original level. All the iron work at the top of a furnace is constructed independent of the shell, so as to liberate it from all strain when longitudinal expansion takes place.
Drying a furnace becomes necessary before it is charged for <c blowing in." There are several methods of doing this. One is by building a fire inside the furnace; another by constructing a fire-place outside, at the breast portion, and letting the heat from the same pass into the furnace; still another by the admission of natural gas, or the gas from the ovens of another furnace, should two or more furnaces be near each other. The objection to building a fire inside a furnace is that the dirt and ash which it creates requires considerable labor to clean out, and requiresis a practice now followed in lining furnaces. This spaceare               jf|V