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14                          METALLURGY   OF   CAST   IRON.
to have a cell structure of about fifty per cent greater than exists in coal. The quality of hardness is one of much importance, especially in blast furnace practice, as the coke should possess a certain strength to sustain the weight of the stock which is charged on top of it. If it is not strong enough to resist the load, it can be crushed into a mass so compact as to prevent the free passage of blast through its body, which is necessary to create proper combustion and make the furnace work well. To a degree it has the same effect on passage of blast in cupolas. Then again a soft coke can crush so as to lower a bed, cause dull iron, and make a cupola bung up much more readily than hard coke. (vSee close of chapter.) Oven coke can be light and porous as well as heavy and dense, and is often spoken of as hard or soft. The terms hard and dense do not mean the same thing. Coke can be dense but soft. The following table, No. 2, of physical tests, by Mr. John Fulton, will illustrate the crushing strength of coke with other properties. A chemical analysis of the same coke by Mr. A. S. McCreath and Mr. T. T. Morrel is seen in Table 3, and which is taken from an article by the late Joseph D. Weeks of Pittsburg, which appeared in the Pennsylvania Annual Report of the vSecretary of Internal Affairs, 1893. In referring to the coke tested, Mr. Fulton says: " These tests show a compact, hard-bodied coke, harder than the average Connellsville standard. This coke has been carefully prepared and cannot be distinguished from Connellsville coke. The cells are a little less than the Connellsville, but the difference is not large enough to induce any marked change in blast furnace. It has proved an excellent fuel for this and kindred uses.''