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2. A study of the microhardness impression of coal;Das;Trans. Inst. Min. Met. Ostrava, Czechoslovakia, Mining and Geology Series,1971
3. Hardness of coal;Honda;Fuel, Lond.,1956
4. Development of optical anisotropy in vitrains during carbonization;Patrick;Fuel, Lond.,1973
5. Chemical structure and properties of coal VII-elastic constant;Schuyer;Fuel, Lond.,1954