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2. O'Neill, "The Hardness of Metals and its Measurement" (London).
3. Pfeil, Carnegie Schol. Mem., 16, 153 (1927).
4. Norbury, J. Inst. Met., 29, 407 (1923).
5. O'Neill and Cuthbertson, J. Inst. Met., 46, 273 (1931).