1. C. Palache, H. Berman, and C. Frondel:Dana’s System of Mineralogy, 7th ed., pp. 207 and 231, Wiley, New York, 1944.
2. H. M. Howe:The Metallurgy of Steel, Scientific Publishing Co., p. 52, New York, 1890.
3. C. Benedicks and H. Löfquist:Nonmetallic Inclusions in Iron and Steel, p. 32, John Wiley, New York, 1931.
4. R. Vogel and W. Hotop:Arch. Eisenhuettenw., 1937, vol. 11, p. 41.
5. H. Wentrup:Tech. Mitt. Krupp, 1937, vol. 5, p. 131.