1. A. SUMER, Bulletin of the Turkish Physical Soc. No. 23 (1955).
2. R. V. G. SUNDARA RAO, Proc. Ind. Acad. Sci. 32 (1955) 275.
3. R. F. S. HEARMON, Advan. Phys. 5 (1956) 323.
4. LANDOLT and BÖRNSTEIN, GROUP III, “Crystal and Solid State Physics,” Vol. 1 (Springer-Verlag, 1966) p. 5.
5. P. W. BRIDGMAN, Proc. Amer. Acad. Sci. 62 (1927) 207.