1. Westbrook, J. H., Trans. Amer. Soc. Metals, 45, 221 (1953).
2. Osipov, K. A., and Fedotov, S. G., Izv. Akad. Nauk. S.S.S.R. Otd. tekhn. nauk., 2, 98 (1955).
3. Goldschmidt, V. M., Skrift. Norske vid-akad., Oslo, 2 (8), 102 (1926).
4. Wolff, G. A., Toman, L., Field, N. J., and Clark, J. C., “Semiconductors and Phosphors”, Proc. Intern. Colloq., 463 (Garmisch-Partenkirchen, 1956).
5. Petty, E. R. (to be published).