1. von Knorring, O., Brindley, G. W., and Hunter, K., Min. Mag., 29, 963 (1952).
2. Edelman, C. H., and Favejee, J. Ch. L., Z. Krist., A, 102, 417 (1940); and Edelman, C. H., Verres et Silicates Indust., 12 (Supp.), 3 (1947).
3. For example, see Nature, 168, 107 (1951).
4. Robertson, R. H. S., and Emödi, B. S., Nature, 152, 539 (1943).
5. Meldau, R., and Robertson, R. H. S., Ber. deut. keram. Ges., 29, 27 (1952).