1. Mügge, O., Neues Jahrb. f. Miner. 1, 71 (1898).
2. Mark, H., Polanyi, M., and Schmid, E., Z. Phys., 12, 58 (1922).
3. Ewald, P. P., in "The Physics of Solids and Fluids" (Blackie and Son, 1936), 121.
4. Brilliantow, N. A., and Obreimow, I. W., Sow. Phys., 6, 587 (1934); 12, 7 (1937).
5. See, for example, Elam, C. F., Proc. Roy. Soc., A, 153, 273 (1936).