1. Hawk, C. O., Schlesinger, M. D., and Hiteshue, R. W., BuMines Rep. Inves., 6264 (1963).
2. Rau, E., and Seglin, L., Preprints, Div. Fuel Chem., 145th Meeting Amer. Chem. Soc., 7, 154, New York (Sept. 1963).
3. Vogel, Sy, and Dulberger, Leon, Electronics, 81 (1961).
4. Honig, R. E., and Woolston, J. R., Appl. Phys. Letters, 2, 138 (1963).
5. Honig, R. E. (personal communication).