1. Welker, H., Z. f. Naturf., 7 a, 744 (1952); 8 a, 248 (1953).
2. For example, The Times, p. 8, Feb. 14 1957.
3. Moissan, H., C.R. Acad. Sci., Paris, 113, 624, 726, 787 (1891). Besson, A., ibid., 113, 78 (1891).
4. Herman, F., J. Electronics, 1, 103 (1955).