1. Alperovitch, E. A., Ph.D. dissertation, Columbia University, New York (1954); unclassified Report NYO-6139, U.S. Atomic Energy Commission (April 1954).
2. Medicus, H., Preiswerk, P., and Scherrer, P., Helv. Phys. Acta, 23, 299 (1950). Boyd, G. E., and Segre, E., Nuovo cimento, 9, 1008 (1952). The latest experimentally determined limit is ⩾ 107 years. Private communication from G. E. Boyd.
3. Jordan, P., Naturwiss., 40, 407 (1953). Gold, T., Mem. Soc. Roy. Sci. Liège, 14, 68 (1954). Greenstein, J. L., ibid., 307. Bidelman, W. P., ibid., 402. St. Temesvary, ibid., 122.
4. Hubenet, H., Jager, C. de, and Zwaan, C., Mem. Soc. Roy. Sci. Liège, 471 (1954). Moore, C. E., Science, 114, 59 (1951).
5. Merrill, P. W., Astrophys. J., 116, 21 (1952).