1. E. Amaldi, E. Fermi and others, Proc. Roy. Soc., A. 149, 522 (1935).
2. S. Sugden, NATURE, 135, 469 (1935).
3. I. K. March and S. Sugden, NATURE, 136, 102 (1935).
4. G. Hevesy and H. Levi, NATURE, 136, 103 (1935).
5. Amaldi, Fermi l.c.; according to Sugden l.c. not active; while McLennan finds a period of 6·4 hours (NATURE, 136, 831 (1935)).