1. J. A. McCarthy:Phys. Rev.,97, 1234 (1955);90, 853 (1953).
2. E. Majorana:Nuovo Cimento,14, 171 (1937);W. H. Furry:Phys. Rev.,56, 1184 (1939).
3. Private communication, Dr.M. Awschalom.
4. Private communication, Dr.B. G. Hogg.
5. Private communication, Dr.E. J. Konopinski. Professor Konopinski has discussed certain theoretical questions relative to double Β-decay inLos Alamos Report LAMS 1949;H. Primakoff:Phys. Rev.,85, 888 (1952).