1. This lecture was summarized in NATURE, 143, 34 (1939): it will be published in full in the Proceedings of the Physical Society.
2. Bernal, Royal Institution, Jan. 27, 1939.
3. Bernal, NATURE, 143, 74 (1939).
4. Neuberger, NATURE [this issue p. 473].
5. Neuberger, NATURE, 142, 1024 (1938).