1. Overbeek, J. Th. G., and Sparnaay, M. J., Disc. Farad. Soc., 18, 12 (1954).
2. See, for example, Kohl, W. H., “Materials Technology for Electron Tubes” (Reinhold, 1951).
3. Williamson, B. P., Greenwood, J. A., and Harris, J., Proc. Roy. Soc., A, 237, 560 (1956).
4. Casimir, H. B. G., Proc. Kon. Ned. Akad. Wetenschap, 51, 793 (1948).
5. Lucretius, “De Rerum Natura”, Lib. 1, vs. 385–400.