1. Gabor, D., Nature, 189, 821 (1961).
2. Bertele, H. von, Nature, 182, 1148 (1958).
3. Hull, A. W., Phys. Rev. (in the press).
4. Carslaw, H. S., and Jaeger, J. C., Conduction of Heat in Solids, 264 (Clarendon Press, Oxford, 1947).
5. For pure mercury the number of electrons per ion was taken as 20 in ref. 3, since the higher temperature, 523° K., causes half the ions formed to be blown away by the evaporating atoms (Hull, A. W., loc. cit., appendix 1c).