N. F. Mott pre-Hitler period before he left; I hardly imagine it could have been Max Born, in view of his own work on alkali halides. t The way that Gurney and I developed the subject is set out in our book(Mott Gurney, Electronic properties of ionic crystals, Oxford University Press, 1940). We did not mean to write a book on semiconductors. We had before us the papers of conducting behaviour could be due to discrete levels produced by impurities. s A.H. wilson, showing the difference between metals and non-metals, and how ser had also, of course, Borns work on the cohesive forces in alkali halides, the concepts of Schottky and Frenkel defects in alkali and silver halides and also that of the Frenkel exciton(Frenkel; 6)see also Peierls. (7) Frenkel's contributions, t particu larly these two, seem to me to be of major importance; I met him in Leningrad in 1934, but i do not think I fully realized the significance of the exciton concept till I started working with Gurney. It is not mentioned in Mott Jones, 9)published in 1936. My most vivid recollection of what started us writing a book is discussions with gurney on the nature of an F-centre. At first we thought it was the 'self- trapped electron, or electron trapped by dig igging its own potential he had been introduced by Landau(8)in 1933, and the hypothesis was put forward by Gurney Motto)in 1937 and also by Von Hippel(3) in 1936. This self-trapped electron would now be called a polaron, and we gave a correct qualitative descrip tion(Mott Gurney, p. 87) of the hopping process for such particles, but not of their band motion at low temperatures, which I think was first proposed by Holstein in 1958. But we ran up against all sorts of difficulties in developing this idea. If F-centres were self-trapped electrons, where was the positive charge We knew that in alkali halides ionic conduction took place through the movement of Schottky defects'-that is vacant cation and anion lattice sites-and surmised that excess metal ought to be taken up as vacant anion sites, with the electron'somewhere The presence of excess metal gave colour to alkali halides and that meant that it produced F-centres. At first we speculated that the Landau self-trapped electron might be held by the field of a vacant anion site, but i think that it was Gurney who came up with the simplifying suggestion that the electron was in the vacancy. I do not know who first used the effective mass approximation for the energy levels in a non-polar semiconductor, though Kohn first set out to justify it. Since Mott gurney is about ionic crystals, it is not mentioned there, though we recog nized that the potential seen by an electron at a considerable distance from an F-centre must be -e2/Kr, where K is the dielectric constant, either static or at high frequencies. This comes out most clearly in our discussions of polarons, and on the relaxation of the medium round a centre after trapping. In fact, the book now ↑ See also article15 t a description of Y. I. Frenkel's work by his son, v.J. Frenkel, is given in Archive fo story of Earact Sciences, vol 13, p 1(1974). Perhaps his main contributions are his book on liquids, with his early realization of the similarities in the atomic packing between liquids and solids, the Frenkel exciton, described in detail long before evidence for it began to ccumulate, work on the electron theory of metals before quantum mechanics, and tI Frenkel defect. See also footnote to paper by Smoluchowski in this volum