74 C. herring was not h radlid as od ar w it bet the tite. theb es whts k obthe ned at this time had already been obtained (though by equally unsatisfactory reasoning) by Carl Because of the importance of surface details, and in particular, of surface patch- ness, for thermionic phenomena, I became interested in the late 1940s in the phenomenon of thermal etching of surfaces. While thermal etching can occur in the course of evaporation or crystal growth, it may also sometimes occur as a normal way of approaching thermodynamic equilibrium; in such a case it must be that the surface can lower its surface free energy by developing a hill-and-valley structure. So I became interested in the surface tension of solids, and its variation with crystallographic orientation. At one of Nottinghams annual physical elec tronics conferences at M I.T., in the late 1940s, i ran into a young metallurgist named George Kuczynski, then(or perhaps soon after)at Sylvania, who was doing fundamental experiments on the sintering of metal particles. I realized that the interpretation of sintering phenomena would have to involve many of the same concepts as were involved with thermal etching, and so was born my interest in sintering. I maintained contact with Kuczynski and his colleagues for several years, and in the course of a number of experimental and theoretical papers it became clear that precise equations could be formulated to describe many sintering ype phenomena, and that from approximate solutions of these equations one could identify which of several mechanisms of transport were operative in any given case. In this period, wire-pulling experiments, as developed by Shaler, Wulff, and others on the basis of similar, cruder, experiments of the 1930s, provided improved quantitative measurements of surface tensions of solids, and verified the phenomenon of diffusional 4. STATISTICAL MECHANICS While I was a graduate student at Princeton-I think it was probably in the Spring of 1937-R. H. Fowler spent a term there and gave some lectures on new and exciting developments in the area of statistical mechanics of solids. I remember particularly his discussion of the theory of order-disorder transformations in alloys, with the Bethe cluster approach being presented as a great improvement over the theory of molecular field type of Bragg and williams. The libration-rotation ransformations were likewise treated In the summer of 1940I took part in the annual summer symposium on theoretical physics at the University of Michigan. I shared a room with Gregory Wannier, whom I had first encountered at Princeton several years earlier. He was hard at work trying to finish up a paper on the statistical mechanics of the two-dimensional ferromagnet, work which he had begun in Holland with Kramers, but which he was having to complete without any contact with his collaborator, because the German occupation had cut all connections. He was very excited over the discovery of a