64 N. f. Mott crude measurements of conductivity as a function of temperature showed that their measurements had actually extended into the intrinsic range and gave a reasonably good value for the band gap Lark-Horovitz and his group at Purdue entered the field after much of our basic work was completed. As I recall, he made a trip to the Radiation Laboratory to learn if there were programs which could be farmed out to his laboratory in Purdue. It was proposed that he cooperate with our group. He and one or two of his colleagues spent several days with us and then returned to Purdue to duplicate the basic equipment The work on silicon and germanium fanned out very rapidly once relatively pure silicon became universally available through the development of an appro- priate production unit at Dupont. One then had a reasonable basis for standar zation of measurement On the Purdue work, Professor Yearian of that university comments as follows By the time we junior members of the research group had received out security clearances and the laboratory was organized in early 1942, it was very clear that Lark-Horovitz was convinced that Ge should be a semiconductor. So far I am aware there was no experimental evidence at that time to confirm this idea. Lark-Horovitz had obtained a few samples of Ge from various chemical supply companies and simple tests with cat whisker'contacts showed that although many such contacts were essentially ohmic, some gave good rectifica- tion, strongly suggestive of a semiconductor. He immediately instituted a pro gram of electrical resistivity and Hall effect measurements as a function of temperature. These, and later thermoelectric power measurements, confirmed that ge is indeed a semiconductor. So far as I can recall, this had been accom plished before the end of 1942 Continuation of the experiments, as the quality of our materials improved showed the p or n effect of different impurities and permitted determination of impurity and intrinsic energy gaps, density of carriers, mobility, etc. The ultimate success of the project was dependent on obtaining high purity Ge for experiments on controlled doping. This became possible through a colla, boration with the Eagle-Pitcher Lead Co. of Joplin, Missouri to obtain high purity GeO2. The oxide was treated by hydrogen furnace reduction in our labo DISLOCATIONS Papers describing plastic flow and work-hardening of solids were published inde pendently by Orowan, Polanyi and G. I. Taylor in 1934. I have always been struck y the fact that it was after the discovery of the nucleus, nuclear disintegration ind the neutron that physicists first turned their attention to the explanation in terms of atomic movements of so familiar a phenomenon as the ductility of metals I was in Bristol at the time, already committed to 'solid state, and I remember