Opportunities missed and opportunities seized Subsequent developments are either well known or would be better described by others. I shall therefore conclude my personal reminiscences by posing two questions, namely why were my discoveries not made earlier, and why was so little done in the 1930s to follow them up? As regards the first question, both Bloch and especially Peierls might well have forestalled me, and they may be able to remember why they did not do so. The only clue that I can offer is a remark made by Peierls in his paper of 193o on electrical and thermal conductivity in which he states: ' Die Eigenfunktionen sind in zwei Fallen bekannt: erstens in dem Grenzfall freier Elektronen... und zweitens in dem Grenzfall stark gebundener Elektronen, wo man sie nach Bloch durch ein Storungs- verfahren aus den Eigenfunktionen des einzelnen Atoms verhalten kann... Von dem zweiten Grenzfall(stark gebundenen Elektronen) weiss man wegen seinen Konse- quenzen fur die magnetische Suszeptibilitat dass er praktisch nie realisiert ist With the concept of nearly free electrons uppermost in his mind, it is understandable that Peierls did not give much attention to insulator As regards opportunities missed or seized during the remainder of the 1930s I can speak only about Cambridge with first-hand knowledge. On my return there n October 1931 considerable interest was shown in my discoveries, but, when I suggested that germanium might be a very interesting substance to study in detail the silence was deafening. However, I eventually got J. D. Bernal to consider the project, though he saw little hope in it. In a paper published in 1927, H. J Seemann had investigated the temperature coefficient of the resistance of ten single crystals of silicon between-80C and room temperature. He found that not only was it positive but that it was of the same order of magnitude as for normal metals. (This was confirmed by Schulze in 193I ) In view of this fact, that silicon, in its pure state and freed from oxide films, was a metal, germanium would also be a metal and of no more interest than grey tin. But, since Bernal possessed a large single crystal of silicon of outstanding purity, he agreed to examine its properties. After some six months Bernal told me that he thought that Seemann was probably right, though he did not reveal his grounds for saying so. It is likely that he was merely being polite, and that the subject held no interest for him The canard that silicon is a good metal continued to be believed in many circles for a long time. For example, in the Handbuch der Metallphysik, published in 1935 both U. Dehlinger and G. Borelius stressed that all the elements which had at one time or another been classed as semiconductors were, in the pure state, true metals Borelius ends his review ruling out the existence of semiconductors with the follow ing statement(p. 354): 'Man kann qualitativ nur zwischen metallischen Leitern (Elektronenleiter)und elektrolytischen Leitern(Ionenleiter)unterscheiden. Whether anything interesting would have emerged from a proper study of germanium in the 1930s is extremely doubtful. Methods of purification would almost certainly not have been established because there were no incentives, either acade- mic or industrial, to do so, and impure germanium is one of the least interesting ofOppo "8 issed αnd opportunities seized 47 Subsequent developments well known Qr would be be er described by others. 1 shall therefore conclude my personal reminiscences by posing tWQ que ons namely why were my discoveries no de earlier, and why was 80 Ii灿le don in 1930s to follow them up ? As rcgards fìrst question, both Bloch and especially Peierls might well have re lled me, and they be ble to remember why they did not do 80. The only c1 ue tha也 1 can offer is a. remark made by Peierls in his paperof 1930 on ec ri ca and thermal conductivity in which he states: ' Die Eigenfunk onen sind in zw Fällen bekannt: in dem Grenzfal! freier Elektronen, ... und zweitens in dem Grenzfal! stark gebundener Elektronen, wo man sie nach Bloch durch ein rungs￾verfahren aus den Eigenfunktionen des einzelnen Atoms verhalten kann ... Von dem zweiten Grenzfall (stark gebundenen Elektronen) weiss man wegen nen Konse quenzen für die magnetische Suszeptibîlîtät dass er praktisch nie eali ier .' Ni怕也 concept of nearly free electrons uppermos也 in rns mind, itis understanda.ble at Peierls did not give much attention to insuJators As regards opportunities missed or seîzed during le remainder of 1930s 1 can speak onJy about Cambridge with first-hand knowledge. On my re um here in October 193 1 considerable inter.曲也 shown in my d>scoveries, but, when 1 suggeste 出岛 germ üum a. very nteres ng subs 也a.nce study in detail, the silence was deafening eve 1 even ual1 go J. D. Bernal consider the project ，也 hough he saw li le hope in it. In a er published in 1927, H. J. Seemann had in ve ig ated he mp atur coefficient of he resis nceof single crystals of silicon between - 80 oC ld room mperature. He found that nly was positive bu at asof he same order of nitude as for normal 回国 s. (This was confirmed by Schulze in 1931.) In view of this fact, that silicon, in ur state and freed from oxide films, was a metal , germanium would al50 be a metal and of no more in忧而睛也han grey tin. But, since Bcrnal po回国回 a large singlc crystal of silicon of outstanding puri 'y hc ag ed to examine its proper cs. After some six months Bernal told me 创陆也 he thought that Soomann was probably right, ough he did not reveal his grounds for saying so. It is likely tha也 he was merely being poli阳， and t hat the suçject held no inter困也 fo him The canard 出叫 ili con is a good metal continued to be believed in many circles for a ong im e. For example, in the Randbuch der Metallphysik, published in 1935, both U. Dehlinger and G. Borelius stressed that lJ乞 element8 which had at one u