process of phase coherence between such fluctuating pairs at different sites induced by the kinetic energy. The chemistry and physics leading to this ”negative” U is not well understood. It doubt- less depends on the fact that the valence on either side of the skipped valence forms closed shells so that U calculated from the gas-phase ionization and affinity energy is much lower than for adjacent elements [97], although still positive. This is likely to be reduced in a low energy model by polarization processes and by nearest neighbor repulsions [97]. But a quan- titative understanding of why in the solid state, irrespective of their chemical surroundings, such elements always skip valence needs further investigation. The phase diagram of the form in fig. (17) and some of its other properties were obtained in a model with negative U and repulsive nearest neighbor interactions [97]. A detailed investigation on this model has been carried out [100]. I wish to highlight only the experi- mental features which suggest that this compound has unusual properties, besides the high T$_{c}$ indicative of pairing due to electron-electron interactions. A limitation on our knowl- edge of this compound is that the extrapolated residual resistivity of the samples measured suggests a mean-free path due to impurities of only a few lattice constants. I hope better samples of the compound can be made and its properties in the pure limit studied more thoroughly. In the insulating state at low doping, transport and optical gaps differ by nearly an order of magnitude. In BaBiO$_{3}$ , the transport gap is 0.24 eV while the optical gap is about 2 eV [100]. This remarkable deviation has been well explained in the model of local electronic attraction [100] by the formation of Cooper pairs in the insulating state. The ac conductivity in the metallic state deviates from the Drude form and resembles in some experiments the form in the Cuprates. Tunneling measurements [101] show a conductance G ( V ) ∝ | V | for | V | >> T . This is characteristic of a single particle scattering rate ∝ | ω | , the energy measured from the chemical potential. This is of the same form as observed through ARPES in Cuprates near ’optimal’ doping, as well as with the peculiarity in the optical conductivity. What is even more interesting is that a systematic study of the slope G ( V ) / | V | as a function of doping shows a correspondence with the measured T$_{c}$ . These results are shown in fig. (18). This suggests, unsurprisingly, that the fluctuations responsible for the anomalous normal state scattering are also responsible for pairing. There is evidence for anomalous fluctuations in Raman scattering, shown in Fig. (14). 54