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Some systems for which the binding energy increases more rapidly than linearly with the number of partieles, are shown to exhibit negative specific heat c for some energies. In thermal contact with larger systems, c < 0 creates an instability, and in the canonical ensemble one sees only a phase transition. It is argued that supernovae are, in essence, a phase transition of this origin.

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The energy level shifts and level widths of the s states of the '11"-mesonic atoms are discussed. The discussion is limited to fairly light nuclei. On the basis of Orear's determination of the scattering lengths for meson-nucleon scattering, semiquantitative predictions are made. It is pointed out that even a knowledge of the algebraic sign of the level shift would be of value.

The limitations on scattering amplitudes imposed by causality requirements are deduced from the demand that the commutator of field operators vanish if the operators are taken at points with space-like separations. The problems of the scattering of spin-zero particles by a force center and the scattering of photons by a quantized matter field are discussed. The causality requirements lead in a natural way to the