  A stability criterion is worked out for the superconducting phase. The
validity of a prerequisite, established previously for persistent currents, is
thereby confirmed. Temperature dependence is given for the specific heat and
concentration of superconducting electrons in the vicinity of the critical
temperature $T_c$. The isotope effect, mediated by electron-phonon interaction
and hyperfine coupling, is analyzed. Several experiments, intended at
validating this analysis, are presented, including one giving access to the
specific heat of high-$T_c$ compounds.
