  A novel mechanism of prompt nuclear fragmentation is proposed. Assuming
micro-canonical or canonical equilibrium, it is shown that a strong enhancement
of the accessible phase space volume, due to the diffuceness of the nuclear
surface, leads to dynamical instabilities of hot nuclei and to prompt
fragmentation. Equations are derived for the transition temperature T_T, for
which the thermodynamical surface tension vanishes, as well as for the
thermodynamical fissility parameter chi_td.
