  The evolution of the nuclear matter density distribution with excitation
energy is studied within the framework of a finite-range interacting Fermi gas
model and microcanonical thermodynamics in Thomas-Fermi approximation. It is
found that with increasing excitation energy, both infinite and finite systems
become unstable against infinitesimal matter density fluctuations, albeit in
different ways. In modeling, this instability reveals itself via an apparent
negative heat capacity of the system and is seen to result in the volume
boiling in the case of infinite matter and surface boiling in the case of
finite systems. The latter phenomenon of surface boiling is unique to small
systems and it appears to provide a natural explanation for the observed
saturation-like patterns in what is commonly termed caloric curves and what
represents functional dependence of nuclear temperature on the excitation
energy.
