  The kinetics of nucleation of a core-shell composite nucleus that consists of
a core of stable final phase surrounded by a wetting layer of intermediate
metastable phase is studied using the kinetic theory of binary nucleation not
only in the size and composition space but also in the component space. The
steady-state solution of the Fokker-Planck equation is considered. Various
formulas for the critical nucleus at the saddle point as well as for the
post-critical nucleus are derived. The kinetics of nucleation at the saddle
point is more appropriately characterized in the size and composition space,
while the kinetics of the post-critical nucleus is more appropriately described
in the component space. Although both the free-energy landscape and the
reaction rates play decisive role to determine the kinetics of nucleation at
the saddle point, the details of the free energy landscape are irrelevant to
the kinetics of the post critical nucleus.
