  Phase diagrams of some globular proteins have a fluid-fluid transition as
well as a fluid-crystal transition. Homogeneous nucleation of the crystal from
the fluid phase near the critical point of the fluid-fluid transition is
examined. As the fluid-fluid critical point is approached, the number of
molecules in the critical nucleus, the nucleus at the top of the free energy
barrier to nucleation, is found to diverge as the isothermal compressibility.
This divergence is due to a layer of the fluid phase of width equal to the
fluid's correlation length which surrounds the core of the nucleus; the number
of molecules in a crystalline environment in the nucleus does not diverge. The
free energy barrier to nucleation remains finite but its derivative with
respect to the chemical potential is equal to minus the number of molecules in
the critical nucleus and so diverges.
