  The density profile and Gibbs adsorption of a near-critical fluid confined
between two identical planar walls is studied by means of
  Monte Carlo simulation and by density functional theory for a Lennard-Jones
fluid. By reducing the strength of wall-fluid interactions relative to
fluid-fluid interactions we observe a crossover from behaviour characteristic
of the normal surface universality class, strong critical adsorption, to
behaviour characteristic of a 'neutral' wall. The crossover is reminiscent of
that which occurs near the ordinary surface transition in Ising films subject
to vanishing surface fields. For the 'neutral' wall the density profile, away
from the walls, is almost constant throughout the slit capillary and gives rise
to an adsorption that is constant along the critical isochore. The same
'neutral' wall yields a line of capillary coexistence that is almost identical
to the bulk coexistence line. In the crossover regime we observe features in
the density profile similar to those found in the magnetisation profile of the
critical Ising film subject to weak surface fields, namely two smooth maxima,
located away from the walls, which merge into a single maximum at midpoint as
the strength of the wall-fluid interaction is reduced or as the distance
between walls is decreased. We discuss similarities and differences between the
surface critical behaviour of fluids and of Ising magnets.
