  We show that condensation in a capped capillary slit is a continuous
interfacial critical phenomenon, related intimately to several other surface
phase transitions. In three dimensions (3d), the adsorption and desorption
branches correspond to the unbinding of the meniscus from the cap and opening,
respectively and are equivalent to 2d-like complete-wetting transitions. For
dispersion forces, the singularities on the two branches are distinct, owing to
the different interplay of geometry and intermolecular forces. In 2d we
establish precise connection, or covariance, with 2d critical-wetting and
wedge-filling transitions, i.e. we establish that certain interfacial
properties in very different geometries are identical. Our predictions of
universal scaling and covariance in finite capillaries are supported by
extensive Ising model simulation studies in 2d and 3d.
