  The entropy-temperature curves are calculated for non-interacting bosons in a
3D optical lattice and a 2D lattice with transverse harmonic confinement for
ranges of depths and filling factors relevant to current experiments. We
demonstrate regimes where the atomic sample can be significantly heated or
cooled by adiabatically changing the lattice depth. We indicate the critical
points for condensation in the presence of a lattice and show that the system
can be reversibly condensed by changing the lattice depth. We discuss the
effects of interactions on our results and consider non-adiabatic processes.
