  We have experimentally investigated the free expansion of a Bose-Einstein
condensate in an array of two-dimensional traps created by a one-dimensional
optical lattice. If the condensate held in a magnetic trap is loaded
adiabatically into the lattice, the increase in chemical potential due to the
additional periodic potential is reflected in the expansion of the condensate
after switching off the magnetic trap. We have calculated the chemical
potential from measurements of the transverse expansion of the condensate as a
function of the lattice parameters.
