  We present a method of adiabatically changing the local phase-space density
of an ultracold gas, using a combination of magnetic and optical forces.
Applying this method, we observe phase-space density increases in a gas of
sodium atoms by as much as 50-fold. The transition to Bose-Einstein
condensation was crossed reversibly, attaining condensate fractions of up to
30%. Measurements of the condensate fraction reveal its reduction due to
interactions.
