  We report on the observation of controllable spatial separation in a
dual-species Bose-Einstein condensate (BEC) with $^{85}$Rb and $^{87}$Rb.
Interparticle interactions between the different components can change the
miscibility of the two quantum fluids. In our experiments, we clearly observe
the immiscible nature of the two simultaneously Bose-condensed species via
their spatial separation. Furthermore the $^{85}$Rb Feshbach resonance near 155
G is used to change them between miscible and immiscible by tuning the
$^{85}$Rb scattering length. Our apparatus is also able to create $^{85}$Rb
condensates with up to $8\times10^4$ atoms which represents a significant
improvement over previous work.
