  Optical measurements were performed on thin films of Rb$_{x}$C$_{60}$,
identified by X-ray diffraction as mostly $x=1$ material. The samples were
subjected to various heat treatments, including quenching and slow cooling from
400K. The dramatic increase in the transmission of the quenched samples, and
the relaxation towards the transmission observed in slow cooled samples
provides direct evidence for the existence of a metastable insulating phase.
Slow cooling results in a phase transition between two electrically conducting
phases.
