  We have analyzed the spectral variations of the superluminal black-hole X-ray
binary GRS 1915+105 by using data obtained with the PCA on the Rossi XTE. We
find that, despite the marked differences in the structure and the time scale
of variability, all spectral changes can be attributed to the rapid
disappearing of the inner region of an accretion disk, followed by a slower
re-filling of the emptied region. The time scale for each event is determined
by the extent of the missing part of the disk. The observed relation between
the duration of an event and the radius of the disappearing region matches
remarkably well the expected radius dependence of the viscous time scale for
the radiation-pressure dominated region of an accretion disk.
