  We reported previously that for Cyg X-1 there is a settling period following
the transition from hard to soft state (astro-ph/9610071). During the
transiton, The low energy spectrum (below ~10 keV) varies significantly from
observation to observation while the high energy portion changes little. The
source reaches nominal soft-state brightness during the settling period. It can
be characterized by a soft low-energy spectrum and significant low-frequency
1/f noise and white noise on the power density spectrum (PDS). The low-energy
spectrum becomes even softer, and the PDS is completely dominated by the 1/f
noise, when the ``true'' soft state is reached. In this paper, subsequent RXTE
observations of Cyg X-1 in the soft state are examined, and the results confirm
our earlier conclusions. Furthermore, we show the results from observations
taken during a soft-to-hard transition. As expected, the white noise appears
again, and accordingly, the 1/f noise becomes less dominant, similar to the
settling period at the end of the hard-to-soft transition. The low-frequency
1/f noise has not been observed when Cyg X-1 is in the hard state. Therefore,
it seems to be positively correlated with the disk mass accretion rate which is
low in the hard state and high in the soft state. The difference in the
observed spectral and timing properties between the hard and soft states is
qualitatively consistent with a simple ``fluctuating corona'' model
(astro-ph/9610071). Here we present more evidence for it.
