  We announce a new facility in the spectral code CLOUDY that enables tracking
the evolution of a cooling parcel of gas with time. For gas cooling from
temperatures relevant to galaxy clusters, earlier calculations estimated the
[Fe XIV] {\lambda}5303 / [Fe X] {\lambda}6375 luminosity ratio, a critical
diagnostic of a cooling plasma, to slightly less than unity. By contrast, our
calculations predict a ratio ~3. We revisit recent optical coronal line
observations along the X-ray cool arc around NGC 4696 by Canning et al. (2011),
which detected [Fe X] {\lambda}6375, but not [Fe XIV] {\lambda}5303. We show
that these observations are not consistent with predictions of cooling flow
models. Differential extinction could in principle account for the
observations, but it requires extinction levels (A_V > 3.625) incompatible with
previous observations. The non-detection of [Fe XIV] implies a temperature
ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent
pressure support, we estimate the gas mass at ~1 million solar masses. The
coronal gas is cooling isochorically. We propose that the coronal gas has not
condensed out of the intracluster medium, but instead is the conductive or
mixing interface between the X-ray plume and the optical filaments. We present
a number of emission lines that may be pursued to test this hypothesis and
constrain the amount of intermediate temperature gas in the system.
