  NGC 4151 is one of the brightest and best-studied Seyfert galaxies. Here, we
present a two-phase model of the narrow-line region (NLR) of NGC 4151. This
study is motivated by (i) the fact that the X-ray spectrum of NGC 4151 is among
the flattest known for Seyferts, and (ii) the recent Chandra detection of an
X-ray narrow-line region in this galaxy. X-ray spectra as flat as that of NGC
4151 (Gamma_x = -1.5) are expected to favor the presence of two gas phases in
pressure equilibrium (Krolik, McKee & Tarter 1981). In the present study, we
show that a pronounced two-phase equilibrium develops in the extended
emission-line region if we use the observed multi-wavelength spectrum of NGC
4151 to ionize the clouds. The material is stable to isobaric perturbations
over a wide range of temperatures. We therefore propose that such a condition
has arisen in the NLR of NGC 4151, and that it explains the detection of hot,
extended X-ray gas which we identify as the NLR-cloud confining medium.
