  Two long-slit spectra of the diffuse ionized gas in NGC 891 are presented.
The first reveals variations parallel to the major axis in emission line ratios
in the halo gas at z=700 pc. It is found that filaments of Halpha emission show
lower values of [NII]/Halpha, [SII]/Halpha and [OI]/Halpha. Although this
result is expected if the filaments represent the walls of evacuated chimneys,
it merely reflects a more general correlation of these ratios with Halpha
surface brightness along the slit, and may simply arise from radiation dilution
effects. Much more interesting information on the nature of this gaseous halo
comes from the second observation, which shows the vertical dependence of
[NII]/Halpha, [SII]/Halpha, [OI]/Halpha, and [OIII]/Hbeta through the brightest
region of the DIG halo. The most surprising result, in complete contradiction
to models in which the DIG is ionized by massive stars in the disk, is that
[OIII]/Hbeta rises with height above the plane for z>1 kpc (even as
[NII]/Halpha, [SII]/Halpha, and [OI]/Halpha are rising, in line with
expectations from such models). The run of [SII]/[NII] is also problematic,
showing essentially no contrast with z. The [OIII] emission probably arises
from shocks, turbulent mixing layers, or some other secondary source of
ionization. Composite models in which the line emission comes from a mix of
photo-ionized gas and shocks or turbulent mixing layers are considered in
diagnostic diagrams, with the result that many aspects of the data can be
explained at a qualitative level.
