We present ultraviolet interstellar absorption-line measurements for the si
ghtline toward the O9.5 V star mu Columbae (l = 237.degrees 3, b = -27.degr
ees 1; d approximate to 400 pc, z approximate to 180 pc; [n(H I)] approxima
te to 0.06 cm(-3)) obtained with the Goddard High Resolution Spectrograph (
GHRS) on board the Hubble Space Telescope. These archival data represent th
e most complete GHRS interstellar absorption-line measurements for any line
of sight toward an early-type star. The 3.5 km s(-1) resolution of the ins
trument allows us to accurately derive the gas-phase column densities of ma
ny important ionic species in the diffuse warm neutral medium, including ac
counting for saturation effects in the data and for contamination from ioni
zed gas along this sightline. For the low-velocity material (-20 less than
or similar to v(LSR) less than or similar to +15 km s(-1)), we use the appa
rent column density method to derive column densities. For the individual a
bsorbing components at v(LSR) approximate to -28.8, +20.1, +31.0, and +41.2
km s(-1), we apply component fitting techniques to derive column densities
and b-values. We have also used observations of interstellar Ly alpha abso
rption taken with the GHRS intermediate resolution gratings to accurately d
erive the H I column density along this sightline. The resulting interstell
ar column density, log N(H I) = 19.86 +/- 0.015, is in agreement with other
determinations but is significantly more precise. The low-velocity materia
l shows gas-phase abundance patterns similar to the warm cloud (cloud A) to
ward the disk star zeta Ophiuchi, while the component at v(LSR) approximate
to +20.1 km s(-1) shows gas-phase abundances similar to those found in war
m halo clouds. We find that the velocity-integrated gas-phase abundances of
Zn, P, and S relative to H along this sightline are indistinguishable from
solar system abundances. We discuss the implications of our gas-phase abun
dance measurements for the composition of interstellar dust grains. We find
a dust-phase abundance [(Fe + Mg)/Si](d) = 2.7-3.3 in the low-velocity gas
; therefore the dust cannot be composed solely of common silicate grains, b
ut must also include oxides or pure iron grains. The low-velocity material
along this sightline is characterized by T approximate to 6000-7000 K with
n(e) approximate to 0.3 cm(-3), derived from the ionization equilibrium of
Mg and Ca. The relative ionic column density ratios of the intermediate-vel
ocity components at v(LSR) = +31.0 and +41.2 km s(-1) show the imprint both
of elemental incorporation into grains and (photo)ionization. These clouds
have low total hydrogen column densities [log N(H) similar to 17.4-17.7],
and our component fitting b-values constrain the temperature in the highest
velocity component to be T = 4000 + 700 K. The electron density of this cl
oud is n(e) approximate to 0.6 cm(-3), derived from the P-2(1/2) to P-2(3/2
) fine structure excitation of C II. The components at v(LSR) approximate t
o -30 and -48 km s(-1) along this sightline likely trace shocked gas with v
ery low hydrogen column densities. The v(LSR) approximate to -30 km s(-1) c
omponent is detected in a few strong low-ionization lines, while both are e
asily detected in Si III. The relative column densities of the -30 km s(-1)
suggest that the gas is collisionally ionized at moderate temperatures (T
approximate to 25,000 K).
This is consistent with the measured b-values of this component, though non
thermal motions likely contribute significantly to the observed breadths.