We present ultraviolet interstellar absorption line measurements for the sightline towards the O9.5 V star \mu Columbae ( l = 237 \fdg 3 , b = -27 \fdg 1 ; d \approx 400 pc , z \approx 180 pc ; \langle n _ { HI } \rangle \approx 0.06 cm ^ { -3 } ) obtained with the Goddard High Resolution Spectrograph ( GHRS ) on board the Hubble Space Telescope .
These archival data represent the most complete GHRS interstellar absorption line measurements for any line of sight towards an early-type star .
The 3.5 km s ^ { -1 } resolution of the instrument allow us to accurately derive the gas-phase column densities of many important ionic species in the diffuse warm neutral medium , including accounting for saturation effects in the data and for contamination from ionized gas along this sightline .
We find that the effects of an H II region around \mu Col itself do not significantly affect our derivation of gas-phase abundances .
For the low velocity material ( -20 \lesssim v _ { LSR } \lesssim + 15 km s ^ { -1 } ) we use the apparent column density method to derive column densities .
For the individual absorbing components at v _ { LSR } \approx - 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 absorption taken with the GHRS intermediate resolution gratings to accurately derive the H I column density along this sightline .
The resulting interstellar column density \log N ( \mbox { \ion { H } { 1 } } ) = 19.86 \pm 0.015 is in agreement with other determinations but is significantly more precise .

The low-velocity material shows gas-phase abundance patterns similar to the warm cloud ( cloud A ) towards the disk star \zeta Ophiuchi , while the component at v _ { LSR } \approx + 20.1 km s ^ { -1 } shows gas-phase abundances similar to those found in warm halo clouds . We find 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 abundance measurements for the composition of interstellar dust grains .
We find a dust-phase abundance \left [ { ( Fe + Mg ) } / { Si } \right ] _ { d } = 2.7 - 3.3 in the low-velocity gas ; therefore the dust can not be composed solely of common silicate grains , but must also include oxides or pure iron grains .
The low velocity material along this sightline is characterized by T \approx 6 , 000 - 7 , 000 K with n _ { e } \approx 0.3 cm ^ { -3 } , derived from the ionization equilibrium of Mg and Ca .

The relative ionic column density ratios of the intermediate velocity 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 } ) \sim 17.4 - 17.7 ) , and our component fitting b -values constrain the temperature in the highest velocity component to be T = 4 , 000 \pm 700 K. The electron density of this cloud is n _ { e } \approx 0.6 cm ^ { -3 } , derived from the ^ { 2 } P _ { 1 / 2 } to ^ { 2 } P _ { 3 / 2 } fine structure excitation of \ion C2 .
The components at v _ { LSR } \approx - 30 and -48 km s ^ { -1 } along this sightline likely trace shocked gas with very low hydrogen column densities .
The v _ { LSR } \approx - 30 km s ^ { -1 } component is detected in a few strong low-ionization lines , while both are easily detected in \ion Si3 .
The relative column densities of the -30 km s ^ { -1 } suggest the gas is collisionally ionized at moderate temperatures ( T \approx 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 .