We use new Space Telescope Imaging Spectrograph ( STIS ) and archival Goddard High Resolution Spectrograph ( GHRS ) observations to study interstellar B ii \lambda 1362 and O i \lambda 1355 absorption along seven sightlines .
Our new column density measurements , combined with measurements of four sightlines from the literature , allow us to study the relative B/O abundances over a wide range of interstellar environments .
We measure sightline-integrated relative gas-phase abundances in the range [ { B / O } ] = -1.00 to -0.17 , and our data show the B/O abundances are anticorrelated with average sightline densities over the range \log \langle n _ { H } \rangle \approx - 1.3 to +0.7 .
Detailed comparisons of the B ii and O i line shapes show that the B/O ratio is significantly higher in warm interstellar clouds than in cool clouds .
These results are consistent with the incorporation of boron into dust grains in the diffuse ISM .
Since boron is likely incorporated into grains , we derive a lower limit to the present-day total ( gas+dust ) interstellar boron abundance of { B / H } \gtrsim ( 2.5 \pm 0.9 ) \times 10 ^ { -10 } .
The effects of dust depletion and ionization differences from element to element will make it very difficult to reliably determine ^ { 11 } B/ ^ { 10 } B along most interstellar sightlines .