  We present a comprehensive survey of boron abundances in diffuse interstellar
clouds from observations made with the Space Telescope Imaging Spectrograph
(STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is
the result of a complete search of archival STIS data for the B II 1362
resonance line, with each detection confirmed by the presence of absorption
from O I 1355, Cu II 1358, and Ga II 1414 (when available) at the same
velocity. Five previous measurements of interstellar B II from Goddard High
Resolution Spectrograph observations are incorporated in our analysis, yielding
a combined sample that more than quadruples the number of sight lines with
significant boron detections. Our survey also constitutes the first extensive
analysis of interstellar gallium from STIS spectra and expands on previously
published results for oxygen and copper. The observations probe both high and
low-density diffuse environments, allowing the density-dependent effects of
interstellar depletion to be clearly identified in the gas-phase abundance data
for each element. In the case of boron, the increase in relative depletion with
line-of-sight density amounts to an abundance difference of 0.8 dex between the
warm and cold phases of the diffuse interstellar medium. The abundance of boron
in warm, low-density gas is found to be B/H = (2.4+/-0.6)*10^-10, which
represents a depletion of 60% relative to the meteoritic boron abundance.
Beyond the effects of depletion, our survey reveals sight lines with enhanced
boron abundances that potentially trace the recent production of 11B, resulting
from spallation reactions involving either cosmic rays or neutrinos. Future
observations will help to disentangle the relative contributions from the two
spallation channels for 11B synthesis.
