  Ultraviolet spectra of stars recorded by orbiting observatories since the
1970's have revealed absorption features produced by atoms in their favored
ionization stages in the neutral ISM of our Galaxy. Most elements show
abundances relative to hydrogen that are below their values in stars,
indicating their removal by condensation into solid form. The relative amounts
of these depletions vary from one location to the next, and different elements
show varying degrees of depletion. In a study of abundances along 243 different
sight lines reported in more than 100 papers, Jenkins (2009) characterized the
systematic patterns for the depletions of 17 different elements, and these
results in turn were used to help us understand the compositions of dust
grains. Since the conclusions are based on differential depletions along
different sightlines, they are insensitive to errors in the adopted values for
the total element abundances. Some of the more remarkable conclusions to emerge
from this study are that (1) oxygen depletions in the denser gas regions (but
not as dense as the interiors of molecular clouds) are stronger than what we
can expect from just the formation of silicates and metallic oxides, and (2)
the chemically inert element krypton shows some evidence for weak depletion,
perhaps as a result of trapping within water clathrates or binding with H_3^+.
