  We present interstellar elemental abundance measurements derived from Space
Telescope Imaging Spectrograph echelle observations of 47 sight lines extending
up to 6.5 kpc through the Galactic disk. These paths probe a variety of
interstellar environments, covering ranges of nearly four orders of magnitude
in molecular hydrogen fraction f(H_2) and more than two in mean hydrogen sight
line density <n_H>. Coupling the current data with Goddard High Resolution
Spectrograph data from 17 additional sight lines and the corresponding Far
Ultraviolet Spectroscopic Explorer and Copernicus observations of H_2
absorption features, we explore magnesium, phosphorus, manganese, nickel,
copper, and germanium gas-phase abundance variations as a function of <n_H>:
density-dependent depletion is noted for each element, consistent with a smooth
transition between two abundance plateaus identified with warm and cold neutral
interstellar medium depletion levels. The observed scatter with respect to an
analytic description of these transitions implies that total elemental
abundances are homogeneous on length scales of hundreds of parsecs, to the
limits of abundance measurement uncertainty. The probable upper limit we
determine for intrinsic variability at any <n_H> is 0.04 dex, aside from an
apparent 0.10 dex deficit in copper (and oxygen) abundances within 800 pc of
the Sun. Magnesium dust abundances are shown to scale with the amount of
silicon in dust and, in combination with a similar relationship between iron
and silicon, these data appear to favor the young F and G star values of Sofia
& Meyer (2001; ApJ 554, L221) as an elemental abundance standard for the
Galaxy.
