  High-resolution spectroscopic measurements of blue horizontal-branch stars in
six metal-poor globular clusters -- M3, M13, M15, M68, M92, and NGC 288 --
reveal remarkable variations in photospheric composition and rotation velocity
as a function of a star's position along the horizontal branch. For the cooler
stars (Teff < 11200 K), the derived abundances are in good agreement with the
canonical cluster metallicities, and we find a wide range of v sin i rotation
velocities, some as high as 40 km/s. In the hotter stars, however, most metal
species are strongly enhanced, by as much as 3 dex, relative to the expected
cluster metallicity, while helium is depleted by 2 dex or more. In addition,
the hot stars all rotate slowly, with v sin i < 8 km/s. The anomalous
abundances appear to be due to atomic diffusion mechanisms -- gravitational
settling of helium, and radiative levitation of metals -- in the non-convective
atmospheres of these hot stars. We discuss the influence of these photospheric
metal enhancements on the stars' photometric properties, and explore possible
explanations for the observed distribution of rotation velocities.
