  There is a fortuitous coincidence in the positions of the quasar TonS210 and
the compact H I high velocity cloud CHVC224.0-83.4-197 on the sky. Using Far
Ultraviolet Spectroscopic Explorer observations of the metal-line absorption in
this cloud and sensitive H I 21cm emission observations obtained with the
multibeam system at Parkes Observatory, we determine a metallicity of (O/H)
<0.46 solar at a confidence of 3 sigma. The metallicity of the high velocity
gas is consistent with either an extragalactic or Magellanic Cloud origin, but
is not consistent with a location inside the Milky Way unless the chemical
history of the gas is considerably different from that of the interstellar
medium in the Galactic disk and halo. Combined with measurements of highly
ionized species (C III and O VI) at high velocities, this metallicity limit
indicates that the cloud has a substantial halo of ionized gas; there is as
much ionized gas as neutral gas directly along the Ton S210 sight line. We
suggest several observational tests that would improve the metallicity
determination substantially and help to distinguish between possible origins
for the high velocity gas. Additional observations of this sight line would be
valuable since the number of compact HVCs positioned in front of background
sources bright enough for high resolution absorption-line studies is extremely
limited.
