  Abridged.
  Low-metallicity star-forming dwarf galaxies are prime targets to understand
the chemical enrichment of the interstellar medium. The HI region provides
important constraints on the dispersal and mixing of heavy elements released by
successive star-formation episodes. Our primary objective is to study the
enrichment of the HI region and the interplay between star-formation history
and metallicity evolution.
  We observed the most metal-poor star-forming galaxy in the Local Universe, I
Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from
far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the
abundances in the HII region. Models are constructed to calculate the
ionization structure and the thermal processes. We investigate the gas cooling
in the HI region through physical diagnostics drawn from the fine-structure
level of C+.
  We find that HI region abundances are lower by a factor of ~2 as compared to
the HII region. There is no differential depletion on dust between the HI and
HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of
1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we
propose that C, N, O, and Fe are mainly produced in massive stars. We argue
that the HI envelope may contain pockets of pristine gas with a metallicity
essentially null. Finally, we derive the physical conditions in the HI region
by investigating the CII* absorption line. The cooling rate derived from CII*
is consistent with collisions with H atoms in the diffuse neutral gas. We
calculate the star-formation rate from the CII* cooling rate assuming that
photoelectric effect on dust is the dominant gas heating mechanism. Our
determination is in good agreement with the values in the literature if we
assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).
