  We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI ($\lambda$
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
  The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
  The average number density ratio [HI]/[\h2] is $1.5\times10^{-3}$. The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H$_2$ and destruction via formation of H$_2$ on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.
