  Hydrogen column densities inferred from X-ray absorption are typically 5 - 30
times larger than the neutral atomic hydrogen column densities derived from
21cm HI absorption toward radio-loud active galactic nuclei. Some part of the
difference is ascribed to uncertainty in the spin temperature \Tsp\ = 100 K
that is often used to convert 21cm HI HI absorption to N(HI). Here we propose
another way to infer the gas column from HI absorption. In our Galaxy there is
a nearly linear correlation between the inteferometrically-measured integrated
21cm HI absorption \WHI\ and reddening, \WHI\ $\propto$ \EBV$^{1.10}$ for \WHI\
$\ga 0.7$ \kms\ or \EBV\ $\ga 0.04$ mag. Scaling \EBV\ then provides the total
gas column density N(H) from the same dust column that is responsible for
optical obscuration and X-ray absorption, without calculating N(HI). Values of
N(H) so derived typically exceed N(HI) by a factor 4 because the ubiquitous
Galactic 21cm HI HI absorption samples only a portion of the interstellar gas.
If the well-studied case of Hydra-A is a guide, even very large disparities in
X-ray and 21cm HI gas column densities can be explained by resolving the core
radiocontinuum and inferring N(H) from 21cm HI absorption. Milky Way conditions
are often invoked in discussion of obscured AGN, so the empirical relationship
seen in the Milky Way should be a relevant benchmark.
