  There is a robust polyatomic chemistry in diffuse, partially-molecular
interstellar gas that is readily accessible in absorption at radio/mm/sub-mm
wavelengths. Accurate column densities are derived owing to the weak internal
excitation, so relative molecular abundances are well known with respect to
each other but not with respect to H2. Here we consider the use of proxies for
hydrogen column densities N(H2) and N(H) = N(HI)+2N(H2) based on measurements
of HCO+ absorption and CO emission and absorption, and we compare these with
results obtained by others when observing HI, H2 and CO toward stars and AGN.
We consider the use of HCO+ as a proxy for H2 and show that the assumption of a
relative abundance N(H2) = N(HCO+)/3x10^{-9} gives the same view of the
atomic-molecular hydrogen transition that is seen in UV absorption toward
stars. CO on the other hand shows differences between the radio and optical
regimes because emission is always detected when N(\hcop) > 6x10^{11}\pcc or
N(H2) > 2x10^20\pcc. Wide variations in the integrated CO {J=1-0} brightness
W_CO and N(CO)/N(H2) imply equivalent variations in the CO-H2 conversion factor
even while the ensemble mean is near the usual Galactic values. Gas/reddening
ratios found in absorption toward stars, N(H)/E(B-V) = 6.2x10^21 H \pcc/mag
overall or 6.8x10^21 H \pcc/mag for sightlines at E(B-V) <= 0.08 mag lacking H2
are well below the Galactic mean measured at low reddening and high Galactic
latitude, 8.3x10^21 H \pcc/mag.
