Several of the full LWS spectra of IR bright galaxies observed to
date are dominated by clear absorption and possible faint emission in a
number of molecular species including OH, HD, and CH (or HeH+).  These
spectra are indicative of very high column densities/extinction, as is
also found by the SWS Instrument team (eg.  Lutz et al. 1996, A&A ISO
Issue).  This proposal is one of several follow-up proposals designed to
verify and study the cloud conditions, chemistry, and column densities
in a sample of bright galaxies.
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Here we request time for deep LWS grating observations of the
HD 112 um (J = 1-0), HD 56 um (J = 2-1) (for two galaxies), and
CH 149 um lines in a small sample of bright galaxies in which we believe
they are bright enough to be detected at the level seen in M82 and for which
LWS and SWS spectra have been obtained.  Estimation of the molecular column
densities toward these heavily obscurred galactic nuclei can be used to
infer the column density of H_2 with minimum dependence on the gas
metallicity, using the current estimates of the abundance of D/H.