We propose to use ISO-SWS & LWS in their Fabry-Perot modes to observe
the H2 and HD pure rotational lines at 28.2 and 112.1 um due to cold
(ground-state) molecular hydrogen in the plane of our Galaxy, as seen in
absorption against strong IRAS sources. These transitions must be
observed from space, and ISO is the only current or future mission with
adequate capabilities. The combination of choice background-sources with
an optimum observing strategy will yield the most sensitive search ISO
is capable of. The resulting data will significantly constrain the
relation between the cold molecular component and the dark matter
required by dynamical studies of spiral and irregular galaxies.