The SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) s
atellite has provided an unprecedented view of the mid-infrared emission (8
-25 mu m) of the Galactic plane. An initial analysis of images from MSX Gal
actic plane survey data reveals dark clouds seen in silhouette against the
bright emission from the Galactic plane (Egan et al.). These clouds have mi
d-infrared extinctions in excess of 2 mag at 8 mu m. We probed the physical
properties of 10 of these MSX dark clouds using millimeter-wave molecular
rotational lines as an indicator of dense molecular gas. All 10 clouds were
detected in millimeter spectral lines of H2CO, which confirms the presence
of dense gas. The distances to these clouds range from 1 to 8 kiloparsecs
and their diameters from 0.4 to 15.0 pc. Excitation analysis of the observe
d lines indicates that the clouds are cold (T < 20 K) and dense [n(H-2) > 1
0(5) cm(-3)]. Some of the clouds have nearby H II regions, H2O masers, and
other tracers of star formation at comparable spectral line velocities; how
ever, only one cloud contains embedded centimeter or infrared sources. The
lack of mid- to far-infrared emission associated with these clouds suggests
that they are not currently forming high-mass stars. If star formation is
present in these clouds, it is clearly protostellar class 0 or earlier.