  We investigate the propagation of a shock wave into a warm neutral medium and
cold neutral medium by one-dimensional hydrodynamic calculations with detailed
treatment of thermal and chemical processes. Our main result shows that thermal
instability inside the shock-compressed layer produces a geometrically thin,
dense layer in which a large amount of hydrogen molecules is formed. Linear
stability analysis suggests that this thermally-collapsed layer will fragment
into small molecular cloudlets. We expect that frequent compression due to
supernova explosions, stellar winds, spiral density waves, etc., in the galaxy
make the interstellar medium occupied by these small molecular cloudlets.
