We have made an extensive mapping of the 13CO 13CO J=1-0 transition line in the dark cloud L1535. We also constructed the 100μm 100μm IRAS map in the region. We found a semi-detached cloud component of 13CO 13CO in the northeast direction of the 13CO 13CO main cloud which forms a dumbbell-like structure. This additional component with an angular size of 20′×16′ 20′×16′ has not been observed before in any molecular surveys of the cloud. The IRAS map shows a similar structure with two intensity peaks whose positions coincide with those of the 13CO 13CO clouds.

The 4.8GHz formaldehyde absorption line in the dark clouds in M17 and NGC 2024 regions has been mapped. In both nebulae, we detected two H2CO line components. In M17, the 24km S-1 cloud is closely associated with the HII region located in front of the radio continuum source, and the 19km S-1 cloud is associated with the visual dark clouds with a larger extent which are closer to us. The 19km S-1 cloud has a mass motion approaching to the HII region. In both clouds, a velocity gradient from the north-east to the south-west directions is observed. The linewidth has no variation indicating no collapsing motion. In NGC 2024, the 9km S-1 feature is extended along the dark bar in front of the bright nebula and a weak second component at 13km S-1 is confined to the immediate vicinity of the radio source. Indications are that the 9km S-1 cloud is physically associated with the dark bar and the 13km S-1 cloud is located behind the radio source. The angular extent, the column density, and the total mass of the clouds are derived. The radial velocities of other molecular lines observed in these clouds are compared.

The abundances of simple molecules are examined in terms of the time-dependent cloud evolution. The formation and destruction mechanisms of H 2 C O are reviewed. The average value of the fractional abundance of H 2 C O is derived to be in the range of 10 − 10 t o 5 × 10 − 9 . This is comparable to the observed values. The expected variations of the molecules formed from or destroyed by CO, CI, and C + whose abundances depend on the evolutionary state of the cloud are discussed.