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We study four lines of sight that probe the transition from diffuse moleculargas to molecular cloud material in Taurus. Measurements of atomic and molecularabsorption are used to infer the distribution of species and the physicalconditions toward stars behind the Taurus Molecular Cloud (TMC). Newhigh-resolution spectra at visible and near infrared wavelengths ofinterstellar Ca II, Ca I, K I, CH, CH^+, C2, CN, and CO toward HD28975 andHD29647 are combined with data at visible wavelengths and published CO resultsfrom ultraviolet measurements for HD27778 and HD30122. Gas densities andtemperatures are inferred from C2, CN, and CO excitation and CN chemistry. Ourresults for HD29647 are noteworthy because the CO column density is 10^{18}cm^{-2} while C2 and CO excitation reveals a temperature of 10 K and densityabout 1000 cm^{-3}, more like conditions found in dark molecular clouds.Similar results arise from our chemical analysis for CN through reactionsinvolving observations of CH, C2, and NH. Enhanced potassium depletion and areduced CH/H2 column density ratio also suggest the presence of a dark cloud.The directions toward HD27778 and HD30122 probe molecule-rich diffuse clouds,which can be considered CO-dark gas, while the sight line toward HD28975represents an intermediate case. Maps of dust temperature help refine thedescription of the material along the four sight lines and provide an estimateof the distance between HD29647 and a clump in the TMC. An Appendix providesresults for the direction toward HD26571; this star also probes diffusemolecular gas.

The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus