  We study four lines of sight that probe the transition from diffuse molecular
gas to molecular cloud material in Taurus. Measurements of atomic and molecular
absorption are used to infer the distribution of species and the physical
conditions toward stars behind the Taurus Molecular Cloud (TMC). New
high-resolution spectra at visible and near infrared wavelengths of
interstellar Ca II, Ca I, K I, CH, CH^+, C2, CN, and CO toward HD28975 and
HD29647 are combined with data at visible wavelengths and published CO results
from ultraviolet measurements for HD27778 and HD30122. Gas densities and
temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our
results 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 density
about 1000 cm^{-3}, more like conditions found in dark molecular clouds.
Similar results arise from our chemical analysis for CN through reactions
involving observations of CH, C2, and NH. Enhanced potassium depletion and a
reduced 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 HD28975
represents an intermediate case. Maps of dust temperature help refine the
description of the material along the four sight lines and provide an estimate
of the distance between HD29647 and a clump in the TMC. An Appendix provides
results for the direction toward HD26571; this star also probes diffuse
molecular gas.
