  We present further observational evidence in support of our earlier proposal
(Houde et al. 2000) for detecting the presence of the magnetic field in
molecular clouds by comparing spectra of molecular ions with those of neutral
molecules. The ion lines tend to be narrower and do not show the wings due to
flows, when the magnetic field is sufficiently strong. We obtained spectra for
the optically thin lines of the H13CN and H13CO+ species in a sample of ten
molecular clouds and found the results to be in agreement with our previous
observations of the main isotopic species, HCN and HCO+, made in OMC-1, OMC-2,
OMC-3 and DR21OH, thus eliminating the possibility of optical depth effects
playing a role in the ion line narrowing. HCS+ was also detected in four of
these star forming regions. We also discuss previously published results by
(Benson et al. 1998) of N2H+ detections in a large sample of dark clouds. We
show that the similarity in line widths between ion and neutral species in
their sample is consistent with the relatively small amount of turbulence and
other flows observed in these clouds.
