  We report the detection of the 1560 A and 1657 A ground-state C I absorption
features in the z(abs)=1.6748 system toward the QSO PKS 1756+237. We find no
associated C I* lines with a resulting 3-sigma excitation temperature
upper-limit of T(ex) <= 8.54 (+0.65,-0.56) K, which is consistent with the
predicted CMBR temperature of T(CMBR)=7.291 K. Because the redshifted CMBR
populates the J=1 level and leaves little room for additional local excitation
through either collisions or UV pumping, our data place 2-3 times more
stringent limits on particle densities and UV fields than existing Copernicus
observations of similar column density sightlines in the Milky Way. We also
detect several Ni II lines and the weak Fe II 1611 A line. From the Ni/Fe
column density ratio we find evidence for dust at a dust-to-metals ratio
similar to that seen toward warm Galactic disk clouds. Based on these findings
and supported by our Ly-alpha spectrum we propose to reclassify this system as
a damped Ly-alpha absorber.
