We propose to observe the C+ 158-micron line emission in order
 to trace the phase transitions of C in the translucent cloud
 G236+39 (spring launch) or the edges of the dark cloud L183
 (autumn launch). G236+39 has been shown to contain the phase
 transitions from H/H2 and C+/CO, and we will propose to observe
 the CI 609-micron line for comparison to the ISO observations.
 L183 has already be observed in CI line. Both G236+39 and L183
 have been observed in CO and its 13CO isotopomer.  The C+
 observations will be combined with CI observations in order to
 distinguish the C+/CI and CI/CO transitions, which must lie
 between the cloud edge and the CO cores. These phase transitions
 are a key both to the cloud thermodynamics (governed by the
 cooling through the C+ line) and the cloud chemistry of
 C-bearing species (governed by the abundance of C+).