Estimated emission measures of gas emitting in the [C II] λ158 μm (FIR) line and in the C IIλ1335 (FUV) triplet in our sightline. The latter curve is identical to the curve for C II in Figs. 8 and 13, and the FIR curve is derived as described in Sect. 8.4, assuming nearly all the C in the filament is in the C⁺ state, and that collisions with atomic Hydrogen excite the [C II] λ158 μm transition. The red point shows our fiducial choice of temperature for the cold clumps, T = 1000 K, although we expect any value from 100 ≲ T ≲ 5000 K is possible. We have also drawn lines of constant filament volume filling factor f_c, as in Fig. 8 and 13, using Eq. (5). We note that the FIR-emitting gas must have a higher f_c than the FUV-emitting gas assuming the latter is at its peak emissivity at T ≈ 4 × 10⁴ K. This result holds for both choices of pressure, indicated by the upper and lower panels. This supports our model of cold FIR-emitting clumps in the filament surrounded by thin conductive boundary layers which emit the FUV lines.