Neutral atomic carbon column density related to H I and H₂. Left: N(C I) vs. N(H I); Right: N(C I) vs. N(H₂). Superposed are results for models that include grain-assisted atomic-ion neutralization, at two densities n(H) = 32   cm^-3   and   256   cm^-3, at two values of the scaling factor for the interstellar radiation field, G₀ = 1.0 (solid curves) and 1.7 (dashed (orange) curves). Sources of C I measurements are labelled as; WTHK08: Wolfire et al. (2008); JT2001: Jenkins & Tripp (2001); JS1979: Jenkins & Shaya (1979); BFJ2010: Burgh et al. (2010). Sightlines having two values of N(C I) (see Table 1) are enclosed by dotted rectangles at both values.