Molecular column densities (top) and column density ratios (bottom) in foreground clouds along the sight-lines to W49N (blue), W31C (red), G34.3+0.1 (magenta) and G29.96–0.02 (black). The error bars indicate 1σ statistical errors only. Dashed curves indicate the regions spanned by the range of column density ratios measured in this set of foreground clouds, with the dotted curve plotted midway between the dashed curves. The green point applies to the 62−75 km s^-1 velocity range in W51, which probably lies in close proximity to the background continuum source.