HCO⁺ emission plotted against absorption. Left: channel values of emission brightness and absorption for the new data shown in Figure 6. The excitation temperature corresponding to the regression line is 2.758 K. Right: integrated emission brightness against integrated absorption ϒ_HCO⁺ for the new and old data. Superposed at the right are the locii of constant number density n(H) for diffuse purely molecular gas using results for the weak excitation limit (including electrons) described by Liszt & Pety (2016) (see Section 5.1 for an explanation of the underlying physics).