I_H2([C ii]), the intensity of [C ii] in the H₂ layer plotted against I(¹³CO) on a log-log scale. There is a trend towards larger I_H2([C ii]) with increasing I(¹³CO). The solid line shows a linear fit to the data, I_H2([C ii]) = 1.08 + 0.49 I(¹³CO), which appears curved when transformed to the log-log plot shown here. The 1-σ noise for I_H2([C ii]) is indicated on three points; those for I(¹³CO) are too small to be visible on this figure and have been omitted. It can be seen on the scale of this figure that the S/N is high for most of points in this scatter plot.