nominal source position. Our goal was to get good signal-to-noise ratio in the outer parts of the core in order to estimate the H$_{2}$ column density, $N_{H_{2}}$, as far from the core centre as possible (typically $\sim$ 200$\arcsec$). The total (ON+OFF) integration time was 2 minutes on average and up to 10 or 25 minutes in the outer parts of the cores in order to reach an rms sensitivity better than $\sim$ 0.1 K km s$^{-1}$. The spectral resolution was 20 kHz and the observing frequency 109 GHz. Additional N$_{2}$H$^{+}$(1-0), H$_{2}$CO(2-1), and DCO$^{+}$(2-1) spectra were taken at a few positions for selected cores.

During these continuum and line runs at the 30 m telescope, the pointing, as well as the focus, was checked every $\sim$ 1 hour and found to be better than $\sim 5\arcsec$ in both azimuth and elevation.

Results and analysis {#images}
====================

ISOCAM images {#isoimages}
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Out of the 24 dark cloud fields in our programme, 23 show absorption-like features in the mid-IR.

The fields presenting such features can be divided into two broad categories, those that show deep ($\simgt$ 10% - cf. column 9 of Table \[sources\]), compact absorption features likely associated with dense cores, and 