I have attempted to obtain an answer to the question “In which gas (neutral hydrogen) clouds do we find molecules?” By limiting the investigation to those clouds which might be considered normal (i.e. specifically excluding the class of clouds which emit by maser action) it is possible to obtain a definitive answer to the question namely, those clouds which have the highest gas concentration. Further data on the correlation between the distribution of neutral hydrogen and of different molecules can be found.

The clouds which are seen in absorption against background radio sources are most ideally suited for this purpose and I have used the absorption spectra of neutral hydrogen (21 cm wavelength), of the OH radical (18 cm, the 1667 MHz line) and the formaldehyde molecule (6 cm). Only the absorption spectra of the three strongest non-thermal sources (Cas A, Cyg A, and Tau A) were used because only these have unambiguous neutral hydrogen absorption spectra and also because they, unlike the HII regions, do not have 1667 MHz OH spectra which may be confused by anomalous emission. In the absorption spectra of these three sources individual clouds at specific velocities can be recognized on the H, OH and H2CO spectra. We can then investigate the OH and H2CO content of each cloud relative to its neutral hydrogen content.