  We have used data taken as part of the Herschel infrared Galactic Plane
survey (Hi-GAL) to study 3171 infrared-dark cloud (IRDC) candidates that were
identified in the mid-infrared (8 {\mu}m) by Spitzer (we refer to these as
'Spitzer-dark' regions). They all lie in the range l=300 - 330 \circ and |b| 6
1 \circ. Of these, only 1205 were seen in emission in the far-infrared (250-500
{\mu}m) by Herschel (we call these 'Herschel-bright' clouds). It is predicted
that a dense cloud will not only be seen in absorption in the mid-infrared, but
will also be seen in emission in the far-infrared at the longest Herschel
wavebands (250-500 {\mu}m). If a region is dark at all wavelengths throughout
the mid-infrared and far-infrared, then it is most likely to be simply a region
of lower background infrared emission (a 'hole in the sky'). Hence, it appears
that previous surveys, based on Spitzer and other mid-infrared data alone, may
have over-estimated the total IRDC population by a factor of 2. This has
implications for estimates of the star formation rate in IRDCs in the Galaxy.We
studied the 1205 Herschel-bright IRDCs at 250 {\mu}m, and found that 972 of
them had at least one clearly defined 250-{\mu}m peak, indicating that they
contained one or more dense cores. Of these, 653 (67 per cent) contained an
8-{\mu}m point source somewhere within the cloud, 149 (15 per cent) contained a
24-{\mu}m point source but no 8-{\mu}m source, and 170 (18 per cent) contained
no 24-{\mu}m or 8-{\mu}m point sources. We use these statistics to make
inferences about the lifetimes of the various evolutionary stages of IRDCs.
