  We attempt to determine the nature of the bubbles observed by Spitzer in the
Galactic plane, mainly to establish if possible their association with massive
stars. We take advantage of the very simple morphology of these objects to
search for star formation triggered by HII regions, and to estimate the
importance of this mode of star formation. We consider a sample of 102 bubbles
detected by Spitzer-GLIMPSE, and catalogued by Churchwell et al.(2006). We use
mid-infrared and radio-continuum public data to discuss their nature. We use
the ATLASGAL survey at 870 micron to search for dense neutral material
collected on their borders. Results: We find that 86% of the bubbles contain
ionized gas detected by means of its radio-continuum emission at 20-cm. Thus,
most of the bubbles observed at 8.0 micron enclose HII regions ionized by O-B2
stars. Ninety-eight percent of the bubbles exhibit 24 micron emission in their
central regions. The ionized regions at the center of the 8.0 micron bubbles
seem to be devoid of PAHs but contain hot dust. Among the 65 regions for which
the angular resolution of the observations is high enough to resolve the
spatial distribution of cold dust at 870 micron, we find that 40% are
surrounded by cold dust, and that another 28% contain interacting
condensations. The former are good candidates for the collect and collapse
process, as they display an accumulation of dense material at their borders.
The latter are good candidates for the compression of pre-existing
condensations by the ionized gas. Eighteen bubbles exhibit associated
ultracompact HII regions and/or methanol masers in the direction of dust
condensations adjacent to their ionization fronts. Our results suggest that
more than a quarter of the bubbles may have triggered the formation of massive
objects.
