Context :

Aims : The aims are to investigate the clustering of the far-infrared sources from the Herschel infrared Galactic Plane Survey ( Hi-GAL ) in the Galactic longitude range of -71 to 67 deg .
These clumps , and their spatial distribution , are an imprint of the original conditions within a molecular cloud .
This will produce a catalogue of over-densities .

Methods : The minimum spanning tree ( MST ) method was used to identify the over-densities in two dimensions .
The catalogue was further refined by folding in heliocentric distances , resulting in more reliable over-densities , which are cluster candidates .

Results : We found 1,633 over-densities with more than ten members .
Of these , 496 are defined as cluster candidates because of the reliability of the distances , with a further 1,137 potential cluster candidates .
The spatial distributions of the cluster candidates are different in the first and fourth quadrants , with all clusters following the spiral structure of the Milky Way .
The cluster candidates are fractal .
The clump mass functions of the clustered and isolated are statistically indistinguishable from each other and are consistent with Kroupa ’ s initial mass function .

Conclusions :