  We have studied the FIR/{\it mm} spectrum of IR galaxies by combining IRAS
photometry with new {\it mm} data on a complete southern IRAS galaxy sample.
The observed spectra and a dust model emphasize a dicothomy in the galaxy
population: half of the objects with a lot of warm dust are characterized by
higher values of the bolometric (UV-FIR) luminosity, of the dust-to-gas mass
ratio, of the dust optical depths and extinction, while those dominated by cold
({\it cirrus}) dust show opposite trends. From these data we derive the {\it
mm} luminosity function of galaxies and estimate their contribution to the
sub-{\it mm} background (BKG).
