  We use the radiation transfer simulation of Xilouris et al. (1999) to
constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC
5907 and NGC 4565). The predicted visual optical depth from the model is
compared with the thermal continuum radiation detected from NGC 4013 and NGC
5907 at 850 microns and from NGC 4565 at 1.2 mm. The former is based on SCUBA
images of NGC 4013 and NGC 5907, reduced and presented for the first time in
this work. The comparison of visual optical depth and 850 microns (1.2 mm)
emission yields the emissivity of dust grains in the submillimeter (millimeter)
waveband. We infer a value of 1.2x10^{4} for the emissivity at 850 microns
which is a factor 4 higher than the benchmark, semi-empirical model of Draine &
Lee (1984). At 1.2 mm our values are a factor 1.5 higher than this model. Our
estimates are more closely aligned with recent measurements carried out in the
laboratory on amorphous carbon and silicate particulates. A comparison between
the distribution of 850 microns (1.2 mm) surface brightness and the intensity
levels in the ^{12}CO(1-0) and 21 cm lines underlines the spatial association
between dust detected in the submillimeter/millimeter waveband and molecular
gas clouds. We suggest that the relatively high emissivity values that we
derive may be attributable to amorphous, fluffy grains situated in denser gas
environments.
