  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 891). The predicted visual optical depth from the model is
compared with the thermal continuum radiation detected from these galaxies at
850 microns. This comparison yields the emissivity of dust grains in the
submillimeter waveband which is a factor 4 higher than the benchmark,
semi-empirical model of Draine & Lee (1984). 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 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 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.
