  Using data obtained by the DIRBE and FIRAS instruments on the COBE spacecraft
we present the mean 3.5-1000 um dust spectrum from the high latitude cirrus
clouds. These data represent the most comprehensive wavelength coverage of dust
emission, and first strong evidence for the presence of PAHs in cirrus. The
COBE data are well fit with a dust model consisting of a mixture of PAH
molecules and bare astronomical silicate and graphite grains. From the model we
derive the size distribution, abundances relative to the total hydrogen column
density, and relative contribution of each dust component to the observed IR
emission. The model provides a good fit to the FIRAS spectrum in the 140-500 um
wavelength regime, but leaves an excess Galactic emission component at 500-1000
um. The nature of this component is still unresolved. The model requires about
85% of the solar carbon abundance to be locked up in dust, a third of that in
PAHs. The remaining ~ 15% is in the gas as inferred from the [C II] 158 um line
emission detected by the FIRAS. PAHs emit about 25% of the total IR power, and
must therefore be responsible for the same fraction of the total UV-optical
extinction.
