  Physical dust models are presented for 65 galaxies in the SINGS survey that
are strongly detected in the four IRAC bands and three MIPS bands. For each
galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass
contributed by PAHs, and (3) the intensity of the starlight heating the dust
grains. We find that spiral galaxies have dust properties resembling the dust
in the local region of the Milky Way, with similar dust-to-gas ratio, and
similar PAH abundance. The observed SEDs, including galaxies with SCUBA
photometry, can be reproduced by dust models that do not require "cold" (T<10K)
dust.
  The dust-to-gas ratio is observed to be dependent on metallicity. In the
interstellar media of galaxies with A_O=12+log(O/H)>8.1, grains contain a
substantial fraction of interstellar Mg, Si and Fe. Galaxies with A_O<8.1 and
extended HI envelopes in some cases appear to have global dust-to-gas ratios
that are low for their measured oxygen abundance, but in the regions where
infrared emission is detected, the dust-to-gas ratio generally appears to be
consistent with a substantial fraction of interstellar Mg, Si, and Fe being
contained in dust. The PAH index q_PAH -- the fraction of the dust mass in the
form of PAHs -- correlates with metallicity. The nine galaxies in our sample
with A_O<8.1 have a median q_PAH=1.0%, whereas galaxies with A_O>8.1 have a
median q_PAH=3.55%. The derived dust masses favor a value X_CO approx 4e20
cm^{-2}(K kms)^{-1} for the CO to H_2 conversion factor. Except for some
starbursting systems (Mrk33, Tolo89, NGC3049), dust in the diffuse ISM
dominates the IR power.
