  We analyze far-infrared (10-650 cm$^{-1}$) emissivity spectra of Saturn's
main rings obtained by the Cassini Composite Infrared Spectrometer (CIRS). In
modeling of the spectra, the single scattering albedos of regolith grains are
calculated using the Mie theory, diffraction is removed with the
delta-Eddington approximation, and the hemispherical emissivities of
macroscopic free-floating ring particles are calculated using the Hapke's
isotropic scattering model. Only pure crystalline water ice is considered and
the size distribution of regolith grains is estimated. We find that good fits
are obtained if the size distribution is broad ranging from 1 $\mu$m to 1-10 cm
with a power law index of $ \sim 3$. This means that the largest regolith
grains are comparable to the smallest free-floating particles in size and that
the power law indices for both free-floating particles and regolith grains are
similar to each other. The apparent relative abundance of small grains
increases with decreasing solar phase angle (or increasing mean temperature).
This trend is particularly strong for the C ring and is probably caused by
eclipse cooling in Saturn's shadow, which relatively suppresses warming up of
grains larger than the thermal skin depth ($\sim$ 1 mm) under subsequent solar
illumination.
