  Variations in the dust emissivity are critical for gas mass determinations
derived from far-infrared observations, but also for separating dust foreground
emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow
us for the first time to study the dust emissivity variations in the inner
regions of the Galactic plane at resolution below 1 degree. We present maps of
the emissivity spectral index derived from the combined Herschel PACS 160 \mu
m, SPIRE 250 \mu m, 350 \mu m, and 500 \mu m data, and the IRIS 100 \mu m data,
and we analyze the spatial variations of the spectral index as a function of
dust temperature and wavelength in the two Science Demonstration Phase Hi-GAL
fields, centered at l=30{\deg} and l=59{\deg}. Applying two different methods,
we determine both dust temperature and emissivity spectral index between 100
and 500 \mu m, at an angular resolution of 4'. Combining both fields, the
results show variations of the emissivity spectral index in the range 1.8-2.6
for temperatures between 14 and 23 K. The median values of the spectral index
are similar in both fields, i.e. 2.3 in the range 100-500 \mu m, while the
median dust temperatures are equal to 19.1 K and 16.0 K in the l=30{\deg} and
l=59{\deg} field, respectively. Statistically, we do not see any significant
deviations in the spectra from a power law emissivity between 100 and 500 \mu
m. We confirm the existence of an inverse correlation between the emissivity
spectral index and dust temperature, found in previous analyses.
