  We report on spectra of two positions in the XA region of the Cygnus Loop
supernova remnant obtained with the InfraRed Spectrograph on the Spitzer Space
Telescope. The spectra span the 10-35 micron wavelength range, which contains a
number of collisionally excited forbidden lines. These data are supplemented by
optical spectra obtained at the Whipple Observatory and an archival UV spectrum
from the International Ultraviolet Explorer. Coverage from the UV through the
IR provides tests of shock wave models and tight constraints on model
parameters. Only lines from high ionization species are detected in the
spectrum of a filament on the edge of the remnant. The filament traces a 180
km/s shock that has just begun to cool, and the oxygen to neon abundance ratio
lies in the normal range found for Galactic H II regions. Lines from both high
and low ionization species are detected in the spectrum of the cusp of a
shock-cloud interaction, which lies within the remnant boundary. The spectrum
of the cusp region is matched by a shock of about 150 km/s that has cooled and
begun to recombine. The post-shock region has a swept-up column density of
about 1.3E18 cm^-2, and the gas has reached a temperature of 7000 to 8000 K.
The spectrum of the Cusp indicates that roughly half of the refractory silicon
and iron atoms have been liberated from the grains. Dust emission is not
detected at either position.
