  Quintessence, a time-varying energy component that may account for the
accelerated expansion of the universe, can be characterized by its equation of
state and sound speed. In this paper, we show that if the quintessence density
is at least one percent of the critical density at the surface of last
scattering the cosmic microwave background anisotropy can distinguish between
models whose sound speed is near the speed of light versus near zero, which
could be useful in distinguishing competing candidates for dark energy.
