  We report Hubble Space Telescope (HST) optical to near-infrared transmission
spectroscopy of the hot Jupiter WASP-6b, measured with the Space Telescope
Imaging Spectrograph (STIS) and Spitzer's InfraRed Array Camera (IRAC). The
resulting spectrum covers the range $0.29-4.5\,\mu$m. We find evidence for
modest stellar activity of WASP-6b and take it into account in the transmission
spectrum. The overall main characteristic of the spectrum is an increasing
radius as a function of decreasing wavelength corresponding to a change of
$\Delta (R_p/R_{\ast})=0.0071$ from 0.33 to $4.5\,\mu$m. The spectrum suggests
an effective extinction cross-section with a power law of index consistent with
Rayleigh scattering, with temperatures of $973\pm144$ K at the planetary
terminator. We compare the transmission spectrum with hot-Jupiter atmospheric
models including condensate-free and aerosol-dominated models incorporating Mie
theory. While none of the clear-atmosphere models is found to be in good
agreement with the data, we find that the complete spectrum can be described by
models that include significant opacity from aerosols including Fe-poor
Mg$_2$SiO$_4$, MgSiO$_3$, KCl and Na$_2$S dust condensates. WASP-6b is the
second planet after HD189733b which has equilibrium temperatures near
$\sim1200$ K and shows prominent atmospheric scattering in the optical.
