  A particularly interesting line for exploring the physical conditions of the
quiet solar chromosphere is H$\alpha$, but its intensity profile is
magnetically insensitive and the small circular polarization signatures
produced by the longitudinal Zeeman effect come mainly from the underlying
photosphere. Here we show that the Hanle effect in H$\alpha$ provides
quantitative information on the magnetism of the quiet chromosphere. To this
end, we calculate the response function of the emergent scattering polarization
to perturbations in the magnetic field.
