  Given an indefinite binary quaternionic Hermitian form $f$ with coefficients
in a maximal order of a definite quaternion algebra over $\mathbb Q$, we give a
precise asymptotic equivalent to the number of nonequivalent representations,
satisfying some congruence properties, of the rational integers with absolute
value at most $s$ by $f$, as $s$ tends to $+\infty$. We compute the volumes of
hyperbolic 5-manifolds constructed by quaternions using Eisenstein series. In
the Appendix, V. Emery computes these volumes using Prasad's general formula.
We use hyperbolic geometry in dimension 5 to describe the reduction theory of
both definite and indefinite binary quaternionic Hermitian forms.
