  We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate
(\QTR{group}{Cu(NO$_{3}$)$_{2}\cdot 2.5$D$_{2}$O}). Using inelastic neutron
scattering, we have studied the one- and two-magnon excitation spectra in a
large single crystal of this material. We compare this new data with
perturbative expansions of the alternating Heisenberg chain and find good
agreement with these calculations. The data may also show evidence for the
recently proposed S=1 two-magnon bound state (Phys. Rev. B54, R9624 (1996)).
