  Magnetic susceptibility, high-field magnetization and inelastic neutron
scattering experiments are used to study the magnetic properties of a new S=1
quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are
shown to be almost, but not quite, strong enough to destroy the nonmagnetic
singlet ground state and the energy gap in the magnetic excitation spectrum.
Substituting nonmagnetic Mg$^{2+}$ (S=0) ions for Ni$^{2+}$ (S=1) induces a
magnetically ordered state at low temperatures. To our knowledge this is the
first observation of doping-induced long-range order in a Haldane-gap system.
