  For the traditional low T_c superconductors, the superconducting condensation
energy is proportional to the change in energy of the ionic lattice between the
normal and superconducting state, providing a clear link between pairing and
the electron-ion interaction. Here, for the t-J model, we discuss an analogous
relationship between the superconducting condensation energy and the change in
the exchange energy between the normal and superconducting states. We point out
the possibility of measuring this using neutron scattering and note that such a
measurement, while certainly difficult, could provide important evidence for an
exchange interaction-based pairing mechanism.
