The electric susceptibility of a lattice of partly permanent dipoles is determined as a function of the temperature and molecular parameters. The calculation is based on the spherical model approximation. The susceptibility has a discontinuous slope at a critical temperature Tc. Numerical techniques are developed for the evaluation of Tc which may have application elsewhere in the statistical theory of lattices. The theory predicts a critical temperature of 98°K for solid hydrogen bromide which corresponds to a measured value of 89°K.