  We calculate the longitudinal response of 208Pb using a relativistic
random-phase approximation to three different parameterizations of the Walecka
model with scalar self-interactions. From a nonspectral calculation of the
response-that automatically includes the mixing between positive- and
negative-energy states-we extract the distribution of strength for the
isoscalar monopole, dipole, and high-energy octupole resonances. We employ a
consistent formalism that uses the same interaction in the calculation of the
ground state as in the calculation of the response. As a result, the
conservation of the vector current is strictly maintained throughout the
calculation. Further, at small momentum transfers the spurious dipole
strength-associated with the uniform translation of the center-of-mass-gets
shifted to zero excitation energy and is cleanly separated from the sole
remaining physical fragment located at an excitation energy of about 24 MeV; no
additional dipole strength is observed. The best description of the collective
modes is obtained using a ``soft'' parameterization having a compression
modulus of K=224 MeV.
