  Despite suggestions to the contrary, we show in this paper that the usual
dispersive form of the electromagnetic energy must be used to derive the
Lifshitz force between parallel dielectric media. This conclusion follows from
the general form of the quantum vacuum energy, which is the basis of the
multiple-scattering formalism. As an illustration, we explicitly derive the
Lifshitz formula for the interaction between parallel dielectric semispaces,
including dispersion, starting from the expression for the total energy of the
system. The issues of constancy of the energy between parallel plates and of
the observability of electrostrictive forces are briefly addressed.
