  The Abraham--Minkowski momentum controversy is the outwardly visible symptom
of an inconsistency in the use of the energy-momentum tensor in the case of a
plane quasimonochromatic field in a simple linear dielectric. We show that the
Gordon form of the electromagnetic momentum is conserved in a thermodynamically
closed system. We regard conservation of the components of the four-momentum in
a thermodynamically closed system as a fundamental property of the
energy--momentum tensor. Then the first row and column of the energy--momentum
tensor is populated by the electromagnetic energy density and the Gordon
momentum density. We derive new electromagnetic continuity equations for the
electromagnetic energy and momentum that are based on the Gordon momentum
density. These continuity equations can be represented in the energy-momentum
tensor using a material four-divergence operator in which temporal
differentiation is performed with respect to ct/n.
