  Given the present status of the problem of the electromagnetic energy tensor
in matter, there is perhaps use in recalling a forgotten argument given in 1923
by W. Gordon. Let us consider a material medium which is homogeneous and
isotropic when viewed in its rest frame. For such a medium, Gordon's argument
allows to reduce the above mentioned problem to an analogous one, defined in a
general relativistic vacuum, in presence of a suitably determined effective
metric. For the latter problem the form of the Lagrangian is known already,
hence the determination of the energy tensor is a straightforward matter. One
just performs the Hamiltonian derivative of the Lagrangian chosen in this way
with respect to the true metric. Abraham's tensor is thus selected as the
electromagnetic energy tensor for a medium which is homogeneous and isotropic
in its rest frame.
