  A macroscopic theory for the dynamics of elastic, isotropic matter in
presence of electromagnetic fields is proposed here. We avail of Gordon's
general relativistic derivation of Abraham's electromagnetic energy tensor as
starting point. The necessary description of the elastic and of the inertial
behaviour of matter is provided through a four dimensional generalisation of
Hooke's law, made possible by the introduction of a four dimensional
``displacement'' vector. As intimated by Nordstroem, the physical origin of
electrostriction and of magnetostriction is attributed to the change in the
constitutive equation of electromagnetism caused by the deformation of matter.
The part of the electromagnetic Lagrangian that depends on that deformation is
given explicitly for the case of an isotropic medium and the resulting
expression of the electrostrictive force is derived, thus showing how more
realistic equations of motion for matter subjected to electromagnetic fields
can be constructed.
