  Historically, the thermodynamic behavior of gasses was described first and
the derived equations were adapted to solids. It is suggested that the current
thermodynamic description of solid phase is still incomplete because the
isothermal work done on or by the system is not counted in the internal energy.
It is also suggested that the isobaric work should not be deducted from the
internal energy because the system does not do work when it expands. Further
more it is suggested that Joule postulate regarding the mechanical equivalency
of heat -the first law of thermodynamics- is not universal and not applicable
to elastic solids. The equations for the proposed thermodynamic description of
solids are derived and tested by calculating the internal energies of the
system using the equation of state of MgO. The agreement with theory is good.
