  The existing field theories are based on the properties of closed exterior
forms, which correspond to conservation laws for physical fields.
  In the present paper it is shown that closed exterior forms corresponding to
field theories are obtained from the equations modelling conservation (balance)
laws for material sistems (material media).
  The process of obtaining closed exterior forms demonstrates the connection
between field-theory equations and the equations for material sistems and
points to the fact that the foundations of field theories must be conditioned
by the properties of equations conservation laws for material sistems.
