We characterize the functionals which are Mosco-limits, in the

topology, of some
sequence of functionals of the kind
where

is a bounded domain of

(

). It is known that this family of functionals is included in the
closed set of Dirichlet forms. Here, we prove that the set of Dirichlet forms is actually the
closure of the set of diffusion functionals. A crucial step
is the explicit construction of a composite material whose
effective energy contains a very simple non-local interaction.