  A simple and {\it innocent} modification of Poisson's equation leads to a
modified Newtonnian theory of gravitation where a localized and {\it positive}
energy density of the gravitational field contributes to its own source. The
result is that the total {\it active gravitational mass} of a compact object is
the sum of its {\it proper mass} and an {\it evanescent gravitational mass}
which is a mass equivalent to the gravitational energy.
