  The physical property of mass has two distinct aspects, gravitational mass
and inertial mass. The weight of a particle depends on its gravitational mass.
According to the weak form of the equivalence principle, the gravitational and
inertial masses are equivalent. But, we show here that they are correlated by a
dimensionless factor, which can be different of one . The factor depends on the
electromagnetic energy absorbed or emitted by the particle and of the index of
refraction of the medium around the particle. This theoretical correlation has
been experimentally proven by means of a very simple apparatus, presented here.
