The recently obtained Piola transformations for second gradient continua are presented.
They prove that edge and surface contact forces in Lagrangian description do not depend only on their Eulerian counterparts but also
on Eulerian contact double forces.
A careful discussion about the limited validity of so-called Cauchy postulate for stress induces some remarks on 
relevant aspects of history of mechanics and in particular on the development of the concepts of force, stress and couples.