It is impossible to discover by this means any absolute characteristic which establishes a separation between the two classes.

Modern researches prove this clearly.

It is not without use, in order to well understand them, to state precisely the meaning of a few terms generally rather loosely employed.
If a conjunction of forces acting on a homogeneous material mass happens to deform it without compressing or dilating it, two very distinct kinds of reactions may appear which oppose themselves to the effort exercised.

During the time of deformation, and during that time only, the first make their influence felt.

They depend essentially on the greater or less rapidity of the deformation, they cease with the movement, and could not, in any case, bring the body back to its pristine state of equilibrium.