wever, in
reality, to a difference in the value of certain coefficients. 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. The existence of these reactions leads
us to the idea of viscosity or internal friction.

The second kind of reactions are of a different nature. They continue
to act when the deformation remains stationary, and, if the external
forces happen to disappear, they are capable of causing the body to
return to its initial form, provided a certain limit has not been
exceeded. These last constitute rigidity.

At first sight a solid body appears to have a finite rigidity and an
infinite viscosity; a liquid, on the contrary, presents a certain
viscosity, but no rigidity. But if we examine the matter more closely,
beginning either with the solids or with the liquids, we see this
distinction vanish.

Tresca showed long ago that internal friction is not infinite in a
solid; certain bodies can, so to speak, at once flow and be moulded.
M.W. Spring has given many examples of such phenomena. On the other
hand, viscosity in liquids is never non-existent; for were it so for
water, for example, in the celebrated experiment effected by Joule for
the determination of the mechanical equivalent of the caloric, the
liquid borne along by the floats would slide without friction on the
surrounding liquid, and the work done by movement would be the same
whether the floats did or did not plunge into the liquid mass.

In certain cases observed long ago with what are called pasty bodies,
this viscosity attains a value almost comparable to that observed by
M. Spring in some solids. Nor does rigidity allow us to establish a
barrier between the two states. Notwithstanding the ext