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.