The skill shown by M.Bouasse authorises us to hope that, thanks to his researches, a strong light will some day illumine these unknown corners.
A particularly interesting chapter on elasticity is that relating to the study of crystals; and in the last few years it has been the object of remarkable researches on the part of M.Voigt.

These researches have permitted a few controversial questions between theorists and experimenters to be solved: in particular, M.Voigt has verified the consequences of the calculations, taking care not to make, like Cauchy and Poisson, the hypothesis of central forces a mere function of distance, and has recognized a potential which depends on the relative orientation of the molecules.

These considerations also apply to quasi-isotropic bodies which are, in fact, networks of crystals.
Certain occasional deformations which are produced and disappear slowly may be considered as intermediate between elastic and permanent deformations.

Of these, the thermal deformation of glass which manifests itself by the displacement of the zero of a thermometer is an example.

So also the modifications which the phenomena of magnetic hysteresis or the variations of resistivity have just demonstrated.
Many theorists have taken in hand these difficult questions.