This law has, however, during some years been frequently disputed.

Certain mechanicians or physicists freely admit it to be incorrect, especially as regards extremely weak deformations.

According to a theory in some favour, especially in Germany, i.e.the theory of Bach, the law which connects the elastic deformations with the efforts would be an exponential one.

Recent experiments by Professors Kohlrausch and Gruncisen, executed under varied and precise conditions on brass, cast iron, slate, and wrought iron, do not appear to confirm Bach's law.
Nothing, in point of fact, authorises the rejection of the law of Hoocke, which presents itself as the most natural and most simple approximation to reality.
The phenomena of permanent deformation are very complex, and it certainly seems that they cannot be explained by the older theories which insisted that the molecules only acted along the straight line which joined their centres.

It becomes necessary, then, to construct more complete hypotheses, as the MM.