r instance, he finds that
ordinary ice transforms itself, under a given pressure, at the
temperature of -80 deg. C. into another crystalline variety which is
denser than water.

The statics of solids under high pressure is as yet, therefore, hardly
drafted, but it seems to promise results which will not be identical
with those obtained for the statics of fluids, though it will present
at least an equal interest.

Sec. 4. THE DEFORMATIONS OF SOLIDS

If the mechanical properties of the bodies intermediate between solids
and liquids have only lately been the object of systematic studies,
admittedly solid substances have been studied for a long time. Yet,
notwithstanding the abundance of researches published on elasticity by
theorists and experimenters, numerous questions with regard to them
still remain in suspense.

We only propose to briefly indicate here a few problems recently
examined, without going into the details of questions which belong
more to the domain of mechanics than to that of pure physics.

The deformations produced in solid bodies by increasing efforts
arrange themselves in two distinct periods. If the efforts are weak,
the deformations produced are also very weak and disappear when the
effort ceases. They are then termed elastic. If the efforts exceed a
certain value, a part only of these deformations disappear, and a part
are permanent.

The purity of the note emitted by a sound has been often invoked as a
proof of the perfect isochronism of the oscillation, and,
consequently, as a demonstration _a posteriori_ of the correctness of
the early law of Hoocke governing elastic deformations. 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 explain