reme mobility
of their particles, liquids contain, in fact, vestiges of the property
which we formerly wished to consider the special characteristic of
solids.

Maxwell before succeeded in rendering the existence of this rigidity
very probable by examining the optical properties of a deformed layer
of liquid. But a Russian physicist, M. Schwedoff, has gone further,
and has been able by direct experiments to show that a sheath of
liquid set between two solid cylinders tends, when one of the
cylinders is subjected to a slight rotation, to return to its original
position, and gives a measurable torsion to a thread upholding the
cylinder. From the knowledge of this torsion the rigidity can be
deduced. In the case of a solution containing 1/2 per cent. of
gelatine, it is found that this rigidity, enormous compared with that
of water, is still, however, one trillion eight hundred and forty
billion times less than that of steel.

This figure, exact within a few billions, proves that the rigidity is
very slight, but exists; and that suffices for a characteristic
distinction to be founded on this property. In a general way, M.
Spring has also established that we meet in solids, in a degree more
or less marked, with the properties of liquids. When they are placed
in suitable conditions of pressure and time, they flow through
orifices, transmit pressure in all directions, diffuse and dissolve
one into the other, and react chemically on each other. They may be
soldered together by compression; by the same means alloys may be
produced; and further, which seems to clearly prove that matter in a
solid state is not deprived of all molecular mobility, it is possible
to realise suitable limited reactions and equilibria between solid
salts, and these equilibria obey the fundamental laws of
thermodynamics.

Thus the definition of a solid cannot be drawn from its mechanical
properties. It cannot be said, after what we have just seen, that
solid bodies retain their form, nor that they have a limited
elasticity, for M. Spring has made known a case where the elasticity
of solids is without any limit.

It was thought that in the case of a different phenomenon--that of
crystallization--we might arrive at a clear distinction, because here
we should he dealing with a specific quality; and that crystallized
bodies would be the true solids, amorphous bodies being at that time
regarded as liquids viscous in the extreme.

But the studies of a