German physicist, Professor O. Lehmann, seem to
prove that even this means is not infallible. Professor Lehmann has
succeeded, in fact, in obtaining with certain organic compounds--
oleate of potassium, for instance--under certain conditions some
peculiar states to which he has given the name of semi-fluid and
liquid crystals. These singular phenomena can only be observed and
studied by means of a microscope, and the Carlsruhe Professor had to
devise an ingenious apparatus which enabled him to bring the
preparation at the required temperature on to the very plate of the
microscope.

It is thus made evident that these bodies act on polarized light in
the manner of a crystal. Those that M. Lehmann terms semi-liquid still
present traces of polyhedric delimitation, but with the peaks and
angles rounded by surface-tension, while the others tend to a strictly
spherical form. The optical examination of the first-named bodies is
very difficult, because appearances may be produced which are due to
the phenomena of refraction and imitate those of polarization. For the
other kind, which are often as mobile as water, the fact that they
polarize light is absolutely unquestionable.

Unfortunately, all these liquids are turbid, and it may be objected
that they are not homogeneous. This want of homogeneity may, according
to M. Quincke, be due to the existence of particles suspended in a
liquid in contact with another liquid miscible with it and enveloping
it as might a membrane, and the phenomena of polarization would thus
be quite naturally explained.[12]

[Footnote 12: Professor Quincke's last hypothesis is that all liquids
on solidifying pass through a stage intermediate between solid and
liquid, in which they form what he calls "foam-cells," and assume a
viscous structure resembling that of jelly. See _Proc. Roy. Soc. A._,
23rd July 1906.--ED.]

M. Tamman is of opinion that it is more a question of an emulsion,
and, on this hypothesis, the action on light would actually be that
which has been observed. Various experimenters have endeavoured of
recent years to elucidate this question. It cannot be considered
absolutely settled, but these very curious experiments, pursued with
great patience and remarkable ingenuity, allow us to think that there
really exist certain intermediary forms between crystals and liquids
in which bodies still retain a peculiar structure, and consequently
act on light, but nevertheless possess considerab