The turbid liquids which were thus obtained were found to possess not only the usual properties of liquids (such as the {52} property of flowing and of assuming a perfectly spherical shape when suspended in a liquid of the same density), but also those properties which had hitherto been observed only in the case of solid crystalline substances, viz. the property of double refraction and of giving interference colours when examined by polarized light; the turbid liquids are _anisotropic_. To such liquids, the optical properties of which were discovered by O. Lehmann,[90] the name _liquid crystals_, or crystalline liquids, was given. Nature of Liquid Crystals.--During the past ten years the question as to the nature of liquid crystals has been discussed by a number of investigators, several of whom have contended strongly against the idea of the term "liquid" being applied to the crystalline condition; and various attempts have been made to prove that the turbid liquids are in reality heterogeneous and are to be classed along with emulsions.[91] This view was no doubt largely suggested by the fact that the anisotropic liquids were turbid, whereas the "solid" crystals were clear. Lehmann found, however, that, when examined under the microscope, the "simple" liquid crystals were also clear,[92] the apparent turbidity being due to the aggregation of a number of differently oriented crystals, in the same way as a piece of marble does not appear transparent although composed of transparent crystals.[93] Further, no proof of the heterogeneity of liquid crystals has yet been obtained, but rather all chemical and physical investigations indicate that they are homogeneous.[94] No separation {53} of a solid substance from the milky, anisotropic liquids has been effected; the anisotropic liquid is in some cases less viscous than the isotropic liquid formed at a higher temperature; and the temperature of liquefaction is constant, and is affected by pressure and admixture with foreign substances exactly as in the case of a pure substance.[95] [Illustration: FIG. 12.] Equilibrium Relations in the Case of Liquid Crystals.--Since, now, we have seen that we are dealing here with substances in two crystalline forms (which we may call the solid and liquid[96] crystalline form), which possess a definite transition point, at which, transformation of the one form into the other occurs in both directions, we can represent the conditions of equilibrium by a diagram in all respects similar to that employed in the case of other enantiotropic substances, _e.g._ sulphur (p. 35). {54} In Fig. 12 there is given a diagrammatic representation of the relationships found in the case of _p_-azoxyanisole.[97]