FIG. 107.—Ammonium Oleate under considerable pressure. ] [Illustration: FIG. 108.—Diagrammatic Representation of Arrangement of Molecules. ] Lehmann believes the explanation of these singular phenomena to be that the “liquid crystals” of ammonium oleate are composed of piles or layers of thin plates perpendicular to the optic axis. Disturbance detaches the plates from their piled positions over one another, and sets them parallel to the glass plate, except in places, the oily strips, where the plates stand upright, perpendicularly to the micro-slip and cover-glass. Lehmann, indeed, goes further, and asserts that the molecules themselves are anisotropic, and probably take the form of plates. [Illustration: FIG. 109.—A Crystal of Ammonium Oleate A, broken at B, each part repairing itself at C, perfect again at D. ] An extremely interesting experimental observation of Lehmann’s with the bipyramids of ammonium oleate is, that if one of them, for instance A in Fig. 109, be broken into two parts, as at B, each part grows again and completely repairs itself, becoming once more a perfect double pyramid, as indicated in stages at C and D in the figure. [Illustration: FIG. 110. ] [Illustration: FIG. 111. Cruciform, Boomerang, and Arrow-head Twins of Ammonium Oleate. ] [Illustration: _PLATE XXII._ FIG. 104. Dichroic Crystal Drops of Para-azoxy-anisol. ] [Illustration: FIG. 112. Rectilinear Liquid Prisms of Para-azoxy-benzoic Acid. ] [Illustration: FIG. 115.—Tetragonal Astatic Magnet-system. ] [Illustration: FIG. 116.—Cubic Astatic Magnet-system. ] Twins of ammonium oleate are also shown in Figs. 110 and 111, the former figure representing a twin of cruciform character, and the latter exhibiting twins resembling a boomerang and an arrowhead respectively. This substance, ammonium oleate, thus appears to be one of the most remarkable and interesting of all the bodies yet observed to afford liquid crystals. Many other oleates produce liquid crystals also, but the ammonium salt is by far the most striking, and very convincing as to the reality of Lehmann’s discovery. Another substance of a different nature was discovered by Vorländer in the year 1904, namely, the ethyl ester of para-azoxy-benzoic acid. A characteristic microscope slide of it in ordinary light is shown in Fig. 112, Plate XXII., which is a reproduction of an actual photograph most generously sent to the author by Prof. Lehmann.