Fig. 102.—Lehmann’s Crystallisation Microscope.
The form of Lehmann’s “Crystallisation Microscope,” as now constructed by Zeiss, is shown in Fig. 102. Its essential features are that the glass object-plate, which is somewhat wider than the usual microscope 3 by 1 inches slip, is supported by little metallic columns at a height an inch or more above the ordinary stage, and may be heated from below by a miniature Bunsen burner, which is provided with a delicate graduated gas-tap and is adjustable for its position, swinging in or out as desired. The small Bunsen flame may be converted into a blowpipe flame if necessary, an air-blast attachment to a mixing reservoir being provided, to which the arm of the burner is hinged. Two cooling blasts, connected with a gas-holder of air, are also provided, and are adjustable to the most suitable symmetrical positions above the slide for directing the cooling air on the part of the latter where the liquid is situated. These arrangements enable the substance on the slide to be rapidly or slowly heated or cooled at will. Electric connections are also provided, in the event of the observer desiring to study the behaviour of the liquid crystals under the influence of the electric current.
Considerably later, in the year 1889, the attention of Lehmann was called by Reinitzer to another similarly singular substance, cholesteryl benzoate, which appeared to consist of an aggregate of minute crystals which flow as readily as oil, while preserving many of the characters of crystals.
In the next year, 1890, the substance para-azoxyphenetol, then recently discovered by Gattermann, was observed by Lehmann to form a turbid “melt” on fusion, which consisted of an aggregate of crystals flowing with a mobility equal to that of water, and which take the form of spherical drops showing a dark kernel inside, as shown at a in Fig. 103, quite unlike a drop of ordinary liquid. The kernel disappears on shaking, but reappears on coming to rest again. In polarised light the drops show dichroism, that is two different colours in different parts or directions, being divided into white and yellow parts, the yellow as a pair of opposite approximately 60°-sectors, as indicated at c in Fig. 103. Under crossed Nicols they show a black cross, as represented at d in Fig. 103.
Now obviously these drops are doubly refractive, and their whole optical behaviour corresponds to the arrangement of the molecules in concentric circles, such as that suggested at b in Fig. 103.
Fig. 103.—Liquid Crystals of Para-azoxy-phenetol arranged in Spherical Drops.
Another substance of like character, para-azoxy-anisol, was subsequently found to behave similarly, and forms an excellent substance to use for demonstration purposes. A reproduction of a photograph, kindly sent to the author by Prof. Lehmann, of a slide of this substance is given in Fig. 104, Plate XXII. It shows a characteristic field of such drops, exhibiting white parts and yellow sectorial parts which photograph dark, of para-azoxy-anisol mixed with a little para-azoxy-phenetol, oil and resin (colophony), as seen under the polarising microscope with crossed Nicols.
The next and possibly most interesting step in this remarkable series of discoveries was made by Lehmann himself in the year 1894. He alighted on the fact that ammonium oleate, crystallised from solution in alcohol, affords a splendid example of flowing crystals, which are sufficiently large to enable their habits to be studied in detail. The individuals are almost invisible in ordinary light, owing to the refractive index of the crystals and of the mother liquor being approximately the same. But in polarised light, using crossed Nicols, they are clearly revealed as steep double pyramids with more or less rounded edges. Their section is nearly circular in consequence, and they exhibit optical properties of a uniaxial character, the optic axis being that of the double cone or bipyramid. A characteristic individual is shown at e in Fig. 105. When two of these flowing crystals approach each other, as at a in Fig. 105, they coalesce to form a larger single individual, as is indicated in stages at b, c, and d in the illustration.
