Pointing out that the study of alkali halides by X-rays indicates their structure to be holosymmetric, while the etch-figures of some of them are recorded to be gyroidal or plagihedral, A. Hettich, a student under Fajans and Herzfeld at Munich, has recently published the results of a somewhat elaborate investigation on the influence of various substances on the symmetry of these etch-figures.1 Most of his observations were made on pure potassium chloride, fused to destroy organic matter, and then cleaved into small blocks or plates. When these blocks were etched with nearly saturated solutions of KCl, for periods of a few minutes or hours, series of tiny pyramids appeared on the surfaces, and their symmetry could be readily recognized with a low power microscope. Upon taking special precautions to exclude organic matter, such as doubly distilling the water over permanganate, cleaning all vessels with bichromate-sulfuric acid mixture, heating the forceps used to handle the crystals in a flame, and excluding dust as far as practicable, the etch-figures were found to be entirely symmetrical. However, when dust was admitted, when the crystals were handled, or when the water was stirred with animal charcoal, filter paper, etc., the etch-figures always showed a certain amount of rotation with respect to the crystal axes, corresponding to plagihedral (gyroidal) symmetry. The sense of this rotation is always clockwise. When minute amounts of optically active organic compounds were added to the etching solution, the same rotation was produced in some cases, although most of the substances tried showed no effect. Of the substances, both optical isomers of which could be used, only lactic acid had a consistent asymmetric effect, but both right and left forms produced the same etch-figures. The author concludes that the merosymmetry (he uses the less apt term hemihedrism) shown by the etch figures of potassium chloride, potassium bromide, (which was also tried to some extent), and presumably the rest of the alkali halides, is due to a trace of optically active organic matter contaminating the etching solution and being differentially adsorbed on the surface of the crystal.

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