Banded Spherulites of Hippuric Acid

Abstract

IT has long been known that under certain conditions many substances crystallize in the form of spherulites showing concentric extinction bands between crossed polars. The explanation of this effect, originally put forward by Michel-Levy and Munier Chaimas¹ for chalcedony, and later confirmed by other workers^2–5 for a number of substances of low molecular weight, was that the structure of the spherulite has a twist around any given radius, and that the bands correspond to those orientations in which an optic axis is parallel to the light beam. Thus, Gaubert⁵ obtained spherulites of hippuric acid in which the radial twist was around the β-axis of the indicatrix, a fact which he established conoscopically, that is, by a study of the interference figures along the radii. More recently there has been a revival of interest in banded spherulites, since they are formed by many polymers (for example, polythene and terylene). These cases have been the subject of much investigation by, among others, Keller⁶, and Point⁷, who have shown convincingly, from the optical effects of tilting the preparations with respect to the direction of the light, that the spherulites have a radially twisted structure as for the low molecular weight substances referred to above.