Topography of Crystal Faces

IN Nature of May 4, Tolansky and Wilcock1 report various features of the topography of the face of a diamond crystal, and conclude that the triangular pits often seen on the faces are due to growth, not to solution: a layer, spreading across the crystal face, does not at first cover a particular region but grows all round it, afterwards closing in on it from three directions to form a triangular pit. We wish to report that, in observing certain crystals growing from solution, we have often seen this sort of thing actually happening—not of course on diamond, but on crystals of water-soluble substances, notably potassium dihydrogen phosphate. The layers we have seen, since they are easily visible under the microscope, are much thicker than those seen by Tolansky and Wilcock on diamond. Two further points are worth mentioning. One is that these pits are often irregular in shape; this is natural enough when the advancing edges of the layers are irregular, as is the case more often than not when growth is rapid. Irregular pits found on finished crystals may therefore be due to growth, no less than regular ones. The second point is that the pits often become completely filled in, there being afterwards no visible trace of their existence.