Recrystallisation accompanying an Allotropic Change

Abstract

DR. W. G. BURGERS and Dr. van Amstel are to be congratulated on their success in obtaining a cinematograph record of the change in iron by the very beautiful method they describe in NATURE of November 2 (136, 721). Their photographs show that recrystallisation at an allotropic change point is similar to that occurring on heating coldworked metal. Some years ago, in order to settle this same point, I examined a number of salts which undergo allotropic changes of a similar character. Ammonium, potassium and silver nitrates are particularly suitable as they can be melted on a slide and, if crossed Nicol prisms are used, the whole process can be studied. In every case the change began at the crystal boundaries or round an inclusion or blow-hole, by the appearance of new crystal nuclei which rapidly spread across the field. I then found that the rate of change could be slowed down by the presence of a cover-slip and by regulating the thickness of the fused salt, and obtained a series of photographs—some of them in colour—sillustrating the change over.