376 SCIENCE PROGRESS 



the layers differ in composition, but the temperature may not 

 be uniform, and hence there must be a differential contraction 

 due to variations in the coefficient of dilatation of the various 

 layers. Thus the conditions likely to give rise to curved crystals 

 are high vectoriality of the crystals, the formation of lamellae 

 of varying composition, and variable temperature, giving rise to 

 unequal contraction of the lamellae. An analogous explanation 

 may be applied to the case of the curved crystals of augite 

 described by the present writer {Min. Mag., 17, loo, 1914), where 

 the mineral shows a fine lamellar structure approximately 

 " parallel " to the curved faces. The tendency of certain 

 organic compounds to develop crystals with curved faces when 

 grown in solutions containing various dyes which are absorbed 

 by the growing crystals has been noticed by P. Gaubert {Bull. 

 Soc. fr. Min., 28, 286, 1905 ; Compt. Rend., 151, 11 34, 1910 ; 

 157, 1531, 1 91 3, Recherches recentes sur le fades des cristaux, 

 Paris, 191 1 ), and it is again possible that the phenomenon may 

 be explained by Benedick's hypothesis. 



An elaborate study of the figures which appear on the 

 surface of hydrated crystals during the process of efflorescence 

 has been made by C. Gaudefroy {Bull. Soc. fr. Min., 42, 284, 

 1 91 9.) It is found that while many of the dehydration figures 

 obtained bear some relation to the structure of the original 

 crystal, others are connected with the product of dehydration 

 while some are more complex and difficult to explain. On the 

 basis of the exterior form the figures may be divided into three 

 groups : 



{a) Polygonal figures determined by the structure of the 

 original crystal. The salt is partly dissolved in its own water 

 of crystallisation, and then, as the latter'evaporates, the poly- 

 hedron which is formed intersects the surface of the original 

 crystal in a polygon whose form is determined by certain 

 structural planes in the original crystal. The formation of a 

 liquid phase during the process of dehydration is probably 

 general^ since any hydrated mineral, if sufficiently fine grained, 

 will cohere to a " solid " mass on standing in a desiccator. 



{b) Polygonal figures determined by the secondary product, 

 that is, by the structure of the salt formed by the dehydration. 

 In this case each figure represents a single crystal of the product 

 of dehydration ; for example, on the surface of the rhombic 

 heptahydrated zinc sulphate monoclinic crystals of the hexa- 

 hydrate develop. There is no essential coincidence between 

 the orientation of the two crystals, and various forms may 

 occur in the same original crystal. 



{c) Sectoral Figures. — In the most usual form of this type, 

 the figure is divided into four sections resembling an hour glass 

 structure. In general, each section is composed of an aggregate 



