TUTTON: x-ray ANAIvYSIS and assignment of CRYSTAIvS 95 



certain substances — notably the diamond, sylvine (potassium 

 chloride), pyrites and its analogues hauerite and cobaltite, 

 the nitrates of barium, strontium, and lead, and the rutile group 

 of minerals — are in a sense intermediate in structure between 

 two crystal classes, possessing some of the attributes of each. 

 It is advocated that the difficulty be overcome in each of such 

 cases by assigning the substance to both classes. It is further 

 assumed that they are usually cases of weak hemihedrism. 



The diamond, although formerly classed as hexakis-tetra- 

 hedrally hemihedral (class 31), has for some time now, largely 

 owing to the very definite proof of the absence of electric polarity 

 by Van der Veen, been considered as holohedral, that is, as 

 belonging to the class 32 of full cubic symmetry. This view has 

 now been shown to be correct by W. H. and W. h. Bragg, as the 

 result of their X-ray structural analysis. The memoir of Dr. 

 Wherry, however, regards the Bragg result as indecisive, and as 

 indicating holohedrism as a whole, but a tetrahedral structure- 

 unit symmetry. Etch-figures are cofisidered to bring out partial 

 symmetries when equilibrium is delayed, and in the case of 

 diamond they are at first hemihedral, but given longer time 

 become eventually holohedral. It is concluded in the memoir, 

 therefore, that while the system of diamond is cubic, and the 

 space-lattice structure is holohedral, the structure-unit is tetra- 

 hedral. 



In the case of sylvine, also found holohedral by X-ray analysis, 

 the etch-figures indicate gyrohedral (class 29) symmetry, which 

 it is assumed is due to the difference in atomic volume of potas- 

 sium and chlorine; whereas in the case of rock-salt, which is 

 undoubtedly holohedral (qf class 32), although class 29 etch 

 figures are also produced at first, they are subsequently further 

 developed into class 32 figures and this is supposed to be due to 

 the near equality of the atomic volumes of sodium and chlorine. 

 The memoir finally concludes that this family of halides belongs 

 to the cubic system, with a holohedral space-lattice and a gyro- 

 hedral structure-unit. 



