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XLII. The Crystalline Structure of Copper, By W. Law- 

 eence Bragg, B.A., Allen Scholar of the University of 

 Cambridge *. 



-rriHE copper crystals used in this investigation were some 

 -L natural specimens, for which I am indebted to 

 Mr. Hutchinson, of the Mineralogical Laboratory at Cam- 

 bridge. In their natural state these specimens are obviously 

 rough crystals, and have some faces of large dimensions 

 (1 cm. each way), but these faces are very much warped and 

 distorted. An attempt was made to obtain an X-ray reflexion 

 from various natural faces of such crystals, but it was not 

 successful. Apparently the outer surface of the crystal has 

 been so battered and distorted that little regular crystalline 

 arrangement is left. Any attempt to grind crystal faces 

 artificially also destroys the crystalline character of the 

 surface and so prevents the reflexion of X rays from the face. 

 It was observed, however, that when the crystal was placed 

 in nitric acid until the surface was eaten away to an extent 

 of perhaps \ millimetre, the faces were etched deeply into 

 numerous parallel facets, which all reflected the light simul- 

 taneously in the usual way. This suggested that, internally 

 the crystal structure was perfect, and showed further that in 

 some cases the whole specimen was composed of a single 

 cryst.il. Moreover, in this case the surface layers are not 

 pulled about, and so are capable of reflecting the X rays 

 falling on them. This method of obtaining a crystal surface 

 was suggested by some previous experiments on natural zinc 

 oxide, zincite. Zincite occurs very rarely as crystals, and 

 the specimens used had merely a platy structure. However, 

 by partly dissolving a block of the mineral in hydrochloric 

 acid, the etched mass showed indications of crystalline 

 structure sufficient to serve as a guide in the preparation of 

 various faces. The faces reflected the X rays and led to the 

 determination of the arrangement of zinc and oxygen atoms. 

 Copper crystallizes in the cubic system, holohedral class. 

 The natural crystals of copper used in the experiments were 

 mostly of one type, being composed of two individuals 

 twinned about the plane (111). The faces of the simplest 

 crystals approximated to those of the rhombic dodecahedron 

 {110}. The face first investigated was that parallel to the 

 twin plane (111) of the best of the crystals. The crystal 

 appeared like two triangular pyramids joined base to base, 

 and the apex of one pyramid was ground down on a carbo- 

 rundum wheel until a triangular face (111) was formed. 



* Communicated by the Author. 

 2 A2 



