CHS 



Dr. A. E. H. Tutton 



[March 14, 



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Fig. 7. — Space-lattice 



C ENTRED-FACE CUBK. 



on the screen (Fig. 7), and a pair of models of the arrangement are 

 illustrated in the next two pictures, in the first of which the points 

 are expanded into spheres of considerable size, and in the second they 

 a])!iear still further expanded into actual contact. The third stage, 

 in wliich the expansion proceeds until all interstices are filled up and 

 the spheres are converted into polyhedra, 

 is left to the imagination. In the second 

 picture (reproduced in black and white in 

 Fig. 8) the mutual arrangement of the 

 spheres of the two elements in zinc blende, 

 zinc and sulphur, is indicated by the yellow 

 colouring of the sulphur spheres and the 

 grey tinting of those of zinc. The tetra- 

 hedral mode of derivation of the structure, 

 accounting for the observed hemihedrism, 

 is also shown in another slide (Fig. 9). 

 The eight larger cubes which together 

 form the grosser unit are each supposed to 

 be occupied by four smaller cubes of the same element, arranged tetra- 

 hedrally, and of zinc and of sulphur alternately in different larger 

 cubes ; on replacing the little cubes by spheres in contact the model 

 represented in the second picture (Fig. 8, Plate II.) is produced. 

 Barlow's conception of quartz affords us an example in which the 



symmetry is trigonal, 

 and of which there are 

 two kinds possible, one 

 of which is the mirror- 

 image of the other, the 

 two being helical in 

 character and of the 

 nature of right- and left- 

 handed screws respec- 

 tively. The two struc- 

 tures, as thus conceived, 

 are represented in the 

 next two slides (Figs. 10 

 and 11), the white 

 spheres representing 

 silicon atoms and the 

 black ones oxygen 

 atoms, of which there 

 are twice as many, corre- 

 sponding to the formula 

 SiO.,. The helical cha- 

 racter is clearly shown, 

 the white spheres being obviously arranged in a right-handed 

 screw in one picture and a left-handed screw in the other. Right- 



FiG. 9. — Scheme of Tetrahedral Arrange- 

 ment OF Zinc (B) and Sulphur (R) Atoms in 

 Zinc Blende. Unshaded Cubes unoccupied. 



