CRYSTALLOGRAPHY 559 



trigonal. A number of compounds of silver and copper have 

 also been examined by the work of the Geophysical Laboratory. 

 Wyckoff finds silver oxide to be similar to copper oxide 

 (Amer. Journ. Sci., (5), 3, 184, 1922), but some forms of 

 ammonium chloride {ibid., (5), 3, 177, 1922 ; (5), 4, 469, 1922) 

 give a different degree of symmetry from that indicated 

 by the etch figures and external form ; e.g., in one case the 

 X-ray structure indicates tetrahedral symmetry, while the 

 form and etch figures suggest hemihedral symmetry. Wyckoff 

 and Posnjak show that the cuprous haloids have a structure 

 resembling that of zinc sulphide so far as the chloride, bromide, 

 and iodide are concerned. The same authors (Journ. Wash. 

 Acad. Set., 12, 248, 1922) have shown that calcium chloride, 

 bromide, and iodide are composed of body-centred lattices 

 (cf. W. P. Davey and F. G. Wick, Phys. Rev., 17, 403, 1921), 

 and R. M. Bozorth (Journ. Amer. Chem. Soc, 44, 2232, 1922) 

 that cadmium iodide is not hexagonal but trigonal. G. G. 

 Dickenson has examined phosphonium iodide (ibid., 44, 1489, 

 1922), and M. L, Huggins, calcium carbonate (Phys. Rev., 19, 

 354, 1922). W. H. Bragg (Proc. Phys. Soc, 34, 918, 1922) dis- 

 cusses the question of the structure of ice and finds that his results 

 agree with those of Dennison (Phys. Rev., 17, 20, 1921), but 

 not with those of J. John (Proc. Nat. Acad. Sci., 4, 192, 191 8). 

 W. H. Bragg (Proc. Phys. Soc, 34, 33, 192 1) has also examined 

 by the powder method the structure of a number of organic 

 compounds such as naphthaline, a-naphthylamine, benzoic 

 acid, acenaphthene, yS-naphthol, while a large number of 

 carbon compounds have been investigated by K. Becker 

 and W. Jancke (Zeit. phys. Chem., 99, 242, 267, 1921) ; P. 

 Niggli (Zeit. Kryst., 67, 253, 1922) has assigned specific space- 

 groups to cuprite, tenorite, and silver oxide, while F. Rinne 

 and his co-workers have published a series of papers in Abhand. 

 Sachs. Akad. Wiss. (Abs. in Chem. Cent., 1,626, 1 921) assigning, 

 on the basis of the X-ray structure and the physical properties 

 of the cr3^stals, specific space-groups to such minerals as olivine, 

 tourmaline, cobalt-glance, and carborundum. In each case 

 the dimensions of the lattice have been determined as well as 

 the number of atoms in the elementary parallelepiped. There 

 are four molecules of the orthosilicate in the case of olivine in 

 the unit structure, and four of CoAsS in the unit of the cobalt- 

 glance. The three forms of carborundum, which on the basis 

 of the etch-figures are supposed to differ, are shown from the 

 X-ray examination to be the same since only type of lattice, 

 prismatic hexagonal, is present. 



Although it may not be possible at present to elucidate 

 completely the structure of complex silicates by means of 

 X-ray analyses, the method has been successfully applied in 



