RECENT ADVANCES IN SCIENCE 549 



twinning. Examination of the X-ray spectrum, supplemented 

 by Laue radiograms, shows the rhombohedral modification to 

 have as its structural unit a centred rhombohedron consisting 

 of one caesium, one iodine, and two chlorine atoms. The 

 chlorine atoms are situated on the long diagonal of the 

 rhombohedron about one-third way from the corners, while 

 the caesium and iodine atoms occupy the corner and centre, 

 the discrimination of the latter two not being possible so far. 

 The structure is considered to be analogous to that of the simple 

 halides, the deformation of the cube into a rhombohedron being 

 due to the replacement of the simple halogen by the group IClg. 



The same writer {Phys. Rev., 16, 149, 1920) has subjected 

 sodium nitrate to X-ray examination, and finds the structure 

 to be composed of interpenetrating rhombohedral lattices very 

 similar to those in calcite. Bragg's suggestions as to the struc- 

 ture of sodium nitrate are thus verified. R. G. Dickenson 

 {Jour. Amer. Chem. Soc, 42, 85, 1920) has examined the X-ray 

 spectra of wulfenite (lead molybdate) and scheelite (calcium 

 tungstate). Although the elucidation of the structure is 

 incomplete, the results show that the heavier atoms are 

 arranged in face-centred lattices. 



The general results of X-ray work on crystals is discussed in 

 several recent papers. From considerations of the spatial 

 relationships of the atoms in crystals, W. L. Bragg {Phil. Mag. 

 [vi], 40, 169, 1920) concludes that each type of atom has a 

 characteristic constant, and the space which separates it from 

 an atom of another type is a function of such constants of the 

 two atoms. By graphing the constants of a number of elements 

 a periodic curve analogous to the atomic volume curve is 

 obtained. In the case of electronegative atoms, certain electrons 

 are common to both, and hence a closer packing than is obtained 

 in the case of electropositive atoms is possible. The paper 

 includes calculated data for the diameter of the outer " electron 

 shell " of many elements. H. Tertsch, in a paper entitled, 

 " Crystallographic Observations on Atomic Structure " {Sitzber. 

 Akad. Wien, 129, 24, Jour. Chem. Soc, 120, ii, 24, 1921) discusses 

 the relationship between crystal and atomic symmetry, and 

 concludes that the latter has an important influence on the 

 crystal form. In a mathematical paper on the atomic constitu- 

 tion of crystal surfaces, E. Madelung {Physikal. Zeit., 20, 494, 

 1919) shows that in binary compounds there is a displacement 

 of one type of atom relative to the other so far as the surface 

 layers are concerned. 



The question of the existence of molecules in crystalline 

 compounds is treated by A. Reis {Zeit. Physik, 1, 204, 1920), 

 who deduces, on geometrical grounds, an essential difference 

 between those compounds which are of the nature of salts and 



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