196 SCIENCE PROGRESS 



To get over the difficulty, A. Lande {ibid., 20, 217, 1918; 21, 

 2, 644, 1919 ; Ber. preuss. Akad. Wiss., loi, 1919 ) cf. M. 

 Born, Verh. deut. phys. Ges., 20, 230, 191 8 ; 21, 13, 1919) 

 assumes that the electronic orbits are symmetrically placed, and 

 that, in the examples considered, the symmetry of the orbits 

 is cubic. Some criticisms of the theory by L. Vegard {ibid., 21, 

 383, 1919) are replied to by Born and Lande {ibid., 21, 385, 1919)- 

 W. Voigt {Ann. Phys., [4], 60, 638, 1919) gives an exhaustive dis- 

 cussion of the same work, especially with reference to Land^'s 

 view that the forces of cohesion are due to ionic electrical 

 attractions. In connection with the relation between force and 

 deformation, he points out that the difficulty in anticipating 

 the rupture-point may possibly be explained by the symmetrical 

 arrangement of the orbits. The theory has been further tested 

 by M. Born and E. Bormann {Verh. deut. phys. Ges., 21, 733, 

 191 9), using zinc blende as an example; and although the 

 calculated and observed results do not agree very closely, yet 

 they are of the same order of magnitude (cf. M. Born, Ann. 

 Phys., [4], 61, 87, 1920). 



In a paper on a " Kineto-electro-magnetic Theory of Crys- 

 tals," J. Beckenkamp {Verh. phys. Ges. Wurzburg, 45, 135, 

 191 8, Abs. in Journ. Chem. Soc, 116, 273, 1919) extends Bohr's 

 theory and applies it to cubic crystals. In a lecture to the 

 Royal Institution, W. L. Bragg {Nature, 105, 646, 1920) sum- 

 marises the work which has been done on the relation between 

 the structure of the atoms and that of the crystal. On the basis 

 of X-ray work and glide-plane experiments, A. Johnsen {Cent. 

 Min., 385, 1 9 1 6) discusses the deformation of crystals of bismuth, 

 in which the molecules are considered to be diatomic and to 

 have the same symmetry as the crystals. In a further paper, 

 A. Johnsen and A. Gruhn {ibid., 366, 433, 191 7) conclude that, 

 in deformation, the atoms tend to move in groups or " com- 

 plexes " (cf. A. Gruhn, ibid., 85, 191 8). F. Rinne {ibid., 

 161, 172, 191 9), on the basis of the absolute atomic volumes, 

 calculated from the crystal structural units of such metals as 

 aluminium, silver and gold, of the rhombohedral carbonates 

 and the alkali haloids, comes to the conclusion that the chemical 

 nature and the valency of the atoms are more important than the 

 atomic volume. For example, the atomic volumes of the three 

 metals named are approximately equal, but while silver and 

 gold form solid solutions, aluminium does not do so with the 

 others. Mixed crystals are supposed to be intermediate 

 between compounds and physical mixtures. According to 

 A. Johnsen {ibid., 97, 1919), zircon is probably hemimorphous, 

 and hence not in the same symmetry class as rutile, xenotime, 

 etc., which are holohedral (cf. L. Vegard, Phil. Mag., [6], 32, 

 65, 505, 1916). 



