RECENT ADVANCES IN SCIENCE 381 



cussed by C. Runge (Phys. Zeit. 18, 5°9, 191 7 ; Sei. A ^s. 21a, 

 20, 191 8). A. W. Hull (Phys. Rev. 10, 661, 191 7) has devised, 

 apparently independently, a similar method, and gives theresults 

 of investigations of several metallic elements and of silicon, 

 graphite, and diamond. In the case of diamond the structure 

 assigned is the same as that originally given by Bragg. 



Several further investigations have also been made by Bragg's 

 method. C. M. Williams has examined cassiterite and rutile, 

 and has obtained results, in the latter case, which differ from 

 those of Vegard (Phil. Mag. (6), 32, 65, 1916). Vegard appears 

 not to have observed a fairly intense reflection at 3 52' for 

 the (100) face, and consequently the centred tetragonal lattice 

 suggested by him for the titanium atoms does not hold, but, 

 according to Williams, should be substituted by a more com- 

 plex structure consisting of eight interpenetrating lattices of 

 the simple tetragonal type. In the case of the basal plane the 

 intensity of the second order reflections indicates the proba- 

 bility of the oxygen atoms lying in the planes parallel to this. 

 The symmetry of this structure is holohedral tetragonal. 



L. Vegard and H. Schjelderup (Ann. Phys. (iv.), 54, 146, 

 l 9 l 7'> Jour. Chem. Soc. 114, ii. 156, 191 8) have examined crys- 

 tals of normal ammonium and potassium alums, chrome alum 

 and ferric ammonium alum, and from the reflections from the 

 cube, octahedron, and rhombic dodecahedron have arrived at a 

 structure for the metallic, sulphur, and oxygen atoms. The 

 twenty-four molecules of water present are apparently all of 

 the same type, and are symmetrically disposed with reference 

 to the sulphur atoms. The crystalline structure is destroyed 

 when the crystals are dehydrated. The effect of dehydration 

 has also been examined in the case of chabazite (CaAl 2 Si 4 0i2, 

 6H 2 0), the X-ray structure being determined before and after 

 dehydration. Although the relative intensities remain the same 

 the absolute intensities diminish. Hence it is assumed that 

 dehydration destroys the crystalline structure, the reflections 

 obtained in the second case being ascribed to the presence 

 of still undehydrated portions. The results would probably 

 have been more definite if the mineral had been completely 

 dehydrated. 



The above structure for the alums is criticised by C. Schaefer 

 and M. Schubert (Ann. Phys. (iv.)55, 397, 191 8 ; Jour. Chem. Soc. 

 114, ii. 315, 191 8), who also claim that the theory regarding 



