RECENT ADVANCES IN SCIENCE 39 



zoisite through a temperature range extending from I5°C. to 

 400 C. The different results obtained in the two cases are con- 

 sidered evidence in favour of the individuality of the two 

 minerals. 



Crystal Structure. — L. Vegard {Phil. Mag. 32, 65, 505, 1916, 

 33» 395, 191 7) has determined the crystal structure of a number 

 of substances. Gold, silver, and lead all have the same lattice 

 as copper, the face-centred cube, and an examination of the 

 spinel group confirms the results obtained, independently, by 

 Bragg and Nishikawa. Interesting results are obtained in the 

 case of the closely-related tetragonal group comprising zircon, 

 rutile, cassiterite, xenotime, and anatase, all of which crystallise 

 in the ditetragonal bipyramidal class. While the lattices, as 

 usual, are atomic, the atoms form groups of the type R0 2 (where 

 R = Si, Ti, Zr or Sn), the three atoms in a group being arranged 

 in a linear fashion, and the distance between the R- and the 

 O-atoms varying with the nature of the former. The structure 

 of zircon is represented by twelve face-centred lattices of which 

 two are composed of zirconium, forming a lattice of the diamond 

 type, and two of silicon similarly associated, the remaining 

 eight being composed of oxygen atoms of which every two are 

 associated with one R-atom. In rutile the titanium is considered 

 to replace both silicon and zirconium so that the metallic atoms 

 form the centred-prism tetragonal lattice. Cassiterite is 

 like rutile, but in anatase the titanium atoms form a lattice of 

 the diamond type with the " molecular " axes parallel to the 

 tetragonal axis and not perpendicular to it as in the other cases. 

 Xenotime was at first thought to have a structure such that the 

 oxygen atoms were all associated with the phosphorus, but a 

 more careful examination showed that it was like zircon, and 

 of the structure Y0 2 P0 2 . In all these cases the departure from 

 the cubic form is due to the arrangement of the oxygen atoms, 

 the ratio c : a being greater or less than unity according as the 

 molecular axis is parallel or perpendicular to the tetragonal 

 axis. 



No reflections could be obtained from thorite on account 

 of its metamict nature, the crystals being physically amorphous 

 though the external tetragonal form persisted. 



These structures appear to afford interesting evidence on 

 the question of the existence of molecules in the solid state. In 

 the case of such substances as rock-salt, examined by Bragg, it 



