370 SCIENCE PROGRESS 



would suffice to produce one having the above constants of 

 sulphur. The nature of this distortion cannot be definitely 

 stated but Barlow and Pope show that a minute diminution in 

 size of one-third of the spheres, selected in a particular manner, 

 in a cubic assemblage of equal spheres will cause a distortion 

 sufficient to give the assemblage the above constants. A similar 

 shrinking of every third sphere in a hexagonal assemblage of 

 equal spheres, followed by a slight distortion, gives a structure 

 having axial ratios corresponding to those of orthorhombic 

 sulphur, a:b'.c — o"8io8 : i : 1*9005. The remaining monosym- 

 metric form of sulphur can be accounted for in a somewhat 

 similar manner; and by adopting such devices it is possible to 

 mimic the divergence of the crystals of selenium, tin and iodine 

 from the ideal cubic and hexagonal forms. It is interesting to 

 note in this connection that whilst the colourless elements are 

 all cubic or hexagonal, all the highly coloured elements belong 

 to systems of lower symmetry. Since colour may be taken as 

 conditioned by molecular complexity, it is not unjustifiable to con- 

 clude that the abnormality of crystalline form of coloured elements 

 may also be the result of the formation of molecular aggregates. 

 It must, be understood that the preferential treatment of a 

 proportion of the spheres in an assemblage of equal spheres in 

 order to explain the divergence of certain elements in their 

 crystalline forms from the cubic or hexagonal symmetry in- 

 dicated by theoretical considerations can at present be regarded 

 merely as a geometrical device for obtaining the desired result. 

 At most it is but a slight distortion which has to be accounted 

 for; as was pointed out by Prof. Pope in a lecture recently 

 delivered at the Royal Institution, such a distortion may arise 

 from some want of symmetry in the individual atoms or in a 

 reduction of the symmetry caused by some grouping of the 

 atoms. From a chemical point of view the latter seems, perhaps, 

 a more rational explanation, since some elements, as for example 

 sulphur, are known to have a complex molecule in the gaseous 

 state ; but the former explanation is adequate to account for the 

 observed geometrical facts. However, until there is more direct 

 evidence regarding the internal structure of crystalline sub- 

 stances — and it is hard to say at present whence such evidence 

 is to come — the matter must be allowed to rest. 



{To be continued) 



