1910] oil the Chemical Significance of Crystal Structure. 829 



tions approximates to the value a : c = 1 : 0*8165, deduced for the 

 model assemblage. 



The task of accounting for the 15 per cent, of the crystalline ele- 

 ments which have been examined and found to crystallise in systems 



The proportion of substances crystallising in each system is stated 

 above as a percentage. 



other than the cubic or hexagonal still remains. A little inspection 

 shows that the crystal forms of these elements in every case approach 

 very closely to one or other of the tAvo of highest symmetry, namely 

 the cubic or the hexagonal ; one example of this will now suffice. 

 The values of corresponding dimensions in three directions in space 

 for the monosymmetric form of the element sulphur are given by the 

 axial ratios alh: c = 0' 9958 : 1 : • 9998, ^ = 95° 46'. The sUght 

 departure of these dimensions from the corresponding values for the 

 cubic closest-packed assemblage, in which a : h : c = 1 : 1 : 1, (^ = 90°, 

 at once suggests that the monosymmetric modification of sulphur is 

 derived from the latter assemblage by some minute distortion. Such 

 a distortion indicates a very trifling departure from uniformity in the 

 influence exerted in different directions from each atomic centre, and 

 may either arise from some want of symmetry in the individual atoms, 

 or in a reduction of the symmetry caused by some grouping of the 

 atoms ; two or more atoms might thus be more closely connected in 

 some way with one another than Avith other next neighbouring atoms. 



Having shown that the crystalline forms of the elements are in 

 complete harmony with the conception that crystal structures can be 

 homogeneously divided into similar cells of polyhedral shapes 

 approximating closely to the spherical, reference may now be made 

 to some simple compounds, those, namely, in which the molecule 

 consists of two dissimilar atoms. 



The conception of the equilibrium of centred forces which has 



