298 



Prof. W. J. Sollas. 



halves of the primitive octahedra, and so much -of interstitial space as 

 must be assigned to them in an equal partitioning of space. The 

 volume of this cube will therefore be given by dividing the molecular 

 weight of the crystalline substance by its specific gravity and multi- 

 plying the molecular volume so obtained by 6. The cube-root of this 

 number will give the edge of the cube. 



FIG. 6 



Plan of assemblage of primitive octahedra dyad atoms indicated by three concentric 

 circles, monad atoms by single circles. 



The mineral chosen for our first essay is silver sulphide (AgoS), 

 because we have already obtained what appear to be very trustworthy 

 values for the diameter of the atom of silver, which is 2*172, both in 

 the free state and in its haloid compounds, and for sulphur, which in 

 galena has a value of 2*408. The molecular weight of silver sulphide 

 is 247*936, its specific gravity has been variously determined as from 

 7*27 to 7*32, we take the mean, which is 7*285, the molecular volume 

 is thus 34*03, this multiplied by 6 is 204*18; extracting the cube 

 root we have 5*889 as the length of the edge of the cube, to which all 

 subsequent calculations must be referred. 



Given the dimensions of the atoms of silver and sulphur as stated 

 above, then if they are arranged as in Case I, the edge of the cube 



