Page:Proceedings of the Royal Society of London Vol 69.djvu/314

298 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