ARRANGEMENT OF ATOMS IN A CRYSTAL. 135 



The salt crystallises in rhombohedra; the author assumes 



that "the molecule must conform to the symmetry of the 



rhombohedral system, and for this at least three sodium 



nitrate molecules must be conjoined, while to produce a 



rhombohedron six will be required". He supposes then 



that these six molecules are distributed into two groups, 



one consisting of oxygen, six atoms of which are vertically 



linked to a nitrogen atom, and the other of six rays of 



-O-N-O-Na, the chemical bonds of which lie more or less 



parallel to the equatorial plane. Rays of light travelling 



perpendicular to the optic axis determine the two principal 



indices of refraction ; and the next supposition made is 



that the ordinary index relates to the horizontal group of 



atoms, and the extraordinary to those which are vertically 



linked ; in other words, the ordinary index relates to 



Na N 3 2 and the extraordinary index to N 2 O. It will 



T 5 



be observed that this introduces a fresh assumption, 



namely, that we are justified in assigning three-fifths of the 



molecular weight of the nitrogen to the equatorial members 



of the molecule, because only three out of the five bonds 



are horizontally linked. 



Applying the formulae 



n 1 — 1 



n 2 — 1 





K 



where «, is the ordinary, n % the extraordinary index, 

 k, is the refractive energy of NaN 3 2 



5 



k NO 



we get d 1 = 2-693, l/ z = !'474- 



What may be the physical meaning of these two 

 quantities the author does not venture to suggest, but he 

 next assumes that the molecular weight of the first atomic 

 group divided by d t gives its volume v- v and that the 

 molecular weight of the second divided by d 2 gives similarly 

 z> 2J the volume of the group N 2 O, and that the sum of 



these partial volumes is equal to that of the whole salt. 



