Sottas—On the Law of Gladstone and Dale. 159: 
It will be observed that the maximum and median indices of 
potassium nitrate, which are nearly equal, differ considerably from 
the maximum index of sodium nitrate; on the other hand, the 
minimum indices of the two salts are of nearly identical value. 
If, as we suppose, the refractive indices stand in close connexion 
‘with the chemical constituents of the salt, we shall naturally 
conclude that the lower indices, which are almost the same in 
both compounds, are connected with the radicle, which is the same 
in both, 7.e. the nitric oxide; while the indices, which are 
markedly different, will stand in direct relation to the basic 
radicles, sodium and potassium oxyles. 
In other words, between these two salts there exist two simi- 
larities and two differences. The two similarities are correspon- 
dences between the like chemical composition and the like 
refractive indices; the two differences are also correspondences, 
but in their case between the unlike chemical constituents and the 
unlike refractive indices. 
To these two may indeed be added a third, since an equally 
striking correspondence is to be noted between the similar disper- 
sion for the minimum index in both salts (the value of pq — pp. 
being 0-0108 for KNO;, and 0:009 for NaNO;); and again, 
between the very different dispersion for the maximum index in 
both, the value of gx — gz for KNO, being 0:041, and for NaNO,, 
0:047. 
‘Let us now consider what is the most probable arrangement 
of atoms in the sodium nitrate molecule. Evidently the pentad 
nitrogen is the centre of the system, and one face looks towards 
sodoxyl and the other towards oxygen, thus’ “oN-O-Na. But, 
again, the molecule must also 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. ‘The following arrangement then results :— 
In the upper half of the molecule three atoms of nitrogen are 
linked together by three atoms of oxygen alternating with them, 
and along three radii, corresponding to the three upper edges of 
the rhombohedron, lie three molecules of sodoxyl, linked each by 
its oxygen atom to an atom of nitrogen which lies on the same 
radius. In the lower half of the molecule the same arrangement 
