162 Scientific Proceedings, Royal Dublin Society. 
the case of potassium nitrate. In this case we must make use. of 
the rhombic form, since the refractive indices have not been 
determined for the rhombohedral modification: the maximum and 
median indices which correspond to the ordinary index of sodium 
nitrate are so nearly equal that we may expect to get sufficiently 
approximate results by taking their mean as the index for the group 
K-O-N;-O-. The calculations for this and succeeding salts are 
given in full in the extended Memoir; in this abstract it will be 
sufficient to state results. Thus for K-O-N,-O-, o, = 33°76; and 
for N,-O-, v, = 14°78, but 38°76 + 14°73 = 48-49, and the total 
molecular volume of the salt is 48°8. There is thus as complete 
a correspondence between the sum of the partial volumes and the 
volume of the compound as we found in the case of sodium nitrate. 
Another pair of isomorphous compounds, with not very dissimilar 
extraordinary indices, is met with in the case of dihydric potassium 
arseniate and dihydric ammonium arseniate. Here we have :— 
AsO.KH,; 1, = 1:5674, ne = 1°5179. 
AsO.AmH;; n, = 1:5774, mn, = 1:6117. 
The vertically acting groups will be -O-As,-O-K, and 
-O-As;-O-NH,, the horizontally acting H-O-As,-O-H in both 
cases. . 
Calculating out in the same way as for sodium nitrate we 
obtain the following :— 
As0.K; d,=8:056, 0, = 37-96. 
As,0,H, 5 de = 2°482, Vz, = 25°80. 
The volume of the salt is 63°56, the sum of the volumes of its 
components 37°96 + 25°80 = 63°76. 
As;0,NH, 5 ay = 1:933, Y= 49:15. 
As,0,H, 5 dp» = 2°451, V2 = 26°11.. 
The volume of the whole salt is 75:15, and the sum of the 
volumes of the component groups 49°15 + 26-11 = 75°26. 
A mineral isomorphous with sodium nitrate is calcite, and if 
our hypothesis be true this should possess a similar molecular 
structure, capable of affecting rays of light in the same way. A 
molecule of calcite should then consist of six atoms of carbon—three 
below at the corners of an equilateral triangle, and three above at 
