Sortas—On the Law of Gladstone and Dale. 163 
the corners of a similar triangle, but turned round 60° with respect 
to that below; six atoms of oxygen will link these carbon atoms 
vertically, and to each will be attached a molecule of calcoxyle, 
the atom of calcium in each calcoxyle molecule lying at the 
corner of the primitive rhombohedron. Thus a group ©,0 will 
determine the extraordinary index, and another C.0,Ca, the 
ordinary index. Again our construction gives partial volumes 
which equal in their sum the volume of the whole salt: thus the 
volume of C,0,Ca is 26:12; of C,O it is 10°85; their sum is 36-92; 
and the volume of calcite is 36:9. 
This result is consistent with what is known of the values of 
the atomic volumes of these constituents from independent evidence, 
as will be shown in the detailed Memoir. 
Magnesite, isomorphous with calcite, gives equally satisfactory 
results. The volume of C,0,Mg is 18°16 and of C,0,10; their sum . 
is 28°16, and the volume of magnesite is 28. 
Dolomite falls equally readily into line. The volume of 
CaMg0.C is 45, and of CO,, 20°4, the sum is 65°4, and the volume 
of dolomite is 65°01. Arragonite and cerussite differ slightly in 
constitution, but they equally yield to our interpretation. 
Calcium tungstate and lead molybdenate appear to be similarly 
constituted; atall events, in each, one may suppose that the arrange- 
ment which determines the refractive indices is similar in both; 
thus in calcium tungstate the horizontally acting constituent is 
W.0,Ca, the vertically acting W.0.; in lead molybdenate the 
horizontally acting component is Mo,0.Pb, the vertically acting 
Mo,0,. On this supposition we find :— 
W:0.Ca, d, a 0°388, V1 — 24:74. 
Mo:02, a> = 6 685, V2 = Yolo. 
The sum of the partial volumes is 47°87, and the volume of the 
’ whole salt is 48. 
Mo;0,Pb, ay = 9°775, V4= 30°88. 
Mo,0,, dy = 3°989, V2 = 24:07. 
The sum of the volumes is 54°95, and the volume of the whole 
salt is 54. 
Calomel is a difficult mineral to treat, since the refractive 
equivalent of the mercury in it is abnormal. Taking as the 
