Dimorphism, 79, 87; of antimony oxide, 87, 88; of carbonate of lime, 79, 87; of mercuric iodide, 97; of sodium dihydrogen phosphate, 80, 81; of sulphur, 86; of vitriols, 79 Directive molecular force in crystallisation, 139, 269, 274–277 Dog-tooth spar, 10–12 Double refraction, interference colours due to, 176, 177; measure of, 169, 170; of biaxial crystals, 184; of uniaxial crystals, 183 Double sulphates and selenates, 35, 79, 121, 127, 128, 132 Dulong and Petit’s law, 84 Electronic corpuscles, constituents of atoms, 113, 124 Elements of a crystal, 68, 69 Enantiomorphism and optical activity, 140; 11 classes showing, 150 Epsom salts, 76, 78 Ethyl triphenyl pyrrholone, 105, 106 Eutropic series, definition of, 132 External molecular compensation, 134, 234 Extinction directions, 204, 205 Fedorow, von, discovery of remainder of 230 point-systems, 118, 119, 140; theory of, 291, 292 Ferricyanide, potassium, 44 Ferrocyanide, potassium, 43 Ferrous sulphate, 76, 78 Fletcher, indicatrix of, 184 Fluorspar, single refraction of, 206 Form, definition of a, 11, 60, 61 Frankenheim on morphotropy, 101; discovery of space-lattices, 114 Frankland and Frew, 155 Fuchs, von, researches on sulphates of barium, strontium, and lead, 75, 77 Fuess reflecting goniometer, 64–66 Fundamentalbereich of Schönflies, 112, 113 Gattermann, 258 Gay-Lussac on alums, 77; on ammonia, 83; on racemic acid, 142 Gernez, 238 Gessner, 14 Gmelin, researches on racemic acid, 142 Goniometer, contact, 19, 20; reflecting, 64–66 Graphite, 137 Groth, von, morphotropic researches, 98, 102–104 Growth of a crystal, 1; from solution, 237–254 Guglielmini, researches on crystal structure, 17 Gypsum (selenite), cleavage of, 203; crystals of, 14; extinction directions of, 204; 60°-prism, experiment with, 163, 168; twins of in polarised light, 205, 206 Habit of crystals, 12, 13 Hardness of crystals, 255, 274 Hatchett’s discovery of columbium, 85 Haüy, 22; and Mitscherlich, 70, 75, 77, 88, 132; fundamental forms, 22, 23; law of constancy of form, 23; lattices, 29; law of rational indices, 24, 29, 30; modernisation of theories of, 30–32; molécules intégrantes, soustractives, and élémentaires, 25–28; structural units, 24 Hemihedral classes of crystals, 34 Hexagonal axes of symmetry, 36; prism, 11; system, 36, 53 Hexakis octahedron, 39; indices of, 61 Hjortdahl and morphotropy, 101 Holohedral classes of crystals, 34 Homogeneity, 6, 16; definition of, 114 Homogeneous structures, the 230 types of, 6, 111 Hooke, Robert, 16 Huyghens, 17; discovery of laws of double refraction, 17; investigation of calcite, 17 Ice, crystalline form of, 47 Iceland spar, discovery of, 17; rhombohedron of, 10–12 Inactive tartaric acid, 144 Inactivity, true optical, 235 Indicatrix of Fletcher, 184