Rotation of plane of polarisation by quartz of two varieties and different thickness, 180, 210, 211 Royal Institution experiment with diamonds, 8 Sal-ammoniac, 82, 83 Salol and betol, Miers’ researches on, 242, 243 Sand grains with quartz crystals, 3 Scalenohedron of calcite, 11 Scheele’s discovery of tartaric acid, 142 Schlippe’s salt, 45 Schönflies, discovery of remainder of 230 point-systems, 118, 119, 140 Seebeck, researches on ammonia, 82, 83 Selenates, isomorphous with sulphates, 96; of alkalies, 121 Selenic acid, 95 Selenite, polarisation colours of films of, 203 Selenium, discovery by Berzelius, 96 Sella’s warning against hasty generalisation, 102 Senarmontite, 88 Silver iodide, 256, 292 Single refraction of cubic crystals, 162, 183 Snow crystals, 49 Sobrerol, 153 Sodium chlorate, 151; sulphantimoniate, 45 Sohncke, regular point-systems, 117; mirror-image molecular arrangement, 141; two point-systems of quartz, 151 Sollas, crystal structure, theory of, 292 Solubility and supersolubility, 238–241; curves of, 240 Solutions, optical activity of, 151; metastable and labile, 240, 241 Space-lattices, 50, 114–116; triclinic illustration of, 115 Specific gravity, importance of determinations of, 129 Spheres of influence of atoms, 113 Steno, 16 Stereographic projection, 34, 62, 67; of topaz, 68; of double sulphates, 35 Stereometric arrangement of atoms in molecule, 124, 136 Story Maskelyne, 34 Strain, polarisation colours of glass and diamond due to, 207, 208 Structural units of crystals, 24, 111–113 Sulphantimoniate of sodium, 45 Sulphate, ammonium magnesium, 44 Sulphates of alkalies, 121; of barium, strontium and lead, 74 Sulphur, dimorphism of, 86, 87, 137; monoclinic form of, 4, 86 Symbol of a face or form, 57 Symmetry, axes and planes of, 34, 36, 41, 55; elements of, 34 Systems, the crystal, 6, 7, 33 Tartaric acid, 142–148; dextro, 143, 146–148; lævo, 144, 146–148; pyro-electrical properties of, 149 Tartrate, hydrogen potassium, 43 Tetartohedral classes of crystals, 34 Tetragonal system, 37; axes and planes of, 36, 52 Thallium, relation of to alkali metals, 131 Thénard and Gay-Lussac, research on ammonia, 83 Thomson, J. M., 238 Thomson, Sir J. J., discovery of composition of atoms, 112 Topaz, crystal of, 40; stereographic projection of, 68 Transition tint, 180 Triclinic bipyramid, 56; system, 40; axes and planes of, 53, 54 Trigonal system, 37; axes and planes of, 37, 53, 55 Triphenyl pyrrholone derivatives, 105–108 Triple tartrate of sodium, potassium, and ammonium, 91