Iceland, discovery and settlement, xiv, 261; fault displacement in, 39; foxes of, xii, 344; future manufacturing center, v, 173; ice caps, iii, 61; low pressure area, i, 361; volcanic eruptions, 57, 59; volcanic formation, xiv, 277, 289, 316 Icelandic Language, xv, 162 Iceland Spar, iii, 325; effect on light, 319, iv, 354 Ice Needles, i, 92-3, 96 "Ice Pavement," xiv, 56 Ice Rain, i, 107, 375 Ice Saints, i, 363, 375 Ice Sheets, iii, 60, 61-2; of Great Ice Age, 237-42, xv, 74-6 Ice Storms, i, 108 Ice Water, drinking of, ix, 229; temperature of, viii, 38 Ichneumon Flies, xii, 125 Ichneumons, xii, 352 Ichthyornis, xii, 243 Ichthyosaurs, iii, 286-8, xii, 182, 202 Idaho, lava formations, xiv, 102, 103, 318; mining products, iii, 362-3, 368 Idaho Fire (1910), i, 57 Ideal Metal, resistance, vi, 77 Ideas, psychological meaning, xi, 201-2 (see also Association of Ideas, Dissociation, Repression) I-em-hetep, x, 11 Igneous Rocks, iii, 13, 379, xiv, 17-18; common modes of occurrence, iii, 102 (fig.); illustrations (Pl. 8, 9, 10); intrusive and extrusive, xiv, 105; intrusions in mountain ranges, 228, 230, 232-3, 234; jointing in, 129-30; land forms in, 44, 99-113; oldest by nebular theory, iii, 160; soils from, 28; volcanic and plutonic, 106, 110, xiv, 99-100 Ignis Fatuus, i, 346-9, 375 Ignition, electric, vii, 369; in automobiles, 130-41, 369; in firearms, viii, 145; temperatures of, 53-4 Iguanas, xii, 207; boas and, 216; color of, 204; spiny crest of, 204 Ilkhanic Tables, ii, 39 Illinois, coal beds, iii, 199; prairies of, xiv, 373, 383 Illinois River, sewage effects, viii, 326 Illuminating Gas, acetylene, viii, 60, 231; burning of air in, 55, 56; flame of, 57-9; production, 46, 47, 252; requisites, 60; transfusion of blood in, poisoning from, x, 338 (see also Gas Lighting) Illumination, art of, vi, 273; measurement of, iv, 350-2; unit of intensity, vii, 368 (see also Lighting, Lighting Systems) Illusions, x, 358 (see also Hallucinations) Illusions, Optical (see Optical Illusions) Ilopango, Lake, draining of, xiv, 198 Images, formed by reflection, iv, 335-7; formed by refraction, 337-9; formation of, ix, 106-9; real and virtual, iv, 335, 338, 339 Images (psychology), xi, 218-22 (see also After-Images) Imagination, xi, 218-27; due to conditioned reflexes, 202-3; in science, xvi, 58-9 Imbeciles, reflex action in, xi, 36 Imhoff Tanks, viii, 328 Imitation, education by, xv, 66-7; in language, 153-4; instinct of, xi, 56; suggestion and, 304 Immunity, against disease, ix, 179, x, 204-12; racial, xv, 48-52 Impedance, in alternating currents, vi, 170, 171; in oscillating circuits, vii, 289 "Imponderables," iv, 47 Impressions, first, xi, 211-12