Hyacinth Stone, iii, 341 Hyades, ii, 341-3 Hyalite, iii, 335 Hybrids, defined, ix, 334, xiii, 147; transmission of characters, ix, 334-7, x, 231-2, 233, xiii, 332, 333 Hydra, fresh water, xii, 33-4 Hydraulic Jets, v, 88 (see Water Jets) Hydraulic Machinery, v, 97-108 (see also special heads, as Cranes, Elevators, etc.) Hydraulic Press, v, 97-100, 376; mechanical gain in, iv, 41 Hydraulic Ram, v, 84-6 Hydriodic Acid, formation of, viii, 95-6 Hydro-acids, viii, 98, 114, 377 Hydrocarbons, and derivatives, viii, 51-2, 205-40; molecular complexity and physical state, 298; oxygen affinity for, 36; substitution phenomena, xvi, 162 Hydrochloric Acid, viii, 86-7, 115; action on cellulose, 255; in gastric juice, x, 320, 325, ix, 234-5, 236, 237-8; metal tests by, viii, 288; production, 87, 105, 275, 277; solubility in water, 111; test for, 285 Hydroelectric Plants, v, 79, 83, vi, 351-2, 361-78; on farms, vii, 233-4; use of alternators, vi, 215; (see also Power Plants) Hydroelectric Power, called "white coal," v, 76; in Switzerland, xiv, 242 Hydrogen, viii, 18, 29-33; affinity strength, 128; affinity for halogens, 85, 86; atmospheric, i, 11, 192, ii, 232; atomic weight and symbol, viii, 383; atomic weights based on, 33, 92; basic element, Prout's hypothesis, 177; boiling and freezing points, iv, 173; compressibility and volume, 143; critical temperature and pressure, 173; density of, 110, 113; diffusibility of, viii, 108; discovery, xvi, 120; elimination from body, viii, 353; explosions of, 33, 62; flame of, 57, 58, 59-60; increasing demand for, vii, 321; in acids, viii, 114; in organic compounds, 64, 204; in plants, 336-7, 340-1; in proteins, 351; in water, weight and volume, 39-40; liquefaction of, iv, 171, 191; melting point, 162; molecular speed, viii, 24, iv, 133; percentage in coal series, iii, 345; plant uses and sources, xiv, 64-5; positive ionization, viii, 122; preparation, 30-3, 102; sound speed in, i, 192; uniqueness of, viii, 182-3; use of, in balloons, iv, 108; valence basis, viii, 93; valences to, 178, 179-80; weight, iv, 110 Hydrogen Compounds, viii, 29, 36, 41, 51-2, 68-70, 205-40 Hydrogen Cycle, viii, 334, 350 Hydrogen Peroxide, viii, 41, 97; bleaching by, 86, 256 Hydrogen Sulphide, viii, 77-8; in metal tests, 288; solubility in water, 111 Hydrogenation, viii, 232, 247, 377 Hydroids, xii, 18, 23, 33-7; regeneration in, 170 Hydrology, problems, vi, 365-7 Hydrolysis, viii, 39, 217-18, 377; by enzymes, 357, 358; in cement setting, 280; ionization and, 120 Hydrometers, iv, 113, vi, 147 Hydrophobia, Pasteur's work on, x, 142-3; reduction of mortality in, 217 Hydroplanes, v, 192; of submarines, 197-8 Hydrostatic Pressure, v, 95-6