Absolute Magnitude (stars), ii, 317; Adams' method of determining, 124, 153; spectral type and, 115, 317; used in measuring star distances, 318, 330 Absolute Maximum & Minimum (meteorology), i, 204, 365 Absolute Scale (thermometry), i, 73, iv, 141, viii, 107-8 Absolute Units, iv, 64, 69, 70 Absolute Zero, i, 73, iv, 141, v, 347-8, viii, 107-8; molecular condition at, iv, 142-3, viii, 108; nearest approach, i, 32, iv, 173, xvi, 194; of outer space, vi, 270 Absorption Lines, ii, 111-12 (see Fraunhofer Lines, Spectrum) Abstract Ideas, difficulty of attention to, xi, 228, 233-4; expression of, in primitive language, xv, 144-150 Abul Wefa, Arab astronomer, ii, 38 Acceleration, definition & measurement, iv, 57, 381; force in relation to 59-61, 63-4, 71-2; of gravity, 65 Accidents, from electricity, x, 254; from fatigue, xi, 274; prevention of, vii, 32-3, xi, 365 Accidents (geological), xiv, 188 Accommodation, of vision, ix, 110-11, 113; muscles of, 161, 162 Accumulators, storage batteries, iv, 300 Accumulators, water-pressure, v, 106 Accuracy, habit and, xi, 253; indifferent types of men, 156, 157, 158-9 Acetic Acid, vi, 111, viii, 220; solubility, 112; in vinegar, 218, 249, 293 Acetylene Gas, discovery, xvi, 190; formation, vii, 312; in steel making, 321; luminosity of flame, viii, 60; preparation & uses, 231 Achenes, xiii, 58-9, 344, 345 Acheson, Dr. Edward G., vii, 300-1, 309-10, xvi, 189-90 Acheulean Implements, xv, 105, 107 Achromatic Lenses, iv, 373; invention, xvi, 125-6 Achromatic Refractors, ii, 100-1, 103 Acidosis, x, 280 Acids, viii, 19-20, 114-15, 373; action on saccharides, 226, 228; amino, 230 (see Amino Acids); bases and, 115; defined by ionization theory, 122; electrolytes, 125; formation, 20, 39, 118, 373; formation in body, x, 280-1; hydrogen prepared from, viii, 32-3, 102; ionization in solution, 119-25, 300-1; manufacture of, 275-6; molecular structure, 218; molecular structure & physical state, 298; nomenclature, viii, 98; organic, 52, 219-21; oxygen in, 34; salts formed from, 72, 83, 114, 373; vegetable, 222-3, 336, 349; volumetric analysis of, 292-3 Acid Salts, viii, 116 Acne, causes of, x, 201, 311 Aconite, xiii, 252 Acorns, xiii, 193; dispersal by squirrels, 55-6, 340; survival rate, xv, 21 Acoustic Clouds, i, 190 Acoustics, atmospheric, i, 186-96; of auditoriums, iv, 239 (see also Sound) Acquaintanceships, selection of, xi, 257, 380-1 Acquired Characters, inheritance of, ix, 325-7, x, 230 Acquired Tastes, xi, 72-3 Actinic Rays, iv, 365-6, 381, vii, 250, 361 Actinolite, iii, 321-2 Action & Reaction, iv, 33-4, v, 143; Newton's law, ii, 66, iv, 69