Heat, a material substance, 177; difference between the conceptions of electricity and, 168 et seq.; substantial conception of, 243 et seq.; Carnot on, 156, 160 et seq.; Fourier on the conduction of, 249; not necessarily a motion, 167, 170, 171; mechanical equivalent of, 164, 167; of liquefaction, 178; quantity of, 166; latent, 167, 178, 244; specific, 166, 244; the conceptions of, 160-171; machine, 160; a measure of electrical energy, 133 et seq.; mechanical theory of, 133; where does it come from? 200. Heavy bodies, sinking of, 222. Heights of ascent, 143-151. Helm, 172. Helmholtz, applies the principle of energy to electricity, 184; his telestereoscope, 84; his theory of Corti's fibres, 19 et seq.; on harmony, 35, 99; on the conservation of energy, 165, 247; his method of thought, 247; also 138, 305, 307, 375, 383. Hensen, V., on the auditory function of the filaments of Crustacea, 29, 302. Herbart, 386 et seq. Herbartians, on motion, 158. Herculaneum, art in, 80. Heredity, in organic and inorganic matter, 216, footnote. Hering, on development, 222; on vision, 210. Hermann, E., on the economy of the industrial arts, 192. Hermann, L., 291. Herodotus, 26, 234, 347, 350. Hertz, his waves, 242; his use of the phrase "prophesy," 253. Herzen, 361, footnote. Hindu mathematicians, their beautiful problems, 30. Holtz's electric machine, 132. Horse, 63. Household, physics compared to a well-kept, 197. Housekeeping in science and civil life, 198. Hudson, the, 94. Human beings, puzzle-locks, 27. Human body, our knowledge of, 90. Human mind, must proceed economically, 186. Humanity, likened to a polyp-plant, 235. Huygens, his mechanical view of physics, 155; on the nature of light and heat, 155-156; his principle of the heights of ascent, 149; on the law of inertia and the motion of a compound pendulum, 147-149; on the impossible perpetual motion, 147-148; on work, 173; selections from his works for use in instruction, 368; his view of light, 227-228, 262. Huygens, optical method for detecting imperfections in optical glasses 313. Hydrogen balloon, 199. Hydrostatics, Stevinus's principle of, 141. Hypotheses, their rôle in explanation, 228 et seq. Ichthyornis, 257. Ichthyosaurus, 63. Idea? what is a theoretical, 241. Idealism, 209. Ideas, a product of organic nature, 217 et seq.; and facts, necessary to science, 231; not all of life, 233; their growth and importance, 233; a product of universal evolution, 235; the history of, 227 et seq.; in great minds, 228; the rich contents of, 197; their unsettled character in common life, their clarification in science, 1-2. Ideography, the Chinese, 192. Imagery, mental, 253. Imagination, facts reflected in, 220 et seq.