Shots, double report of, 229 et seq. Similarity, 249. Simony, 280. Simplicity, a varying element in description, 254. Sines, law of the, 193. Sinking of heavy bodies, 222. Sixth sense, 297. Smith, R., on acoustics, 34, 381, 383. Soap-films, Van der Mensbrugghe's experiment with, 11-12. Soapsuds, films and figures of, 7. Social potential, 15. Socrates, the father of scientific criticism, 1, 16. Sodium, 202. Sodium-light, vibrations of, as a measure of time, 205. Solidity, conception of, by the eye, 71 et seq.; spatial, photographs of, 73. Solids, and liquids, their difference merely one of degree, 2. Sonorous bodies, 24 et seq. Soret, J. P., 89. Sounds, symmetry of, 99 et seq.; generally, 22-47, 212. Sound-waves rendered visible, 315 et seq. Sources of the principle of energy, 179 et seq. Space, 205; sensation of, 210. Spark, electric, 117, 127, 132, 133, 190. Spatial vision, 386. Species, stability of, a theory, 216. Specific energies, 291. Specific heat, 166, 244. Specific inductive capacity, 117. Spectral analysis of sound, 27. Spectrum, mental associations of the, 190. Speech, the instinct of, cultivated by languages, 354. Spencer, 218, 222. Sphere, a soft rotating, 2; the figure of least surface, 12; electrical capacity of, 123 et seq. Spherical shell, law of attraction for 124, footnote. Spiders, the eyes of, 67. Spirits, as explanation of the world 186, 243. Spiritualism, modern, 187. Spooks, metaphysical, 222. Squinting, 72. Stability of our environment, 206. Stallo, 336. Stars, the fixed, 90. State, benefits and evils of its control of the schools, 372 et seq.; the Church and, 88. Statical electricity, 134. Stationary currents, 249. Statoliths, 303. Steam-engine, 160, 265. Steeple-jacks, 75. Stereoscope, Wheatstone and Brewster's, 73. Stevinus, on the inclined plane, 140; on hydrostatics, 141; on the equilibrium of systems, 142; discovers the principle of virtual velocities, 150; characterisation of his thought, 142; also 182, 187, 191. Stone Age, 46, 321. Störensen, 306. Stove, primitive, 263. Straight line, a, its symmetry, 98. Straight, meaning of the word, 240. Street, vista into a, 75. Striae, in glass, 313. Striate method, for detecting optical imperfections, 317. Striking distance, 115, 127. Strings, vibrations of, 249. Struggle for existence among ideas, 217. Substance, heat conceived as a, 177, 243 et seq.; electricity as a, 170; the source of our notion of, 199; rôle of the notion of, 203, 244 et seq.; energy conceived as a, 164, 185, 244 et seq. Substitution-value of heat, 178, footnote. Suetonius, 348. Sulphur, specific inductive capacity of, 117. Sun, human beings could not exist on, 3. Swift, 84, 280. Swimmer, Ampère's, 207.