Machinery, oiling of, to decrease friction, 53-54. Magdeburg hemispheres, 15, 16*-17. Magnetism, 190 ff. Magneto, of automobile, 210, 211*; of old-fashioned telephone, 210-211. Magnets, 190-195*. Magnification, 150-157; by concave mirror, 157. Magnifying glass, convex lens in, 149; operation of, 150-156*. Manganese dioxid, an essential in explosives, 347. Megaphone, working of, 184. Melting, freezing and, 96-99. Membrane, diffusion through a, 272. Mercury, cohesion of, 47-48*; use of, in thermometer, 89*, 90-91; an element, 299. Mercury-vapor lamps, 167-168*, 172. Metals, good conductors of heat, 118; good conductors of electricity, 215; as elements, 310; plating of, 336-339*; action of acids on, 351-353. Microscope, 88; working of, 155-157*. Mirrors, concave, 154*, 155*, 157. Mixtures, distinguished from compounds, 309-310. Molecular attraction, 36 ff. Molecules, pull of, on each other, 46-47; explanation of, 88-89; heat defined as the motion of, 90; action of, in evaporation, 102-103*; action of, in boiling water, 107; action of, in conduction of heat, 117; action of, in radiation of heat and light, 125; action of, in magnetizing, 194*-195; made up of atoms, 196, 310; mingling of, 259 ff.; action of, in formation of clouds, rain, and dew, 277. Moon, cause of ring around, 131. Morse telegraph code, 253. Motion-picture machines, lenses of, 143, 148. Motor, the electric, 255-257*. Mountains, rainfall on, 286-287. Musical instruments, pitch of, 185-187*, 188; vibrating devices of, 188. Nail plug, the, 241*, 379*. Needle, magnetizing of, 192*, 193*-195. Negative charges of electricity, 198-200. Neutralization of acids and bases, 360-364. Niagara Falls, electricity generated by, 210. Nickel, an element, 299. Nickel-plating copper, process of, 336-339*. Night, reason for coolness at, 127-128. Nitric-acid, etching copper with, 352*-353; action of, on cloth, 354*. Nitrogen, an element, 299; a non-burning gas, 308; used in electric lamps, 317. Northern Lights, cause of, 193. Ocean, why salt, 104-105. Oil, reason for floating of, 26-27; decreasing of friction by, 53-54; softening due to, 290-292; carbon and hydrogen in crude, 315; why water will not put out burning, 317. Oil heaters, action of, 319. Orange, litmus test of, 359. Osmosis, process called, 272-274. Osmotic pressure, 272-273*. Oxidation, 312-322. Oxygen, an element, 293, 299; an element of water, 295-296; experiments in getting, from two solids, 305-308*; function of, in burning, 308; part taken by, in burning (oxidation), 312-313; released in manufacture of plant food, 333-335; a compound of, an essential in explosives, 347. Pancakes, made from sour milk, 362. Paper, carbon and hydrogen in, 315.