Heat, 9. absorbed in boiling, 31-32. and disease germs, 250. and food, 252. and friction, 47. and light, 142, 147. and oxidation, 45, 48, 49. and wave motion, 145-147. conduction, 25. convection, 24, 25. from burning hydrogen, 80. from electricity, 312-315, 339. needed to melt substances, 39. of fusion, 40. of vaporization, 32. produced by compression, 96. relation of water to weather, 29, 40. set free by freezing water, 40. sources of, 29-30. specific, 28-29. temperature, 27. unit of, 27, 28. Heating effect of electric current, 312-315. Heating of buildings: central heating plant, 19. fireplaces, 17-18. Heating, furnaces, 19. hot water, 19-22. Helix, 318. Horse power, 173, 351. Hot water heating, 19-22. Hues, primary, 135. Humidity, 38. proper percentage for health and comfort, 38, 39. Humus, 216, 217. Hydrocarbons, 149. Hydrochloric acid, composition, 227. in bleaching, 241. to make a salt, 227. to make carbon dioxide, 55. to make chlorine, 238. to make hydrogen, 80. Hydrogen, 65, 66. and chlorine, 239. and water, 79. chemical conduct, 126-127. flame, 80. in voltaic cell, 307. peroxide, 53, 252. preparation, 80. to liquefy, 97. Ice, lighter than water, 42. manufacture of, 98, 99. Ice cream freezers, 44. Illuminating gas, manufacture of, 150, 151. measurement of quantity consumed, 93, 94. test of pressure, 92, 93. Illumination, intensity of, 105, 106. Image, in mirror, 108, 111. Incandescent lighting, 107, 314. Incidence, angle of, 110. Inclined plane, 162-166. screw, 166. wedge, 166. Indigo, 218. Induced current, 346-347. Ink spots, removal of, 243. Insoluble substances, 71. Insulators, electric, 324. Intensity, of light, 105-107. of sound, 270-271. Interval, in musical scale, 283. Iron, forging, 41. filings, 329. galvanizing, 49. oxidation of, 48. Irrigation, 193-194. Isobaric lines, 88, 91. Isothermal lines, 89, 91. Johnstown flood, 214, 215. Kerosene, 149, 150. Kilowatt, 351. Lactic acid, 230. Leaves, 132, 262. Lens, 117-121. concave, 118. converging, 118. crystalline, of eye, 122. focal length, 118. material, 119. refractive power, 119. Lever, 158-162. examples, 160-162. fulcrum, 159, 160. Life, and carbon dioxide, 54. and nitrogen, 261. and oxygen, 49, 54. Lifting pumps, 189-192.