A screw eye (F), large enough to form a rest for the head of another machine bolt (G), is screwed into the back of the box about three fourths of an inch below the head of the suspended bolt (A). Two or three inches away, at a slightly higher level, another screw eye (H) is screwed into the back of the cigar box. This screw eye must have an opening large enough to permit an iron machine bolt (G) to pass through it easily. A nut (I) is screwed down on the threaded end of a machine bolt until about an inch of the bolt projects beyond the nut. This projecting part of the bolt is then passed through the screw eye (H) and another nut (J) screwed on to it to hold it in place. This nut must not be so tight as to prevent the free play of the bolt as its head rises and falls under the influence of the vertical bolt. The head of the horizontal bolt rests upon the screw eye which is immediately below the head of the suspended bolt. You therefore have the wrapped bolt hanging vertically from the top of the box, with its head just over the head of the horizontal bolt. There should be about one quarter inch of space between the heads of the two bolts. An electric current passing through the wires of the vertical bolt will therefore lift the head of the horizontal bolt, which will drop back on to the screw eye when the circuit is broken. [Illustration: Fig. 191. The cigar-box telegraph.] INDEX An asterisk (*) indicates use of one or more illustrations in connection with reference to which appended. Acetylene, carbon and hydrogen in, 315. Acids, 351 ff.; action of, on metals, 351-353*; action of, on cloth, 354*; action of, on nerves of taste, 354-355; distinguished from bases by litmus test, 358-359; neutralization of, by bases, 360-364. Action and reaction, law of, 77-81*. Adhesion, 39, 41-44; cohesion, capillary attraction, and, 47. Air, cooling of, on expanding, 95-96; liquid, 97; heat carried by, by convection, 118-119; absorption of light by, 169; sound produced by vibrations of, 174-181*; pitch due to rapidity of vibrations of, 186; water vapor in, 275-280*; a mixture and not a compound, 309; part taken by, in making automobile go, 344; limewater test for carbon dioxid in, 375. Air pressure, 10 ff., 14*; height water is forced up by, in vacuum, 19; high and low, 20, 282; winds caused by, 20-21. Air pump, 14*, 15. Alcohol, boiling of, 112; distilling, 113*-114. Alkali, 356; in soap, 357-358. Alloys, definition of, 310. Alternating current, defined, 211-212. Alum crystals, experiment with, 265-266*. Aluminum, an element, 299. Alum in water, testing with litmus paper, 359. Amber, electricity produced by rubbing with silk, 196. Ammonia, example of a common base, 356; action of, in cleaning cloth, 358; litmus test of, 359; neutralization of acid by, 363. Ampere, defined, 246. Analysis, chemical, 370-376.