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.

Aneroid barometer, 285*.

Arc, the electric, 233-240*.

Atoms, description of, 196;
electrons and, 197;
everything in the world made of, 310-311;
in molecules, 311.

Aurora Borealis, cause of, 193.

Automobile, reason for cranking, 210;
how made to go, 344-345.

Automobile races, overcoming of centrifugal force in,