ood, light necessary to production of, 332-333.

Force, overcoming of extra motion by, in lever, 63-64*;
reason for, of steam, 110.

Forecasters, weather, 282-285.

Form, elasticity of, 86-87.

Freckles, cause of, 327.

Freezing and melting, 96-99.

Friction, 49-55*;
electricity produced by, 197-198*.

Frost, 97, 275;
explanation of, 287.

Fuel, chief elements in, 315-316.

Fulcrum of lever, 59-60*.

Fuse gap, the, 241*, 379*.

Fuses, short circuits and, 240-245.

Gas, cooling of, on expanding, 94-95;
carbon and hydrogen in, 315;
used for filling electric lamps, 317-318;
will not burn until hot enough, 323-324;
an explosion the sudden release of a confined, 348.

Gases, diffusion of, 269-271;
as elements, 293-294.

Gas heaters, action of, 319, 321*, 322*.

Gasoline, evaporation of, 103;
boiling of, 112;
distilled from petroleum, 114;
elements of, 315;
action of, in making automobiles go, 344-345.

Geysers, cause of, 110.

Glass, a poor conductor of heat, 118;
used as insulator of electricity, 215.

Glowworms, reason for glowing of, 341-342.

Gold, an element, 293, 299;
plating of, 339.

Gravitation, defined, 3.

Gravity, 1;
pull of, opposed to pull of adhesion, 42-43.

Grease, friction diminished by, 53-54;
combined with lye to form soap, 357.

Great Salt Lake, reason for salt in, 104-105.

Greeks, early knowledge of electricity possessed by, 196.

Green color of water, reason for, 169-171*.

Grounded circuits, 225-229*.

Gun, shooting of, caused by explosion, 345-346.

Gunpowder, action of, in shooting of a gun, 345-346;
how made, 347.

Hail, explanation of, 286.

Heat, a result of friction, 53;
is the motion of molecules, 90;
not caused by expansion, 94-95;
cold is absence of, 95, 120;
required to evaporate liquids, 102-103;
conduction of, 116-118;
carried by air, by convection, 118-119;
radiation of, 122-128;
of incandescent lamp, 125-126;
brought to focus by convex lens, 149;
chemical change caused by, 323-325.

Heaters, hot-water, 120*;
electric, 230, 232;
gas, 319, 321*, 322*.

Heat waves, cause of, 141.

Hydrochloric acid, getting hydrogen from, 301-304;
testing for silver with, 373.

Hydrofluoric acid, 351.

Hydrogen, an element, 294, 299;
in water, 295-2