f stars, cause of, 141.

"Up," meaning of word, 4.

Vacuum, defined, 11;
reason for, in electric lamp, 12*, 317;
use of, in manufacture of thermos bottles, 126-127*;
impossibility of producing sound in, 176-177.

Valves, safety, on boilers, 347.

Vaseline, production of, 114.

Vibrations, of air, 174-181*;
pitch due to rapidity of, 186.

Vinegar, litmus test of, 359;
neutralization of lye by, 363.

Violin, tuning of, 187.

Volcanoes, cause of, 110;
explosions and, 346*.

Volume, elasticity of, 86-87.

Washing soda, a common base, 356;
litmus test of, 359.

Water, seeks its own level, 6-10;
gurgling of, when poured from bottle, 13;
experiment with, to show centrifugal force, 73-74;
used for making thermometer, 90*-92;
expansion of, when frozen, 98;
evaporation of, 100-106;
action of, in geysers and volcanoes, 110;
absorption of light by, 169-170;
as conductor of electricity, 216;
use of, for generating electricity, 256-257;
softening due to, 290-292;
elements of, 294-297;
a compound and not a mixture, 308;
formed by burning fuel, 316;
why fire is put out by, but not burning oil, 317;
combining of carbon dioxid and, by plants, 332-333;
rusting of iron by, 349;
acids and bases turned to salt and, by combining, 361-362.

Wear, a result of friction, 53.

Weather, forecasting of, 282-285.

Weight, center of, 30-33*.

Wet battery, 204-205*.

White, a combination of all colors, 162.

Winds, cause of, 20-21.

Winter, reason for cold in, 127-128.

Wiring for arc lamps, 236-239.

Wood, poor conductor of heat, 118;
carbon and hydrogen in, 315.

Yardstick, experiment with, to show leverage, 59*-60.

Yeast, action of, in bread making, 365.

Yellow, in flames, 318.

Yerkes Observatory, telescope of, 156*.

Zinc, in electric battery, 203-206;
an element, 300;
used for driving hydrogen out of acid, 301, 304.

* * * * *

_CONSERVATION SERIES_

[Illustration]

Conservation Reader

_By_ HAROLD W. FAIRBANKS, _Ph. D._

_Lecturer, University of California; Geography Supervisor Berkeley
Public Schools_

A small book bringing out in a simple and interesting manner the
principles of conservation of natural resources has long been wanted,
or there has been little on the subject that could be p