ng, freezing and, 96-99.

Membrane, diffusion through a, 272.

Mercury, cohesion of, 47-48*;
use of, in thermometer, 89*, 90-91;
an element, 299.

Mercury-vapor lamps, 167-168*, 172.

Metals, good conductors of heat, 118;
good conductors of electricity, 215;
as elements, 310;
plating of, 336-339*;
action of acids on, 351-353.

Microscope, 88;
working of, 155-157*.

Mirrors, concave, 154*, 155*, 157.

Mixtures, distinguished from compounds, 309-310.

Molecular attraction, 36 ff.

Molecules, pull of, on each other, 46-47;
explanation of, 88-89;
heat defined as the motion of, 90;
action of, in evaporation, 102-103*;
action of, in boiling water, 107;
action of, in conduction of heat, 117;
action of, in radiation of heat and light, 125;
action of, in magnetizing, 194*-195;
made up of atoms, 196, 310;
mingling of, 259 ff.;
action of, in formation of clouds, rain, and dew, 277.

Moon, cause of ring around, 131.

Morse telegraph code, 253.

Motion-picture machines, lenses of, 143, 148.

Motor, the electric, 255-257*.

Mountains, rainfall on, 286-287.

Musical instruments, pitch of, 185-187*, 188;
vibrating devices of, 188.

Nail plug, the, 241*, 379*.

Needle, magnetizing of, 192*, 193*-195.

Negative charges of electricity, 198-200.

Neutralization of acids and bases, 360-364.

Niagara Falls, electricity generated by, 210.

Nickel, an element, 299.

Nickel-plating copper, process of, 336-339*.

Night, reason for coolness at, 127-128.

Nitric-acid, etching copper with, 352*-353;
action of, on cloth, 354*.

Nitrogen, an element, 299;
a non-burning gas, 308;
used in electric lamps, 317.

Northern Lights, cause of, 193.

Ocean, why salt, 104-105.

Oil, reason for floating of, 26-27;
decreasing of friction by, 53-54;
softening due to, 290-292;
carbon and hydrogen in crude, 315;
why water will not put out burning, 317.

Oil heaters, action of, 319.

Orange, litmus test of, 359.

Osmosis, process called, 272-274.

Osmotic pressure, 272-273*.

Oxidation, 312-322.

Oxygen, an element, 293, 299;
an element of water, 295-296;
experiments in getting, from two solids, 305-308*;
function of, in burning, 308;
part taken by, in burning (oxidation), 312-313;
released in manufacture of plant food