its vibratory motion. Thus the particles of
air are very, very small, and consequently air is found to be very
elastic, and allows sound to be transmitted through it with
comparatively great velocity, some sounds travelling at the rate of over
1000 feet per second.

A most important factor in determining the propagation of any
wave-motion, through a gas or solid, is the relationship of the
elasticity of the gas or solid to its density. Suffice to say, that the
velocity of any wave-motion is determined by the relation of the
elasticity to the density. For example, sound, which is a wave-motion of
the air, can not only be transmitted through gaseous bodies as air, but
also through liquids and solids. Sound travels faster through solids
than through liquids, and faster through liquids than through gases. In
liquids, the relation of the elasticity to density is greater than in
air, and in solids the relation is greater still. Therefore sound
travels much faster in liquids than in gases, and faster in solids than
in liquids.

This is the reason why a train can be heard coming if the ear is put to
the railway-line, when no indication of its approach is given to the ear
by the atmosphere. Some examples of the velocities of sound through
different substances are as follows--

Gases O. C. Liquids. Solids.
FEET FEET FEET

Air 1090 per sec. Water 4708 per sec. (8 deg. C.). Gold 5717 per sec.
Oxygen 1040 " " Alcohol 4218 " " (20 deg. C.). Silver 8553 " "

ART. 40. _Matter possesses Inertia._--Inertia is that property of
matter, by which matter cannot of itself alter, or change its state of
motion, or of rest.

Newton's first law of motion states that a body at rest remains at rest
until some force or motion acts upon it. If a stone be dropped from a
balloon, the stone does not fall because of any property which it
possesses, but because the force of gravity acts upon it. If it were
possible to eliminate this force of gravity, then if there were no other
force which could act upon the stone, it would remain suspended in
space.

The inertia of a body is equal to the mass of that body, or the amount
of matter in the body as measured by gravity, so that if a body is
halved, its inertia will be halved also, and if doubled, its inertia
will be doubled also. As the inertia of matter opposes all kinds of
motion, the