of gold. Water also is porous, but the spaces between the molecules are
very small.

COMPRESSIBILITY.--It follows from the foregoing statement, that if there
are little interstices between the molecules, the various bodies can be
compressed together. This can be done in varying degrees with all
solids, but liquids, generally, have little compressibility. Gases are
readily reduced in volume by compression.

ELASTICITY.--This is a property by virtue of which a body resumes its
original form when compressed. India rubber, ivory and glass are
examples of elasticity; whereas, lead and clay do not possess this
property. Air is the most elastic of all substances.

INERTIA.--This is a property of matter by virtue of which it cannot of
itself change its state of motion or of rest.

Newton's first law of motion is, in substance, that matter at rest will
eternally remain at rest, and matter in motion will forever continue in
motion, unless acted on by some external force.

A rider is carried over the head of a horse when the latter suddenly
stops. This illustrates the inertia of movement. A stone at rest will
always remain in that condition unless moved by some force. That shows
the inertia of rest.

MOMENTUM.--This is the term to designate the quantity of motion in a
body. This quantity varies and is dependent on the mass, together with
the velocity. A fly wheel is a good example. It continues to move after
the impelling force ceases; and a metal wheel has greater momentum than
a wooden wheel at the same speed, owing to its greater mass.

If, however, the wooden wheel is speeded up sufficiently it may have the
same momentum as the metal one.

WEIGHT.--All substances have what is called _weight_. This means that
everything is attracted toward the earth by the force of gravity.
Gravity, however, is different from weight. All substances attract each
other; not only in the direction of the center of the earth, but
laterally, as well.

Weight, therefore, has reference to the pull of an object toward the
earth; and gravity to that influence which all matter has for each other
independently of the direction.

CENTRIPETAL FORCE.--This attraction of the earth, which gives articles
the property of weight, is termed centripetal force--that is, the
drawing in of a body.

CENTRIFUGAL FORCE.--The direct opposite of centripetal, is centrifugal
force, which tends to throw outwardly. Dirt flying from a rapidly
moving wheel illustrat