twelve times as many hydrogen atoms to
weigh a pound as compared with the number of atoms of carbon. This is
only another way of stating that carbon has twelve times the density of
hydrogen. If we compare lead and silver with hydrogen in the same way,
we find that the density is 206 times and 107 times greater than that of
hydrogen.

Thus, it may be seen, that all matter possesses density, and that that
density depends partly upon its atomic constitution. If the molecule of
matter is composed of atoms whose atomic weights are very large compared
with that of hydrogen, as iron, silver, lead and gold, then the
molecules will have a much greater density, than a molecule formed of
oxygen and hydrogen, _i. e._ water. This property of the density of
matter plays a most important part in the transmission of any kind of
wave-motion.

ART. 39. _Matter possesses Elasticity._--Matter possesses elasticity.
Elasticity is that property of matter which enables all bodies to resume
their original shape, when the pressure which has caused the alteration
of shape has been removed.

For example, suppose an ivory ball be dropped upon a marble table, or
any other hard surface. It will then rebound, and rise almost to the
same height from which it was dropped. If the surface upon which it fell
was first covered with blacklead, a circular spot of lead will be found
on the ivory ball. From this fact, we arrive at the conclusion that when
the ball came into contact with the table, at the moment of contact it
was flattened, and then owing to its elasticity it rebounded into the
air again.

Now the measure of the elasticity of a body is proportionate to the
velocity of the wave-motion which it can transmit. A good illustration
of the transmission of wave-motion may be shown with a number of ivory
bagatelle or billiard balls. If eight or more of these be put in a row,
all touching each other, and a single ball be placed about an inch or so
away from the others in a straight line with them, then when the single
ball is struck with a cue against the other eight, the motion of the
single ball is transmitted by each one of the eight successively with
such rapidity, that the end ball would be set in motion in a quicker
time than a single ball would take to reach the end ball, if it had been
free to move along without encountering any opposition.

It is a fact capable of demonstration, that the smaller the particle of
matter, the greater will be