If a large dinner bell is rung, its motion or vibration may be felt on touching it with the finger.  If a tuning fork is made to give forth sound by striking it against the knee, or hitting it with a rubber hammer, and is then touched to the surface of water, small sprays of water will be thrown out, showing that the prongs of the fork are in rapid motion. (A rubber hammer is made by putting a piece of glass tubing through a rubber cork.)

If a light cork ball on the end of a thread is brought in contact with a sounding fork, the ball does not remain at rest, but vibrates back and forth, being driven by the moving prongs.

[Illustration:  FIG. 165.—­The ball does not remain at rest]

These simple facts lead us to conclude that all sound is due to the motion of matter, and that a sounding body of any kind is in rapid motion.

251.  Sound is carried by Matter.  In most cases sound reaches the ear through the air; but air is not the only medium through which sound is carried.  A loud noise will startle fish, and cause them to dart away, so we conclude that the sound must have reached them through the water.  An Indian puts his ear to the ground in order to detect distant footsteps, because sounds too faint to be heard through the air are comparatively clear when transmitted through the earth.  A gentle tapping at one end of a long table can be distinctly heard at the opposite end if the ear is pressed against the table; if the ear is removed from the wood, the sound of tapping is much fainter, showing that wood transmits sound more readily than air.  We see therefore that sound can be transmitted to the ear by solids, liquids, or gases.

Matter of any kind can transmit sound to the ear.  The following experiments will show that matter is necessary for transmission.  Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber tube and pass the rod through one of two openings in a rubber cork.  Insert the cork in a strong flask containing a small quantity of water and shake the bell, noting the sound produced.  Then heat the flask, allowing the water to boil briskly, and after the boiling has continued for a few minutes remove the flame and instantly close up the second opening by inserting a glass stopper.  Now shake the flask and note that the sound is very much fainter than at first.  As the flask was warmed, air was rapidly expelled; so that when the flask was shaken the second time, less air was present to transmit the sound.  If the glass stopper is removed and the air is allowed to reenter the flask, the loudness of the sound immediately increases.

[Illustration:  FIG. 166.—­Sound is carried by the air.]

Since the sound of the bell grows fainter as air is removed, we infer that there would be no sound if all the air were removed from the flask; that is to say, sound cannot be transmitted through empty space or a vacuum.  If sound is to reach our ears, it must be through the agency of matter, such as wood, water, or air, etc.