4. Experiment with an exhausted bell-jar. Place
an electric bell or a small alarm clock on the plate of an air-
pump, and cover it with a bell-jar. If the air is now pumped
out, the sound heard will be somewhat weaker, shewing that
the air took at least some part in the propagation of the
sound. Next repeat the experiment, but place a pad of soft
felt under the bell, or suspend it by india-rubber cords, and it
will be found that now the sound is scarcely audible, when
the air is pumped out.
From this it appears reasonable to conclude that the ether
cannot conduct sound, but that the air can. The sound
cannot be made to disappear entirely, for the bell must be
supported by something, and all material substances carry
sound, though with varying degrees of facility. When the
clock rested directly on the metal plate of the pump, the
sound was conveyed quite readily to the air outside, whilst
the pad of felt cut it off almost entirely.
