Ans. Place a bell jar on the platform of a vacuum pump. Suspend an electric bell in the bell jar with the help of two wires connected to a power supply. When the power supply is switched ON, the electric bell –will start ringing. We can hear the sound of the bell. Now pump out air from the jar by means of a vacuum pump. Again switch ON the power supply. The sound of the bell is not heard, although the bell is ringing. When we put the air back into the jar, we can hear the sound of the bell again. From this, we conclude that sound can only travel through a material medium.