The air particles impart their motion to other particles because of their elasticity. Any elastic substance may become the medium for the transmission of sound, but such a medium is necessary. The elasticity of a body may be measured by the resistance it opposes to compression. The less the compressibility, the greater the elasticity. (a.) That sound is not transmitted in a vacuum. A large glass globe, provided with a stop-cock, contains a small bell suspended by a thread. When the air is pumped from the globe and the globe shaken, no sound is heard, although the clapper of the bell is seen to strike against the bell. Readmitting the air, and again shaking the globe, the sound is plainly heard.