Propagation of sound in different media
Sound needs a medium: in 1660 Robert Boyle showed that the ticking of a clock under a glass bell fades away as the air is pumped out. Sound travels as a wave of compressions and rarefactions of the particles of the medium; in a vacuum there are no particles, so sound does not travel (silence in space). The speed of sound in air was measured in 1636 by the Frenchman Marin Mersenne: at 20 °C it is 343 m/s (≈ 1235 km/h), at 0 °C 331.5 m/s; as temperature rises, the speed rises. Because it is much smaller than the speed of light we see lightning first and hear thunder afterwards. In liquids the particles are denser, sound is faster: in water about 1440–1500 m/s (measured in 1826 by Colladon and Sturm on Lake Geneva). In solids it is fastest: in iron about 5850 m/s, in aluminium 6260 m/s; so by putting an ear to a rail one can hear a train before hearing it through air. Soft, porous materials (wool, foam) conduct sound poorly. A matchbox-and-string “telephone” shows that sound passes through a solid (the string).
Experiment: make a “telephone” from two paper cups or matchboxes joined by a string; pull the string taut and whisper. What happens if you slacken it? Safety: do not wrap string around the neck; check for sharp edges and make holes with a pencil, not a needle.