Longitudinal Waves. To see a animations of spherical longitudinal waves check out: In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation at right shows a one-dimensional longitudinal plane wave propagating down a tube.
The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
It is only an equilibrium if both liquid and vapour are present. The saturated vapour pressure of the liquid will force the mercury level down a bit. You can measure the drop-and this gives a value for the saturated vapour pressure of the liquid at this temperature. liquid boils when its saturated vapour pressure becomes equal to the external pressure on the liquid. When that happens, it enables bubbles of vapour to form throughout the liquid-those are the bubbles you see when a liquid boils.