Why does white phosphorous exhibit chemiluminescence?
From here:


  The best-known and first well-documented example of deliberately
  induced, man-made chemiluminescence was the discovery of elemental
  phosphorus by a German alchemist H. Brand. The waxy white phosphorus
  is slowly oxidized at room temperature giving off faint greenish glow.
  The formation of excited reactive intermediate $\ce{(PO)2}$ and
  $\ce{HPO}$ is responsible for the green glow.


That's where the term phosphorescence come from.