White phosphorus can easily ignite in the air at moderate temperatures (34 °C; 93 °F) and has been used as an incendiary weapon. Due to its high reactivity, it is stored and transported underwater.[2] Elemental phosphorus, exposed to air, glows in the dark with a whitish light due to slow oxidation (chemiluminescence). This light emission was once an important method for igniting street lights prior to the invention of the match; the phrase "burning the midnight oil" originates from this practice.