In barren hills and wild forests, a substance will react with oxygen to form a "ghost fire" (1). What is the phenomenon that it will burn without being ignited? (2)"

It is white phosphorus. The decaying corpses of animals in the barren hills contain this substance. When it meets oxygen in the air, a chemical reaction will occur at a temperature of about 40 degrees (1). This phenomenon is called spontaneous combustion. (2) It's easy to see in summer, of course, because of the hot weather, it will burn. (3) It also exists during the day, but it is not obvious. The product of its combustion is a light blue flame, which can only be seen at night.