Phos·phor·us n.; pl. Phosphori
 1. The morning star; Phosphor.
 2. Chem. A poisonous nonmetallic element of the nitrogen group, obtained as a white, or yellowish, translucent waxy substance, having a characteristic disagreeable smell; this waxy allotropic form is also called yellow phosphorus, to distinguish it from another allotropic form, red phosphorus.  It is very active chemically, must be preserved under water, and unites with oxygen even at ordinary temperatures, giving a faint glow, -- whence its name.  It always occurs combined, usually in phosphates, as in the mineral apatite, in bones, etc.  It is used in the composition on the tips of friction matches, and for many other purposes.  It is an essential component of the genetic material of all living organisms.  The molecule contains four atoms.  Symbol P. Atomic weight 31.0.
 3. Chem. Hence, any substance which shines in the dark like phosphorus, as certain phosphorescent bodies.
 Bologna phosphorus Chem., sulphide of barium, which shines in the dark after exposure to light; -- so called because this property was discovered by a resident of Bologna.  The term is sometimes applied to other compounds having similar properties.
 Metallic phosphorus Chem., an allotropic modification of phosphorus, obtained as a gray metallic crystalline substance, having very inert chemical properties.  It is obtained by heating ordinary phosphorus in a closed vessel at a high temperature.
 Phosphorus disease Med., a disease common among workers in phosphorus, giving rise to necrosis of the jawbone, and other symptoms.
 Red phosphorus, or Amorphous phosphorus Chem., an allotropic modification of phosphorus, obtained as a dark red powder by heating ordinary phosphorus in closed vessels.  It is not poisonous, is not phosphorescent, and is only moderately active chemically.  It is valuable as a chemical reagent, and is used in the composition of the friction surface on which safety matches are ignited.
 Solar phosphori Chem., phosphorescent substances which shine in the dark after exposure to the sunlight or other intense light.
 yellow phosphorus Chem., the waxy yellow allotropic form of elemental phosphorus.  See also phosphorus2.

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