PHOSPHORESCENCE. 383 



phosphorus is exposed to air in the dark it emits a pale light 

 (Expts. 235, 236) ; this effect is due to oxidation simply, as may 

 easily be shown by placing the phosphorus in an atmosphere incap- 

 able of acting chemically upon it, when the luminosity disappears. 

 In the same kind of way decaying fish, certain kinds of rotten wood, 

 and many other substances shine in the dark in virtue of chemical 

 changes taking place between the oxygen of the air and the 

 putrefying or decaying matter. The well-known phosphorescence 

 of the sea on a summer's night, the emission of light by fireflies 

 and glow-worms, and the natural phenomena of "will of the wisp" 

 in marshy places and " corpse candles " in graveyards, are all due 

 to analogous causes.* Besides this chemical phosphorescence, or 

 production of visible light in consequence of chemical action, there 

 exists a physical phosphorescence, consisting of the power of 

 emitting light for a greater or lesser time after having been previ- 

 ously exposed to radiant energy. Some substances can exert this 

 action very powerfully, but only for a short time ; others when 

 once powerfully excited by exposure to a strong light will glow 

 for many hours in the dark. A highly phosphorescent substance 

 of this class is the so-called "Bologna phosphorus," made by 

 strongly heating in a crucible a mixture of powdered heavy spar 

 (natural barium sulphate) and gum tragacanth, or other carbonaceous 

 matter, made into a paste. Canton's phosphorus is similarly pre- 

 pared by ramming into a clay crucible a mixture of 3 parts of 

 finely-powdered calcined oyster shells and 1 of flowers of sulphur, 

 the crucible being well covered and heated to as high a temperature 

 as possible in the centre of a good fire in a kitchen grate, &c. Of 

 late years a somewhat similar preparation has come into use for a 

 variety of purposes under the name of " Balmain's luminous paint." 

 The phosphorescent matter is here mixed with oil, &c., and painted 

 on to the object required to be rendered luminous (a buoy in the 

 channel of a river, a clock face in a bedroom, &c., &c.); during 

 the daytime, the direct sunlight or diffused daylight falling on the 

 phosphorescent substance is, so to speak, absorbed by it, and re- 

 emitted as phosphorescent light subsequently. 



A pretty phosphorescent toy is sold, consisting of a glass slide, 



* Somewhat similar appearances are occasionally due to peculiar forms of 

 electric discharge whereby a glimmering light is produced instead of a 

 lightning flash ; currents of electrified air thus sometimes give rise to the 

 appearance of flickering luminous masses of mist, &c., not unlike some forms 

 of " Jack o' lantern." The formation of crystals from a supersaturated solu- 

 tion, and the sudden conversion of vitreous or glassy substances into 

 crystalline masses, sometimes gives rise to the evolution of light, probably 

 owing to temporary disturbances of electrical equilibrium during the action. 



