CHAPTER VIII 



PHOSPHORESCENCE 



Introduction 



ALTHOUGH the popular use of the word " phosphorescence " implies 

 . any kind of cold light, this term will be restricted here to the 

 lasting luminescence which results from exposure of a substance to 

 visible or ultraviolet radiation— what is more properly designated 

 photoluminescence. When the luminescence is of very short dura- 

 tion—a fluorescence— the history will be given in Chapter XI, since 

 fluorescence has been recognized as a light emission only in fairly 

 recent times. 



When light strikes a body, a number of well-known changes may 

 occur, the most apparent being a chemical change, the photochemi- 

 cal effect. A second effect may be a production of light which is not 

 a reflection, fluorescence, or phosphorescence. In addition there are 

 changes in electrical properties of substances, the photoconductive 

 effect, as when light changes the conductivity of selenium metal, 

 discovered by W. Smith in 1873, and the photoemissive or Hall- 

 wachs ^ effect, the fact that negative charges (electrons) leave a 

 metal when irradiated by light. The question as to whether phos- 

 phorescence may be the result of reversible photochemical or other 

 changes in the phosphorescent body will be considered in subse- 

 quent sections, but it is an important characteristic of phospho- 

 rescence, distinguishing it from chemiluminescence, that no perma- 

 nent chemical change need result from the exposure to light. 



Early Records 



The light from Chinese paintings and the " torches " of the bac- 

 chantes, described in Chapter I, are possible but questionable cases 

 of phosphorescence. The evidence seems to indicate that the " lumi- 

 nous jewels " of ancient and medieval writers excited the admira- 

 tion of observers by the light which they reflected or transmitted, 



^W. Hallwachs (Ann. der Phys. 33:301-312, 1888). H. Hertz had noticed the phe- 

 nomenon at spark gaps (Ann. der Phys. 31:383-421-448, 983-1000, 1887), as well as 

 E. Wiedemann and H. Ebert (Ann. der Phys. 33:241-264, 1888). The fact that light 

 can give rise to a current when it strikes sensitive material in a circuit is called the 

 photovoltaic effect (E. Becquerel, Com. Rend. Acad. Sci. 9: 145-149, 1839) . An E. M. F. 

 is actually produced, whereas the photoemissive and photo-conductive effects require 

 that an E. M. F. be applied in order that current may flow. 



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