Pi^eface 



THE TERM " luminescenz " was first used in 1888 by the great 

 German physicist and historian of science, Eilhardt Wiedemann, 

 for " all those phenomena of light which are not solely conditioned 

 by the rise in temperature." By the rise in temperature, Wiedemann 

 referred to the fact that all liquids and solids emit more and more 

 radiation of shorter and shorter wave-lengths as their temperature 

 is continuously raised above absolute zero. Finally wave-lengths 

 appear which can be perceived by the eye and the material becomes 

 red hot and then white hot. This condition is incandescence or " hot 

 light," in contrast to luminescence or " cold light." For many cen- 

 turies incandescence has been the universal method of practical 

 illumination; the sun, the torch, candle, oil lamp, gas or tungsten 

 filament have served both to heat and to light. 



Examples of luminescence are such dim lights as the glow of 

 phosphorus, a chemiluminescence; the phosphorescence of certain 

 solids (phosphors) after exposure to sunlight, or to X-rays or to 

 electron beams; the transitory fluorescence of many substances, 

 excited by exposure to various kinds of radiation, visible or invisi- 

 ble; the aurora borealis and the electroluminescence of gases when 

 carrying an electric current; the triboluminescence of crystals which 

 are rubbed or broken; or the bioluminescence of many living or- 

 ganisms—the firefly and glowworm, the " burning of the sea," always 

 due to living, mostly microscopic forms, the fungus light of decaying 

 tree trunks, and the bacterial light of dead flesh or fish. 



The purpose of this book is to trace the discovery and the ideas 

 regarding these weak lights without heat from the earliest times 

 until the end of the nineteenth century. As far as possible, the 

 attempt has been made to present the views of the authors in their 

 own words, like a source book of information on the subject. By 

 such quotation a better idea of contemporary beliefs can be obtained, 

 not only regarding luminescence, but also regarding fire, heat, and 

 light in general. These were perplexing problems, and evoked con- 

 tinual discussion among the learned during the entire development 

 of scientific thought. 



The book is divided into three parts. Part I presents a general 

 survey of the increasing knowledge of luminescences, divided into 



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