Introduction 5 



off, it is called phosphorescence,® a persistent luminescence result- 

 ing from the above methods of excitation. Materials which exhibit 

 phosphorescence are often called phosphors, the most famous one, 

 historically, being the calcined Bolognian stone, an impure barium 

 sulphide. 



Thermoluminescence, the emission of light on slight heating of 

 substances. It is characteristic of some samples of fluorspar and has 

 been found to be dependent on a previous storage of energy, usually 

 from light radiation; hence is a delayed phosphorescence. Thermo- 

 stimulation is a more correct term. 



Electroluminescence, a light accompanying an electric discharge, 

 whether this occurs in rarified gas in a vacuum tube, or in the air, 

 as when silk or fur is rubbed or tape is stripped from a roll. It is 

 due to electron bombardment of gas molecules. In more recent 

 terminology the word has been used for light emitted when elec- 

 trons strike solids or liquids. The aurora borealis may for con- 

 venience be placed in this category, even though some modern 

 theories regard it as a radioluminescence. 



Galvanoluminescence, a luminous phenomenon accompanying 

 the passage of electric currents through aqueous solutions, due to 

 chemical reactions of chemiluminescent substances produced during 

 electrolysis, frequently by the formation of " active oxygen." 



SoNOLUMiNESCENCE, light accompanying the passage of intense 

 sound waves through a liquid, owing to electrical discharges in the 

 residual gas of cavities formed by the sound waves in the liquid. 



Triboluminescence or piezoluminescence, the emission of light 

 on rubbing or crushing crystals of various kinds, organic or inor- 

 ganic. It may be an electron bombardment of crystal surfaces or an 

 electroluminescence from separation of surfaces. 



Crystalloluminescence, the light that sometimes appears during 

 crystallization of solutions, thought by some to be due to cleavage 

 during the growth of individual crystals, hence a type of tribolumi- 

 nescence. Lyoluminescence designates the light which appears 

 when crystals dissolve. 



Chemiluminescence, the light accompanying a chemical reaction, 

 whether in a gas or vapor (the various types of flames or pyro- 

 luminescences) ^ or in solution. The best known chemilumines- 



" The word " luminescence " is a general term; the word " phosphorescence " is often 

 used as a general term, but strictly applies to luminescence excited by radiation or 

 streams of particles. 



'E. L. Nichols and Snow (1892) referred to the excess of short wave-lengths in 

 certain heated incandescent oxides (e. g. thorium or magnesium) , as a luminescence. 



