252 History of Luminescence 



tors are rubbed, or by a more simple procedure. Whenever two 

 closely applied surfaces are separated, light may appear, called 

 " Trennungsleucht " in Germany, This light can be observed with 

 thoroughly dark-adapted eyes if mica sheets are split in two, or 

 collodion films are stripped from a glass surface. Another example 

 is the transient greenish luminescence which occurs at the point 

 where electrician's or surgeon's or " Scotch " tape is unrolled. With 

 some samples this luminescence may be so bright that it is visible 

 with only partially dark-adapted eyes. Finally, certain types of 

 " triboluminescence " are undoubtedly due to electric discharges 

 (see Chapter X) . 



The explanation of such luminescences appears to be this: when- 

 ever two surfaces are separated from each other the electrical capa- 

 city diminishes and the voltage rises until a discharge takes place, 

 exciting the surrounding gas to luminesce. That a discharge does 

 actually take place can be readily shown by stripping surgeon's tape 

 or Scotch tape in an atmosphere of low pressure neon gas.^ Then 

 the luminescence is reddish instead of greenish. Red luminescence 

 also occurs when two strips of mica are pulled apart or when col- 

 lodion is stripped from glass in a low-pressure neon atmosphere. 

 The light which appears when ultrasonic waves cause cavitation of 

 liquid appears to be due to electric discharges also. This remarkable 

 example of cold light, sonoluminescence, was first described by J. 

 Frenzel and H. Schultes (1934) . 



Electroluminescences are not only among the oldest natural light 

 phenomena to be observed by man, for example the aurora bore- 

 alis,'^ but in the laboratory controlled electroluminescences have 

 been intimately connected with the rise of modern physics. Since a 

 gas represents the simplest state of matter, study of the conditions 

 for light emission from a gas have supplied the basis for theories of 

 light emission in general, concepts based on present ideas regarding 

 the structure of matter (see Introduction) . 



Meteors 



The word " electroluminescence " is also used in a broad sense 

 to include light from a natural electrical discharge which does not 

 involve the sudden complete surges of electricity, such as occur in 

 lightning flashes or condenser sparks. The silent or glow discharge 

 describes a very diverse group of natural luminous phenomena, 



■See E. N. Harvey, The luminescence of adhesive tape. Science 89:406-461, 

 1939. 

 '' Perhaps the aurora borealis should be considered a radioluminescence. 



