338 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Of the three processes, thermal, chemical, and electrical, at work in 

 the ordinary electric arc, to which is immediately due the light of the 

 arc ? Or is the light partly due to one process and partly due to another ? 

 And, if so, what is the character of the radiation due to high temperature 

 alone, what that due to chemical action, and what that immediately due 

 to the electric current ? 



Some general questions of this character suggested the following experi- 

 ments. But the one definite question which these experiments were 

 intended to answer may perhaps be more clearly stated in terms of the 

 nomenclature which E. Wiedemann * has employed in his suggestive work 

 on the Mechanics of I^uminosity. Using his nomenclature, all sources of 

 luminosity {Licld-entwichlung) are either normal or luminescent. The 

 luminosity of a body is normal when it is jiroduced by high temperature 

 alone, — i. e. the luminosity of a body is normal when it satisfies the con- 

 dition of Kirchhofif's Law. Bodies which become luminous through 

 other causes than that of high temperature are said to be luminescent. 

 The process of producing light without a corresponding high temperature 

 is called "luminescence." Wiedemann distinguishes six different kinds 

 of luminescence. One or two will serve to illustrate the process. Fluo- 

 rescence, in which luminosity is brought about by mere illumination, is 

 called i^hoto-luminescence. The light produced by breaking lump sugar, 

 or by rapidly unwinding bicycle tape, is an instance of frictional-, or triho- 

 luminescence. The radiation which comes from a slightly heated piece of 

 fluorspar illustrates tlier mo-luminescence. 



These and many other instances of luminescence are well known. f 

 But it may be fairly doubted whether •' normal luminosity " has ever been 

 experimentally realized. 



Dr. Paschen,t of Hannover, has recently shown, in the most beautiful 

 and satisfactory manner, that a gas so slightly heated as to preclude the 

 supposition of any chemical change going on within it will emit an invisi- 

 ble spectrum. This spectrum he has determined by means of the bolome- 

 ter, and has shown it to be a characteristic spectrum. Whatever the 

 inferences which may be drawn from Paschen's experiment, no one, so far 

 as we are aware, has shown that high temperature alone will produce a 



* For an excellent re'sume' of Wiedemann's ideas and terrainolbgjs see Winkel- 

 mann's article on Phosphorescenz in his Handbuch der Pliysik. 



t It is here not to be forgotten that even a most excellent nomenclature, such as 

 Wiedemann's, does not of itself add anything to our understanding of the actual 

 processes named. It is, however, a great aid to clear thinking and clear writing. 



X Paschen, Wied. Ann., Bd. L. pp. 409-443 (1893). 



