Mechanics of Luminosity. 151 



this research is to show how the luminous energy may be 

 determined from the intensity. 



I shall speak of brightness when the intensity is measured 

 by photometric methods, that is by a physiological method. 



3. I take as the basis of my investigations the kinetic 

 theory of gases. According to this there exists at constant 

 temperature a perfectly definite relationship between the kinetic 

 energy of the motion of translation, corresponding to the 

 temperature, and that of the intermolecular motions, both as 

 a whole as well as for each kind thereof, and consequently 

 also between those of the motion of translation and the lumi- 

 nous motion ; otherwise no permanent condition would be 

 possible. This relationship may be regarded as the normal 

 one. If, for any reason, the normal relationship is disturbed 

 in any way, it will in time again become normal. 



If, for example, we exalt the luminous motion in a molecule 

 in comparison with the motion of translation, the first will be 

 transformed into the latter ; if the luminous motion becomes 

 lowered by radiation the loss will be partly replaced from the 

 store of energy of the motion of translation by means of the 

 impacts of the molecules. 



Luminescence and Temperature of Luminescence. 



4. In special cases, however, the normal relationship be- 

 tween the motion of translation corresponding to the tempera- 

 ture and the luminous motion does not exist. 



In a former paper I have ventured to employ the term 

 luminescence for all those phenomena of light which are more 

 intense than corresponds to the actual temperature. 



In all phenomena in which luminescence is manifested the 

 energy of the luminous motion is higher than that charac- 

 terized above as corresponding to the relationships determined 

 by the temperature alone. According to the mode of excita- 

 tion I distinguish Photo-, Electro-, Ohemi-, and Tribo-lumi- 

 nescence. In particular, photo-luminescence, including fluores- 

 cence and a number of cases of phosphorescence, is defined as 

 those phenomena in which the incident light excites vibrations 

 within the molecule of a body which produce directly an 

 emission of light. I do not include in this those cases in 

 which the incident light produces primarily chemical pro- 

 cesses, upon which a production of light is secondarily 

 dependent. This occurs for example with a large number of 

 phosphorescent substances, e. g. calcium sulphide. These 



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