152 Prof. E. Wiedemann on the 



phenomena apparently belong to photo-luminescence, but 

 really to chemi-luminescence. 



The temperature of luminescence is defined to be the tem- 

 perature at which a body, heated without decomposition, 

 would give light of a particular wave-length in each case of 

 exactly the same brightness as it does in consequence of the 

 processes of luminescence. 



The production of light in the phenomena of luminescence 

 in gases cannot be explained by assuming that in consequence 

 of the different velocities of the individual molecules, as 

 assumed by the kinetic theory of gases, the individual mole- 

 cules possess a very high temperature, and therefore become 

 luminous. For at the temperature of ignition, defined by the 

 great velocity of the motion of translation, most substances 

 would be decomposed. Certainly all organic substances, 

 fluorescent or phosphorescent, in the gaseous condition. 

 The like holds good for solid bodies and for liquids, only that 

 here the limits within which the velocities of the molecules 

 are included are much narrower than with gases. 



The Temperature of Luminescence and the Second Law of 

 the Thermodynamic Theory. 



5. In a number of phenomena we have to take account of 

 the temperature of luminescence. 



In all mathematical developments which are based upon 

 the second law of the thermodynamic theory expressions 



such as Tp- occur, involving quantities Q of heat communi- 

 cated to or withdrawn from the body, divided by the absolute 

 temperature T at which this takes place. 



If luminescence occurs in consequence of this addition of 

 energy, then the temperature corresponding to certain inter- 

 molecular motions, to be defined later on (39), upon which the 

 luminescence depends, i. e. the temperature of luminescence, 

 is much higher than the temperature of the luminescent 

 body as measured by the thermometer*. 



We must therefore divide the changes of energy which 



* The assumption upon which deductions from the second law rest, 

 viz. that heat cannot be conveyed from a body of lower temperature to 

 one of higher temperature without the expenditure of work, must there- 

 fore be otherwise conceived in accordance with the above considerations 

 upon the temperature of luminescence, since when phenomena of lumines- 

 cence occur such a transference may very well take place, as I shall show 

 more fully further on. 



