Mechanics of Luminosity. 153 



occur into two parts — the first, which corresponds to the 

 preTalent mean temperature as defined by the motion of 

 translation of the molecules ; and the second, which is de- 

 termined by the intermolecular motion. Consequently the 

 expression JtZQ/T must be divided into two parts correspond- 

 ing to these two processes. If the luminescent light is not 

 homogeneous, but if it consists of separate bright lines, or if 

 it yields a continuous spectrum, then for each ray of light 

 of definite wave-length emitted the temperature of lumi- 

 nescence and the corresponding quantity of energy must be 

 determined. Each member of (*^Q/T takes then the form 

 2o4Qo/T a where T a represents the temperatures defined by the 

 motions of translation or by the internal motions, and dQ a 

 represents the corresponding quantities of heat. We must 

 further remark that in the integral all the members whose 

 temperature of luminescence is very high become very small, 

 whilst according to the usual mode of treatment they have a 

 considerable value. 



6. That such phenomena of luminescence actually occur 

 may be directly shown in numerous cases, as in gases which 

 are made luminous by electric discharges without any corre- 

 sponding elevation of temperature ; further, in cases of 

 chemi-luminescence, and indeed in processes where one would 

 not have expected it. The experiments of W. von Siemens* 

 have shown that gases heated far above 1000° C. emit no 

 light ; and yet an alcohol flame is luminous. It produces 

 preeminently ultraviolet rays ; in the combination of the 

 constituents of the alcohol with the oxygen internal motions 

 arise, for which the corresponding temperature of lumi- 

 nescence is situated far above the temperature of the flame. 

 The case is similar with burning carbon disulphide, and 

 sulphur, and in many other cases ; so also with the luminosity 

 of phosphorus at low temperatures, the emission of light by 

 arsenious acid upon crystallization attended with rearrange- 

 ment of its molecules, &c, 



7. In these phenomena of luminescence the occurrence of 

 internal motions of other temperature than that shown by the 

 thermometer may be perceived immediately by the eye. 

 Hence the necessity of dividing the quantities of heat into 

 two parts is at once apparent. But analogous processes occur 

 in many other cases, as in most chemical changes, although 

 they may not be directly perceptible, e. g. when the lumines~ 



* Wied. Ann. xviii. p. 311 (1883). 



