Mechanics of Luminosity. 155 



on the one hand, the coefficient of emission is greater the 

 greater the friction, and on the other hand the coefficient of 

 absorption equally increases with this; the coefficients of 

 absorption and emission must run together; and so for all 

 bodies in which this reciprocity exists KirchhofT's Law must 

 hold good. 



The applicability of Kirchhoff's Law to the phenomena of 

 luminosity thus assumes a uniform transmutability of lumi- 

 nous energy with that of translation, and vice versa, for only in 

 this case can the ratio between absorbed and emitted energy 

 be a function of the wave-length. But if the conditions are 

 such that intermolecular energies produced by the absorp- 

 tion of incident light &c, are not converted back into motions 

 of translation after a few impacts, then the store of luminous 

 energy will gradually increase, and a new emission will be 

 added to that dependent on the temperature — that is to say, 

 luminescence is produced. That Kirchhoff's Law no longer 

 holds good here, and cannot hold good, is clear, since the 

 structure of the molecule is such that the uniform transforma- 

 tion of luminous energy into that of motion of translation, 

 and of that of translation into luminous energy no longer 

 takes place. Indeed, it would seem as if Kirchhoff's Law 

 only held good for an ideal case, viz. only if no increase of 

 the luminous motions could be produced in the luminous 

 body by absorption. Hence Kirchhoff's Law can only hold 

 good for that part of the luminous motion which does not 

 consist in luminescence. Moreover, Kirchhoff's Law has not 

 been quantitatively proved for luminous gases; but we have 

 contented ourselves with verifying certain qualitative- quanti- 

 tative consequences of it. 



Dependence of Luminescence on the Mode of Excitation. 

 9. Luminescent light is in a high degree dependent in 

 colour and intensity upon the mode of production, so that in 

 investigating it it is necessary to consider both of these 

 qualities. In photo-luminescence, and so in fluorescence and 

 phosphorescence, the colour of the emitted light is dependent 

 upon that of the incident light. In electro- luminescence, 

 discharges of various strengths call forth different assemblages 

 of rays. The borders of the stratifications in discharge-tubes, 

 as is well known, are of different colours towards the positive 

 and negative poles. This occurs with hydrogen or air alone, 

 but, as I have observed, more distinctly if the discharge-tube 

 contains hydrogen and sodium vapour. Further, the glow- 

 light and the positive light are, cceteris paribus, differently 

 coloured. 



