154 Prof. E. Wiedemann on the 



cence is restricted to rays of greater or smaller wave-length 

 than those which the eye can perceive. 



Besides these oscillatory internal motions, of other tem- 

 perature than the mean, which produce this luminescence, 

 yet others may occur, rotatory and others, which are not of a 

 nature to produce light-waves in the surrounding sether, as 

 may be the case in the conduction of electricity through 

 electrolytes. But of this I make here only a preliminary 

 mention. 



Luminescence and Kirchhoff's Law. 



8. The production of light may therefore occur in conse- 

 quence of a rise in temperature, as well as in consequence of a 

 rise in luminescence. But these two modes must always be 

 considered separately if we wish to obtain an insight into 

 the mechanics of luminosity. 



For luminosity resulting from a rise of temperature Kirch- 

 hofFs Law as to the relationship of emission and absorption 

 holds good. Upon this rests the well-known reversal of 

 the lines of the spectrum. The light produced by lumi- 

 nescence does not obey the same law, as is shown for example 

 by the behaviour of fluorescent substances, which emit light 

 of a refrangibility different from that of the incident light. 

 In trying whether Kirchhoff's Law holds good or not, we are 

 often able to distinguish the two phenomena. (See further 

 on under 30.) 



To discover the reasons why in glowing bodies Kirchhoff's 

 Law of the ratio between emission and absorption holds good, 

 and why this is not generally the case with luminescent bodies, 

 let us consider the following circumstances : — 



As we have said, there takes place in a gas a constant ex- 

 change between intermolecular energy and that due to 

 motions of translation in consequence of the impact of mole- 

 cules (the luminous energy forming a part of the inter- 

 molecular energy), so that a mean condition ensues. If any 

 molecule suffers an increase of intermolecular energy, e. g. 

 in consequence of the absorption of incident light, this is 

 given up again in the next impact or so, and if it has a 

 deficiency in intermolecular energy this is made good. 

 The emission-coefficient depends upon the ease with which 

 part of this intermolecular energy produced by the impacts, 

 corresponding to the luminous energy, is given up again 

 in the form of light-vibrations, that is upon the friction be- 

 tween the vibrating molecules of the bodj T and the surround- 

 ing sether. The absorption depends upon the same quantity, 

 and thus also upon the structure of the molecule. But since, 



