150 Prof. E. Wiedemann on the 



General. — Production of Light. 



1. According to the newer views of the constitution of 

 bodies, we assume motions of translation of the molecules 

 with their centre of gravity ; further, rotation and oscillation 

 round the centres of gravity considered as fixed, not only of 

 the material parts of the molecules but also of their sether- 

 envelopes. With gases the motions of translation of the 

 centres of gravity produce only feeble emission of light. Sir 

 G. Gr. Stokes* has sought in this the explanation of the feeble 

 continuous spectrum of sodium seen at the same time as the 

 line-spectrum. I have myself f endeavoured to show that 

 the motions of rotation cannot be the cause of the line- and 

 band-spectra. Consequently we must seek the cause of the 

 production of light in gases in the intermolecular move- 

 ments which occur within the molecule — either of their material 

 particles or of their sether-envelopes. We shall endeavour 

 to show later on that it can only be the vibrations of the 

 "material particles to which the emission of light is due. With 

 solid and liquid bodies the emission of light may be due to 

 the vibrations of the whole molecule about its position of 

 equilibrium as well as to the intermolecular motions of the 

 constituent atoms of the molecule. Upon the first depends 

 the uniform continuous spectrum of all solid bodies when 

 heated, and upon the last the differences between the light 

 emitted by different bodies. 



2. The following theoretical considerations rest upon the 

 conception of luminosity introduced by me, and the discrimina- 

 tion of totally different phenomena which have generally been 

 confused. 



I designate as light the whole complex of rays between the 

 infra-red and the extreme ultra-violet. The motions of the 

 molecules which produce the luminosity I will call, for the 

 sake of brevity, luminous motions, in contradistinction to the 

 vibrations of the emitted waves of light. 



I shall always designate as intensity of the light- vibrations 

 the energy measured in gramme-calories per second w r hich 

 the vibrations of the light- waves emitted by the molecules of 

 the body carry with them ; but as luminous energy the energy 

 of those motions of the molecules or their atoms which pro- 

 duce the radiated light. The first energy depends upon the 

 decrease of the second with the time. A chief problem of 



* See a Research of A. Schuster, Phil. Trans. Lond. 1879, p. 37. 

 t Wied. Ann. v. p. 507 (1878). 



