78 M. E. Wiedemann's Investigations on 



forces exist to distances which are great in comparison with 

 the dimensions of the molecules themselves. At very minute 

 distances, however, the molecules must repel one another, 

 since otherwise there would be no reason for the recoil of the 

 particles after each collision. These repulsions probably pro- 

 ceed from the envelopes of aether surrounding the molecules 

 of the bodies, and must diminish more rapidly with increase 

 of distance than the attractive forces. The rotation and oscil- 

 lation of the individual atoms in the molecule about a common 

 centre of gravity j which take place in addition to the transla- 

 tory motions, lead, at a sufficiently elevated temperature, to 

 the division of the molecules into their atoms. These rotatory 

 and oscillatory motions are periodic, and must also call forth 

 periodic vibrations in the surrounding luminiferous aether. 



Line Spectra. — Let a rarefied gas be heated to tho highest 

 possible temperature ; we can then assume that the individual 

 molecules are decomposed into their atoms. On the collision 

 of these, oscillatory motions only will occur, since, according 

 to the experiments of Kundt and Warburg on the specific heat 

 of mercury vapour, as well as the theoretic reflections of Max- 

 well, Watson, and Boltzmann, in monatomic molecules the vis 

 viva of the motions of rotation is nil. The spectra which 

 make their appearance with these elevated temperatures con- 

 sist of separate bright lines, the cause of which we have 

 accordingly to seek in the oscillatory motion of the atoms, 

 since they also occur with the vapours of mercury and cad- 

 mium, which are regarded as monatomic. 



Let the gas be so much rarefied that the time which elapses 

 between two collisions is very great in comparison with that 

 during which the particles are within their reciprocal spheres 

 of action ; then, with the motions of the individual aether par- 

 ticles, at first with small amplitudes, a limited number of rays 

 will be emitted of a different vibration-period, whose wave- 

 lengths will depend on the special arrangement of the aether 

 among the atoms. If the elongations become greater, say by 

 our raising the temperature of the gas, and with it the vis viva 

 of each of the colliding atoms, then to these fundamental vi- 

 brations certain harmonic ones will be added, the vibration- 

 period of which, again, will depend on the arrangement of the 

 aether and the forces in action between it and the material 

 atoms. 



That many of the individual lines in the observed spectra 

 may be regarded as really harmonics of a fundamental vibra- 

 tion, Lecoq de Boisbaudran, Stoney, Soret, and others have 

 proved by careful calculations. Above all must the intensity 



