420 Prof. E. Wiedemann on the Behaviour of Gases 



That there is really such an excess of internal motion in 

 gases rendered luminous by electricity is shown by their low 

 temperature. The change of motion of oscillation into mo- 

 tion of translation takes place with very great rapidity. I 

 have shown on p. 416 that, according to the laws of conduc- 

 tion of heat, in a tube 1 centim. wide, even after jqVo secon d, 

 a perceptible fraction of the original energy must exist in the 

 centre of the tube, yet nevertheless it is seen in the rotating 

 mirror to be discontinuously illuminated, just as a narrow 

 tube would be ; so that we must conclude that the trans- 

 ference of the internal motions produced by the current into 

 heat is much more rapid than the conduction of heat itself in 

 the gas. 



The vibrations produced by the electric discharge may 

 become so energetic that the molecules themselves fall to 

 pieces and are resolved into their constituent atoms, just as 

 we observe decomposition when we receive the chemically- 

 active rays on silver chloride, or an increase of affinity when 

 they fall upon chlorine. 



If the oscillatory motions produce a decomposition of the 

 molecules, the energy necessary for the decomposition is 

 brought to the molecules from the source of electricity, and 

 is again given up to the calorimeter when they reunite. 

 Whether the whole quantity of heat produced in this case 

 results from the latter process cannot be decided off hand. 



ferent names for the same phenomenon. Fluorescence and phosphores- 

 cence differ only in degree. Fluorescent light lasting really for only 

 an instant could never be observed. The time during which the lumino- 

 sity lasts is determined by the rapidity with which the internal motions 

 of the molecules equalize themselves, and consequently by the mutual in- 

 fluence of the molecules in their internal motions. "With solid bodies 

 this is certainly relatively small : the molecules vibrate about their posi- 

 tions of equilibrium ; and the elongations are so small that the actions 

 of neighbouring molecules on the different parts of the same molecule 

 are very little different. Only thus can we explain and understand the ap- 

 pearance of sharp absorption-bands in the case of solid bodies, since light 

 emitted by solid bodies can still interfere when there is a great difference 

 of path (compare Wied. Ann. v. p. 500, 1878). The same holds good for 

 fluids : here also, besides the motion of translation of the molecules, we 

 have vibrations of the whole molecules; for otherwise the law of change 

 of friction with temperature would be the same for gases and for 

 fluids, whereas it increases with the temperature in the former case and 

 decreases in the latter. "With liquids also the time during which a mole- 

 cule vibrates uninfluenced is therefore relatively large, and the transfer- 

 ence of energy follows relatively slowly. This will take place with gases 

 most easily and rapidly, where the mean path-length alone is of account ; 

 and therefore the name fluorescence is first applicable here. But here 

 also the fluorescent light will last at least as long as the mean interval 

 between two collisions. 



