584 Stoney — Cause of Douhle Lines in Spectra. 



the kind of undulation in the aether which is exhibited in Hertz's experiments. 

 The periodic time of this undulation is, as is known, 



T=2TTyiS.secp, 



E'S 

 where sin p = ■— y, 



^S* being the capacity of the molecule, / the co-efficient of self-induction in the 

 current, and R its resistance. It is doubtful whether S, I, and E can be such as 

 will bring the periodic time low enough to correspond to that of any of the 

 observed lines; and even if this be the case, the discharge would probably produce 

 only a single line in the spectrum, or a line and its harmonics. The presence of 

 double lines affords further evidence that the observed spectrum does not arise 

 from these Hertzian -discharges, since they require as their cause some event 

 affecting the lines which operates with a sameness in all the molecules which, we 

 may presume, is inconsistent with the chance conditions under which discharges 

 between molecules would take place. But the most conclusive evidence on this 

 point is furnished by the reversal of the lines of incandescent gases when sur- 

 rounded by their own vapour at a lower temperature. This phenomenon shows 

 that the undulation created in the aether by one set of molecules is capable of 

 effacing itself by transferring back the energy of its special oscillations to 

 another set of the molecules that are more quiescent. This seems incompatible 

 with the event being a Hertzian discharge between pairs of molecules, since this 

 is not a process which would be reversed under the conditions supposed, while it 

 does exactly agree with what would appear to be inevitable if the event is the 

 movement of an electron in that orbit which is its natural swing. 



To explain, therefore, the lines that present themselves in the spectra of 

 incandescent gases, it is probable that we must fall back upon the motions com- 

 municated by the encounters to those non-conducting parts of the molecule in 

 which are lodged the electrons, and upon periodic changes in the distribution of 

 electricity in the conducting part of the molecule consequent upon the movements 

 of these permanent charges. These will be synchronous, and will jointly excite 

 an electro-magnetic undulation in the aether with the periodic times that they 

 have in common. 



There seems but one other point in this connexion that needs elucidation. 

 It may be thought that with a multitude of molecules, each oscillating within 

 itself, the external effect will be nil — that every molecule in which the point 

 P moves one way, will be counteracted by some other molecule, in which the 

 point P moves the opposite way. But this is to overlook the fact that, in addition 



