[ 437 ] 



LVII. On Electro-ethereal Tlieory of the Velocity of Licfht in 

 Gases, L'ujuids, and Solids. By Lord Kelvin, O.ili., 

 G.iW.O,* 



THIS communication is an advance-proof of the last five 

 pages of Lecture XX., as written afresh for a long 

 promised volume of twenty lectures given originally in the 

 Johns Hopkins University of Balthnore, U.S.A., in October 

 1884, and now nearly ready for publication by the Cambridge 

 University Press. It is founded on two recent contributions 

 to electro-ethereal theory referred to as '^ Appendix D,'' and 

 *' Appendix A," previouslv published in the Philosophical 

 Magazine (1902, 1st half-year, and 1900, 2nd half-year) 

 under the titles "Aepiuus Atomized/^ and " On the Motion 

 produced in an Infinite Elastic Solid by the Motion through 

 the Space occupied by it of a Body acting on it only by 

 Attraction or Repulsion.-'^ The long title of Appendix A 

 contains virtually a complete statement of the theory which 

 constitutes its subject. 



In App. A it is temporarily assumed, for the sake of a 

 definite illustration, that the enormous variation of the ethereal 

 density within an atom is due to a purely Boscovichian force 

 actino- on the ether, in lines through the centre of the atom 

 and varying as a function of the distance. This makes no 

 provision for vibrator or vibrators within an atom ; and, for 

 the explanation of molecular vibrators, it only grants such 

 molecular groups of atoms, as we have had for fifty years in 

 the kinetic theory of gases, according to Clausius' impregnable 

 doctrine of specific heats wdth regard to the partition of 

 energy between translational and other than translational move- 

 ments of the molecules. Xow, in App. D, and in applications 

 of it suggested in §§ 162-168 of Lee. XIX., we have Pounda- 

 tiou for something towards a complete electro-ethereal theory, 

 of the Stokes-Kirchhofi' vibrators f in the dynamics of 

 spectrum-analysis, and of the Maxvvell-Sellmeier explanation 

 of dispersion. 



§ 233. In our new theory, every single electrion within a 

 mono-electrionic atom, and every group of two, three, or 

 more, electrions, within a poly-electrionic atom, is a vibrator 

 which, in a source of light, takes energy from its collision 

 with other atoms, and radiates out energy in waves travelling 

 through the surrounding ether. But at present we are not 



* Communicated by the Author, having been read before Section A 

 of the British Association, Southport. 

 t See Lee. IX. pp. 101, 102, 103. 



