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XVIII. The Internal Motions of Gases compared with the Motions 

 of Waves of Light. By G. Johnstone Stoney, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Dublin, July 14, 1868. 



THE following pages are a collateral inquiry in a memoir on 

 the Constitution of the Atmospheres of the Sun and Stars, 

 which was sent to the Royal Society in May of last year. They 

 are now separated, as it has appeared desirable to shorten that 

 memoir. Could you spare space for them in the Philosophical 

 Magazine ? Their aim is to make quantitative estimates in regard 

 to the internal motions of gases, which, though rough, may suf- 

 fice for a preliminary comparison of these motions with the mo- 

 tions of the waves of light. 



For the sake of perspicuity and to avoid wearisome periphrases, 

 free use is made of the hypothesis that the lines of motion within 

 gaseous molecules are or resemble orbits. All that we really know 

 about them is that they are periodic, and that their periods are 

 the same as those of the waves of light to which they give rise. 

 But there is the same convenience in thus speaking in the lan- 

 guage of a definite hypothesis, which has been provisionally 

 adopted for the sake of this convenience, as there is when we 

 speak of waves of light, its transverse vibrations, and thelumini- 

 ferous sether, although we know from Lorenz's and Riemann's 

 investigations that light may be an electrical phenomenon, to 

 which the language of the wave- theory is very imperfectly appli- 

 cable. And it should be distinctly kept in view that the nume- 

 rical estimates at which we arrive, and which are our principal 

 aim, are independent of this hypothesis. 



The second part of the inquiry, in which the conditions which 

 limit the extent of an atmosphere are sought out, will be found 

 in the memoir spoken of above. 



I am, Gentlemen, 



Your obedient Servant, 



G. Johnstone Stone y. 



1. The only class of bodies about the molecular constitution 

 of which we have any satisfactory* information is gases. These 



* The dynamical theory of the molecular constitution of gases, which, if 

 I mistake not, may be ranked in point both of importance and probability 

 along with the wave-theory of light, does not appear to have yet met with 

 that general attention and acceptance which it seems to deserve. It may 

 not be out of place, therefore, to add to the numberless proofs which have 

 been drawn from its interpreting the phenomena of gases by many writers, 

 but especially by Clausius, the following negative proof, which demon- 



