364 Lord Kelvin [April 27, 



to this supposition ; among them one which has been most noticed, 

 though perhaps not really the most serious, that it seems incompatible 

 with the known phenomena of the aberration of light. Referring to it, 

 Fresnel, in his celebrated letter * to Arago, wrote as follows : 



" Mais il parait impossible d'expliquer l'aberration des etoiles 

 " dans cette hypotbese ; je n'ai pu jusqu'a present du moins concevoir 

 " nettcment ce phenoniene qu'en supposant que Tether passe librement 

 " au travers du globe, et que la vitesse communiquee a, ce fluide subtil 

 " n'est qu'une petite partie de celle de la terre ; n'en exceJe pas le 

 " centieme, par exemple. 



" Quelque extraordinaire que paraisse cette hypotbese au premier 

 "abord, elle n'est point en contradiction, ce me semble, avec l'idee 

 " que les plus grands physiciens se sont faite de l'extreme porosite des 

 " corps." 



The same hypothesis was given by Tbomas Young, in his cele- 

 brated statement that ether passes through among the molecules or 

 atoms of material bodies like wind blowing through a grove of 

 trees. It is clear that neither Fresnel nor Young had the idea that 

 the ether of their undulatory theory of light, with its transverse 

 vibrations, is essentially an elastic solid, that is to say, matter which 

 resists change of shape with permanent or sub-permanent force. 

 If they had grasped this idea, they must have noticed the enormous 

 difficulty presented by the laceration which the ether must experience 

 if it moves through pores or interstices among the atoms of matter. 



§ 3. It has occurred to me that, without contravening anything we 

 know from observation of nature, we may simply deny the scholastic 

 axiom that two portions of matter cannot jointly occupy the same 

 space, and may assert, as an admissible hypothesis, that ether does 

 occupy the same space as ponderable matter, and that ether is not 

 displaced by ponderable bodies moving through space occupied by 

 ether. But how then could matter act on ether, and ether act on 

 matter, to produce the known phenomena of light (or radiant heat), 

 generated by the action of ponderable bodies on ether, and acting on 

 ponderable bodies to produce its visual, chemical, phosphorescent, 

 thermal, and photographic effects? There is no difficulty in answer- 

 ing this question if, as it probably is, ether is a compressible and 

 dilatable f solid. "We have only to supposo that the atom exerts force 

 on the ether, by which condensation or rarefaction is produced 

 within the space occupied by the atom. At present J I confine myself, 



* ' Annales de Chimie,' 1818 ; quoted in full by Larmor in his recent book, 

 '.Ether and Matter,' pp. 320-322. 



t To deny this property ia to attribute to ether infinitely great resistance 

 against forces tending to condense it or to dilate it — which seems, in truth, an 

 infinitely difficult assumption. 



X Further developments of the suggested idea have been contributed to the 

 Eoyal Society of Edinburgh, and to the Congres International de Physique, 

 held in Paris in August. (Proc. E.S.E. July 1900 ; vol. of reports, in French, of 

 the Cong. Inter. ; and Phil. Maa?„ Aug., Sept.. 1900.') 



