2 Lord Kelvin on the 



was dealt with by Fresnel and Dr. Thomas Young ; it 

 involved the question, How could the earth move through 

 an elastic solid, such as essentially is the luminiferous ether ? 

 II. The second is the Maxwell-Boltzmnnn doctrine regarding 

 the partition of energy. 



§ 2. — Cloud I. — Relative Motion of Ether and Pon- 

 derable Bodies ; such as movable bodies at the eartlrs 

 surface, stones, metals, liquids, gases ; the atmosphere 

 surrounding the earth ; the earth itself as a whole ; meteo- 

 rites, the moon, the sun, and other celestial bodies. We 

 might imagine the question satisfactorily answered, by 

 supposing ether to have practically perfect elasticity for 

 the exceedingly rapid vibrations, with exceedingly small 

 extent of distortion, which constitute light ; while it behaves 

 almost like a fluid of very small viscosity, and yields with 

 exceedingly small resistance, practically no resistance, to 

 bodies moving through it as slowly as even the most rapid 

 of the heavenly bodies. There are, however, many very 

 serious objections 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 V aberration des 

 " etoiles dans cette hypothese ; je n'ai pu jusqu'a present 

 " du moins concevoir nettement ce phenomene qu'en sup- 

 " posant que Aether 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 excede pas 

 u le centieme, par exemple. 



" Quelque extraordinaire que paraisse cette hypothese au 

 " premier abord, elle n'est point en contradiction, ce me 

 " semble, avec l'idee que les plus grands physiciens se sont 

 u faite de l'extreme porosite des corps." 



The same hypothesis was given by Thomas Young, in his 

 celebrated 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 



* Annates de Chimie, 1818 ; quoted in full bv Larrnor in his recent 

 book, ' iEther and Matter,' pp. 320-322. 



