THE PROPAGATION OP LIGHT. 31 



man with the conviction of his own insignificance, his phys- 

 ical weakness, and the ephemeral nature of his existence ; 

 he is, on the other hand, cheered and invigorated by the 

 consciousness of having been enabled, by the application and 

 development of intellect, to investigate very many important 

 points in reference to the laws of Nature and the sidereal 

 arrangement of the universe. 



Although not only the propagation of light, but also a 

 special form of its diminished intensity, the resisting medium 

 acting on the periods of revolution of Encke's comet, and the 

 evaporation of many of the large tails of comets, seem to 

 prove that the regions of space which separate cosmical bod- 

 ies are not void,* but filled with some kind of matter ; we 

 must not omit to draw attention to the fact that, among the 

 now current but indefinite expressions of " the air of Jieav- 

 en" " cosmical (non-luminous) matter" and " ether" the 

 latter, which has been transmitted to us from the earliest an- 

 tiquity of Southern and Western Asia, has not always ex- 

 pressed the same idea. Among the natural philosophers of 

 India, ether (aka'sa) was regarded as belonging to the pant- 

 scJiata, or five elements, and was supposed to be a fluid of 

 infinite subtlety, pervading the whole universe, and constitu- 

 ting the medium of exciting life as well as of propagating 

 sound.f Etymologically considered, aka'sa signifies, accord- 

 ing to Bopp, " luminous or shining, and bears, therefore, in 

 its fundamental signification, the same relation to the ' ether' 

 of the Greeks as shining does to burning." 



In the dogmas of the Ionic philosophy of Anaxagoras and 

 Empedocles, this ether (alOr^p) differed wholly from the act- 

 ual (denser) vapor-charged air (drjp) which surrounds the 



* Aristotle (Phys. Auseu.lt., iv., 6-10, p. 213-217, Bekker) proves, in 

 opposition to Leucippus and Democritus, that there is no unfilled space 

 no vacuum in the universe. 



t Akd'sa signifies, according to Wilson's Sanscrit Dictionary, " the 

 subtle and ethereal fluid supposed to fill and pervade the universe, and 

 to be the peculiar vehicle of life and sound." " The word dlcd'sa (lu- 

 minous, shining) is derived from the root ka's (to shine), to which is 

 added the preposition d. The quintuple of all the elements is called 

 pantsckatd, or pantschatra, and the dead are, singularly enough, desig- 

 nated as those who have been resolved into the five elements (prdpta 

 pantschatra'). Such is the interpretation given in the text of Amara- 

 koscha, Amarasinha's Dictionary." (Bopp.) Colebrooke's admirable 

 treatise on the Sankhya Philosophy treats of these five elements ; see 

 Transact, of the Asiat. Soc., vol. i., Loud., 1827, p. 31. Strabo refers, 

 according to Megasthenes (xv., $ 59, p. 713, Gas.), to the all-forming 

 fifth element of the Indians, without, however, naming it. 



