1881.] from the time of Newton. 137 



Physical Forces, concluded with regard to the atmospheres of the 

 sun and planets that there is no reason " why these atmospheres 

 should not be, with reference to each other, in a state of equilibrium. 

 Ether, which term we may apply to the highly attenuated matter 

 existing in the interplanetary spaces, being an expansion of some 

 or all of these atmospheres, or of the more volatile portions of 

 them, would thus furnish matter for the transmission of the 

 modes of motion which we call light, heat, etc.; and possibly 

 minute portions of the atmospheres may, by gradual accretions 

 and subtractions, pass from planet to planet, forming a link of 

 material communication between the distant monads of the uni- 

 verse." Subsequently, in his address as President of the British 

 Association for the Advancement of Science, in 1866, Grove 

 farther suggested that this diffused matter may become a source 

 of solar heat, "inasmuch as the sun may condense gaseous matter 

 as it travels in space, and so heat may be produced." 



Humboldt, also, in his Cosmos, considers the existence of a 

 resisting medium in space, and says "of this impeding etherial 

 and cosmical matter," it may be supposed that it is in motion, that 

 it gravitates, notwithstanding its great tenuity, that it is con- 

 densed in the vicinity of the great mass of the sun, and that it 

 may include exhalations from comets; in which connection he 

 quotes from the 42nd proposition of the third book of the Prin- 

 cipia. He farther speaks comprehensively of " the vaporous 

 matter of the incommensurable regions of space, whether, scattered 

 without definite limits, it exists as a cosmical ether, or is con- 

 densed in nebulous masses and becomes comprised among the 

 agglomerated bodies of the universe 1 ." Humboldt also cites in 

 this connection a suggestion made by Arago in the Annuaire du 

 Bureau des Longitudes for 1842, as to the possibility of deter- 

 mining, by a comparison of its refractive power with that of 

 terrestrial gases, the density of " the extremely rare matter occupy- 

 ing the regions of space 2 ." 



In 1854, Sir William Thomson published his Note on the 

 Possible Density of the Luminiferous Ether 3 , wherein he remarks 

 "that there must be a medium of material communication through- 

 out space to the remotest visible body, is a fundamental conception 

 of the undulatory theory of light. Whether or no this medium is 

 (as appears to me most probable) a continuation of our own atmos- 

 phere, its existence cannot be questioned." He then attempts to 

 fix an inferior limit to the density of the luminiferous medium in 

 interplanetary space by considering the mechanical value of sun- 



1 Cosmos. Otte's translation, Harper's Ed., Vol. I. pp. 82, 86. 



2 Ibid. Vol. m. p. 40. 



3 Trans. Eoij. Sue. Edinburgh, Vol. xxi. part 1; and Pltilos. Blag. 18oo, Vol. ix. 

 p. 36. 



