1869.] The Ethereal Eijpothesis of Light. 3 



Is it, as Professor Tyndall supposes, a specific "ether," which 

 serves as the vehicle for hght aud electricity ? Does it enter, as he 

 believes it does, into the constitution of material bodies ? or is it 

 excluded beyond the limits of our atmosphere ? Or again, is it, as 

 Mr. Grrove believes, an attenuated gas or mixture of gases, given 

 off from the atmospheres of the revolving worlds ? 



Let us first endeavour to understand the conceptions of these 

 opposite thinkers, and then to test their respective hypotheses by 

 the best means at our command. Professor Tyndall's conception 

 of a " luminiferous ether " is that it is " a substance almost infinitely 

 elastic," filling all space as with "jelly." * It fills up the interstices 

 between molecules of all kinds of matter, " suffering no rupture of 

 continuity at the surface of the eye, the intermolecular spaces of the 

 various humours being filled with it."t He believes it to form 

 the infinite ocean in which worlds move, and to be the medium for 

 the transmission of hght there, as well as in the intermolecular spaces 

 of material substances — in short he regards it as the medium for 

 the transmission of light — (and probably of electricity) everywhere. 



Mr. Grove objects to this idea of a specific ether, both for the 

 transmission of light and electricity. | His views concerning the 

 latter force we have given generally, § and his ground for refusing 

 to accept the doctrine in regard to light is, that " the more porous 

 bodies, or those most permeable by ether, should he the best con- 

 ductors," II and that " an objection immediately occurs in the opacity 

 of porous, and transparency of certain dense bodies.^F He believes 

 in the universahty of ordinary matter, however attenuated, and 

 considers his hypothesis " the least gratuitous." ** 



There are other writers, who, seeking to reconcile these opposite 

 views, suppose that the ether does not penetrate our atmosphere, 

 being " non-miscible " with it, and that therefore it does not permeate 

 terrestrial matter.tt 



This hypothesis may be at once dismissed, for if the supposed 

 ether is not miscible with our atmosphere, then the latter should 

 itself be the medium upon which light operates ; therefore the first 

 stroke of the piston of an air-j)ump should cause the receiver to 

 darken, and an object in an exhausted receiver should be invisible, 

 just as the sound of a bell striking therein is inaudible. In the 

 present state of the discussion and of our Imowledge, therefore, we 

 are left to consider the resj)ective merits of the two hypotheses, 



* ' Heat as a Mode of Motion,' p. 254. 



t ' On Radiation,' p. 9. Longmans. 



X ' Correlation and Continuity,' p. 133-4. 



§ Tliey -wilt be found detailed in the chapter on " Electricity " in his work on 

 the ' Correlation of the Physical Forces.' 



II ' Correlation,' p. 148. t Ibid., p. 168. ** Ibid., p. 186. 



tt Brooke's edition of Golding Bird's ' Natural Philosophy.' Sixth edition, 

 p. 576. Chmchill. 



B 2 



