ON LIGHT. 285 



bodily movement of each vibrating element will be 

 transverse to the direction of the propagated wave a con- 

 dition which, as we shall hereafter see, is essential to be 

 fulfilled in the luminous undulations. As this hypo- 

 thesis, however, has hitherto received no discussion, and 

 is here suggested only as one not unworthy of consider- 

 ation, however strange its postulates, we shall not dwell 

 on it ; remarking only that every phenomenon of light 

 points strongly to the conception of a solid rather than 

 a fluid constitution of the luminiferous ether, in this 

 sense, that none of its elementary molecules are to be sup- 

 posed capable of 'interchanging places , or of bodily transfer 

 to any measurable distance from their own special and 

 assigned localities in the universe. The constitution 

 above suggested would merely superadd to this abstract 

 idea of a solid structure, the further conception of polar 

 forces bearing some general analogy to those which may 

 possibly subsist among the gross particles of a tesseral 

 crystal 



(68.) This would go to realize (in however unexpected 

 a form), the ancient idea of a crystalline orb. And it 

 deserves notice that under no conception but that of a 

 solid can an elastic and expansible medium be self-con- 

 tained* If free to expand in all directions, it would 

 require a bounding envelope of sufficient strength to 

 resist its outward pressure. And to evade this by sup- 

 posing it infinite in extent, is to solve a difficulty by 

 words without ideas to take refuge from it in the 



* From a liquid the extreme particles would be constantly flying 

 off in vapour and dissipating themselves in space. 



