52 ON MATTER, FOKCE, AND MOTION IN SPACE. 



been reached, save such as are based on a specula- 

 tive foundation, and therefore inadmissible into pure 

 science. 



Another condition may more safely be assumed 

 regarding ether, viz., that it is everywhere equal in 

 quality and manner of distribution through space, ex- 

 cept when coming into connexion with matter of 

 grosser forms. This condition, though not always 

 regarded, is seemingly essential to the functions 

 which ether is invoked to sustain. A medium un- 

 equal in density or mobility of its parts, or fluctuating 

 in other of its properties, could hardly fulfil the con- 

 ditions essential to the theory of light. And the same 

 remark applies to the other offices for which we in- 

 voke this medium of ether, as the sole method of 

 expounding the actions and influences upon each other 

 of bodies widely remote in space, and that retardation 

 in the motion of certain comets which the keen eye of 

 modern astronomy has revealed to us. 



Justified in speculating thus far on the endowments 

 of ether, as the occupant of space, we can scarcely go 

 farther without forcing that barrier which, if passed at 

 all, can only be so by patient progress of research. 

 Mathematical analysis bridges over many chasms, and 

 tells us under what conditions a given elastic medium 

 may fulfil the functions we attribute to ether. But it 

 does not enable us to conceive the infinitesimal minute- 

 ness and mobility of the atoms concerned in these 

 actions, or to comprehend their elementary nature. It 

 tells us nothing of those interstitial spaces which must 

 co-exist with atoms to give them free motion and elas- 



