MATTER AND FORCE, IN PHYSICAL PHENOMENA. 45 



natural world, modified by the matter on which it 

 acts ? We pass here, however, into the region of meta- 

 physical subtleties, and the most daring hypothesis is" 

 lost in a wilderness of words. Yet it is a tempting 

 track ; and the most sober philosophers have exercised 

 themselves on the questions collateral to it. How is 

 this force of gravitation concentrated, and how pro- 

 pagated through space ; giving power to the sun to 

 contain the planets in their orbits, and bring back the 

 comet from its distant aphelion agitating and raising 

 the oceans of our own globe by an influence thus un- 

 seen and remote ? The law of proportion to distance 

 we know ; but this tells us nothing beyond the pre- 

 sence of the power, whatever it be, at each and every 

 point of intervening space. Newton strongly expresses 

 the impossibility of conceiving that one body can act 

 upon another without the interposition of something to 

 convey continuity of force. To satisfy this demand for 

 a material medium for other elemental forces, the aid 

 of ether has been invoked a mighty invocation, since 

 as regards gravitation and light at least, it must ex- 

 tend to all known celestial space. On this subject I 

 have written in another of these papers. It may well 

 be termed a transcendental branch of science, compris- 

 ing some of the most profound problems on which 

 human reason can exercise itself. 



I have spoken above of atomic or molecular attrac- 

 tions and repulsions, and I recur to the latter of these 

 terms, as indicating another of those questions which 

 perplex the theory of forces to* our understanding. 

 We speak of the attraction of gravitation as a force. 



