PHYSICAL SCIENCE. 19 



remote regions of space. There is presumption, approaching 

 to certainty, that heat is associated with light in its origin, 

 as a concomitant, if not convertible force. More doubt 

 exists as to the transmission through space of the electric or 

 magnetic powers ; but numerous observations tend to justify 

 this belief, and such facts, as we shall see hereafter, are 

 every day multiplying around us. 



How then are these forces, or any of them, transmitted 

 to and fro in the universe ? If we say that the tides of the 

 ocean are raised, or the perturbations of a planet produced, 

 without any intervening medium between the bodies affected 

 and those affecting them, we quit the domain of physics 

 altogether, and put an abrupt end to enquiry. Newton has 

 expressed himself strongly on this matter, in saying, 'To 

 suppose that one body may act upon another at a distance, 

 through a vacuum, without the mediation of anything else, 

 by and through which their action and force may be conveyed 

 from one to another, is to me so great an absurdity that I 

 believe no man who has in philosophical matters a competent 

 faculty of thinking, can ever fall into it.' The conviction 

 which his conception of gravity thus impressed on Newton's 

 mind, is enforced upon us not less cogently by the undulatory 

 theory of light. This theory based on mathematical proof, 

 and capable not merely of explaining phenomena before 

 known, but of predicting others evolved by later research 

 presumes of necessity the existence of an elastic medium, 

 whatever its nature, through which these undulations are 

 transmitted. We say of necessity, because it is logically thus 

 to our reason. Not solely on the analogy of air and other 

 elastic media, but as the only conception we can form to the 

 mind of undulation singly considered, the presence of a 

 medium is essential to its existence and effects. And this 

 fully recognised, the inferences become of magnificent kind. 

 The progressive retardation of Encke's comet, the aspects 



c 2 



