RADIANT HEAT AND ITS RELATIONS. 215 



of light require a medium for their formation and propaga- 

 tion, but we cannot see, or feel, or taste, or smell this 

 medium. How, then, has its existence been established ? 

 By showing that by the assumption of this wonderful 

 intangible ether all the phenomena of optics are accounted 

 for with a fulness and clearness and conclusiveness which 

 leave no desire of the intellect unfulfilled. When the law 

 of gravitation first suggested itself to the mind of Newton, 

 what did he do ? He set himself to examine whether it 

 accounted for all the facts. He determined the courses of 

 the planets ; he calculated the rapidity of the moon's fall 

 toward the earth ; he considered the precession of the 

 equinoxes, the ebb and flow of the tides, and found all 

 explained by the law of gravitation. He therefore regarded 

 this law as established, and the verdict of science sub- 

 sequently confirmed his conclusion. On similar, and, if 

 possible, on stronger grounds, we found our belief in the 

 existence of the universal ether. It explains facts far 

 more various and complicated than those on which New- 

 ton based his law. If a single phenomenon could be 

 pointed out which the ether is proved incompetent to 

 explain, we should have to give it up ; but no such phe- 

 nomenon has ever been pointed out. It is, therefore, at 

 least as certain that space is filled with a medium by 

 means of which suns and stars diffuse their, radiant power, 

 as that it is traversed by that force which holds, not only 

 our planetary system, but the immeasurable heavens them- 

 selves, in its grasp. 



There is no more wonderful instance than this of the 

 production of a line of thought from the world of the senses 

 into the region of pure imagination. I mean by imagination 

 here, not that play of fancy which can give to " airy nothing 

 a local habitation and a name," but that power which 

 enables the mind to conceive realities which lie beyond the 

 range of the senses — to present to itself distinct physical 



