RADIANT HEAT AND ITS RELATIONS. 77 



planetary system, but the immeasurable heavens them- 

 selves. 



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 nothings 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 images of processes which, 

 though mighty in the aggregate beyond all conception, 

 are so minute individually as to elude all observation. 

 It is the waves of air excited by a tuning-fork which 

 render its vibrations audible. It is the waves of aether 

 sent forth from those lamps overhead which render them 

 luminous to us ; but so minute are these waves, that 

 it would take from 30,000 to 60,000 of them placed 

 end to end to cover a single inch. Their number, how- 

 ever, compensates for their minuteness. Trillions of them 

 have entered your eyes, and hit the retina at the backs 

 of your eyes, in the time consumed in the utterance 

 of the shortest sentence of this discourse. This is the 

 steadfast result of modern research ; but we never could 

 have reached it without previous discipline. We never 

 could have measured the waves of light, nor even 

 imagined them to exist, had we not previously exercised 

 ourselves among the waves of sound. Sound and light 

 are now mutually helpful, the conceptions of each being 

 expanded, strengthened, and defined by the conceptions 

 of the other. 



The aether which conveys the pulses of light and 

 heat not only fills celestial space, swathing suns, and 

 planets, and moons, but it also encircles the atoms of 

 which these bodies are composed. It is the motion of 

 these atoms, and not that of any sensible parts of 



