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 ether 

 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 utter- 

 ance 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 denned by the conceptions 

 of the other. 



The ether 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 



