RADIANT HEAT AND ITS RELATIONS 83 



by that force which holds in its grasp, not only our plan* 

 etary system, but the immeasurable heavens themselves. 



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 dis- 

 tinct images of processes which, though mighty in the ag- 

 gregate 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, 

 however, 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 imag- 

 ined them to exist, had we not previously exercised our- 

 selves among the waves of sound. Sound and light are 

 now mutually helpful, the conceptions of each being ex- 

 panded, strengthened, and defined 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 



