ON RADIA NT HEAT. 5? 



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, however, compensates for their minuteness. Tril- 

 lions 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 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 

 bodies are composed. It is the motion of these atoms, ami 

 not that of any sensible parts of bodies, that the ether con- 

 veys. This motion is the objective cause of what, in our 

 sensations, are light and heat. An atom, then, sending 

 its pulses through the ether, resembles a tuning-fork 

 sending its pulses through the air. Let us look for a 

 moment at this thrilling medium, and briefly consider its 

 relation to the bodies whose vibrations it conveys. Dif- 

 ferent bodies, when heated to the same temperature, pos- 

 sess very different powers of agitating the ether: some 

 are good radiators, others are bad radiators: which means 

 that some are so constituted as to communicate their 

 atomic motion freely to the ether, producing therein pow- 

 erful undulations; while the atoms of others are unable 

 thus to communicate their motions, but glide through 

 the medium without materially disturbing its repose. 



