230 SIMILARITY OF LIGHT, HEAT, ETC. SECT. XXV. 



deep abyss of the mine, the spirit of light to dispel the 

 midnight darkness. 



It has been observed that heat, like light and sound, 

 probably consists in the undulations of an elastic medium. 

 All the principal phenomena of heat may actually be 

 illustrated by a comparison with those of sound. The 

 excitation of heat and sound are not only similar but 

 often identical, as in friction and percussion ; they are 

 both communicated by contact and radiation ; and Dr. 

 Young observes, that the effect of radiant heat in raising 

 the temperature of a body upon which it falls, resembles 

 the sympathetic agitation of a string when the sound of 

 another string which is in unison with it is transmitted 

 through the air. Light, heat, sound, and the waves of 

 fluids, are all subject to the same laws of reflection, and 

 indeed their undulatory theories are perfectly similar. 

 If, therefore, we may judge from analogy, the undula- 

 tions of some of the heat-producing rays must be less 

 frequent than those of the extreme red of the solar spec- 

 trum ; but the analogy is now perfect, since the inter- 

 ference of heat is no longer a matter of doubt : hence 

 the interference of two hot rays must produce cold ; 

 darkness results from the interference of two undula- 

 tions of light ; silence ensues from the interference of 

 two undulations of sound ; and still water, or no tide, is 

 the consequence of the interference of two tides. The 

 propagation of sound, however, requires a much denser 

 medium than that either of light or heat ; its intensity 

 diminishes as the rarity of the air increases ; so that, at 

 a very small height above the surface of the earth, the 

 noise of the tempest ceases, and the thunder is heard 

 no more in those boundless regions where the heavenly 

 bodies accomplish their periods in eternal and sublime 

 silence. 



A consciousness of the fallacy of our senses is one of 

 the most important consequences of the study of nature. 

 This study teaches us that no object is seen by us in its 

 true place, owing to aberration ; that the colors of sub- 

 stances are solely the effects of the action of matter upon 

 light ; and that light itself, as well as heat and sound, are 

 not real beings, but mere modes of action communicated 

 to our perceptions by the nerves. The human frame 



