264 SIMILARITY OF LIGHT, HEAT, ETO. [SECT. xxv. 



spring ; and, like a magician, he raises from the gloomy and 

 deep abyss of the mine the spirit of light to dispel the mid- 

 night darkness. 



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

 bably 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 un- 

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

 frequent than those of the extreme red of the solar spectrum ; 

 but the analogy is now perfect, since the interference of heat 

 is no longer a matter of doubt : hence the interference of two 

 hot rays must produce cold ; darkness results from the inter- 

 ference of two undulations 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 accom- 

 plish 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 colours of substances are 



