NAT t 'RE OF HEAT. 85 



NATURE OF HEAT. 



It is convenient to adopt the material theory of heat in con- 

 sidering its accumulation in bodies, and in expressing quantities 

 of heat and the relative capacities of bodies for heat. Indeed 

 everything relating to the absorption of heat suggests the idea 

 of its substantial existence ; for heat, unlike light, is never ex- 

 tinguished when it falls upon a body, but is either reflected and 

 may be farther traced, or is absorbed and accumulated in the 

 body, and may again be derived from it without loss. But the 

 mechanical phenomena of heat, which resemble those of light, 

 may be explained with equal if not greater advantage by assuming 

 an undulatory theory of heat, corresponding with the undulatory 

 theory of light. A peculiar imponderable medium or ether is 

 supposed to pervade all space, through which undulations are 

 propagated, that produce the impression of heat. A hot radiant 

 body is a body possessing the faculty to originate or excite such 

 undulations in the ether or medium of heat, which spread on 

 all sides around it, like the waves from a pebble thrown into still 

 water. Sound is propagated by waves in this manner, but the 

 medium in which they are generally produced, or the usual 

 vehicle of sound, is the air ; and all the experiments on the 

 reflection and concentration of heat, by concave reflectors, may 

 be imitated by means of sound. Thus if a watch be placed in the 

 focus of one of a pair of conjugate reflecting mirrors, the waves 

 of air occasioned by its beating emanate from the focus, strike 

 against the mirror, and are reflected from it, so as to break 

 upon the face of the opposite mirror, are concentrated into its 

 focus, and communicate the impression of sound to an ear 

 placed there to receive it. The transmission of heat from the 

 focus of one mirror to the focus of the other may easily be con- 

 ceived to be the propagation of similar undulations through 

 another and different medium from air, but coexisting in the 

 same space. 



In adopting the material theory of heat, we are under the ne- 

 cessity of assuming that there are different kinds of heat, some 

 of which are capable of passing through glass, such as the heat 

 of the sun, while others, such as that radiating from the hand, 

 are entirely intercepted by glass. But on the undulatory theory, 



