Reflections on Heat. 171 



Sound, with which we are better acquainted than we 

 are with light, affords us an example of radiation or 

 undulation in an elastic fluid which most certainly is 

 not an emanation. 



We have sufficiently clear ideas of the mechanical 

 operations by means of which the undulations of the 

 air which constitute sound are excited and propagated ; 

 but We have no conception of any possible mechanical 

 operation by means of which a material substance could 

 be sent forth continually and in all directions from the 

 surface of a body. 



In physics, in order that an hypothesis may be ad- 

 mitted, it must be founded on the supposition of a con- 

 ceivable mechanical operation. 



In order that the theory of heat which is founded on 

 the vibratory hypothesis may be admitted, it is neces- 

 sary to show that the vibrations in question can exist, 

 and that they can cause the rays or undulations which 

 objects emit from their surfaces, and by means of which 

 we suppose that bodies of different temperatures influ- 

 ence each other even at a distance, bringing about 

 reciprocal and simultaneous changes of temperature, so 

 that little by little they arrive at a common and inter- 

 mediate temperature. 



If the particles which compose a body do not touch 

 each other (an opinion which is generally received, and 

 which appears very probable), as there is no doubt that 

 these particles are continually drawn one towards an- 

 other by the recognized force of universal gravitation, 

 it is impossible to conceive how, in an assemblage of 

 particles which form a tangible solid body, these parti- 

 cles can preserve their relative situations without being 

 in motion. 



