178 Reflections on Heat. 



first, that all bodies at all temperatures (cold bodies as 

 well as warm ones) emit continually from their surfaces 

 rays, or rather, as I believe, undulations^ similar to the 

 undulations which sonorous bodies send out into the 

 air in all directions, and that these rays or undulations 

 influence and change, little by little, the temperature of 

 all bodies upon which they fall without being reflected, 

 in case the bodies upon which they fall are either warmer 

 or colder than the body from the surface of which the 

 rays or undulations proceed ; secondly, that the inten- 

 sity of the rays from different bodies at the same tem- 

 perature is very different, and that it is less in bodies 

 which reflect the rays of light than in those which ab- 

 sorb them, less in the metals than in their oxides, less 

 in opaque and polished bodies than in those which are 

 imperfectly transparent and unpolished, (a surface of 

 brass, for instance, emits four times as large a quantity 

 of rays at a given temperature when it is covered with 

 a coating of oxide, and five times as large a quantity 

 when it is blackened by the flame of a candle, as when 

 the surface of the metal is clean and well polished) ; 

 thirdly, that the rays which bodies of the same tem- 

 perature send out to each other have no tendency to 

 bring about any change of temperature in these bodies ; 

 fourthly, that the rays which any body whatever, at a 

 given temperature, sends continually from its surface in 

 all directions, are calorific or frigorific with regard to 

 other bodies on which they fall, according as these latter 

 are less warm or warmer than the body from which the 

 rays come; so that the same rays are calorific as re- 

 gards all bodies less warm than the one from which 

 they proceed, and frigorific as regards all those which 

 are warmer than this body. 



