170 Reflections on Heat. 



We have thus taken one sure step in the investigation 

 of heat. We see that very warm bodies emit from 

 their surfaces rays which, passing (like rays of light) 

 through the air excite, at a distance, heat at the surfaces 

 of the surrounding objects on which they fall without 

 being reflected. 



The existence of the calorific rays, which we are now 

 discussing, being actually proved, and their manner of 

 acting being evidently as I have described it, it is im- 

 portant to ascertain whether the knowledge of these facts 

 be not sufficient to form a theory of heat which will ex- 

 plain all these phenomena. 



A theory which should have the advantage of ex- 

 plaining the communication of heat by a single method, 

 at once simple and easily understood, would be prefera- 

 ble, it seems to me, to one which, in order to explain 

 various phenomena, would be obliged to admit two 

 different modes of the communication of heat. 



In order to form a clear and exact idea of the rays in 

 question and of the effects which they are capable of 

 producing, we must go back to their mechanical cause, 

 and consider them with regard both to their existence 

 and to their operation. 



There are two ways of looking at the radiation from 

 an object ; the first, by conceiving the rays as emana- 

 tions of an actual substance thrown off from the surface 

 of the body ; the second, by considering these rays as 

 undulations which, starting from every point of the sur- 

 face of the radiating object, are propagated in all direc- 

 tions in straight lines in an elastic fluid which surrounds 

 it on every side. 



The system of Newton supposes that the rays of light 

 are real emanations. 



