36 THEORY OF HEAT. [CHAP. I. 



this consideration presupposes the mathematical theory of radiant 

 heat. 



If now between the thermometer and a part of the surface of 

 the enclosure a body M be placed whose temperature is a, the 

 thermometer will cease to receive rays from one part of the inner 

 surface, but the rays will be replaced by those which it will re 

 ceive from the interposed body M. An easy calculation proves 

 that the compensation is exact, so that the state of the thermo 

 meter will be unchanged. It is not the same if the temperature 

 of the body M is different from that of the enclosure. When 

 it is greater, the rays which the interposed body M sends to the 

 thermometer and which replace the intercepted rays convey more 

 heat than the latter; the temperature of the thermometer must 

 therefore rise. 



If, on the contrary, the intervening body has a temperature 

 less than a, that of the thermometer must fall; for the rays which 

 this body intercepts are replaced by those which it gives out, that 

 is to say, by rays cooler than those of the enclosure; thus the 

 thermometer does not receive all the heat necessary to maintain 

 its temperature a. 



49. Up to this point abstraction has been made of the power 

 which all surfaces have of reflecting part of the rays w r hich are 

 sent to them. If this property were disregarded we should have 

 only a very incomplete idea of the equilibrium of radiant heat. 



Suppose then that on the inner surface of the enclosure, main 

 tained at a constant temperature, there is a portion which enjoys, 

 in a certain degree, the power in question ; each point of the re 

 flecting surface will send into space two kinds of rays ; the one go 

 out from the very interior of the substance of which the enclosure is 

 formed, the others are merely reflected by the same surface against 

 which they had been sent. But at the same time that the surface 

 repels on the outside part of the incident rays, it retains in the 

 inside part of its own rays. In this respect an exact compensation 

 is established, that is to say, every one of its own rays which the 

 surface hinders from going out is replaced by a reflected ray of 

 equal intensity. 



The same result would happen, if the power of reflecting rays 

 affected in any degree whatever other parts of the enclosure, or the 



