38 THEORY OF HEAT. [CHAP. I. 



mass M is less than a, the temperature must assume also a tem 

 perature less than a. In fact, it receives again all the varieties of 

 rays which we distinguished in the first case : but there are two 

 kinds of them which contain less heat than in this first hypothesis, 

 that is to say, those which, being sent out by the body M, are 

 reflected by the mirror upon the thermometer, and those which 

 the same body M sends to it directly. Thus the thermometer floes 

 not receive all the heat which it requires to preserve its original 

 temperature a. It gives out more heat than it receives. It is 

 inevitable then that its temperature must fall to the point at 

 which the rays which it receives suffice to compensate those which 

 it loses. This last effect is what is called the reflection of cold, 

 and which, properly speaking, consists in the reflection of too 

 feeble heat. The mirror intercepts a certain quantity of heat, and 

 replaces it by a less quantity. 



51. If in the enclosure, maintained at a constant temperature 

 a, a body M be placed, whose temperature a is less than a, the 

 presence of this body will lower the thermometer exposed to its 

 rays, and we may remark that the rays sent to the thermometer 

 from the surface of the body M, are in general of two kinds, 

 namely, those which come from inside the mass M, and those 

 which, coming from different parts of the enclosure, meet the sur 

 face M and are reflected upon the thermometer. The latter rays 

 have the common temperature a, but those which belong to the 

 body M contain less heat, and these are the rays which cool the 

 thermometer. If now, by changing the state of the surface of the 

 body M, for example, by destroying the polish, we diminish the 

 power which it has of reflecting the incident rays, the thermo 

 meter will fall still lower, and will assume a temperature a&quot; less 

 than a. In fact all the conditions would be the same as in the 

 preceding case, if it were not that the body M gives out a greater 

 quantity of its own rays and reflects a less quantity of the rays- 

 which it receives from the enclosure; that is to say, these last rays, 

 which have the common temperature, are in part replaced by 

 cooler rays. Hence the thermometer no longer receives so much 

 heat as formerly. 



If, independently of the change in the surface of the body M, 

 we place a metal mirror adapted to reflect upon the thermometer 



