SECT. II.] GENERAL NOTIONS. 39 



the rays which have left M, the temperature will assume a value 

 a&quot; less than a&quot;. The mirror, in fact, intercepts from the thermo 

 meter part of the rays of the enclosure which all have the tem 

 perature a, and replaces them by three kinds of rays ; namely, 

 1, those which come from the interior of the mirror itself, and 

 which have the common temperature ; 2, those which the different 

 parts of the enclosure send to the mirror with the same tempera 

 ture, and which are reflected to the focus ; 3, those which, coming 

 from the interior of the body J/, fall upon the mirror, and are 

 reflected upon the thermometer. The last rays have a tempera 

 ture less than a ; hence the thermometer no longer receives so 

 much heat as it received before the mirror was set up. 



Lastly, if we proceed to change also the state of the surface of 

 the mirror, and by giving it a more perfect polish, increase its 

 power of reflecting heat, the thermometer will fall still lower. In 

 fact, all the conditions exist which occurred in the preceding case. 

 Only, it happens that the mirror gives out a less quantity of its 

 own rays, and replaces them by those which it reflects. Now, 

 amongst these last rays, all those which proceed from the interior 

 of the mass M are less intense than if they had come from the 

 interior of the metal mirror ; hence the thermometer receives still 

 less heat than formerly : it will assume therefore a temperature 

 a&quot;&quot; less than a&quot; . 



By the same principles all the known facts of the radiation of 

 heat or of cold are easily explained. 



52. The effects of heat can by no means be compared with 

 those of an elastic fluid whose molecules are at rest. 



It would be useless to attempt to deduce from this hypothesis 

 the laws of propagation which we have explained in this work, 

 and which all experience has confirmed. The free state of heat is 

 the same as that of light ; the active state of this element is then 

 entirely different from that of gaseous substances. Heat acts in 

 the same manner in a vacuum, in elastic fluids, and in liquid or 

 solid masses, it is propagated only by way of radiation, but its 

 sensible effects differ according to the nature of bodies. 



53. Heat is the origin of all elasticity ; it is the repulsive 

 force which preserves the form of solid masses, and the volume of 



