RADIATION OF HEAT. 423 



consequently its temperature would continually fall, and this depression of 

 temperature would continue without any limit. Now this is not supported by 

 observation. It therefore follows, as a necessary consequence, that the power 

 of radiation in every body must be equal to its power of absorption. 



It has likewise appeared that the best reflectors are the worst radiators. 

 This effect might likewise be foreseen on the principle of the theory just ox- 

 plained. A good reflector is a body which reflects the principal part of the 

 rays of heat which strike upon it. Now the heat which is incident on a body 

 must be either reflected or absorbed, and whatever portion of it is not reflected 

 must be absorbed. If, therefore, a great part be reflected, a proportionally 

 small part remains to be absorbed ; consequently it follows, that in the same 

 proportion as a body is a good reflector it must be a bad absorber ; and, vice v>:rsa, 

 if it be a bad reflector, it must in proportion be a good absorber. But it neces- 

 sarily follows, if a body be a powerful absorber of heat, that it must also be a 

 powerful radiator of heat, for otherwise its temperature would rise infinitely by 

 the heat which it absorbs accumulating in it, and not being carried off by radi- 

 ation. A good reflector, therefore, will be a bad radiator, and vice versa. In 

 the experiments of Leslie with the concave reflector, our attention was only 

 directed to the radiation of the hot surface, and we considered only the ray* 

 which, proceeding from it, were collected on the bulb of a thermometer by the 

 concave reflector. It might appear to follow, from an extension of this experi- 

 ment, that bodies radiate cold as well as heat. Let one of the cubical vessels 

 used by Leslie in his experiment be filled with snow, and placed before a re- 

 flector. Immediately the focal ball of the differential thermometer placed in 

 the focus will exhibit a rapid depression of temperature. Are we, therefore, to 

 suppose in this case that rays of cold proceed from sides of the vessel, and are 

 collected on the ball of the thermometer ? On the contrary, it has appeared 

 from previous investigation, that no body is perfectly destitute of heat, and that 

 snow itself, as well as mixtures much colder than it, are capable of imparting 

 heat to other bodies, and therefore possess heat in them. The surface, there- 

 fore, of a vessel containing snow, in this case radiates heat, and these rays of 

 heat are collected on the bulb of the thermometer in the same manner as when 

 that vessel was filled with boiling water. The bulb of the thermometer, how- 

 ever, itself, like all other bodies, radiates heat, and this heat is reflected by the 

 concave reflector toward the surface of the vessel containing the snow. The 

 two bodies, therefore, are radiating heat toward each other ; but the bulb of the 

 thermometer having the higher temperature, radiates more heat than it re- 

 ceives, while the surface of the vessel containing the snow receives more heat 

 than it radiates. The thermometer, therefore, gradually falls in its tempera- 

 ture, while the vessel containing the snow gradually rises. 



In the experiment with the concave reflector already described, the hot 

 body placed in one focus, and the bulb of the thermometer placed in the other, 

 are both radiators and absorbers of heat ; the hot body radiates heat to the 

 bulb, and the bulb radiates heat to it. The hot body absorbs the heat which 

 is radiated by the bulb, and the bulb absorbs the heat radiated by the hot body. 

 But the hot body, radiating more heat than the bulb, necessarily absorbs less, 

 consequently the temperature of this body gradually falls, while that of the 

 bulb of the thermometer rises. Let us now suppose that instead of a hot body, 

 a globe of snow be placed in the focus of the reflector, the bulb of the thermom- 

 eter having a higher temperature, will radiate more heat than it receives from 

 the snow, and it will become a hot body relatively to the snow. Since, there- 

 fore, it radiates more heat than it absorbs, its temperature will fall until it be- 

 comes equal to that of the snow ; the interchange of heat being then equal, no 

 further alteration in temperature will take place. 



