342 



SUPPOSED RADIATPON AND REFLECTION OF COLD. 



The coH body 

 supposed to 

 receive heat 

 from the ther- 

 jjiometer. 



give out radiant 

 heat. 



This curious fact, notwithstanding we are so well ac^ 

 qiiainted with the laws that govern heat during its passage 

 through and impingency upon bodies, has never, I be- 

 lieve, been illustrated with sufficient clearness. 



The explanations that have hitherto been given rest prin- 

 cipally for support on the supposition, that the thermome- 

 ter placed in the focus of one mirror acts as a heated body, 

 and that the heat radiating from it is transmitted to the cold 

 But this cannot body in the opposite focus. The thermometer, however, 

 is in fact not a heated body, since it is not hotter than the 

 surrounding atmosphere, and consequently cannot radiate 

 caloric: but it is said, the surrounding air becomes cooled, 

 and consequently the thermometer in respect to it is a hot 

 body, and radiates caloric accordingly. This however does 

 not explain clearly why the thermometer should be reduced 

 to a temperature lower than the air which surrounds it, 

 which will be found to be the case; or at least, it leaves too 

 much to be supplied by the imagination. I trust I shall be 

 able to render this matter clearer. 



There are only two ways, in which heat can be made to 

 move in one direction through any given body, we will 

 suppose a wire A ^ ^ ~ B; one is the application of 

 a superior temperature to B, causing the heat to move on 

 towards A by the conducting power of the wire, and the 

 tendency of the caloric to establish an equilibrium; the 

 other is, to reduce the temperature at A, and thus cause a 

 partial vacuity of heat, which must af necessity be filled 

 up by a fresh quantity from toward x, which will receive 

 again a fresh supply from toward 3/, and that from towards 

 3f, &c., and by this means induce a current of heat from 

 B to A, till an equilibrium is established. It is upon this 

 principle, the filling up of partial vacuities of heat (if I 

 may be allowed the expression), that the rational explana- 

 tion of the phenomenon in question can be grounded. Fact 

 puts this sufficiently beyond a doubt, and it no,w remains 

 to show how it is eilecled. 

 How t\m is ^^ ^^^^ scarcely be necessary to mention in this place, 



tifectedi. that, when a particle of heat impinges upon a plane re- 



ficctiiig surliice, it is tlyiown off in a,n. angle equal to that 



with 



Another mode 

 of accounting 

 for i^. 



