9S REPORT— 1861. 



plain this experiment as easily as if a hot bulb had been placed in the one 

 focus, raising the temperature of the thermometer in the other. But this 

 explanation being inadmissible, it occurred to Prevost that the theory of 

 a moveable equilibrium would account for the phenomenon. Let us adopt 

 this hypothesis, and suppose, in the first instance, that a body of the same 

 temperature as the thermometer is placed in the other focus. It is obvious 

 that this body will not aflPect the thermometer. Heat is doubtless con- 

 tinually leaving the bulb ; but this receives back precisely as much heat as it 

 radiates, a considerable portion of that which it receives being the heat which 

 leaves the body in the opposite focus, and which by the laws of reflexion is 

 concentrated on the bulb. If we next suppose that the other body is of a 

 higher temperature than the thermometer, it is easy to see that the same laws 

 of reflexion will cause an increase of heat to be especially felt by the bulb, 

 since each of the rays of heat which reach it by virtue of the reflector will 

 be more intense than the corresponding ray which it gives away. Should, 

 however, the body in the opposite focus be of a lower temperature than the 

 thermometer, the rays which the former emits, and which, by virtue of the 

 reflector, reach the bulb, will all be less intense than the corresponding rays which 

 the bulb gives forth, and thus the same cause which formerly made the ther- 

 mometer peculiarly sensitive to an increase in the temperature of the opposite 

 body, will now make it equally sensitive to a diminution of the same. 



We are thus furnished by the theory of exchanges with an explanation of this 

 important experiment, which, it is remarked by Prevost in his first memoir of 

 1791» cannot well be explained by an immoveable equilibrium. 



When Leslie* published his experiments on Heat, the theory of exchanges 

 was not slow to exhibit that appropriating quality which is ever the mark 

 of truth. In the hands of Prevost, these experiments, instead of demand- 

 ing a new hypothesis, were easily explained by means of the old one. Let 

 us take, for instance, the fact discovered by Leslie, that good reflectors of 

 heat, such as metals, are bad radiators. Prevost (in a treatise on Radiant Heat, 

 Paris, 1809) shows how this fact follows from his theory, remarking that in 

 a place of uniform temperature a reflector does not alter the distribution of 

 heat, which it would do if, joined to a good reflecting power, it possessed also 

 that of being a good radiator. It is interesting to note Prevost's mode of 

 expressing himself on this subject, as it shows that he entertained an opinion 

 correct, as far as it went, with regard to internal radiation. He conjectures 

 that a good reflector is a bad radiator, because, as it reflects the heat from 

 without, so it also reflects the heat from within. 



Lambertf of Berlin, and Leslie, both proved by experiment that the radia- 

 tion of a heated surface in any direction is proportional to the sine of the 

 angle which this direction makes with the surface ; and it was demonstrated 

 by FourierJ that this law is the necessary consequence of the theory of ex- 

 changes, in those cases where the reflecting power of the body may be dis- 

 regarded. He shows, in this demonstration, that if we refuse to admit the 

 truth of the law of sines, and suppose that the intensity of the rays emitted 

 does not vary with the obliquity of the surface, a central molecule can 

 only acquire a temperature equal to half that of the surrounding enclosure- 

 Fourier accompanied this proof with an attempt to account for the law of 

 sines, in which he supposes that there is in every case a physical surface of 

 very small thickness, in which surface the radiant heat emitted by a body 

 takes its rise ; but, with the knowledge which we now possess, this cannot, I 

 think, be considered a very happy explanation. 



* Inquiry into the Nature and Propagation of Heat. 1804. f Pyrometrie. 



X Translated in the Philosophical Magazine for February 1833. 



